U.S. patent application number 13/395709 was filed with the patent office on 2012-07-05 for aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Kentaro Ihara, Takahiro Shikama.
Application Number | 20120168045 13/395709 |
Document ID | / |
Family ID | 43826269 |
Filed Date | 2012-07-05 |
United States Patent
Application |
20120168045 |
Kind Code |
A1 |
Ihara; Kentaro ; et
al. |
July 5, 2012 |
ALUMINUM ALLOY EXTRUDATE EXCELLENT IN BENDING CRUSH RESISTANCE AND
CORROSION RESISTANCE
Abstract
Disclosed is an Al--Mg--Si aluminum alloy extrudate which
contains, in terms of mass %, 0.60-1.20% Mg, 0.30-0.95% Si,
0.01-0.40% Fe, 0.30-0.52% Mn, 0.001-0.65% Cu, and 0.001-0.10% Ti
and in which the contents of Mg and Si satisfy
Mg(%)-(1.73.times.Si(%)-0.25).gtoreq.0 and the remainder comprises
Al. The extrudate has an equi-axed recrystallized grain texture in
which the areal proportion of recrystallized grains is 65% or
higher. In examination with a TEM having a magnification of 5,000,
intergranular precipitate grains having a size of 1 .mu.m or more
in terms of center-of-gravity diameter are apart from one another
at an average spacing exceeding 25 .mu.m. The average areal
proportion of Goss-orientation grains is less than 8% throughout
the whole thickness of this extrudate.
Inventors: |
Ihara; Kentaro; (Kobe-shi,
JP) ; Shikama; Takahiro; (Shimonoseki-shi,
JP) |
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
43826269 |
Appl. No.: |
13/395709 |
Filed: |
September 29, 2010 |
PCT Filed: |
September 29, 2010 |
PCT NO: |
PCT/JP2010/066931 |
371 Date: |
March 13, 2012 |
Current U.S.
Class: |
148/690 ;
148/417 |
Current CPC
Class: |
C22C 21/08 20130101;
B21C 23/002 20130101; C22C 21/02 20130101 |
Class at
Publication: |
148/690 ;
148/417 |
International
Class: |
C22F 1/05 20060101
C22F001/05; C22C 21/08 20060101 C22C021/08 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2009 |
JP |
2009-228815 |
Claims
1. An aluminum alloy extrudate excellent in bending crush
resistance and corrosion resistance, being an extrudate of an
Al--Mg--Si aluminum alloy comprising, in terms of percent by mass,
Mg in a content of from 0.60% to 1.20%, Si in a content of from
0.30% to 0.95%, Fe in a content of from 0.01% to 0.40%, Mn in a
content of from 0.30% to 0.52%, Cu in a content of from 0.001% to
0.65%, and Ti in a content of from 0.001% to 0.10%, the contents of
Mg and Si satisfying condition: [Mg (%)]-(1.73.times.[Si
(%)]-0.25).gtoreq.0, with the remainder including Al and inevitable
impurities, wherein the extrudate has an equiaxed recrystallized
structure with an area ratio of recrystallized grains of 65% or
more in a cross section in a thickness direction, wherein the
aluminum alloy extrudate has an average spacing of more than 25
.mu.m between intergranular precipitates each having a size of 1
.mu.m or more in terms of centroid diameter in observation of the
structure under a transmission electron microscope (TEM) of 5000
magnifications, and wherein the aluminum alloy extrudate has an
average area ratio of Goss orientation grains of less than 8%,
throughout the entire thickness region including an outermost grain
growth layer in the cross section in the thickness direction of the
extrudate.
2. The aluminum alloy extrudate excellent in bending crush
resistance and corrosion resistance according to claim 1, further
comprising at least one of Cr in a content of from 0.001% to 0.18%
and Zr in a content of from 0.001% to 0.18% as replacing part of
Mn, and having a total content of Mn, Cr, and Zr of from 0.30% to
0.52%.
3. The aluminum alloy extrudate excellent in bending crush
resistance and corrosion resistance according to claim 1, wherein
the aluminum alloy extrudate has such bending crush resistance as
to have a critical bending radius (R) of 3.0 mm or less without
cracking in a 180-degree bending test according to a press-bending
method prescribed in Japanese Industrial Standards (JIS) Z2248 in
which a plate-shaped specimen is bent in an extrusion
direction.
4. A method for manufacturing an aluminum alloy extrudate excellent
in bending crush resistance and corrosion resistance, the method
comprising the steps of soaking a cast billet of an Al--Mg--Si
aluminum alloy at a temperature of 560.degree. C. or higher, the
aluminum alloy having the chemical composition as defined in claim
1; forcedly cooling the soaked cast billet to a temperature of
400.degree. C. or lower at an average cooling rate of 100.degree.
C./hr or more; reheating the cooled cast billet to a temperature of
500.degree. C. or higher and subjecting the reheated billet to hot
extrusion so that an extrudate reaches a solid solution temperature
of 575.degree. C. or higher at an extruder exit; immediately
forcedly cooling the extrudate from the extruder exit at an average
cooling rate of 5.degree. C./second or more; and subjecting the
cooled extrudate to aging so as to have a 0.2% yield strength of
280 MPa or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to an Al--Mg--Si aluminum
alloy extrudate excellent in bending crush resistance, and to a
method for manufacturing the same (hereinafter "aluminum" is also
simply referred to as "Al"). As used herein the term "aluminum
alloy extrudate" refers to not only any extrudates (extrudates or
materials) manufactured by hot extrusion but also any members
incorporated into automotive bodies as their body reinforcements
(or energy absorbing members) mentioned later. Hereinafter an
"Al--Mg--Si" aluminum alloy is also referred to as an "6000-series"
aluminum alloy.
BACKGROUND ART
[0002] To use such 6000-series aluminum alloy (6xxx aluminum alloy)
extrudates as the reinforcements, much has been suggested about
their metallographic structures for improvements in transverse
crushing performance as reinforcements and in bending workability
(bendability) into reinforcements.
[0003] Typically, Patent literature (PTL) 1 proposes an improvement
in bending workability by allowing a 6000-series aluminum alloy
extrudate to have an equiaxed granular structure. According to the
technique disclosed in this literature, the equiaxed granular
structure is obtained when the aluminum alloy contains Mg and Si in
stoichiometrically equivalent amounts and also contains transition
metal elements that promote the formation of a fibrous structure,
such as Mn, Cr, and Zr, in a controlled total amount of 0.1% or
less, and extrusion is performed at a temperature of 500.degree. C.
or higher, immediately followed by water quenching (forced
cooling), as indicated in the working examples. The resulting
equiaxed granular structure has an average grain size of 100 .mu.m
or less and an aspect ratio of crystal grains of 2 or less. The
aspect ratio is a length-to-thickness ratio of a crystal grain,
with the length being measured in the extrusion direction.
[0004] Independently, PTL 2 proposes a technique of allowing a
hollow extrudate to improve in bending workability by allowing the
same to have a fibrous structure (with crystal grains elongated in
the extrusion direction) in place of the equiaxed granular
structure. According to the technique disclosed in PTL 2, the
extrudate is manufactured from an aluminum alloy containing
transition metal elements such as Mn, Cr, and Zr in a relatively
large total amount of 0.45% to 0.53% by extrusion at 500.degree. C.
or higher, which is immediately followed by water quenching (forced
cooling) in a water bath, as indicated in the working examples.
[0005] It is known that the fibrous structure is effective for such
extrudates as side members and bumper stays to be used as energy
absorbing members requiting good longitudinal crushing performance
in their axial (or lengthwise) direction, so as to resist Euler
buckling (bending in a dogleg shape) but undergo deformation in a
bellow shape. See PTL 3 and PTL4. PTL 3 proposes an extrudate of a
6000-series aluminum alloy containing Mg and Si in
stoichiometrically equivalent amounts so as to have the fibrous
structure. The patent literature also suggests that the tendency
toward transformation into a recrystallization structure due to Mg
and Si contained in stoichiometrically equivalent amounts is
avoided if the extrudate contains transition metal elements such as
Mn, Cr, and Zr in a relatively large total amount of 0.5%, and
extrusion is performed at 500.degree. C. or higher and immediately
followed by water quenching, as indicated in the working
examples.
[0006] PTL 4 proposes an extrudate of a 6000-series aluminum alloy
which contains excess Si and also contains transition metal
elements such as Mn, Cr, and Zr in a relatively large total amount
of from 0.25% to 0.48%, as indicated in the working examples.
According to this patent literature, extrusion is performed at
500.degree. C. and the fibrous structure has a specific thickness
of recrystallized layer (GG layer) and a specific grain size, so as
to exhibit not only good longitudinal crushing performance but also
good transverse crushing performance.
[0007] PTL 5 suggests that an extrudate of 6000-series aluminum
alloy to be used as reinforcements should have not only a fibrous
structure but also an anisotropically elongating structure so as to
have both good bending workability and good crush-cracking
resistance. According to this patent literature, the aluminum alloy
extrudate contains excess Si and also contains transition metal
elements such as Mn, Cr, and Zr in a relatively large total amount
of 0.15% to 0.30% as indicated in the working examples. Moreover,
it mentions that extrusion should be performed at a relatively low
temperature under 500.degree. C. with a high extrusion ratio of
more than 10, so that the extrudate has the fibrous structure
including crystal grains elongating in the extrusion direction,
with the aspect ratio of more than 5. The resulting extrudate has
an anisotropic structure such that the elongation (81) in the
direction deviating by 45 degrees from the extrusion direction is
larger than elongation (82) and (83) in the direction parallel and
perpendicular, respectively, to the extrusion direction.
