U.S. patent application number 15/380011 was filed with the patent office on 2017-07-20 for aluminum alloy structural part, method for producing the same, and 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, Kazufumi SATO.
Application Number | 20170204503 15/380011 |
Document ID | / |
Family ID | 59313582 |
Filed Date | 2017-07-20 |
United States Patent
Application |
20170204503 |
Kind Code |
A1 |
ARUGA; Yasuhiro ; et
al. |
July 20, 2017 |
ALUMINUM ALLOY STRUCTURAL PART, METHOD FOR PRODUCING THE SAME, AND
ALUMINUM ALLOY SHEET
Abstract
Disclosed are a structural part obtained from a 6000 series
aluminum alloy sheet as a shaping raw material and having an
improved crash performance; and a method for producing the sheet.
For the sheet, a 6000 series aluminum alloy sheet is used which has
a specified composition and is produced in the usual way. Even when
this sheet is used, strain is given at a high level thereto by a
cold work, thereby heightening the average dislocation density of a
surface of the resultant structural part, which has been
artificially aged. This density is measured by X-ray diffraction.
Thus, the structural part is improved in strength and in crash
performance, which is estimated in a VDA bending test, when the
automobile collides.
Inventors: |
ARUGA; Yasuhiro; (Kobe-shi,
JP) ; MATSUMOTO; Katsushi; (Kobe-shi, JP) ;
SATO; Kazufumi; (Kobe-shi, 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: |
59313582 |
Appl. No.: |
15/380011 |
Filed: |
December 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21B 3/00 20130101; C22F
1/043 20130101; C22F 1/047 20130101; C22F 1/05 20130101; C22C 21/08
20130101; C22C 21/02 20130101; B21B 2003/001 20130101 |
International
Class: |
C22F 1/047 20060101
C22F001/047; B21B 3/00 20060101 B21B003/00; C22F 1/043 20060101
C22F001/043; C22C 21/08 20060101 C22C021/08; C22C 21/02 20060101
C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2016 |
JP |
2016-005510 |
Claims
1. An aluminum alloy structural part excellent in crash
performance, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the
percent symbols each representing % by mass, and Al and inevitable
impurities as the balance of the part; and the structural part
having an average dislocation density of 3.0.times.10.sup.14 to
8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray
diffraction of the surface.
2. The aluminum alloy structural part excellent in crash
performance according to claim 1, further comprising Cu: 0.05 to
1.0%, the percent symbol representing % by mass; and the amount of
solute Cu in a solution separated from the structural part by a
residue extracting method with hot phenol being from 0.05 to 1.0%
by mass of the solution.
3. The aluminum alloy structural part excellent in crash
performance according to claim 1, further comprising one or more of
the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to
0.15%, the percent symbols each representing % by mass.
4. The aluminum alloy structural part excellent in crash
performance according to claim 1, further comprising one or more of
the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to
0.1%, the percent symbols each representing % by mass.
5. A method for producing an aluminum alloy structural part
excellent in crash performance, comprising: applying homogenization
to an aluminum alloy ingot comprising Mg: 0.30 to 1.5%, and Si:
0.50 to 1.5%, the percent symbols each representing % by mass, and
Al and inevitable impurities as the balance of the ingot, and
subsequently rolling the ingot into a sheet; subjecting the sheet
further to solutionizing and quenching treatments, and subsequently
cold-working the treated sheet to be formed into a structural part
while giving a strain of 5 to 20% to the sheet; thereby adjusting
the artificially aged structural part to have a dislocation density
of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density
being measured by X-ray diffraction of the surface.
6. The method for producing an aluminum alloy structural part
excellent in crash performance according to claim 5, wherein the
aluminum alloy structural part further comprises Cu: 0.05 to 1.0%,
the percent symbol representing % by mass; and the amount of solute
Cu in a solution separated from the structural part by a residue
extracting method with hot phenol is from 0.05 to 1.0% of the
solution.
7. The method for producing an aluminum alloy structural part
excellent in crash performance according to claim 5, wherein the
aluminum alloy structural part further comprises one or more of the
following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to
0.15%, the percent symbols each representing % by mass.
8. The method for producing an aluminum alloy structural part
excellent in crash performance according to claim 5, wherein the
aluminum alloy structural part further comprises one or more of the
following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to
0.1%, the percent symbols each representing % by mass.
9. The method for producing an aluminum alloy structural part
excellent in crash performance according to claim 5, wherein the
strain is given to the sheet when the sheet is formed into the
structural part.
10. An aluminum alloy sheet excellent in crash performance, for a
structural part, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%,
the percent symbols each representing % by mass, and Al and
inevitable impurities as the balance of the sheet; and the
following sheet having, as a microstructure, an average dislocation
density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the
density being measured by X-ray diffraction of the surface: a
surface of the sheet which is obtained, for simulating use of the
structural part, by subjecting the sheet to solutionizing treatment
of keeping the sheet at 550.degree. C. for 30 seconds, water
quenching the sheet immediately down to room temperature at an
average cooing rate of 30.degree. C./s, subjecting the sheet,
immediately after the quenching, to a pre-aging treatment at
100.degree. C. for 5 hours, giving a strain of 10%, after the
treatment, to the sheet through a tensile tester and further aging
the sheet artificially at 210.degree. C. for 30 minutes.
11. The aluminum alloy sheet excellent in crash performance
according to claim 10, further comprising Cu: 0.05 to 1.0%, the
percent symbol representing % by mass; and the amount of solute Cu
in a solution separated from the aluminum alloy sheet by a residue
extracting method with hot phenol being from 0.05 to 1.0% by mass
of the solution.
12. The aluminum alloy sheet excellent in crash performance
according to claim 10, further comprising one or more of the
following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to
0.15%, the percent symbols each representing % by mass.
13. The aluminum alloy sheet excellent in crash performance
according to claim 10, comprising one or more of the following: Ag:
0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%, the percent
symbols each representing % by mass.
Description
BACKGROUND OF THE INVENTION
[0001] Field of the Invention
[0002] The present invention relates to a structural part which is
obtained from a 6000 series aluminum alloy sheet (rolled sheet) as
a shaping raw material and is excellent in crash performance (shock
absorption performance); a method for producing the member; and an
aluminum alloy sheet.
[0003] Description of Related Art
[0004] In recent years, social needs of making automotive bodies
lighter have been further increasing in light of concern for the
global environment and others. In order to respond to the needs,
instead of steel material for steel sheets or others that have been
hitherto used, aluminum alloy material has been used in, out of
automotive body parts, panels (outer panels such as a hood, doors
and a roof, and inner panels), reinforcing material such as a
bumper reinforcement (bumper R/F) and door beams, and other
parts.
[0005] In order to make automotive bodies lighter, it becomes
necessary to enlarge the application of aluminum alloy material to,
out of automotive members, also automotive structural parts
contributing particularly to the weight saving thereof, such as
side members or such members, frames, and pillars. However, it is
necessary to give these automotive structural parts a new crash
performance (crash resistance or crash characteristic), which
results in a further weight saving of raw sheets for the structural
parts, in a high shock absorption performance of the members when
the automotive body collides, and in the protection of vehicle
passengers.
[0006] About high strength reinforcing materials out of automotive
structural parts as described above, it has been already popular to
use, as raw material thereof, extruded shapes each produced by
hot-extruding an JIS or AA7000 series aluminum alloy. In the
meantime, about large-sized structural parts such as frames or
pillars, it is preferred to use, as raw material thereof, rolled
sheets each produced by an ordinary method, such as a method of
homogenizing an ingot, and then hot-rolling the workpiece, or
optionally cold-rolling the hot-rolled workpiece further. However,
the 7000 series aluminum alloy is not easily produced into any
rolled sheet because of high-level alloying thereof. Thus, the
rolled sheets have not been very much put into practical use.
[0007] For this reason, as alloys for rolled sheets that are
produced by an ordinary rolling method (in the usual way),
attention has been paid to JIS or AA6000 series aluminum alloys,
which are Al--Mg--Si aluminum alloys produced easily because these
alloys are lower-level alloying metals than the 7000 series
alloys.