[0008] PTL 6 suggests that an extrudate of a 6000-series aluminum
alloy to be used as side members and bumper reinforcements should
have a fine equiaxed granular structure (with the aspect ratio of
crystal grains of 3 or less) instead of the fibrous structure so as
to have both good bending workability and good impact absorbing
performance. The aspect ratio is a ratio in length of the long axis
to the short axis of a crystal grain. According to this patent
literature, the fine equiaxed granular structure contributes to
improved elongation and bending workability and also restricts the
amount and size of intergranular precipitates, thereby preventing
fragmentation of crystal grains from occurring at intergranular
precipitates under an impact.
[0009] PTL 1: Japanese Unexamined Patent Application Publication
(JP-A) No. 2002-241880
[0010] PTL 2: Japanese Unexamined Patent Application Publication
(JP-A) No. H05-171328
[0011] PTL 3: Japanese Unexamined Patent Application Publication
(JP-A) No. H09-256096
[0012] PTL 4: Japanese Unexamined Patent Application Publication
(JP-A) No. 2003-183757
[0013] PTL 5: Japanese Unexamined Patent Application Publication
(JP-A) No. 2005-105317
[0014] PTL 6: Japanese Unexamined Patent Application Publication
(JP-A) No. H06-25783
SUMMARY OF INVENTION
Technical Problem
[0015] When a 6000-series aluminum alloy extrudate is practically
used as automotive body reinforcements, such as bumper
reinforcements and door guard bars, they usually receive a
concentrated collision force in the approximately horizontal
direction. In such a situation, the 6000-series aluminum alloy
extrudate is poor in bending crush resistance, which is important
for improvement in transverse crushing performance, even though it
has the fibrous structure or anisotropic structure (as suggested in
PTL 2 to 5) or it has the equiaxed granular structure (as suggested
in PTL 1 and PTL 6).
[0016] To cope with collision under more critical conditions, it is
necessary to improve a 6000-series aluminum alloy extrudate in
bending crush resistance. Meeting this requirement, however, is
limited even with the relatively strong extrudate having the
fibrous structure, as well as the extrudate having the equiaxed
granular structure disclosed in the PTL 1 and PTL 6.
[0017] It is also effective for improving the bending crush
resistance of reinforcements to suitably design the cross sectional
shape of extrudates (reinforcements), in addition to improvements
in material strength. However, insufficiency in bending crush
resistance, which is important for improvements in transverse
crushing performance, can obviously happen, depending on the
magnitude of the load, not only to aluminum alloy bumper
reinforcements having an approximately rectangular, simple hollow
cross-section but also to aluminum alloy bumper reinforcements
provided with one or more inner ribs for further reinforcement,
such as one with a single inner rib provided in a central part of
the cross section in parallel with the upper and lower side walls,
or one with double inner ribs provided at a certain spacing in
parallel with the upper and lower side walls, or one with cross
inner ribs connected to four sides of the cross section.
[0018] For these reasons, 7000-series aluminum alloy extrudates,
which have higher strength than that of 6000-series aluminum alloy
extrudates, have been still used mainly in energy absorbing
members, such as bumper reinforcements and door guard bars, which
should laterally crush (which require transverse crushing
performance). However, the 7000-series aluminum alloy extrudates,
as containing large amounts of alloy components, are not suitable
for the recycling and have high production cost. They also have
corrosion resistance inferior to that of 6000-series aluminum alloy
extrudates.
[0019] The present invention has been made under these
circumstances, and an object of the present invention is to provide
a 6000-series aluminum alloy extrudate and a manufacturing method
thereof; which aluminum alloy extrudate excels both in bending
crush resistance and corrosion resistance which are required of
reinforcements of automotive bodies subject to collision under more
severe conditions.
Solution To Problem
[0020] To achieve the object, the present invention provides an
aluminum alloy extrudate excellent in bending crush resistance and
corrosion resistance, being an extrudate of an Al--Mg--Si aluminum
alloy including, in terms of percent by mass, Mg in a content of
from 0.60% to 1.20%, Si in a content of from 0.30% to 0.95%, Fe in
a content of from 0.01% to 0.40%, Mn in a content of from 0.30% to
0.52%, Cu in a content of from 0.001% to 0.65%, and Ti in a content
of from 0.001% to 0.10%, the contents of Mg and Si satisfying
condition [Mg (%)]-(1.73.times.[Si (%)]-0.25).gtoreq.0, with the
remainder including Al and inevitable impurities. The aluminum
alloy extrudate has an equiaxed recrystallized structure with an
area ratio of recrystallized grains of 65% or more in a cross
section in a thickness direction. The aluminum alloy extrudate has
an average spacing of more than 25 .mu.m between intergranular
precipitates each having a size of 1 .mu.m or more in terms of
centroid diameter in observation of the structure under a
transmission electron microscope (TEM) 4'5000 magnifications, and
has an average area ratio of Goss orientation grains of less than
8%, throughout the entire thickness region including an outermost
grain growth layer in the cross section in the thickness direction
of the extrudate.
[0021] The aluminum alloy extrudate may further selectively contain
at least one of Cr in a content of from 0.001% to 0.18% and Zr in a
content of from 0.001% to 0.18%, as replacing part of Mn, within
such a range that a total content of Mn, Cr, and Zr be from 0.30%
to 0.52%.
[0022] The aluminum alloy extrudate according to the present
invention, as having the specific chemical composition and
structure, may have such bending crush resistance as to have a
critical bending radius (R) of 3.0 mm or less without cracking in a
180-degree bending test according to a press-bending method
prescribed in Japanese Industrial Standards (JIS) Z2248 in which a
plate-shaped specimen is bent in an extrusion direction; and the
aluminum alloy extrudate may have such corrosion resistance that a
specimen does not suffer from intergranular corrosion in an
alternating immersion corrosion test prescribed in International
Organization for Standardization/Draft of International Standard
(ISO/DIS) 11846B. The aluminum alloy extrudate will find use as
energy absorbing members which crush under a load in a direction
perpendicular to the extrusion direction.
[0023] The aluminum alloy extrudate having the equiaxed
recrystallized structure, intergranular precipitate distribution,
and texture and excelling in bending crush resistance and corrosion
resistance may be manufactured by soaking a cast billet of an
Al--Mg--Si aluminum alloy at a temperature of 560.degree. C. or
higher, the aluminum alloy having the chemical composition as
defined above; forcedly cooling the soaked cast billet to a
temperature of 400.degree. C. or lower at an average cooling rate
of 100.degree. C./hr or more; reheating the cooled cast billet and
subjecting the reheated billet to hot extrusion so that an
extrudate reaches a solid solution temperature of 575.degree. C. or
higher at an extruder exit; immediately forcedly cooling the
extrudate from the extruder exit at an average cooling rate of
5.degree. C./second or more; and subjecting the cooled extrudate to
aging. The aging is preferably performed under such conditions that
the aged extrudate has a 0.2% yield strength of 280 MPa or
more.
Advantageous Effects of Invention
[0024] The present inventors paid attention to the texture of the
6000-series (Al--Mg--Si) aluminum alloy extrudate which had not
attracted attention so much in the past, and they investigated anew
the effect of the texture on the bending crush resistance. As a
result, they found that, of such textures, an equiaxed
recrystallized structure with less Goss orientation grains
significantly effectively improves the bending crush
resistance.
[0025] Much has been studied about the texture of 6000-series
aluminum alloy in the field of rolled plates to elucidate its
effect on the press formability and bending workability of
automotive panels. Bending workability includes hem formability,
particularly flat hem formability. There are a number of patent
literature based on such studies. Typically, it is known that the
texture of 6000-series aluminum alloy is more effective in
improvement of flat hem formability with an increasing ratio of the
crystal grains having the Cube orientation. As known well, the Cube
orientation is the major orientation of the texture in the rolled
sheet of 6000-series aluminum alloy.
[0026] Unlike extrudates (for use as reinforcements), the rolled
sheets of 6000-series aluminum alloy are used as automotive body
panels and therefore they are very thin (about 1 mm or less) for
weight saving. Moreover, when they undergo press forming (stamping)
and bending, they receive a bending load which is different from
collision load the extrudates receive. The bending load is applied
by molds and punches almost uniformly over a broad area of the
sheet. In addition, the rolled sheet has a relatively low strength
(150 MPa or less in terms of 0.2% yield strength), even in the case
of T4 material, in consideration of formability into automotive
body panels.
[0027] The present invention, however, is intended to be adopted to
an extrudate (reinforcement) which has a relatively large wall
thickness of 2 mm or more than that (about 1 mm or less) of the
rolled sheet and also has a rectangular hollow cross section This
extrudate is a high-strength one having a 0.2% yield strength of
280 MPa or more. The above-mentioned rolled sheet receives a
bending load upon hemming, but the bending load basically differs
in deformation mechanism and pattern from that which the extrudate
according to the present invention experiences upon vehicle
collision (such as pole collision or offset collision) involving
locally concentrated loads. The relation between flat hem
formability and Cube orientation in the texture of rolled thin
sheet of 6000-series aluminum alloy is useless for predicting how
bending crush resistance varies depending on Goss orientation in
the texture of the 6000-series aluminum alloy extrudate according
to the present invention.
[0028] In the field of 6000-series aluminum alloy extrudate, it has
been common practice to cause the extrudate to have the fibrous
structure elongating in the extrusion direction in order that the
resulting hollow extrudate has good crushing performance in the
axial (lengthwise) direction (longitudinal crushing) and lateral
(or crosswise) direction (transverse crushing), as disclosed in PTL
2 mentioned above. This technique fails to pay attention on the
texture itself. Also for this reason, it is very difficult to
predict how the bending crush resistance varies depending on the
Goss orientation in 6000-series aluminum alloy extrudate.
[0029] According to the present invention, the 6000-series aluminum
alloy extrudate is designed to have the texture in the form of
equiaxed recrystallized structure with less growth of Goss
orientation (grains having the Goss orientation) so as to improve
bending crush resistance. Thus, the 6000-series aluminum alloy
extrudate can be used as energy absorbing members, such as bumper
reinforcements and door guard bars, which crush under loads in the
lateral or transverse direction, in the same way as the extrudate
of 7000-series aluminum alloy which has a relatively higher
strength, with the former outperforming the latter in corrosion
resistance.