[0008] Sheets of the 6000 aluminum alloys have been already used as
large-sized automotive body panels (outer panels such as hoods,
fenders, doors, a roof and a trunk lid; and inner panels). Thus, in
order for the alloy sheets to have both of press formability and BH
response (bake hardenability), which are required for these
large-sized automotive body panels, or to be improved in both the
properties, many suggestions have been made about metallurgically
remedial measures about, e.g., component composition or
microstructure.
[0009] For reinforcing materials as described above and others,
6000 series aluminum alloy extruded shapes have been hitherto
suggested and put into practical use. However, for automotive
structural parts, few examples of an aluminum alloy rolled sheet
have been suggested.
[0010] Only Patent Literature 1 (JP 2001-294965 A) and other
literatures suggest 6000 series aluminum alloy sheets in which
controls are made about the size and the aspect ratio of crystal
grains that are related to an aluminum alloy rolled sheet
microstructure, whereby after the sheets are artificially aged, the
sheets are improved to have a yield strength of 230 MPa or more and
are heightened in crash performance.
[0011] In the meantime, as is well known, about means for
controlling the composition or the microstructure of a 6000 series
aluminum alloy raw sheet in order to improve the formability or
strength properties of this sheet for a panel as described above,
many suggestions have been hitherto made about controls of the
grain diameter of crystal grains, controls of the texture, and
controls of clusters of atoms in the sheet.
[0012] These microstructure-controlling means also include various
means of controlling the amount of Mg, Si or Cu solid-solutionized
in the alloy sheet, and of controlling the dislocation density
thereof.
[0013] For example, Patent Literature 2 (JP 2008-174797 A) suggests
that in order to gain, for a panel, a 6000 series aluminum alloy
sheet which is excellent in stability at ordinary temperature and
is not be easily lowered in BH response and other
material-qualities by natural aging at room temperature, the solute
Si amount and the solute Mg amount therein are set into the range
of 0.55 to 0.80% by mass and that of 0.35 to 0.60% by mass,
respectively, and the ratio of the solute Si amount to the solute
Mg amount is set into the range of 1.1 to 2.
[0014] Patent Literature 3 (JP 2008-266684 A) also suggests, for a
panel as described above, a 6000 series aluminum alloy sheet that
is for being warm-formed and is excellent in BH response in which
the amount of solute Cu that is measured by a residue extracting
method is set into the range of 0.01 to 0.7% and further the
average crystal grain diameter is set into the range of 10 to 50
.mu.m.
[0015] Furthermore, Non-Patent Literature 1 (Journal of Japan
Institute of Metals and Materials, vol. 75, No. 5 (2011), pp.
283-290, "Experimental and Computationally Scientific Research on
Competitive Precipitation Observed in Al--Mg--Si alloy Having High
Dislocation Density and Ultrafine Grain Microstructure" Tetsuya
Masuda, Shoichi Hirosawa, Zenji Hotta, and Kenji Matsuda) suggests
that the following are forecasted in order to make a 6000 series
aluminum alloy sheet higher in strength: microstructural parameters
(dislocation density and crystal grain diameter) for combining
dislocation strengthening or crystal grain refinement strengthening
optimally with precipitation strengthening.
[0016] This literature states that: about samples each obtained by
subjecting a 6000 series aluminum alloy sheet to cold rolling, or
HPT working, which is a high-pressure torsion method, the
dislocation densities thereof have been inspected. As a result,
samples not subjected to the working have a dislocation density of
about 10.sup.11 m.sup.-2, and the samples cold-rolled at a rolling
ratio of 30% (corresponding strain: 0.36) have a dislocation
density of about 10.sup.14 m.sup.-2. Measurements of the
dislocation densities are made by an equal thickness interference
method in which 5 visual fields in a 100000-magnification TEM
photograph of each of the samples are used in an intersection
analysis manner.
[0017] According to this Non-Patent Literature 1, an inspection has
been made about conventional technique reports each stating that
when the microstructure of a 6000 series aluminum alloy sheet is
controlled for dislocation strengthening or crystal grain
refinement strengthening, the artificial age-hardenability of the
sheet is frequently restrained in a subsequent artificial aging of
the sheet, so it is difficult to attain consistency between the two
strengthening mechanisms.
[0018] Results of the test demonstrate that: as the artificial
aging period elapses, the non-worked alloy sheets and the
cold-rolled alloy sheets are increased in hardness; about the
non-worked alloy sheets, the value obtained by subtracting the
hardness thereof before the artificial aging treatment from the
peak hardness thereof is 75 HV while about the cold-rolled alloy
sheets, the value is 43 HV, which has become reversely smaller;
thus, the cold rolling makes the artificial age-hardenability low.
The literature states that as the aging period elapses, the HPT
alloy sheets are monotonously decreased in hardness not to exhibit
an aging hardening behavior.
[0019] Structural parts of automobiles and others in which the
present invention is to be used are required to have properties
peculiar to the use, for example, are required to be further
heightened in strength, and to be newly caused to have a shock
absorption performance, that is, crash resistance when the
automotive body collides.
[0020] For example, according to a matter that collision safety
standards of automobiles have been raised (or made severer) in
recent years, in Europe and others, structural parts of an
automobile, such as frames and pillars, have been required to
satisfy crash performance (crash resistance or shock absorption
performance) when the automobile collides, this property being
evaluated in a "VDA 238.100 plate bending test for metallic
materials (hereinafter referred to as a VDA bending test)", which
is standardized by Verband der Automobilindustrie e.V. (VDA).
[0021] Against such severe safety standards, structural parts of
any automobile that are obtained from 6000 series aluminum alloy
sheets as shaping raw materials, these sheets being produced by an
ordinary rolling method, are short in the following property when
the automobile collides: crash performance that has been obtained
by making the alloy sheets higher in strength. As means for causing
such automotive structural parts, which are obtained from 6000
series aluminum alloy sheets as shaping raw materials, to satisfy
the crash performance, an effective means has not yet been clear
even when the existence of the above-mentioned non-patent
literature is known. Thus, there remains a room for realizing such
a means.
SUMMARY OF THE INVENTION
[0022] In light of such a situation, an object of the present
invention is to provide a structural part which is obtained from a
6000 series aluminum alloy sheet as a shaping raw material and is
improved in crash performance; a method for producing the member;
and an aluminum alloy sheet.
[0023] For attaining this object, a subject matter of the aluminum
alloy structural part of the present invention excellent in crash
performance is a member, comprising Mg: 0.30 to 1.5%, and Si: 0.50
to 1.5%, the percent symbols each representing % by mass, and Al
and inevitable impurities as the balance of the member; and this
structural part having an average dislocation density of
3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density
being measured by X-ray diffraction of the surface.
[0024] For attaining this object, a subject matter of the method of
the invention for producing an aluminum alloy structural part
excellent in crash performance is a method including: applying
homogenization to an aluminum alloy ingot comprising Mg: 0.30 to
1.5%.COPYRGT., and Si: 0.50 to 1.5%, the percent symbols each
representing % by mass, and Al and inevitable impurities as the
balance of the ingot, and subsequently rolling the ingot into a
sheet; subjecting this sheet further to solutionizing and quenching
treatments, and subsequently cold-working the treated sheet to be
formed into a structural part while giving a strain of 5 to 20% to
the sheet; thereby adjusting the artificially aged structural part
to have a dislocation density of 3.0.times.10.sup.14 to
8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray
diffraction of the surface.
[0025] For attaining this object, a subject matter of the aluminum
alloy sheet of the invention excellent in crash performance is a
sheet for a structural part comprising Mg: 0.30 to 1.5%, and Si:
0.50 to 1.5%, the percent symbols each representing % by mass, and
Al and inevitable impurities as the balance of the sheet; and the
following sheet having, as a microstructure, an average dislocation
density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the
density being measured by X-ray diffraction of the surface: a
surface of a sheet which is obtained, for simulating use of the
structural part, by subjecting the sheet to solutionizing treatment
of keeping the sheet at 550.degree. C. for 30 seconds, water
quenching the sheet immediately down to room temperature at an
average cooing rate of 30.degree. C./s, subjecting the sheet,
immediately after the quenching, to a pre-aging treatment at
100.degree. C. for 5 hours, giving a strain of 10%, after the
treatment, to the sheet through a tensile tester and further aging
the sheet artificially at 210.degree. C. for 30 minutes.
[0026] In the present invention, a cold work, such as press forming
to be applied to a structural part of an automobile or some other,
is used to give (add) a large quantity of strain beforehand to a
raw sheet (rolled sheet) subjected to a tempering treatment, such
as solutionizing treatment, thereby making a surface of the formed
structural part higher in dislocation density than in the prior
art.
[0027] Such a structural part, the dislocation density of which has
been made high, is artificially aged to cause the finally obtained
structural part, which is to be used, to exhibit a high crash
performance of showing a high 0.2% yield strength of 250 MPa or
more and a bending angle of 90.degree. or more according to a VDA
bending test.
[0028] Detailed mechanism of the exhibition (reasons therefor) have
not yet been clear. However, it is presumed that: the structural
part surface is made higher in dislocation density level than any
raw sheet or structural part used for an ordinary panel, thereby
increasing remarkably an effect of preventing dislocation-movement
when the structural part is collapsed to be deformed, for example,
when the automobile undergoes a collision accident; thus, the
structural part is improved in balance between strength and crash
performance.
[0029] This effect is increased also by an increase in the Cu
amount solid-solutionized in the structural part, so that this
member is increased in solute strengthening level to be heightened
in strength and be further restrained from undergoing
dislocation-localization when collapsed to be deformed, thereby
being also improved in crash performance.
[0030] The present invention makes it possible to improve a 6000
series aluminum alloy raw rolled sheet, which has already been
standardized as a structural part, in crash performance without
changing the composition and the production process of the rolled
sheet largely and without lowering the formability and other
properties of the aluminum alloy raw rolled sheet.
[0031] Moreover, by giving (adding) a large quantity of strain
beforehand to the aluminum alloy raw rolled sheet when the sheet is
subjected to press forming or some other cold work to be made into
a structural part, the structural part can be heightened in
dislocation density in a production process of the structural part
without increasing the number of steps for the cold work.
[0032] Thus, the present invention makes it possible to apply a raw
rolled sheet to a structural part that is a security member
important for automobiles even when this sheet is a 6000 series
aluminum alloy raw rolled sheet used for an ordinary panel.
BRIEF DESCRIPTION OF THE DRAWING
[0033] FIG. 1 is a perspective view illustrating a manner of a VDA
bending test for evaluating the shock absorption performance of a
metallic test specimen.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0034] A structural part referred to in connection with the present
invention denotes a structural part having a relatively large
thickness of about 2 to 10 mm to function as a skeleton of, in
particular, a transporting machine such as an automobile or a
railroad vehicle, and is strictly distinguished from any
large-sized body panel having a relatively small thickness less
than 2 mm, such as an outer or inner panel.
[0035] The dislocation density of a surface of the structural part,
which is specified in the present invention, is decreased by an
artificial aging treatment of the structural part, such as
paint-baking treatment thereof.
[0036] Accordingly, in order to ensure the crash performance of the
structural part when the member is used, the dislocation density
stipulated in the present invention is specified about the
structural part after the member is artificially aged.
[0037] An aluminum alloy raw sheet referred to in connection with
the present invention denotes an aluminum alloy raw sheet that is a
hot-rolled sheet, a cold-rolled sheet or any other rolled sheet
subjected to a tempering (T4) treatment, such as solutionizing
treatment or quenching treatment, and that is a sheet which has not
yet been formed into an automotive structural part to be used. In
the following description, aluminum may be represented also as Al.
Hereinafter, embodiments of the present invention will be
specifically described in accordance with each requirement of the
invention.
Aluminum Alloy Composition:
[0038] Initially, a description is made about not only the chemical
component composition of an aluminum alloy sheet in the present
invention, but also reasons why elements to be used therein and the
respective contents by percentage of the elements are limited. The
percent symbol(s) for showing the content of each of the elements
(each) represent(s) % by mass.
[0039] The chemical component composition of the aluminum alloy
sheet in the present invention functions as a presupposition for a
purpose that a structural part obtained finally by aging a 6000
series aluminum alloy artificially, or a raw sheet to which strain
or heat treatment is given or applied in such a manner that this
raw sheet simulates the structural part described just above can
satisfy a specified dislocation density and further gain a required
strength and crash performance, and can preferably have, together
therewith, formability into structural parts.
[0040] From these viewpoints, the chemical component composition of
the aluminum alloy sheet in the present invention is rendered a
composition including Mg: 0.30 to 1.5% and Si: 0.50 to 1.5%, and Al
and inevitable impurities as the balance of the composition.
[0041] In order to improve the sheet in strength, this composition
may further optionally include Cu in a proportion of 0.05 to 1.0%,
or solute Cu in a solution separated from the alloy sheet by a
residue extracting method with hot phenol in a proportion of 0.05
to 1.0% of the solution.
[0042] Moreover, in order to improve the strength, the composition
may optionally include one or more of the following: Mn: 0.05 to
0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%.
[0043] Furthermore, in order to improve the strength, each of the
above-mentioned compositions may optionally include one or more of
the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to
0.1%.
[0044] The percent symbol(s) for showing the content of each of the
elements (each) represent(s) % by mass.
Si: 0.50 to 1.5%
[0045] Si is combined with Mg to produce a Mg--Si precipitate,
which contributes to solute strengthening, and an improvement in
the strength of the structural part when the aluminum alloy is
subjected to an artificial aging treatment such as paint-bake
treatment, thereby exhibiting aging hardenability. Thus, this
element is an element essential for causing this structural part to
gain a strength (yield strength) necessary for automobiles and
others.
[0046] If the Si content by percentage is too small, the solute Si
amount is decreased before the paint-bake treatment (before the
artificial aging treatment) so that the amount of the produced
Mg--Si precipitate is insufficient. Thus, the structural part is
remarkably lowered in BH response to be short in strength or crash
performance.
[0047] In the meantime, if the Si content by percentage is too
large, coarse crystallized and precipitated products are produced
so that the aluminum alloy is lowered in ductility to be cracked
when rolled. Thus, the Si content is set into a range from 0.50 to
1.5%, preferably from 0.70 to 1.5%.
Mg: 0.30 to 1.5%
[0048] Mg is combined with Si to produce a Mg--Si precipitate,
which contributes to solute strengthening, and an improvement in
the strength of the structural part when the aluminum alloy is
subjected to an artificial aging treatment such as paint-bake
treatment, thereby exhibiting aging hardenability. Thus, this
element is an element essential for causing this structural part to
gain a yield strength necessary for automobiles and others.
[0049] If the Mg content by percentage is too small, the solute Mg
amount is decreased before the artificial aging treatment so that
the quantity of the produced Mg--Si precipitate is insufficient.
Thus, the structural part is remarkably lowered in BH response to
be short in strength or crash performance.
[0050] In the meantime, if the Mg content by percentage is too
large, the aluminum alloy easily undergoes the formation of shear
zones therein when cold-rolled, to be cracked in the rolling. Thus,
the Mg content is set into a range from 0.3 to 1.5%, preferably
from 0.7 to 1.5%.COPYRGT..
Cu: 0.05 to 1.0%
[0051] Cu makes the structural part high in strength by solute
strengthening, and further improves the crash performance by a
restraint of dislocation-localization when the member is collapsed
to be deformed. If the Cu content by percentage is too small, this
advantageous effect is small. If the content is too large, the
advantageous effect is saturated, and the corrosion resistance and
others are conversely deteriorated. Thus, Cu is optionally
incorporated in a range from 0.05 to 1.0% into the aluminum
alloy.
Solute Cu Amount: 0.05 to 1.0%
[0052] When Cu is incorporated to be caused to ensure (exhibit) the
strength-heightening or the crash-performance-improving effect by
the solute strengthening of Cu, the solute Cu amount in a solution
separated from the structural part by a residue extracting method
with hot phenol is set into a range from 0.05 to 1.0% of the
solution. As the solute Cu amount is larger, the structural part is
made better in work hardenability, smaller in yield ratio and
larger in elongation to be improved in crash performance.