Best Modes For Carrying Out the Invention
[0030] The 6000-series aluminum alloy extrudate according to the
present invention will be illustrated in detail with reference to
embodiments thereof.
Texture
[0031] The 6000-series aluminum alloy extrudate as a reinforcement
improves in, bending crush resistance with a decreasing average
area ratio of crystal grains having the Goss orientation (Goss
orientation grains).
[0032] The present invention should meet the following requirements
in order that the extrudate used as a reinforcement has improved
bending crush resistance. The extrudate should have the
metallographic structure whose cross section in the thickness
direction shows the equiaxed recrystallized structure and also the
average area ratio of Goss orientation grains is less than 8%, and
preferably less than 5%, throughout the entire thickness region
including an outermost grain growth layer in the cross section in
the thickness direction of the extrudate.
[0033] The extrudate, if having an average area ratio of Goss
orientation grains of 8% or more, may fail to improve in bending
crush resistance when the extrudate is designed to have a high
strength, and may fail to satisfy requirements (specifications) as
automotive reinforcements.
[0034] The relation between the Goss orientation and the bending
crush resistance may be described as follows.
[0035] The yield stress .sigma.y of a polycrystal is indicated by
the equation: .sigma.y=M.tau..sub.CRSS wherein M is the Taylor
factor, and .tau..sub.CRSS is the critical resolved shear stress of
crystal. The Taylor factor M is a constant corresponding to the
crystal orientation, reaches a maximum of 3.674 when the tension
axis is in parallel with [110] and [111], and reaches 2.449, near
to the minimum 2.300, when the tension axis is in parallel with
[100]. The critical resolved shear stress .tau..sub.CRSS has a
constant value. It has been pointed out that the bending
workability correlates with the Taylor factor.
[0036] The Taylor factor M reaches a maximum of 3.674 when the
tension axis is in the Goss orientation, to increase the stress
(yield stress cry) required upon deformation, and this often causes
formation of a shear zone upon lateral bending deformation. As a
result, crystal grains having the Goss orientation, if present in a
large quantity, lead to deteriorated bending crush resistance.
[0037] As used herein the term "average area ratio of Goss
orientation grains" is defined as the average area ratio throughout
the entire thickness region including an outermost grain growth
layer in a cross section in the thickness direction (cross section
in a direction perpendicular to the extrusion direction;
perpendicular cross section) of the extrudate. In the cross section
in the thickness direction of the extrudate, there exist usually on
both sides of the outermost surface the grain growth layer (GG
layer or layer of coarse recrystallized structure) with a thickness
of several hundred micrometers which inevitably occurs as the
outermost surface comes into contact with the extrusion die. The
outermost GG layer can have an orientation distribution different
from that of internal equiaxed recrystallized structure other than
the GG layer. Accordingly, in the present invention, the average
area ratio of Goss orientation grains is prescribed in terms of the
average area ratio throughout the entire thickness region including
an outermost grain growth layer in a cross section in the thickness
direction of the extrudate.
[0038] In addition, with the help of the crystal orientation
analyzing method (SEM/EBSP method) mentioned later, it is possible
to measure Goss orientation grains throughout the entire thickness
region including the outermost grain growth layer, for example,
over the region broader than a thickness of 2 mm of the extrudate,
and it is also possible to obtain the average of the area ratios.
In contrast, X-ray diffractometry (such as one for X-ray
diffraction intensity), which is commonly used for measurement of
the texture, is designed to measure the structure (or texture) in a
relatively microscopic (small) region for each crystal grain as
compared to the crystal orientation analyzing method that employs
SEM/EBSP. Therefore, the X-ray diffractometry needs a large number
of measurements to cover the area larger than 2 mm over the entire
region in the thickness direction of the extrudate, and it is
practically incapable of measuring the average area ratio of Goss
orientation grains over the entire region in the thickness of the
extrudates, as defined in the present invention.
Equiaxed Recrystallized Structure
[0039] The reason why the present invention is intended for the
extrudate to have an equiaxed recrystallized structure is that the
fibrous structure as disclosed in PTL 2 to 5, in which the crystal
grain have an aspect ratio of more than 5 and elongate in the
extrusion direction, may seldom allow an extrudate to have high
strength and to excel in bending crush resistance. As used herein
the term "equiaxed recrystallized structure" refers to an equiaxed
granular structure in which crystal grains have an average aspect
ratio of less than 5 even if crystal grains elongate in the
extrusion direction. The term "aspect ratio of crystal grain"
refers to the ratio of the major axis to the minor axis, with the
major axis being measured in the extrusion direction and the minor
axis being measured in the thickness direction.
[0040] The equiaxed recrystallized structure should have an area
ratio of recrystallized grains of 65% or more in a cross section in
the thickness direction. The equiaxed recrystallized structure, if
having an area ratio of recrystallized grains of smaller than this
range, may have insufficient bending crush resistance. The area
ratio of recrystallized grains is preferably 80% or more.
[0041] The customary techniques disclosed in PTL 1 and PTL 6
mentioned above are intended for the 6000-series aluminum alloy
extrudate to have the equiaxed recrystallized structure. However,
though being able to provide the equiaxed recrystallized structure,
the techniques fail to make Goss orientation grains be in an
average area ratio of less than 8% throughout the entire region of
the thickness direction of the cross section of the extrudate.
[0042] The technique disclosed in PTL 1 is designed to manufacture
the extrudate in such a manner that Mg and Si are contained in
stoichiometrically equivalent contents so that the equiaxed
granular structure develops; the total amount of transition metal
elements, such as Mn, Cr, and Zr, that promote the formation of
fibrous structure is limited to 0.1% or less; and extrusion is
performed at 500.degree. C. or higher, immediately followed by
water quenching for forced cooling, as in indicated in the working
examples. The extrudate manufactured in this manner has an equiaxed
granular structure having an average grain size of 100 .mu.m or
less and an aspect ratio of the crystal grain of 2 or less. PTL 6
discloses in its working examples an 6000-series aluminum alloy
extrudate which has an excess-Si composition and optionally
contains transition metal elements, such as Mn, Cr, and Zr, in a
relatively large total amount of 0.34%. The extrudate does not
undergo forced cooling such as water quenching (that immediately
follows extrusion at 500.degree. C.) on-line but undergoes
separately solution treatment and quench hardening off-line.
[0043] In contrast, in order for the extrudate to have the grain
structure such that growth of Goss orientation is suppressed to
such an extent that its area ratio is less than 8% throughout the
entire region of the thickness direction of the cross section of
the extrudate, as intended in the present invention, it is
necessary to control manufacturing conditions, typified by soaking
treatment temperature, forced cooling after the soaking treatment,
and extruder exit temperature. In addition, the chemical
composition of the extrudate which does not contain a significantly
excess amount of Si, unlike the technique disclosed in PTL 6, and
contains transition metal elements, such as Mn, Cr, and Zr, in a
content controlled within a specific range, is one condition for
Goss orientation grains to be suppressed, as mentioned later. For
these reasons, the extrudates disclosed in PTL 1 and PTL 6, which
were manufactured by the process merely involving an ordinary
soaking treatment without positive control mentioned above, do not
suppress Goss orientation grains even when the other conditions
than those mentioned above are the same as in the present
invention. In other words, the extrudates according to the
techniques disclosed in PTL 1 and PTL 1 and PTL 6 have the texture
with random crystal orientations and hence they have an average
area ratio of Goss orientation grains being inevitably higher than
that in the extrudate according to the present invention.
Specifically, the ordinary manufacturing method gives extrudates
which have an equiaxed granular structure but fail to have an
equiaxed granular structure in which Goss orientation grains are
suppressed as intended in the present invention.
Measurement of Goss Orientation
[0044] The area ratio (the existence ratio) of Goss orientation
grains (or orientation components of each crystal grain) is
measured on the cross section (cross section in the thickness
direction) typically of the flange (front wall) of the extrudate by
a crystal orientation analyzing method (SEM/EBSP method) employing
an EBSP (electron backscatter diffraction pattern) with help of a
SEM (scanning electron microscope).
[0045] The crystal orientation analyzing method employing EBSP is
carried out in such a manner that a specimen placed in the lens
barrel of an SEM is irradiated with electron beams so that an EBSP
is projected onto a screen. The image on the screen is photographed
by a high-sensitivity camera and captured into a computer. In the
computer, the image is analyzed and compared with patterns which
have been obtained by simulation with known crystal systems, so
that the crystal orientation is identified.
[0046] The crystal orientation analysis employing EBSP is not
performed on individual crystals but is performed on a specific
region of the specimen through scanning at certain intervals.
Therefore, the above-mentioned process is performed on all the
points of measurement automatically, and there are obtained tens to
hundreds of thousands of data for crystal orientation at the end of
measurement. This measuring method offers the advantage of
permitting observation over a wide field of view and providing
information about a large number of crystal grains, including
average grain size, standard deviation of average grain size, and
orientation analysis, within a few hours. Therefore, the method is
most suitable for the measurement of the texture which is to be
analyzed over the entire region, such as a broad region of 2 mm or
more in thickness, in the thickness direction of the extrudate,
including the outermost GG layer, as in the present invention.
[0047] The crystal orientation analysis employing EBSP uses a
specimen for observation of structure which is taken from the cross
section of the extrudate. The cross section covers all the
directions in the thickness, including the outermost GG layer. The
specimen is prepared by mechanical polishing, buffing, and
electrolytic polishing. The resulting specimen is examined
typically by JEOL JSM 5410 (supplied by Japan Electron Optics
Laboratory Ltd) typically with the EBSP measuring and analyzing
system "OIM" (Orientation Imaging Macrograph) bundled with an
analyzing program called OIMAnalysis, supplied by TexSEM
Laboratories, Inc. The analysis judges whether or not each crystal
grain has a target orientation (or within 15 degrees from the ideal
orientation) and then determines the density of an orientation
(area ratio of grains having the orientation) in the field of view
for which measurement has been carried out. Measurement is carried
out at suitable several points at intervals of 3 .mu.m or less in
the cross section of the extrudate, and the measured values are
averaged.