[0053] If the solute Cu amount is less than 0.05% regardless of the
Cu content by percentage, the advantageous effect thereof is
insufficient. The upper limit of the solute Cu amount is
substantially equal to that of the added amount of Cu.
Mn: 0.05 to 0.5%; Zr: 0.02 to 0.20%; and Cr: 0.02 to 0.15%
[0054] One or more of Mn, Zr and Cr may be optionally incorporated
into the aluminum alloy, as the same advantageous-effect element(s)
for making crystal grains in the ingot or raw sheet finer, to
contribute to an improvement of the finally obtained structural
part in strength.
[0055] These elements are present in a dispersed particle form to
contribute to crystal grain refinement to produce also an
advantageous effect of improving the raw sheet in formability. If
the content by percentage of each of the elements is too small, the
strength- or formability-improving effect based on the crystal
grain refinement is insufficient. If the content is too large,
coarse compound grains are produced to deteriorate the aluminum
alloy in ductility.
[0056] Thus, when one or more of Mn, Zr and Cr are optionally
incorporated, the incorporation is attained as follows: Mn: 0.05 to
0.5%, Zr: 0.02 to 0.20%, and/or Cr: 0.02 to 0.15%.
Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%
[0057] One or more of Ag, Sn and Sc may be optionally incorporated
into the aluminum alloy as the same advantageous-effect element(s)
for improving the strength thereof.
[0058] Ag causes an aging precipitate contributing to an
improvement of the structural part in strength to be densely and
minutely precipitated by an artificial aging treatment after the
raw sheet is formed into the structural part, thereby producing an
advantageous effect of promoting the enhancement of the strength.
Thus, as required, Ag is optionally incorporated. If the Ag content
is less than 0.01%, the strength-improving effect is small. If the
Ag content is too large, the aluminum alloy is conversely lowered
in various properties such as rollability and weldability, and
further the strength-improving advantageous effect is also
saturated merely to increase costs. Thus, when Ag is optionally
incorporated, the Ag content is set into a range from 0.01 to
0.2%.
[0059] Sn has an advantageous effect of restraining the production
of clusters at room temperature to keep an excellent formability or
workability of the raw sheet after a solutionizing/quenching
treatments thereof, and further improving the strength when the
sheet is subsequently subjected to an artificial aging treatment,
such as paint-bake treatment. Sn is therefore an element essential
for giving the structural part a yield strength and crash
performance necessary for structural parts of automobiles. If the
Sn content is less than 0.001%, the advantageous effects are small.
If the content is more than 0.1%, the advantageous effects are
saturated, and the aluminum alloy conversely undergoes hot
brittleness to be remarkably deteriorated in hot workability (heat
stretchability). Thus, when Sn is optionally incorporated, the Sn
content is set into a range from 0.001 to 0.1%.
[0060] Sc makes crystal grains in the ingot and the finally
obtained product fine to contribute to an improvement in the
strength thereof. Moreover, Sc is dispersed in a dispersed particle
form to contribute to crystal grain refinement to improve the raw
sheet also in formability. If the Sc content by percentage is too
small, these advantageous effects are short. If the content is too
large, coarse compound grains are produced to deteriorate the
aluminum alloy in ductility. Thus, when Sc is optionally
incorporated, Sc is incorporated in a range from 0.02 to 0.1%.
Other Elements:
[0061] Elements other than the above-mentioned elements, for
example, Ti, B, Fe, Zn and V are inevitable impurities. The
aluminum alloy may contain each of these elements in a content
range specified in the JIS Standard and others for 6000 series
alloys.
Dislocation Density:
[0062] Under the presupposition of the above-mentioned alloy
composition, about the microstructure of each surface
(microstructure obtained by observing the surface) of the
structural part subjected to an artificial aging treatment, or each
surface of a raw sheet to which strain or heat treatment is given
or applied in such a manner that the raw sheet simulates this
structural part, the dislocation density measured by X-ray
diffraction is set into a range from 3.0.times.10.sup.14 to
8.0.times.1024 m.sup.-2, preferably from 4.0.times.10.sup.14 to
8.0.times.10.sup.14 m.sup.-2 on average.
[0063] About the surface of the artificially aged structural part,
or the surface of the raw sheet to which the strain or heat
treatment is given or applied in such a manner that the raw sheet
simulates this structural part, the dislocation density is set into
the specified range, whereby the structural part can have a 0.2%
yield strength of 250 MPa or more, and further have such a crash
performance that a bending angle of 90.degree. or more is obtained
according to a VDA bending test thereof.
[0064] It can be presumed that: the dislocation density level of
the structural part surface is made higher than that of any surface
of a raw sheet for an ordinary panel, or a panel obtained from this
raw sheet as raw material, whereby the structural part produces a
remarkably increased effect of hindering dislocation-movement when
collapsed to be deformed, for example, when the automobile meets
with a collision accident; and thus, the member is improved in
balance between strength and crash performance.
[0065] In connection with this point, if the dislocation density is
less than 3.0.times.10.sup.14 m.sup.-2 to be too small, the
structural part is equivalent to conventional members (such as
panels) not to exhibit required strength and crash performance.
[0066] In the meantime, if the dislocation density exceeds
8.0.times.10.sup.14 m.sup.-2 on average to be too large, the
aluminum alloy is lowered in elongation to be conversely lowered in
crash performance.
[0067] In the present invention, when a raw sheet (rolled sheet)
subjected to a tempering treatment, such as solutionizing
treatment, is subjected to a cold work, such as press forming, so
as to be made into a structural part, or before or after this press
forming, the sheet is further cold-worked to add (give) strain
beforehand to the structural part. In this way, the dislocation
density of any surface of the formed structural part is heightened
into the above-mentioned specified range.
[0068] For reference, when an aluminum alloy raw sheet is made into
a panel by an ordinary press forming, strain given thereto is
generally a small value less than 5%. Consequently, the dislocation
density of any surface of a structural part obtained from the raw
sheet cannot be adjusted into the range specified in the present
invention after the member is artificially aged.
[0069] Considering conditions for an artificial aging treatment
such as an ordinary paint-baking treatment at high temperature for
a short period, in order to set the dislocation density of the
structural part surface into the range specified in the present
invention, it is necessary to set, into 5% or more, preferably 10%
or more, the strain given to the structural part by press forming
into this structural part, or the strain given thereto by a
combination of the press forming with cold work performed before or
after the press forming.
[0070] However, if the given strain is more than 20%, the
dislocation density of the structural part surface may exceed
8.0.times.10.sup.14 m.sup.-2 on average to become too large under
conditions for an artificial aging treatment of the structural
part, such as an ordinary paint-baking treatment thereof at high
temperature for a short period. Thus, the structural part may be
lowered in elongation to be conversely lowered in crash
performance.
[0071] Considering that strain is given to the raw sheet when the
sheet is press-formed into a structural part, it is actually
difficult to give strain or dislocation in a quantity more than
described in the above.
[0072] Accordingly, in order to control the average dislocation
density into the range specified in the present invention under
conditions for the artificial aging treatment, such as the
high-temperature and short-period ordinary paint-baking treatment,
it is preferred to select an optimal strain to be given from a
range from 5 to 20%, preferably from 10 to 20% while the artificial
aging conditions are also considered.
[0073] For reference, the Non-Patent Literature 1 suggests
dislocation (density) strengthening for making a 6000 series
aluminum alloy sheet higher in strength. The 6000 series aluminum
alloy sheet is artificially aged through cold rolling, or HPT
working, which is a high-pressure torsion method.
[0074] However, as described in the conventional technique reports,
the test results support only the fact that even when the 6000
series aluminum alloy sheet is dislocation-strengthened (increased
in dislocation density), the sheet is restrained from having
aging-hardenability while subsequently artificially aged. The
literature never describes any relationship between this
dislocation density and the crash performance of the structural
part obtained by formation of the sheet and artificial aging
treatment thereof.