[0048] The region of specimen for measurement is usually divided
into, for example, hexagonal sections and each section is
irradiated with electron beams so that reflected beams form a
Kikuchi pattern. Two-dimensional scanning with electron beams on
the specimen surface to measure the crystal orientation at
predetermined intervals gives the distribution of orientations of
the specimen surface. The thus obtained Kikuchi pattern is analyzed
to define the crystal orientation at the position for incident
beams. Specifically, the resulting Kikuchi pattern is compared with
the data of known crystal structures to identify the crystal
orientation at the measurement point, based on which the average
area ratio of Goss orientation grains is determined.
Texture
[0049] The texture including Goss orientation grains of the
extrudate is examined in accordance with prescriptions and
measurement procedure for rolled sheets, with the measurement
portions being regarded as a plate.
[0050] Each orientation is represented as follows according to
"Texture" written and edited by NAGASHIMA Shinichi (published by
MARUZEN Co., Ltd.) and "Light Metals" edited by The Japan Institute
of Light Metals, vol. 43 (1993), pp. 285-293. Cube orientation:
{001}<100>; Goss orientation: {011}<100>; CR
orientation: {001}<520>; RW orientation:
{001}<110>[corresponding to Cube orientation turned with
respect to the (100) plane]; Brass orientation: {011}<211>; S
orientation: {123}<634>; Cu orientation: {112}<111>(or
D orientation: {4411}<11118>); SB orientation:
{681}<112>
Intergranular Precipitates
[0051] In order for the 6000-series aluminum alloy extrudate to
exhibit good bending crush resistance and corrosion resistance when
used as reinforcements, the present invention specifies that the
extrudate has the texture in which intergranular precipitates each
having a size of 1 .mu.m or lager in terms of centroid diameter are
separate from one another at an average spacing of 25 .mu.m or more
in the observation under a TEM of 5000 magnifications. The larger
the average spacing between intergranular precipitates is, the
better.
[0052] As used herein the term "intergranular precipitates"
(precipitates present at grain boundaries) denotes compounds such
as MgSi or elementary Si, which are expected from the chemical
composition of the 6000-series aluminum alloy. MgSi forms, for
example, .beta.' phase to increase the strength (yield strength) of
the 6000-series aluminum alloy extrudate used as reinforcements.
Intergranular precipitates, however, are harmful if they are
excessively coarse and they exist excessively densely (in an
excessively large amount); they will cause rupture and propagate
rupture even when the texture is controlled as mentioned above,
thereby deteriorating the bending crush resistance and corrosion
resistance of the extrudate used as automotive reinforcements. The
distance between intergranular precipitates can be said as a
precondition to allow the 6000-series aluminum alloy extrudate to
exhibit good bending crush resistance and corrosion resistance.
[0053] In the case where intergranular precipitates 1 .mu.m or
lager in terms of the centroid diameter less than 25 .mu.m separate
from one another on average in the observation under a TEM of 5000
magnifications, intergranular precipitates are coarse and
excessively close to one another and are distributed densely.
Therefore, they cause intergranular rupture and propagate the
rupture when they receive bending loads (crush-induced loads) upon
collision. Therefore, the extrudate used as automotive
reinforcements decreases in bending crush resistance even though
the texture is controlled as mentioned above. Incidentally,
intergranular precipitates smaller than 1 .mu.m in terms of the
centroid diameter do not affect bending crush resistance and
corrosion resistance so much. Therefore, such small intergranular
precipitates are not taken into account herein in order to clarify
the relation between the distance between intergranular
precipitates as specified in the present invention and the
aforementioned properties.
Measurement of Average Spacing Between Intergranular Precipitates
And Size of Intergranular Precipitates
[0054] Measurement of the average spacing between intergranular
precipitates and the size of intergranular precipitates is
performed on the equiaxed recrystallized structure in the cross
section of the extrudate. Unlike in the observation of the texture
as mentioned above, this structure excludes the outermost GG layer
and includes an inner portion of the extrudate, such as the central
part in the thickness direction of the extrudate. A specimen for
equiaxed recrystallized structure is made into a thin film, whose
structure is subsequently observed under a TEM of 5000
magnifications to measure the parameters.
[0055] The observation under a TEM is designed to examine the
structure in a more microscopic (much smaller) region than in the
crystal orientation analysis by means of SEM/EBSP mentioned above.
It needs a huge number of measurements if observation is performed
over the entire region in the thickness direction of the extrudate.
To avoid this, in actual practice, observation is performed at one
point in the center of the thickness such that the total field of
view is 800 .mu.m.sup.2 or more, and this procedure is repeated at
ten points an adequate distance apart in the lengthwise direction
of the extrudate, and the resulting data are averaged. The centroid
diameter of each intergranular precipitate is expressed in terms of
the diameter of an equivalent circle (equivalent circle diameter,
projected area diameter). All the intergranular precipitates in the
field of view are examined for the diameter of equivalent circle
(centroid diameter), and those each having a centroid diameter of 1
.mu.m or more are selected. The average spacings between all the
adjacent intergranular precipitates thus selected are measured, and
the resulting data are averaged.
Chemical Composition
[0056] The chemical composition of the 6000-series aluminum alloy
to which the present invention is applied will be described below.
The 6000-series aluminum alloy requires properties such as
satisfactory bending crush resistance and good corrosion resistance
so as to be used as automotive reinforcements as mentioned
above.
[0057] To meet this requirement, the 6000-series aluminum alloy
extrudate (or the cast billet as a raw material thereof) to which
the present invention is applied is an Al--Mg--Si aluminum alloy
containing, as its chemical composition in percent by mass, Mg in a
content of from 0.60% to 1.20%, Si in a content of from 0.30% to
0.95%, Fe in a content of from 0.01% to 0.40%, Mn in a content of
from 0.30% to 0.52%, Cu in a content of from 0.001% to 0.65%, and
Ti in a content of from 0.001% to 0.10%, the contents of Mg and Si
satisfying the condition: [Mg (%)] -(1.73.times.[Si
(%)]-0.25).gtoreq.0, with the remainder including Al and inevitable
impurities. The chemical composition may further selectively
include at least one of Cr in a content of from 0.001% to 0.18% and
Zr in a content of from 0.001% to 0.18% as replacing part of Mn,
with a total content of Mn, Cr, and Zr being from 0.30% to 0.52%.
All percentages (%) for the contents of respective elements herein
are in terms of percent by mass.
[0058] Any other elements than listed above are basically
impurities. The content of such impurities should be lower than the
level allowed by the Aluminum Association (AA) standards and
Japanese Industrial Standards (JIS). However, contamination with
impurities is liable to occur when the molten metal (melt) is
prepared from not only high-purity aluminum ingots but also scraps
of 6000-series alloys and other aluminum alloys in large amounts
for the purpose of recycling. Reducing these impurity elements, for
example, below the detection limit increases production cost, and a
certain level of their content should be permitted. Therefore,
other elements than listed above may be permitted according
typically to the AA standards or JIS.
[0059] Preferred content and meaning thereof, or permissible
content of each element in the 6000-series aluminum alloy will be
illustrated below.
Si
[0060] The Si content should be from 0.30% to 0.95% on the
precondition that the aforementioned condition between Si and Mg
contents is satisfied. The Si content is preferably from 0.40% to
0.70%, and more preferably from 0.40% to 0.60% so as to give the
balanced alloy mentioned above. Silicon (Si) is, as well as Mg, an
essential element which contributes to solid-solution strengthening
and forms aging precipitates (which contribute to strengthening) in
crystal grains upon artificial aging treatment at a low
temperature, thereby exhibiting the ability of age hardening and
imparting strength (yield strength) of 280 MPa or more necessary
for reinforcements. If in an excessively low content, Si may not
form the compound phase upon artificial aging treatment, without
attaining the age hardening and desired strength. If in an
excessively high content, Si may not give the balanced alloy which
has the texture specified in the present invention. An excessively
high content of Si may increase intergranular precipitates and be
detrimental typically to bending workability and weldability.
Mg
[0061] The Mg content should be from 0.60% to 1.20%, on the
precondition that the condition between Si and Mg contents is
satisfied. The Mg content is more preferably from 0.70% to 1.1% so
as to give the balanced alloy. Magnesium (Mg) is an essential
element which causes solid-solution strengthening and forms,
together with Si, aging precipitates (which contribute to
strengthening) in crystal grains upon artificial aging treatment,
thereby exhibiting the ability of age strengthening and imparting
strength (yield strength) of 280 MPa or more necessary for
reinforcements. If in an excessively low content, Mg may not form
the compound phase upon artificial aging treatment, without
attaining the age hardening and desired strength. If in an
excessively high content, Mg may not give the balanced alloy. An
excessively high content of Mg is detrimental to bending
workability.
Contents of Mg And Si
[0062] In order that the 6000-series aluminum alloy extrudate has
the equiaxed recrystallized structure having an average area ratio
of Goss orientation grains of less than 8% and having an average
spacing of 25 .mu.m or more between intergranular precipitates
having a size of 1 .mu.m or more in terms of centroid diameter, the
contents of Mg and Si satisfies the condition Mg (%)-(1.73.times.Si
(%)-0.25).gtoreq.0. This relationship was established for the
6000-series aluminum alloy specified in the present invention to be
a balanced alloy which contains Mg and Si in stoichiometrically
equivalent amounts or an Si-excess aluminum alloy with a relatively
low content of Si.
[0063] A 6000-series aluminum alloy which contains Si in an amount
more than specified by the condition [Mg.gtoreq.1.73.times.Si], or
an aluminum alloy of Si-excess type which contains Si in a more
excessively large amount, increases in yield strength upon
artificial age hardening treatment at a relatively low temperature
and exhibits good age hardening performance (baking hardening
performance; BH performance) that imparts necessary strength.