[0075] The microstructure or mechanical properties of this
(original) structural part subjected to the artificial aging
treatment can be evaluated by examining microstructure or
mechanical properties of the following even when a raw sheet
therefor is actually subjected to formation into a structural part
followed by an artificial aging treatment: the microstructure and
mechanical properties of a product obtained, for simulating this
original structural part, by subjecting a 6000 series aluminum
alloy raw sheet subjected to tempering treatments, such as
solutionizing and quenching treatment, to a cold work for giving
strain thereto, such as press forming, followed by an artificial
aging treatment.
[0076] About preferred treatment conditions for simulating this
original structural part, in order to simulate a specific usage of
the original structural part, a 6000 series raw aluminum alloy
sheet is subjected to solutionizing treatment at a temperature
selected from the range of a temperature of 550.degree. C. to the
melt temperature of the sheet (both inclusive), this range being
one out of preferred producing conditions that will be also
described later, for about 0.1 sec to several tens of seconds in a
continuous furnace, or for about several tens of minutes in a batch
furnace; immediately, the sheet is rapidly cooled to room
temperature at an average cooling rate of 20.degree. C./sec or more
and, is subjected, immediately after the cooling, to a pre-aging
treatment of keeping the sheet at 60 to 120.degree. C. for 2 to 10
hours; and then a strain of 10 to 20% is given to the resultant
sheet through a tensile tester, and subsequently the sheet is
further artificially aged at 210 to 270.degree. C. for 10 to 30
minutes. By examining the microstructure and mechanical properties
of the product obtained in this way, the original structural part
can be evaluated with a high correlation and a good
reproducibility.
[0077] In order to make this reproducibility stricter in the
aluminum alloy sheet of the present invention, specific treatment
conditions for simulating the original structural part are rendered
one-point conditions of subjecting an aluminum alloy sheet to a
solutionizing treatment of keeping the sheet at 550.degree. C. in a
batch furnace for 30 seconds; immediately water quenching the sheet
to room temperature at an average cooling rate of 30.degree.
C./sec; subjecting the sheet, immediately after the quenching, to a
pre-aging treatment at 100.degree. C. for 5 hours; and subsequently
giving the resultant sheet a strain of 10% through a tensile
tester, and further aging this sheet artificially at 210.degree. C.
for 30 minutes.
Method for Measuring Dislocation Density
[0078] As described in the Non-Patent Literature 1 and others, it
is widely used to measure the dislocation density of, e.g., a
metallic sheet through, e.g., a transmission electron microscope.
In the present invention, the dislocation density is more easily
measured with a better reproducibility by X-ray diffraction.
Regions (cell walls and shear zones) where linear and streak
dislocations, out of dislocations, gather densely are not easily
discriminated through any transmission electron microscope, so that
the regions cause measurement accidental errors when the
dislocation density p is analyzed and gained. In contrast, X-ray
diffraction produces an advantage that even such dislocations
gathering in large numbers cause only small accidental errors since
the dislocation densities p are calculated out from the respective
half-value widths of diffraction peaks obtained from individual
surfaces of the texture, as will be detailed later.
[0079] In the microstructure of a sheet into which dislocations are
introduced by adding plastic deformations to the sheet by, e.g.,
cold rolling or a tensile test, lattice strains are generated to
centralize the dislocations. Moreover, by the arrangement of the
dislocations, low-angle grain boundaries, cell structures, and
others are developed. When such dislocations, or domain structures
following the dislocations are caught from the resultant X-ray
diffraction pattern, in accordance with diffraction indexes thereof
characteristic breadths and shapes make their appearance in the
diffraction peaks of the pattern. When the shapes (line profile) of
the diffraction peaks are analyzed (line profile analysis), the
dislocation density can be gained.
[0080] Specifically, from a structural part artificially aged or a
raw sheet simulating this structural part, a sample is collected in
such a manner that surfaces of the member or sheet are to be
observing surfaces. Microstructures of the surfaces of the sample
are then subjected to X-ray diffraction. The half-value width of a
diffraction peak of each of the (111), (200), (220), (311), (400),
(331), (420) and (422) planes (orientation planes), which are main
orientations of the texture of the surfaces of the structural part,
are gained.
[0081] As the dislocation density p is higher, the half-value width
of the diffraction peak of each of these planes is larger. The
surfaces of the structural part as the sample, which are targets to
be measured by X-ray diffraction, may be in the state of the
surfaces of the sample not subjected to any further treatment, or
may be washed without being etched.
[0082] Next, from the respective half-value widths of the
diffraction peaks of these individual surfaces, the lattice strain
(crystal strain) a is gained by the Williamson-Hall method.
Furthermore, the dislocation density p of the sample can be
calculated out in accordance with the following expression:
.rho.=16.1.epsilon..sup.2/b.sup.2
wherein .rho. represents the dislocation density; .epsilon., the
lattice strain of the sample; and b, the magnitude of Burger's
vector.
[0083] As the magnitude of the Burger's vector,
2.8635.times.10.sup.-10 m is used.
[0084] The Williamson-Hall method is a known line profile analyzing
method used widely to gain the dislocation densities or crystal
grain diameters of a metal sample from a relationship between
respective half-value widths of plural diffraction peaks of the
sample, and the diffraction angles thereof. Known is also a series
of manners for gaining the dislocation densities by X-ray
diffraction. The dislocation densities obtained by the manner
series for gaining the dislocation densities by X-ray diffraction
are generically named the "dislocation density measured by X-ray
diffraction" in the present invention.
[0085] About the dislocation density of any structural part, 10
samples collected from arbitrarily-selected sites of the structural
part are measured, and the resultant dislocation densities are
averaged.
Producing Method:
[0086] The following describes a preferred method for producing the
structural part of the present invention. Initially, a preferred
method for producing a raw rolled sheet is described hereinafter in
the order of steps thereof.
[0087] A 6000 series aluminum alloy sheet which is a raw material
for the structural part is a hot-rolled sheet obtained by
subjecting an ingot to homogenization followed by hot rolling, or a
cold-rolled sheet obtained by subjecting the hot-rolled sheet to
cold rolling, and is produced in a usual manner of subjecting the
hot-rolled sheet or cold-rolled sheet further to a tempering
treatment such as solutionizing treatment. Specifically, the raw
material is an aluminum alloy hot-rolled sheet produced through
ordinary individual producing steps composed of casting,
homogenization and hot rolling, and having a sheet thickness of
about 2 to 4 mm; or a cold-rolled sheet obtained by cold-rolling a
hot-rolled sheet having a larger thickness and produced through the
same steps into a thickness of about 2 to 4 mm.
[0088] The 6000 series aluminum alloy sheet in the present
invention may be produced by an especial producing method or
rolling method in which after continuous casting into a thin sheet
in, e.g., a twin roll manner, the thin sheet is cold-rolled with an
omission of any hot rolling, or is warm-rolled.
[0089] Accordingly, the producing method has an advantage that a
raw sheet can be produced without making a large change of 6000
series aluminum alloy compositions standardized already for
structural parts as described above, and without making a large
change of a rolling step in the usual way.
Melting, and Casting:
[0090] A raw alloy is initially molten and cast. In the melting and
casting steps, a molten aluminum alloy adjusted into the
above-mentioned 6000 series component composition range, as the
molten raw alloy, is cast by an appropriately selected ordinary
melting and casting method such as a continuous casting method or a
semi-continuous casting method (DC casting method).
Homogenization:
[0091] Next, the cast aluminum alloy ingot is subjected to
homogenization in the usual way before subjected to hot rolling. A
purpose of this homogenization is to homogenize the microstructure,
that is, to remove segregation inside crystal grains in the ingot
microstructure. Conditions for this homogenization are
appropriately selected from the range of 500.degree. C. or higher
and less than the melting point of the alloy, and the holding
period range of 2 hours or longer.
Hot Rolling:
[0092] The sheet is then hot-rolled. Under a condition that the
starting temperature of the hot rolling is higher than the solid
phase line temperature of the alloy, burning is caused not to
conduct the hot rolling itself easily. If the hot rolling starting
temperature is lower than 350.degree. C., an excessively high load
is generated in the hot rolling not to conduct the hot rolling
itself easily. Thus, the hot rolling is performed at a hot rolling
starting temperature selected from the range of 350.degree. C. to
the solid phase line temperature to produce a hot-rolled sheet
having a thickness of about 2 to 10 mm. This hot-rolled sheet is
not necessarily annealed before cold-rolled; however, the sheet may
be annealed.