Therefore, it is commonly used in the field of 6000-series aluminum
alloy sheets which need good formability and high strength after
forming so that it is made into automotive panels by press forming
(stamping) or bending.
[0064] However, if the 6000-series aluminum alloy extrudate
according to the present invention has the Si-excess composition,
Si remains undissolved during extrusion and becomes nuclei having
various crystal orientations, resulting in the texture with random
orientations, possibly resulting in relatively developed Goss
orientation grains. The resulting extrudate also tends to have the
fibrous structure elongating in the extrusion direction.
[0065] For this reason, any extrudate manufactured from an
6000-series aluminum alloy containing excess Si would not have the
equiaxed recrystallized structure having an average area ratio of
Goss orientation grains of less than 8% over the entire region in
the thickness direction of the extrudate, the thickness region
including the outermost grain growth layer, though this depends on
the manufacturing conditions such as extrusion conditions.
Moreover, Si, if in an excess content, may cause a larger number of
coarse intergranular precipitates derived from Si, which would
prevent the formation of the above-mentioned texture and the
structure having an average spacing between intergranular
precipitates having a size of 1 .mu.m or more in terms of centroid
diameter, which is necessary for the extrudate to exhibit good
bending crush resistance and corrosion resistance as
reinforcements. Therefore, if the Si content exceeds such an amount
as to satisfy the condition: Mg (%)-(1.73.times.Si
(%)-0.25).gtoreq.0, the extrudate would not exhibit good bending
crush resistance and corrosion resistance when used as
reinforcements. This depends on the manufacturing conditions such
as extrusion conditions.
Fe
[0066] Iron (Fe) functions in the same way as Mn Cr, and Zr to form
dispersed particles (dispersoids), impedes grain boundary migration
after recrystallization, prevents crystal grains from becoming
coarse, and makes crystal grains fine. Fe is an element which
inevitably contaminates in a certain amount (substantial amount)
from scraps as a raw material for molten metal. For these reasons,
the content of Fe should be from 0.01% to 0.40%. Fe, if in an
excessively low content, may not exhibit these effects. Fe, if in
an excessively high content, may tend to cause coarse crystals such
as Al--Fe--Si crystals, which impair properties such as fracture
toughness and fatigue characteristics. The Fe content is more
preferably from 0.1% to 0.3%.
Mm
[0067] Manganese (Mn) is a transition metal element like Cr and Zr
and is necessary for prevention of crystal grains from becoming
coarse. These elements selectively combine with other alloy
elements to form dispersed particles (dispersoids) of Al--Mn and
other intermetallic compounds upon soaking treatment and subsequent
hot extrusion The dispersed particles are fine and dispersed
densely and uniformly (to varied degrees depending on the
manufacturing conditions), so that they effectively hinder grain
boundary migration (as pinning effect) after recrystallization and
highly effectively prevent crystal grains from becoming coarse and
make crystal grains fine. Mn, if in an excessively low content, may
exhibit insufficient pinning force on grain boundaries, allow the
growth of Goss orientation to an average area ratio of Goss
orientation grains of 8% or more to thereby tend to deteriorate
bending workability. Mn is also expected to dissolve in the matrix
to increase strength.
[0068] Excess Mn, however, may cause the extrudate to have the
fibrous structure elongating in the extrusion direction. This
prevents the formation of the equiaxed recrystallized structure
having an average area ratio of Goss orientation grains of less
than 8%. Moreover, excess Mn tends to form, upon melting and
casting, coarse intermetallic compounds and crystals which cause
rupture and cause the extrudate to decrease in required properties
as reinforcements, such as bending crush resistance and corrosion
resistance, and bending workability of the extrudate. Therefore,
the Mn content should be from 0.3% to 0.52% (in the case where Cr
and Zr are not added).
Cu
[0069] Copper (Cu) contributes to improved strength through
solid-solution strengthening and also remarkably promotes age
hardening of the final product upon aging treatment. The content of
Cu should therefore be from 0.001% to 0.65%. Cu, if in an
excessively low content, may not exhibit the effects mentioned
above. In contrast, excess Cu may cause the extrudate to be highly
susceptible to stress corrosion cracking and intergranular
corrosion, thereby deteriorating corrosion resistance and
durability. The Cu content should therefore be as specified above.
The Cu content is more preferably from 0.2% to 0.5%.
Ti
[0070] Titanium (Ti) makes crystal grains in an ingot fine and
allows the extrudate to have the structure including such fine
crystal grains. The extrudate should contain Ti in a content of
from 0.001% to 0.10%. When the source of Ti contains boron (B), the
content of B should be from 1 to 300 ppm. Ti, if in an excessively
low content, may not exhibit the above-mentioned effects. In
contrast, excess Ti may form coarse crystals and may cause the
extrudate to decrease in required properties as reinforcements,
such as bending crush resistance and corrosion resistance, and in
bending workability of the extrudate. Therefore, an adequate
content of Ti is in the range specified above.
At Least One of Cr And Zr
[0071] Chromium (Cr) and zirconium (Zr) form dispersed particles
(dispersoids) of Al--Cr, Al--Zr, and other intermetallic compounds,
thereby effectively preventing crystal grains from becoming coarse
(as pinning effect), as with Mn. However, excess Cr and Zr, like
excess Mn, may cause the extrudate to have the fibrous structure
which elongates in the extrusion direction. When the aforementioned
effects are necessary, part of Mn is replaced with at least one of
Cr in a content of from 0.001% to 0.18% and Zr in a content of from
0.001% to 0.18% with a total content of Mn, Cr, and Zr of from
0.30% to 0.52%. Mn is desirably contained in a content of 0.13% or
more so as to cause a preferential growth of Cube orientation
grains and to relatively suppress a growth of Goss orientation
grains. Within this range, Zr is preferably contained in a content
of from 0.1% to 0.18%. This enables the extrudate to have a further
lower average area ratio of Goss orientation grains of less than 5%
to further improve bending workability while having an area ratio
of recrystallized grains of 65% or more. The improved bending
workability means that, even when the extrudate has an identical
critical bending radius R to that of one having a Zr content out of
the above-specified range, shows smaller cracking when the
extrudate is bent over the critical bending radius R and suffers
from cracking. These are achieved by the manufacturing method
according to the present invention, in which the cast billet is
soaked at a high temperature, the soaked billet is reheated to a
high temperature, extruded at a high rate and at a high extruder
exit temperature. Specifically, the heating at a high temperature
slightly weakens the pinning force derived from Al--Zr
intermetallic compound particles, allows a preferential growth of
Cube orientation grains having a high growth rate, and relatively
impedes a growth of Goss orientation grains.
Zn
[0072] Zinc (Zn) is contained in the 6000-series aluminum alloy as
an impurity. Zn, if present in a content of 0.001% or more,
effectively improves the strength due to solid-solution
strengthening and accelerates age hardening, as with Cu. In
contrast, Zn, if present in an excessively high content, may cause
the extrudate structure to have remarkably high susceptibility to
stress corrosion cracking and intergranular corrosion and to have
insufficient corrosion resistance and durability. For these
reasons, an acceptable Zn content is 0.25% or less, as with a JIS
6061 alloy.
Sectional Shape of Extrudate
[0073] The 6000-series aluminum alloy extrudate may have a suitable
sectional shape selected so as to exhibit good bending crush
resistance when used as reinforcements. The sectional shape is
preferably hollow so that the extrudate has a light weight and good
bending crush resistance required of reinforcements. The hollow
sectional shape may typically (basically) be rectangular. The
rectangular shape includes two flanges (front and rear walls) and
two webs (upper and lower walls connecting the both flanges). The
rectangular cross section may additionally have one or more inner
ribs for reinforcement (and for improvement in bending crush
resistance). Possible arrangements using such inner ribs include
one with a single inner rib provided in a central part of the cross
section in parallel with the upper and lower side walls, or one
with double inner ribs provided at a certain spacing in parallel
with the upper and lower side walls, or one with cross ribs
connected to four sides of the cross section.
[0074] The sectional shape may be modified such that the flanges
are wider than the distance between the webs (or the edges of the
flanges extend beyond the webs horizontally or vertically), or the
flanges and webs may be curved inward or outward instead of being
straight. The hollow sectional shape may be uniform over the entire
length of the extrudate (reinforcement) or may vary from one place
to another along the length partially or sequentially. Such
variation may be chosen freely in view of reinforcement design. The
extrudate to be used as the bumper reinforcement may have a hollow
sectional shape which is not completely closed but is partially
opened in any of walls and sides, instead of being the hollow shape
with completely closed cross section as described above. The
partially opened sectional shape is, however, less strong than the
completely closed one and disadvantageous for weight saving and
bending crush resistance.
Wall Thickness of Extrudate
[0075] The extrudate should have an adequate wall thickness in
relation to the sectional shape so as to exhibit better bending
crush resistance required of reinforcements. Since the present
invention is intended for automotive reinforcements that absorb
energy upon collision, the extrudate should have a certain wall
thickness unlike body panels of rolled thin sheets, so as to
exhibit good bending crush resistance required of reinforcements. A
larger wall thickness is desirable for good bending crush
resistance, but an excessively large thickness increases weight,
which is contrary to weight saving. Therefore, an adequate wall
thickness may be selected within a range of 2 to 7 mm. It is not
always necessary that the parts such as flanges, webs, and inner
ribs constituting the above-mentioned sectional shape have the same
thickness, but they may have different thicknesses. For example,
the flanges which receive loads upon collision may be thicker than
other parts.
Manufacturing Method
[0076] The method for manufacturing the 6000-series aluminum alloy
extrudate will be illustrated below. The extrudate according to the
present invention refers to one which undergoes refining (thermal
refining), such as quenching and artificial age hardening
treatment, after hot extrusion. The manufacturing process itself is
ordinary and known, except typically for the conditions for
controlling the texture.