[0093] The hot rolling of the ingot subjected to the homogenization
is composed of a rough rolling step of the ingot (slab) and a
finish rolling step. In these rough and finish rolling steps, a
rolling machine of, e.g., a reverse type or a tandem type is
appropriately used.
[0094] During the hot rolling from the start of the hot rough
rolling to the end thereof, it is preferred to keep the solute
amounts of Si and Mg surely without lowering the temperature to
450.degree. C. or lower. If the lowest temperature of the rough
rolled sheet in the middle of the rolling path is lowered to
450.degree. C. or lower, for example, by making the rolling period
long, one or more compound precipitate easily. Thus, even when
strain is given thereto before the sheet is artificially aged, the
dislocation density may not be sufficiently increased. Moreover,
the possibility is great that the solute Cu amount is also
lowered.
[0095] After the hot rough rolling, the sheet is subjected to hot
finish rolling the end temperature of which is preferably set into
the range of 300 to 360.degree. C. If the end temperature of the
hot finish rolling is lower than 300.degree. C. to be too low, the
rolling load becomes high to lower the producing performance of
this method. In the meantime, in the case of heightening the end
temperature of the hot finish rolling to make the alloy into a
recrystallized phase without leaving a large quantity of the
deformed microstructure, coarse transition-element-dispersed
particles may probably precipitate if the end temperature is higher
than 360.degree. C.
[0096] From the temperature of the material (sheet) just after the
end of the hot finish rolling to a material temperature of
150.degree. C., the average cooling rate is controlled into at
lowest 5.degree. C./hour or more by forcible cooling using, e.g.,
fans. If this average cooling rate is less than 5.degree. C./hour,
the quantity of a precipitate produced during the cooling becomes
large. Thus, even when strain is given to the sheet before the
sheet is artificially aged, the dislocation density does not
increase sufficiently. Moreover, the solute Cu amount is decreased
in the resultant product sheet.
[0097] It is therefore preferred that the average cooling rate is
larger just after the end of the hot finish rolling. The rate is
set to at lowest 5.degree. C./hour or more, preferably to 8.degree.
C./hour.
[0098] For reference, according to any ordinary hot finish rolling,
after this rolling, the resultant sheet is wound into the form of a
coil. Thus, when the coil diameter is an ordinary diameter, the
average cooling rate according to natural cooling just after the
end of the hot finish roll easily turns into less than 5.degree.
C./hour as far as the rolled sheet is not forcibly cooled by, e.g.,
fans.
[0099] When the resultant hot-rolled sheet is further cold-rolled,
the sheet does not need to be annealed before the cold rolling.
However, the annealing may be performed.
Cold Rolling:
[0100] The sheet is then cold-rolled. In the cold rolling, the
hot-rolled sheet is cold-rolled to produce a cold-rolled sheet
(that may be in the form of a coil) having a desired final
thickness. In order to make the crystal grain finer, the cold
rolling ratio is desirably 30% or more. Moreover, to attain the
same purpose that the above-mentioned annealing does, the
hot-rolled sheet may be subjected to intermediate annealing in the
middle of the cold rolling path.
Solutionizing and Quenching Treatments:
[0101] After the cold rolling, the rolled sheet is subjected to
solutionizing treatment followed by quenching treatment down to
room temperature. For the solutionizing and quenching treatments,
an ordinary continuous heat treatment line may be used. In order
for the treated sheet to gain a sufficient solute amount of each of
Mg, Si, and other elements, it is preferred to perform the
solutionizing treatment at the molten temperature of the sheet or
lower, followed by the quenching down to room temperature at an
average cooling rate of 20.degree. C./second or more. If the
solutionizing treatment temperature is lower than 550.degree. C.,
compounds of Mg--Si and others that have been produced before this
solutionizing treatment are insufficiently re-solid-solutionized so
that the solute amounts of Mg and Si are lowered.
[0102] If the average cooling rate is less than 20.degree.
C./second, the possibility becomes great that during the cooling,
Mg--Si precipitates are produced to lower the solute amounts of Mg
and Si so that sufficient solute amounts of Mg and Si cannot be
ensured. In order to ensure the cooling rate in the quenching, a
cooling means is selected from fans and other air-cooling means or
manners, and mist, spraying, immersion and other water-cooling
means or manners, as well as cooling conditions are selected.
Pre-Aging Treatment: Reheating Treatment:
[0103] After the solutionizing treatment followed by the quenching
treatment, resulting in the cooling of the sheet to room
temperature in this way, it is preferred to subject the cold-rolled
sheet to pre-aging treatment (reheating treatment) within one hour
of the cooling end. If the room temperature holding period from the
end of the quenching down to room temperature to the pre-aging
start (heating start) is too long, Mg--Si clusters rich in Si
amount are unfavorably produced by natural aging at room
temperature, so that an increase cannot easily be made in the
amount of Mg--Si clusters good in balance between the Mg and Si
proportions. It is therefore more preferred that this room
temperature holding period is shorter. The solutionizing and
quenching treatments, and the reheating treatment may be
continuously conducted without having any time lag substantially
therebetween. The lower limit of the period is not particularly
specified.
[0104] About this pre-aging treatment, a holding period at 60 to
120.degree. C. is adjusted preferably into the range of 2 to 40
hours both inclusive. In this case, Mg--Si clusters good in balance
between the Mg and Si proportions are produced.
[0105] If the pre-aging temperature is lower than 60.degree. C., or
the holding period is shorter than 2 hours, the same results as in
the case of not conducting this pre-aging are produced to restrain
the production of Mg--Si clusters rich in Si amount. Thus, the
Mg--Si clusters good in balance between the Mg and Si proportions
are not easily increased in quantity, so that the alloy sheet is
easily lowered in yield strength after paint-baked.
[0106] In the meantime, if the pre-aging temperature is higher than
120.degree. C., or the holding period is longer than 40 hours, the
quantity of produced precipitation nuclei is too large so that the
alloy sheet is too high in strength when subjected to bending work
before the bake-painting. Thus, the sheet is easily deteriorated in
bendability.
Production of Structural Part
Strain Addition:
[0107] The raw sheet subjected to these tempering treatments (T4)
is formed into a product, such as a side member or such a member, a
frame, a pillar or any other structural part, mainly by press
forming.
[0108] At this time, the raw sheet is formed into a structural part
while a strain of 5 to 20% is given thereto by cold work. In
addition thereto, the resultant structural part is artificially
aged, thereby making it possible to set, into the range of
3.0.times.10.sup.14 or 8.0.times.10.sup.14 m.sup.-2, the
dislocation density of any surface of the structural part subjected
to the artificial aging treatment. This density is measured by
X-ray diffraction.
[0109] At this time, it is allowable before the artificial aging
treatment to apply a cold work for giving the above-mentioned
strain beforehand to the raw sheet when the raw sheet is
press-formed into the structural part without conducting cold work
in a separate step.
[0110] In accordance with the shape of the structural part, the
strain may be added thereto by not only the press forming but also
a method or means for the cold work, such as tension, cold rolling,
a leveler or stretch. In this case, the total strain added by the
press forming and the cold work is adjusted into the
above-mentioned range.
[0111] The strain is made larger than that added at the time of
press forming for producing, e.g., an automotive panel in the usual
way, and then the strain is beforehand added (given) to the alloy
sheet before the sheet is artificially aged. In order to adjust the
average dislocation density into the above-mentioned range of
3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the strain is
added thereto in a proportion of 5% or more, preferably of from 10
to 20% both inclusive.
[0112] As described above, if the strain is less than 5%, this
strain, which depends on the artificial aging treatment conditions,
is not largely different from that given when any conventional
press forming or bending working is conducted. Thus, the average
dislocation density cannot be adjusted to 3.0.times.10.sup.14
m.sup.-2 or more.