[0077] The manufacturing method for the extrudate according to the
present invention starts with preparing a billet from the
6000-series aluminum alloy. The billet is subjected to soaking,
followed by cooling approximately to room temperature. The billet
is, heated again to a temperature for solution treatment and then
subjected to hot extrusion. The extrudate is immediately cooled to
a temperature of 190.degree. C. or lower (or down to room
temperature) for forced cooling on-line. In this way there is
obtained the extrudate having the specific sectional shape. The
extrudate that has passed through the series of hot extrusion steps
also has undergone solution and quenching treatments. Subsequently,
the extrudate undergoes cutting and straightening treatment and
optional suitable refining such as artificial age hardening.
Alternatively, the artificial age hardening may be performed
simultaneously with paint baking after the extrudate (as a
reinforcement) has been built into the automotive body and the
automotive body has been painted, instead of being performed
preliminarily while the extrudate still remains as such.
Melting And Casting
[0078] In the melting and casting step, an aluminum alloy having
the above-mentioned chemical composition conforming to 6000 series
is melted, and the molten metal is cast according to a suitable
common procedure, such as continuous casting or semicontinuous
casting (e.g., direct chill casting (DC casting)).
Soaking Heat Treatment
[0079] The billet of aluminum alloy which has been cast as
mentioned above subsequently undergoes soaking heat treatment.
Soaking temperature itself is chosen within high temperatures of
560.degree. C. or higher and lower than the melting point, and
optimally chosen within temperatures of from 560.degree. C. to
590.degree. C. The soaking heat treatment (soaking) is intended to
homogenize the structure, specifically, to eliminate segregation
from the crystal grains in the structure of the billet, thereby
making alloy elements and coarse compounds into a complete solid
solution. Soaking at a lower temperature than specified above may
not completely eliminate segregation from crystal grains; and
residual segregation may cause rupture. Soaking at a temperature of
lower than 560.degree. C. may fail to give an equiaxed
recrystallized structure (particularly in the case where the alloy
contains Zr in a content of from 0.1% to 0.18%), or even if giving
the equiaxed recrystallized structure, may cause the extrudate to
have a high area ratio of Goss orientation grains and to have
insufficient bending crush resistance.
[0080] After the soaking, the billet undergoes forced cooling to
400.degree. C. or lower (or further down to room temperature) at an
average cooling rate of 100.degree. C./hr or more. Forced cooling
is preferably accomplished at a higher cooling rate by air blowing
or with water. Once 400.degree. C. or lower is reached after
soaking, forced cooling may be stopped at the temperature, or
switched to self-cooling down to mom temperature, or continued down
to mom temperature, as arbitrarily chosen.
[0081] The cooling rate mentioned above is quite different from one
employed in a customary procedure where billets are allowed to cool
outside the soaking pit. In this case the cooling rate is usually
about 40.degree. C./hr at the highest, depending on the size of
billets; it never exceeds 100.degree. C./hr as mentioned above.
Such slow cooling results in that MgSi compounds, which have
dissolved temporarily into solid solution during soaking treatment
at a high temperature, combine, during cooling, with FeAl
compounds, which remain undissolved because of their high melting
points, to form other composite compounds (precipitates). Such
composite compounds (precipitates), once formed, remain undissolved
in the extrusion process and become nuclei having various crystal
orientations like excess Si mentioned above, thereby altering the
structure into a texture with random orientations. This impedes the
development of Cube orientation grains, relatively increases the
ratio of Goss orientation grains, and impairs bending crush
resistance.
Hot Extrusion
[0082] Next, the billet undergoes reheating and hot extrusion so
that the temperature of the extrudate (at the exit of the extruder)
be 575.degree. C. or higher, which is high enough to keep the
extrudate in solution form. Immediately after extrusion, the
extrudate undergoes forced cooling to 190.degree. C. or lower (or
further down to room temperature) at an average cooling rate of
5.degree. C./min or higher. This forced cooling is necessary to
achieve T5 refining, which is preferably combined with subsequent
artificial aging to achieve T6 refining (aging) or T7 refining
(over aging). Once the temperature of the extrudate reaches
190.degree. C. or below through cooling immediately after
extrusion, forced cooling may be stopped at the temperature, or
switched to self-cooling down to room temperature, or continued
down to mom temperature, as arbitrarily chosen. For T5 refining,
the extrudate at the exit of the extruder is kept at 575.degree. C.
or higher, which is high enough to keep the extrudate in solution
form. The extrudate undergoes solution treatment on-line (as the
result of extrusion) and, immediately thereafter, undergoes forced
cooling (for quenching) down to the neighborhood of room
temperature on-line (on the exit side of the extruder).
[0083] The temperature upon hot extrusion may be rather low so that
cube orientations develop easily and the texture of the extrudate
becomes the equiaxed recrystallized structure in which Cube
orientation grains become a domination, and the average area ratio
of Goss orientation grains is less than 8% over the entire region
in the thickness direction of the cross section of extrudate.
However, if the temperature of the extrudate at the exit of the
extruder is lower than 575.degree. C. (which is the solution
temperature), an equiaxed recrystallized structure having an area
ratio of recrystallized grains of 65% or more may not be obtained,
coarse intergranular precipitates (e.g., Mg--Si compounds
(precipitates) and elementary Si) remain undissolved in the matrix
and they start rupture to impair bending crush resistance and
corrosion resistance. To meet the trade-off requirements, it is
desirable to select a lower possible temperature of 575.degree. C.
or higher (solution temperature) for the extrudate at the exit of
the extruder. However, it is not always necessary to reheat the
cast billet to 500.degree. C. or higher for extrusion, as long as
the temperature of the extrudate at the extruder exit be
575.degree. C. or higher by the action of heat generation during
hot extrusion. Extrusion at an excessively low rate may fail to
allow the extrudate to have a temperature within the solution
temperature range. However, extrusion at a low rate tends to allow
Cube orientation grains to accumulate, whereas extrusion at a high
rate tends to allow Goss orientation grains to accumulate. For
these reasons, it is preferred to select such an extrusion rate
that the temperature of the extrudate can be risen to a solid
solution temperature without decrease in Cube orientation grains
and increase in Goss orientation grains.
[0084] The temperature of the extrudate at the extruder exit refers
to the temperature of the extrude surface immediately downstream
from the die exit (at a distance of 0 mm from the exit). When the
temperature immediately downstream from the die exit is difficult
to be measured, the aforementioned temperature may be determined by
measuring the temperature of the extrudate surface at a certain
distance from the die exit (the position where temperature
measurement can be performed varies depending on the extrusion
press to be used) with a contact type thermometer; and, from the
measured temperature, calculating backward the temperature
immediately downstream from the die exit using a cooling curve of
the extrudate which has been measured in advance.
[0085] Forced cooling for quenching down to 190.degree. C. or below
(or further down to room temperature) at an average cooling rate of
5.degree. C./second or more immediately after extrusion is intended
for the extrudate as reinforcements to improve in bending crush
resistance. Specifically, forced cooling alters the texture of the
extrudate into the equiaxed recrystallized structure having an
average area ratio of Goss orientation grains of less than 8%. In
addition, forced cooling also gives rise to intergranular
precipitates having a size of 1 .mu.m or more in terms of centroid
diameter and are separated from one another at an average spacing
of 25 .mu.m or more so as to improve bending crush resistance and
corrosion resistance. Forced cooling immediately after extrusion is
preferably performed through cooling with water. In this process,
the forced cooling may be performed on-line with any forced cooling
means arranged near the exit of the extruder, such as mist, shower
or spray typically of water, or water bath, alone or in
combination. The cooling rate for forced cooling means with water
can generally be 10.degree. C./second or more, while varying
depending on the specifications of facilities.
[0086] The T5 refining treatment saves post-extrusion steps such as
reheating, solution treatment, and quenching for the extrudate.
Under certain circumstances or for the sake of convenience, the T5
refining treatment may be replaced by the T6 refining treatment
which includes separate reheating of the extrudate after hot
extrusion to 500.degree. C. or higher, and subsequent solution
treatment, quenching, and artificial aging treatment, to give a T6
refining extrudate.
Aging Treatment
[0087] The extrudate undergoes artificial aging treatment after
cutting to a predetermined length or straightening treatment.
Artificial aging treatment is preferably carried out at a
temperature of from 150.degree. C. to 250.degree. C. for a
necessary period of time. Duration of aging treatment controls age
hardening, and may be properly selected to maximize strength (peak
aging) or extended for overaging that improves corrosion
resistance.
EXAMPLES
[0088] Next, some working examples of the present invention will be
illustrated below. Samples of extrudates were prepared from
6000-series aluminum alloys varying in composition as given in
Tables 1 and 2 and under different conditions as given in Tables 3
and 4. The extrudates each have a rectangular sectional shape with
a center inner rib. Each sample was examined for structure and
properties (such as mechanical properties and bending crush
resistance) as shown in Tables 5 and 6.