[0113] As the strain is larger, the average dislocation density can
be made larger. However, if the strain is more than 20%, the
average dislocation density exceeds 8.0.times.10.sup.14 m.sup.-2 so
that the resultant structural part is remarkably lowered in
elongation to be poor in crash performance.
Artificial Aging Treatment:
[0114] The artificial aging treatment of the strain-given
structural part or a sheet to which a strain of 5 to 20% is given
to simulate this structural part may be conducted by paint-bake
treatment or an ordinary artificial aging treatment (T6 or T7).
[0115] Conditions for the heating temperature and the holding
period are freely decided in accordance with, e.g., a desired
strength of the structural part or the natural-aging-advancing
degree thereof at room temperature. When the artificial aging
treatment is, for example, a one-stage treatment, the artificial
aging treatment is conducted preferably at a heating temperature of
200 to 270.degree. C. for a holding period of 5 to 30 minutes.
[0116] If the heating temperature is too low or the holding period
is too short, the structural part may undergo insufficient aging
hardening not to have a strength or crash performance that is a
target of the present invention. Also if the heating temperature is
too high or the holding period is too long, the structural part may
undergo over-aging not to have a strength or crash performance that
is a target of the present invention.
EXAMPLES
[0117] In each of invention examples and comparative examples, as
shown in Table 2 described below, one out of variously changed
strains was added to a cold-rolled sheet of a tempering-applied
6000 series aluminum alloy having one out of component compositions
shown in Table 1 to simulate the structural part concerned. The
resultant was artificially aged, and then measurements and
evaluations were made about the microstructure of any surface of
this artificially-aged test material (the average dislocation
density thereof and the solute amount of Cu therein), and the
strength and the crash performance evaluated in a VDA bending test.
The results are shown in Table 2. In the indication of the content
by percentage of each element in Table 1, the symbol "-" shown in a
numerical value cell about the element denotes that the content by
percentage is not more than the limit of detection.
[0118] The above-mentioned cold-rolled sheet as a raw sheet was
specifically produced as follows:
[0119] An aluminum alloy ingot having the composition shown in
Table 1 was molten and cast into an ingot by a DC casting method.
Subsequently, the ingot was subjected to homogenization at a
temperature-raising rate of 150.degree. C./hour and at a
homogenization temperature of 550.degree. C. for a holding period
of 3 hours.
[0120] Thereafter, hot rough rolling of the workpiece was started
at 500 to 520.degree. C., and the lowest temperature of the hot
rough rolling was set to one out of variously changed temperatures
shown in Table 2. Furthermore, the workpiece was subjected to hot
finish rolling the end temperature of which was set into the range
of 300 to 350.degree. C. to produce a hot-rolled sheet of 4.0 mm
thickness.
[0121] At this time, the average cooling rate (.degree. C./hour)
from the material (sheet) temperature just after the end of the hot
finish rolling to a material temperature of 150.degree. C. was set
to one out of variously changed values as shown in Table 2.
[0122] This hot-rolled sheet was cold-rolled at a rolling ratio of
50% without being subjected to heat treatment after the hot rolling
and intermediate annealing in the middle of the cold rolling path.
Thus, the cold-rolled sheet was obtained, which had a thickness of
2.0 mm.
[0123] Furthermore, the respective cold-rolled sheets in the
examples were subjected to a tempering treatment (T4) under
conditions common to the examples in heat treatment facilities.
Specifically, the sheets were each subjected to solutionizing
treatment at 550.degree. C. for a holding period of 30 seconds. At
this time, the average heating rate up to the solutionizing
treatment temperature was set to 50.degree. C./second. After the
solutionizing treatment, at an average cooling rate of 30.degree.
C./second, the workpiece was subjected to water quenching down to
room temperature. Just after the quenching, the workpiece was
subjected to pre-aging treatment at 100.degree. C. for a holding
period of 5 hours. After the pre-aging treatment, the workpiece was
gradually (naturally) cooled to yield a T4 material.
[0124] In each of the examples, from the T4 material, a #5 tensile
test specimen (25 mm.times.25 mm GL.times.sheet thickness)
according to a JIS Z 2201 was collected. In order to simulate the
addition of strain to the T4 material when the material is formed
into the structural part concerned, one out of variously changed
pre strains was added to the #5 test specimen in a tensile test
that will be detailed later. The strain-added #5 test specimen was
artificially aged under conditions shown in Table 2 to make a
tensile test. Thereafter, from this test specimen, sheet-form test
specimens each having a required size were cut out, and then
evaluated about the solute amount of Cu therein, and the
dislocation density and the shock absorption performance thereof as
follows:
Measurement of Solute Amount of Cu:
[0125] In a measurement of the solute amount of Cu, one of the
above-mentioned sheet-form test specimens, which was a target to be
measured, was dissolved by a residue extracting method with hot
phenol. The resultant solid and solution were filtrated and
separated from each other through a filter having a mesh (particle
catching diameter) of 0.1 .mu.m. The Cu content by percentage in
the separated solution was measured as the solute Cu amount.
[0126] This residue extracting method with hot phenol was
specifically performed as follows: Initially, phenol was put into a
decomposing flask and then heated. The sheet-form test specimen to
be measured was transferred into the decomposing flask to be heated
and decomposed. Next, benzyl alcohol was added thereto, and then
the content was filtrated under reduced pressure to be separated
into a solid and a solution. The Cu content by percentage in the
separated solution was quantitatively determined.
[0127] This quantitative determination appropriately made use of,
e.g., atomic absorption spectrophotometry (AAS), or inductively
coupled plasma emission spectrometry (ICP-OES). As described above,
for the filtration under reduced pressure, a membrane filter was
used which had a mesh of 0.1 .mu.m and a diameter of 47 mm.
[0128] The measurement and calculation were made about three
samples collected from arbitrarily-selected three sites of the
sheet-form test specimen. The respective solute amounts (% by mass)
of Cu in these samples were averaged and the resultant value was
defined as the solute Cu amount.
Measurement of Dislocation Density:
[0129] A surface of one of the sheet-form test specimens was caused
to simulate a surface of the structural part concerned, and the
dislocation density (.times.10.sup.14 m.sup.-2) of the sheet-form
test specimen surface was measured by X-ray diffraction under the
above-mentioned conditions. The measurement was made about
arbitrarily-selected 5 sites of the sheet-form test specimen. The
respective dislocation densities of these sites were averaged and
the resultant value was defined as the average dislocation density
(.times.10.sup.14 m.sup.-2).
Tensile Test:
[0130] The artificially-aged #5 tensile test specimen was used to
make a tensile test at room temperature. At this time, the tensile
direction of the test specimen was made parallel to the rolling
direction. The test was made at room temperature, 20.degree. C., on
the basis of JIS Z 2241 (1980) at a supporting point distance of 50
mm and a constant tensile speed of 5 mm/minute until the test
specimen was broken. If the test specimen had a 0.2% yield strength
of 250 MPa or more, the specimen was judged to be acceptable as an
artificially-aged structural part.
Shock Absorption Performance:
[0131] A bending test for evaluating shock absorption performance
was made in accordance with the following VDA bending test: "VDA
238-100 plate bending test for metallic materials", which is
standardized by Verband der Automobilindustrie e.V. (VDA). This
test method is illustrated in FIG. 1 as a perspective view.
[0132] As represented by dot lines in FIG. 1, initially, one of the
sheet-form test specimens is put onto two rolls arranged in
parallel to each other and having a roll gap therebetween, so as to
make right and left parts of the specimen equal in length to each
other and be horizontally stretched.
[0133] Specifically, the sheet-form test specimen is put onto the
two rolls, so as to make right and left parts of the specimen equal
in length to each other and be horizontally stretched in such a
manner that the rolling direction of the specimen is made
perpendicular to the extending direction of a plate-form
pushing/bending member arranged to stand upward and vertically, and
that the center of the specimen is positioned at the center of the
narrow roll gap.
[0134] The pushing/bending member is pushed from the above onto the
center of the sheet-form test specimen to apply a load thereto. In
this way, this sheet-form test specimen is pushed (thrusted) toward
the narrow roll gap to be bent. Thus, the bent and deformed center
of the sheet-form test specimen is pushed into the narrow roll
gap.