TABLE-US-00001 TABLE 1 Chemical Composition (percent by mass)
Number Si Fe Cu Mn Mg Cr Zn Ti Zr Mn + Cr + Zr Mg--(1.73Si-0.25)
Examples 1 0.544 0.189 0.352 0.343 0.945 0.001 0.014 0.022 0.143
0.49 0.25 2 0.544 0.189 0.352 0.343 0.945 0.001 0.014 0.022 0.143
0.49 0.25 3 0.544 0.189 0.352 0.343 0.945 0.001 0.014 0.022 0.143
0.49 0.25 4 0.534 0.197 0.335 0.352 0.914 0.000 0.012 0.020 0.141
0.49 0.24 5 0.538 0.215 0.348 0.202 0.955 0.052 0.008 0.022 0.145
0.40 0.27 6 0.521 0.206 0.348 0.350 0.959 0.000 0.008 0.002 0.000
0.35 0.31 7 0.581 0.200 0.344 0.351 0.927 0.050 0.012 0.023 0.001
0.40 0.17 8 0.564 0.222 0.358 0.350 0.910 0.000 0.015 0.022 0.154
0.50 0.18 9 0.549 0.194 0.331 0.361 0.903 0.000 0.012 0.023 0.156
0.52 0.20 10 0.542 0.192 0.364 0.344 0.959 0.000 0.009 0.022 0.144
0.49 0.27 11 0.450 0.210 0.352 0.351 0.720 0.000 0.010 0.021 0.150
0.50 0.19 12 0.400 0.210 0.600 0.350 0.796 0.053 0.000 0.019 0.000
0.40 0.35 13 0.700 0.150 0.150 0.350 1.100 0.000 0.010 0.020 0.020
0.37 0.14 Comparative 1 0.534 0.197 0.335 0.352 0.914 0.001 0.012
0.020 0.141 0.49 0.24 Examples 2 0.560 0.182 0.349 0.204 0.925
0.047 0.014 0.023 0.000 0.25* 0.21 3 0.553 0.187 0.344 0.204 0.940
0.052 0.008 0.022 0.000 0.26* 0.23 4 0.553 0.187 0.344 0.204 0.940
0.052 0.008 0.022 0.000 0.26* 0.23 5 0.538 0.215 0.348 0.202 0.955
0.052 0.008 0.022 0.145 0.40 0.27 6 0.581 0.200 0.344 0.351 0.927
0.050 0.012 0.023 0.001 0.40 0.17 *Data out of the conditions
specified in the present invention
TABLE-US-00002 TABLE 2 Chemical Composition (percent by mass)
Number Si Fe Cu Mn Mg Cr Zn Ti Zr Mn + Cr + Zr Mg--(1.73Si-0.25)
Comparative 7 0.584 0.214 0.347 0.354 0.934 0.049 0.000 0.022 0.140
0.54* 0.17 Examples 8 0.584 0.214 0.347 0.354 0.934 0.049 0.000
0.022 0.140 0.54* 0.17 9 0.549 0.194 0.331 0.350 0.903 0.000 0.012
0.023 0.156 0.51 0.20 10 0.900 0.080 0.160 0.100 0.570 0.050 0.000
0.020 0.000 0.15* -0.74* 11 0.410 0.200 0.170 0.000 0.750 0.050
0.000 0.020 0.000 0.05* 0.29 12 0.588 0.224 0.356 0.358 0.922 0.000
0.013 0.023 0.156 0.51 0.15 13 0.564 0.222 0.358 0.350 0.910 0.000
0.015 0.022 0.154 0.50 0.18 14 0.588 0.224 0.356 0.358 0.922 0.000
0.013 0.023 0.156 0.51 0.15 15 0.970* 0.140 0.160 0.000 0.590*
0.050 0.010 0.020 0.000 0.05* -0.84* 16 0.574 0.500* 0.353 0.198
0.954 0.050 0.012 0.018 0.007 0.26* 0.21 17 0.400 0.200 0.800*
0.050 0.800 0.050 0.100 0.020 0.000 0.10* 0.36 18 0.400 0.200 0.150
0.550* 0.800 0.050 0.020 0.020 0.000 0.60* 0.36 19 0.850 0.200
0.150 0.050 1.300* 0.050 0.100 0.020 0.000 0.10* 0.08 20 0.400
0.200 0.150 0.150 0.800 0.200* 0.000 0.020 0.200* 0.55* 0.36 21
0.400 0.200 0.150 0.050 0.800 0.050 0.350* 0.020 0.000 0.10* 0.36
22 0.400 0.200 0.150 0.050 0.800 0.050 0.000 0.200* 0.000 0.10*
0.36 23 0.220* 0.200 0.150 0.100 0.400* 0.050 0.000 0.020 0.000
0.15* 0.27 24 0.549 0.194 0.331 0.361 0.903 0.000 0.012 0.023 0.156
0.52 0.20 *Data out of the conditions specified in the present
invention
TABLE-US-00003 TABLE 3 Manufacturing Method Soaking temper- Rate of
cooling Billet heat- Extrusion Die exit Rate of cooling Num- ature
(.degree. C.) after soaking ing temper- rate temperature
immediately after ber (for 4 hrs) (.degree. C./hr) ature (.degree.
C.) (m/min) (.degree. C.) extrusion (.degree. C./s) Artificial
aging Examples 1 590 100 or more 500 10 578 13 190.degree. C. for 3
hrs 2 580 100 or more 500 10 579 14 190.degree. C. for 3 hrs 3 570
100 or more 500 10 578 13 190.degree. C. for 3 hrs 4 560 100 or
more 500 10 576 13 190.degree. C. for 3 hrs 5 580 100 or more 500
10 575 13 190.degree. C. for 3 hrs 6 580 100 or more 500 10 576 13
190.degree. C. for 3 hrs 7 580 100 or more 500 10 578 13
190.degree. C. for 3 hrs 8 580 100 or more 530 6 578 9 190.degree.
C. for 3 hrs 9 580 100 or more 520 6 579 9 190.degree. C. for 3 hrs
10 580 100 or more 500 10 578 13 190.degree. C. for 3 hrs 11 580
100 or more 500 10 581 13 190.degree. C. for 3 hrs 12 560 100 or
more 500 10 580 13 190.degree. C. for 3 hrs 13 580 100 or more 500
10 579 13 190.degree. C. for 3 hrs Comparative 1 550* 100 or more
500 10 575 13 190.degree. C. for 3 hrs Examples 2 580 100 or more
500 10 575 13 190.degree. C. for 3 hrs 3 560 100 or more 500 10 575
13 190.degree. C. for 3 hrs 4 550* 100 or more 500 10 575 13
190.degree. C. for 3 hrs 5 550* 100 or more 500 10 575 13
190.degree. C. for 3 hrs 6 550* 100 or more 500 10 575 13
190.degree. C. for 3 hrs *Data out of the conditions specified in
the present invention
TABLE-US-00004 TABLE 4 Manufacturing Method Soaking temper- Cooling
rate Billet heat- Extrusion Die exit Cooling rate Num- ature
(.degree. C.) after soaking ing temper- rate temperature
immediately after ber (for 4 hrs) (.degree. C./hr) ature (.degree.
C.) (m/min) (.degree. C.) extrusion (.degree. C./s) Artificial
aging Comparative 7 580 100 or more 500 10 575 13 190.degree. C.
for 3 hrs Examples 8 550* 100 or more 500 10 575 13 190.degree. C.
for 3 hrs 9 580 100 or more 480* 6 564* 9 190.degree. C. for 3 hrs
10 580 100 or more 500 10 575 13 190.degree. C. for 3 hrs 11 550*
100 or more 500 3 530* 4* 190.degree. C. for 3 hrs 12 580 100 or
more 500 7 570* 11 190.degree. C. for 3 hrs 13 580 100 or more 530
5 564* 8 190.degree. C. for 3 hrs 14 580 100 or more 495* 10 574*
13 190.degree. C. for 3 hrs 15 580 100 or more 500 10 575 13
190.degree. C. for 3 hrs 16 580 100 or more 500 10 575 13
190.degree. C. for 3 hrs 17 560 100 or more 500 10 575 13
190.degree. C. for 3 hrs 18 560 100 or more 500 10 575 13
190.degree. C. for 3 hrs 19 580 100 or more 500 10 575 13
190.degree. C. for 3 hrs 20 560 100 or more 500 10 575 13
190.degree. C. for 3 hrs 21 560 100 or more 500 10 530* 10
190.degree. C. for 3 hrs 22 560 100 or more 500 10 530* 10
190.degree. C. for 3 hrs 23 560 100 or more 500 10 530* 10
190.degree. C. for 3 hrs 24 580 40* 520 6 577 13 190.degree. C. for
3 hrs *Data out of the conditions specified in the present
invention
[0089] More specifically, each sample of the extrudate was prepared
in the following manner. Initially, the aluminum alloy whose
chemical composition is given in Table 1 or 2 was melted and east
into a billet. The billet underwent soaking treatment at a
temperature given in Table 3 or 4, followed by cooling to room
temperature at an average cooling rate (.degree. C./hr) given in
Table 3 or 4 through forced air cooling by a blower. The cooled
billet was heated again to a temperature given in Table 3 or 4, and
immediately subjected to hot extrusion at an extrusion rate (m/min)
given in Table 3 or 4. A temperature (.degree. C.) measured at the
exit of the extruder (temperature of the extrudate attained at the
die exit) is shown in Table 3 or 4. Immediately after extrusion,
the extrudate underwent forced cooling to the neighborhood of room
temperature through water spraying (samples other than Comparative
Example 11) and air cooling with a blower (Comparative Example 11).
Thus there was obtained an extrudate having a rectangular sectional
shape with a central inner rib. The average cooling rate of each of
samples according to the examples and comparative examples
immediately after extrusion to a surface temperature of 190.degree.
C. is shown in Table 3 or 4. The resulting extrudate underwent
artificial age hardening treatment under conditions given in Table
3 or 4.
[0090] The extrudates having a rectangular sectional shape with a
central inner rib have outer dimensions corresponding to those for
bumper reinforcements and have the following dimensions. The
flanges (the front and rear walls) are each 40 mm long and 2.3 mm
thick. The webs (the side walls) and the central inner rib are each
40 mm long and 2.0 mm thick. The extrudate was cut to a length of
1300 mm.
[0091] After artificial age hardening treatment, the web (side
wall) of the extrudate was cut into a specimen in sheet form. The
specimen was examined for structure and characteristic properties.
The results are shown in Tables 5 and 6.
Specimen Structure
Average Area Ratio of Goss Orientation Grains
[0092] After refining as mentioned above and being left stand at
room temperature for 15 days, the specimen was examined for texture
by means of the SEM-EBSP. The texture was analyzed to obtain the
average area ratio (%) of Goss orientation grains over the entire
region of the cross section in the thickness direction, including
the outermost grain growth layer.