[0135] When the load F from the above through the pushing/bending
member turns maximum in this case, the angle of the outside of the
bent center of the sheet-form test specimen is measured as the
bending angle (.degree.) of the specimen. In accordance with the
value of the bending angle, the shock absorbance performance is
evaluated. As this bending angle is larger, the sheet-form test
specimen is higher in shock absorbance performance (crash
performance) to continue to have the bending deformation without
being collapsed in the middle.
[0136] Test conditions of this VDA bending test are shown
hereinafter, using symbols described in FIG. 1. The sheet-form test
specimen is made into a square shape having a width "b" of 60 mm
and a length "1" of 60 mm. The diameter D of each of the two rolls
is set to 30 mm; and the roll gap L, to 4 mm, which is two times
the sheet thickness of the sheet-form test specimen. The symbol S
represents the pushed-depth of the center of the sheet-form test
specimen into the roll gap when the load F becomes maximum.
[0137] As illustrated in FIG. 1, about the plate-form
pushing/bending member, a side of the member at the low-end-side
thereof, which is to be pushed onto the center of the sheet-form
test specimen, is made into a tapered form such that the tip (lower
end) thereof has a radius of 0.2 mm.
[0138] In each of the examples, the VDA bending test was made about
three of the sheet-form test specimens (made three times). The
average thereof was used as the bending angle (.degree.) of the
example. The results are shown in Table 2.
[0139] As is evident from Table 2, the above-specified preferred
strain or artificial aging treatment is applied to each of
Invention Examples 1 to 13, which make use of respective aluminum
alloys represented by alloy numbers 1 to 10 (within the composition
range in the present invention) in Table 1.
[0140] Thus, these examples are in an artificially aged sheet state
which simulates the structural part concerned, and satisfy the
average dislocation density specified in the invention.
[0141] As a result, also about the crash performance evaluated in
the VDA bending test, the bending angle is 90.degree. or more to
satisfy an excellent property required for the structural part.
Moreover, the 0.2% yield strength thereof is also a high strength
of 250 MPa or more to satisfy a property required for the
structural part.
[0142] In contrast, about each of the comparative examples, the
alloy composition thereof is out of the range in the present
invention, or the strain requirement thereof is out of the
preferred range although the alloy composition is within the range
in the invention.
[0143] Thus, each of the comparative examples does not satisfy the
average dislocation density specified in the invention.
[0144] As a result, about the comparative example, the 0.2% yield
strength or the crash performance evaluated in the VDA bending test
is poorer than that about the invention examples not to satisfy a
property required for the structural part.
[0145] About Comparative Examples 14 to 20, the alloy composition
is within the range in the present invention as shown about alloy
number 2 or 5 in Table 1, but producing conditions for their raw
sheet are out of the preferred producing-conditions, or the
preferred pre-strain is never or insufficiently added to the
structural part that has not yet been artificially aged. Thus, on
the whole, the average dislocation density range thereof is
downwards out of the range specified in the present invention.
[0146] Comparative Example 14 does not satisfy the average
dislocation density specified in the present invention since the
lowest temperature in the hot rough rolling is too low in the
production of the raw sheet.
[0147] About Comparative Example 15, the average dislocation
density range thereof is downwards out of the range specified in
the present invention because the average cooling rate is too small
from the material (sheet) temperature just after the hot finish
rolling to a material temperature of 150.degree. C. in the
production of the raw sheet.
[0148] About Comparative Examples 16 and 17, the pre-strain is
never or insufficiently added thereto.
[0149] About Comparative Example 18, the added pre-strain is too
large.
[0150] About Comparative Examples 19 and 20, the artificial aging
treatment temperature is too high or the holding period is too long
relatively to the added pre-strain.
[0151] About Comparative Examples 21 and 22, their raw sheet is
produced under the above-mentioned preferred conditions, and the
strain is added thereto under the preferred conditions. However,
their alloy composition is downwards out of the range about the Mg
or Si content by percentage in the present invention, as shown
about the alloy numbers 11 and 12 in Table 1.
[0152] Thus, their average dislocation density range is downwards
out of the range specified in the invention, so that the BH
response is remarkably lowered and the strength and the crash
performance are too low.
[0153] The above-mentioned results support respective critical
significances of the requirements of the present invention for
causing the aluminum alloy structural part of the present invention
to have both of a crash performance estimated in the VDA bending
test, and a high strength.
TABLE-US-00001 TABLE 1 6000 Series aluminum alloy
chemical-component-composition (% by mass) (balance: Al) Alloy No.
Mg Si Fe Cu Me Zr Cr Ag Sn Sc Ti 1 0.58 0.95 0.17 -- -- -- -- -- --
-- -- 2 0.45 1.1 0.15 -- -- -- -- -- -- -- 0.02 3 0.35 1.2 0.08 --
-- -- -- -- -- -- 0.02 4 0.95 0.54 -- -- 0.35 -- -- -- -- -- 0.02 5
0.43 1.1 -- 0.08 0.12 -- -- -- -- -- -- 6 0.60 1.0 -- 0.72 -- -- --
-- -- -- 0.02 7 1.0 1.4 0.20 -- 0.08 -- 0.12 -- -- -- -- 8 0.70
0.95 0.12 0.18 -- 0.15 0.05 -- -- -- 0.02 9 1.4 1.1 0.14 -- -- --
-- 0.15 -- 0.07 0.02 10 0.45 0.66 0.15 0.45 0.07 -- 0.05 -- 0.05 --
0.02 11 0.22 0.88 0.14 -- 0.08 0.05 -- -- -- -- 0.02 12 0.75 0.43
-- 0.12 0.08 -- -- -- -- -- 0.02
TABLE-US-00002 TABLE 2 Producing method Microstructure and
properties after pre-stain Hot finish rolling addition and
artificial aging treatment (T6) Hot rough Average cooling rate
Artificial aging Microstructure Properties rolling (.degree.
C./hr.) treatment Average 0.2% VDA Lowest from material Pre-
End-point Holding dislocation Solute Cu Yield bending No. in
temperature temperature just after strain temperature period
density quality strength angle Classification No. Table 1 (.degree.
C.) rolling-end to 150.degree. C. % (.degree. C.) (min)
(.times.10.sup.14 m.sup.-2) (% by mass) (MPa) (.degree.) Invention
1 1 480 10 10 210 30 4.5 -- 253 98 Examples 2 2 470 7 5 250 10 3.4
-- 255 107 3 2 470 8 10 230 20 4.6 -- 258 101 4 2 470 7 15 230 15
5.6 -- 265 94 5 2 460 10 20 200 10 7.3 -- 276 90 6 3 460 7 10 230
20 4.1 -- 257 103 7 4 470 6 20 270 5 5.2 -- 254 95 8 5 460 10 10
230 20 5.2 0.07 265 96 9 6 460 15 10 230 20 6.7 0.63 280 97 10 7
450 7 15 230 20 7.6 -- 277 90 11 8 460 10 10 230 20 6.0 0.16 272 98
12 9 450 7 5 210 20 6.2 -- 270 92 13 10 470 15 10 250 20 3.1 0.42
254 110 Comparative 14 2 430 7 5 250 10 2.7 -- 244 88 Examples 15 5
460 4 10 250 20 2.9 0.03 248 85 16 2 470 7 -- 230 20 1.5 -- 228 98
17 2 470 7 2 230 20 2.4 -- 239 93 18 2 470 7 25 230 20 8.5 -- 290
75 19 5 460 10 10 280 15 2.6 0.03 243 89 20 5 460 10 10 230 40 2.8
0.04 247 88 21 11 480 10 10 210 30 2.7 -- 242 88 22 12 480 10 10
210 30 2.8 0.11 245 86
INDUSTRIAL APPLICABILITY
[0154] As described above, the present invention can provide a
structural part obtained from a 6000 series aluminum alloy sheet as
a shaping raw material, and having an improved crash performance;
and a method for producing the sheet. The present invention is
therefore suitable for a structural part contributing to weight
saving for, e.g., an automobile, a bicycle or a railroad
vehicle.
* * * * *