[0093] Each specimen was also examined by the SEM-EBSP for the
recrystallized structure in terms of the aspect ratio of crystal
grains. The structure having an average aspect ratio of crystal
grains of 5 or less was identified as an equiaxed granular
structure, and one having an average aspect ratio of crystal grains
of more than 5 was identified as a fibrous structure. In the column
of "Recrystallization" in Tables 5 and 6, one having an equiaxed
granular structure content of 50% or more is indicated as an
"equiaxed recrystallized structure", and one having an equiaxed
granular structure content of less than 50% is indicated as a
"fibrous structure". In all the examples and comparative examples,
recrystallized portions were fully composed of equiaxed granular
structure.
Properties of Specimen
[0094] After refining mentioned above and being left stand at room
temperature for 30 days, the specimen was examined for properties,
such as 0.2% yield strength ("As yield strength" in MPa),
elongation (%), as well as bending crush resistance and corrosion
resistance. The results are also shown in Tables 5 and 6.
Tensile Test
[0095] The specimen was cut into a sample, No. 13 B sample
conforming to JIS Z22201, which measures 12.5 mm in width, 50 mm in
gauge length, and 2.0 mm in thickness as extruded, and the sample
underwent tensile test at room temperature. The sample length and
tensile force are parallel with the extrusion direction. The
tensile force was applied at a rate of 5 mm/min up to the 0.2%
yield strength and 20 mm/min thereafter. Five measurements (N=5)
were averaged to determine respective mechanical properties.
Test For Bending Crush Resistance (Bending Workability)
[0096] The specimen (in sheet form) was bent to 180 degrees
according to the press-bending method prescribed in JIS Z2248, in a
direction perpendicular to the extrusion direction (so that the
bending line be in parallel with the extrusion direction). The
bending test was repeated ten times, and a critical bending radius
R (mm) without rupture due to cracking in the outside of the bent
corner (or in the stretched side) in all the ten bending tests was
determined. A specimen with a decreasing critical bending radius R
was evaluated as having more satisfactory bending crush resistance.
Any specimen having a critical bending radius of more than 3.0 mm
is regarded as good in bending crush resistance and advantageously
usable as automotive reinforcements.
Test For Corrosion Resistance
[0097] The specimen was tested for corrosion resistance by dipping
under the following conditions according to ISO/DIS 11846B. The
test method includes dipping the specimen in an aqueous solution
containing 30 g/L of NaCl and 10 mL/L of HCl for 24 hours at room
temperature and subsequently observing the cross section of the
specimen to detect intergranular corrosion cracking. The specimen
was rated by the following criteria: .times.: Intergranular
corrosion cracking occurred; .DELTA.: Intergranular corrosion
occurred but intergranular corrosion cracking did not occur;
.smallcircle.: Neither intergranular corrosion cracking nor
intergranular corrosion occurred (including the case where
surficial general corrosion occurred).
TABLE-US-00005 TABLE 5 Properties of Extrudate Average spacing Area
ratio Area ratio Critical Corro- Tensile Yield Elon- between of
Goss of recrys- bending sion strength strength gation intergranular
orientation talized radius R resis- No. (MPa) (MPa) (%)
precipitates (.mu.m) Recrystalization grains (%) grains (%) (mm)
tance Examples 1 328 301 12 >25 100% Equiaxed recrystalized
structure 3 100 3.0 .largecircle. 2 330 302 12 >25 87% Equiaxed
recrystalized structure 3 87 3.0 .largecircle. 3 329 303 11 >25
88% Equiaxed recrystalized structure 1 88 3.0 .largecircle. 4 341
313 11 >25 67% Equiaxed recrystalized structure 2 67 3.0
.largecircle. 5 337 317 11 >25 72% Equiaxed recrystalized
structure 2 72 3.0 .largecircle. 6 327 303 12 >25 100% Equiaxed
recrystalized structure 6 100 3.0 .largecircle. 7 333 308 12 >25
100% Equiaxed recrystalized structure 6 100 3.0 .largecircle. 8 332
306 12 >25 91% Equiaxed recrystalized structure 7 91 3.0
.largecircle. 9 322 294 10 >25 100% Equiaxed recrystalized
structure 6 100 3.0 .largecircle. 10 323 290 12 >25 100%
Equiaxed recrystalized structure 3 100 3.0 .largecircle. 11 304 281
11 >25 100% Equiaxed recrystalized structure 3 100 3.0
.largecircle. 12 325 287 14 >25 100% Equiaxed recrystalized
structure 7 100 2.0 .largecircle. 13 336 306 11 >25 100%
Equiaxed recrystalized structure 6 100 3.0 .largecircle. Compar- 1
347 319 12 >25 Fibrous structure* 3 28 5.0* .largecircle. ative
2 331 309 12 >25 100% Equiaxed recrystalized structure 8* 100
5.0* .largecircle. Exam- 3 331 305 12 >25 100% Equiaxed
recrystalized structure 11* 100 5.0* .largecircle. ples 4 327 304
12 >25 81% Equiaxed recrystalized structure 9* 81 6.0*
.largecircle. 5 348 324 10 >25 Fibrous structure* 3 40 5.0*
.largecircle. 6 329 306 10 >25 87% Equiaxed recrystalized
structure 8* 87 5.0* .largecircle. *Data out of, or inferior to,
the conditions specified in the present invention.
TABLE-US-00006 TABLE 6 Properties of Extrudate Average spacing Area
ratio Area ratio Critical Corro- Tensile Yield Elon- between of
Goss of recrys- bending sion strength strength gation intergranular
orientation talized radius R resis- No. (MPa) (MPa) (%)
precipitates (.mu.m) Recrystalization grains (%) grains (%) (mm)
tance Compar- 7 346 315 12 >25 Fibrous structure* 3 46 4.0*
.largecircle. ative 8 348 318 11 >25 Fibrous structure* 3 37
4.0* .largecircle. Exam- 9 283 248* 10 10.0* Fibrous structure* 4
40 3.0 .largecircle. ples 10 343 328 10 5.0* 98% Equiaxed
recrystalized structure 13* 98 10 or .DELTA.* more* 11 290 260* 14
3.0* 100% Equiaxed recrystalized structure 10* 100 2.0 .DELTA.* 12
307 271* 11 10.0* 60% Equiaxed recrystalized structure 2 60* 3.0
.largecircle. 13 336 303 11 10.0* Fibrous structure* 3 40 5.0*
.largecircle. 14 315 286 12 10.0* 98% Equiaxed recrystalized
structure 7 98 4.0* .largecircle. 15 343 327 10 2.8* 100% Equiaxed
recrystalized structure 10* 100 10 or .largecircle. more* 16 319
288 13 2.8* 100% Equiaxed recrystalized structure 11* 100 4.0*
.largecircle. 17 365 313 15 2.0* 100% Equiaxed recrystalized
structure 10* 100 4.0* X* 18 299 266* 12 2.8* Fibrous structure* 4
40 5.0* .largecircle. 19 312 288 12 2.8* 100% Equiaxed
recrystalized structure 11* 100 10.0* .largecircle. 20 263 238* 11
2.8* Fibrous structure* 4 45 4.0* .largecircle. 21 298 277* 10 2.0*
100% Equiaxed recrystalized structure 13* 100 6.0* X* 22 247 236*
10 >30 100% Equiaxed recrystalized structure 9* 100 10.0*
.largecircle. 23 182 151* 15 >30 100% Equiaxed recrystalized
structure 11* 100 1.0 .largecircle. 24 327 296 12 10.0* 100%
Equiaxed recrystalized structure 6 100 5.0* .largecircle. *Data out
of, or inferior to, the conditions specified in the present
invention.
[0098] As shown in Tables 1 to 4, specimens in Examples 1 to 10
contain Mg and. Si in contents specified by the present invention
and undergo soaking and hot extrusion under preferred conditions
with regard to soaking temperature, forced cooling after soaking,
billet heating temperature, temperature at the extruder exit, and
forced cooling immediately after extrusion. Therefore, as
demonstrated in Table 5, they have the equiaxed recrystallized
grain structure (area ratio of recrystallized grains of 65% or
more) having the area ratio of Goss orientation grains and the
average spacing of intergranular precipitates, as specified in the
present invention. The specimens in the examples therefore excel in
bending crush resistance and corrosion resistance, and also excel
in mechanical properties such as strength (280 MPa or more) and
elongation. These outstanding characteristic properties suggest
that the extrudates have such good bending crush resistance
required of reinforcements as to be adaptable as automotive
reinforcements which might encounter more serious collisions such
as pole collision and offset collision.
[0099] In contrast, specimens in Comparative Examples 1, 5, and 6,
as undergoing soaking at low temperatures, are poor in bending
crush resistance. Specifically, specimens in Comparative Examples 1
and 5 (having the chemical composition containing Zr in the
specific content) fail to have an equiaxed recrystallized structure
(having an area ratio of recrystallized grains of 65% or more); and
the specimen in Comparative Example 6 (having the chemical
composition containing no Zr) suffers from large accumulation of
Goss orientation grains due to insufficient pining force.
[0100] The specimens in Comparative Examples 2 to 4, as having an
excessively low total content of Mn, Cr, and Zr, suffer from the
growth of Goss orientation grains due to insufficient pinning force
and are thereby poor in bending crush resistance, although they
have an equiaxed recrystallized structure (having an area ratio of
recrystallized grains of 65% or more).
[0101] The specimens in Comparative Examples 7 and 8, as having an
excessively high total content of Mn, Cr, and Zr, have a fibrous
structure (having an area ratio of recrystallized grains of less
than 50%) and are poor in bending crush resistance, both in the
case where the specimen was manufactured according to the method of
the present invention (Comparative Example 7) and the case where
the specimen underwent soaking at a temperature lower than the
range specified in the present invention (Comparative Example
8).
[0102] The specimen in Comparative Example 9, as undergoing
extrusion at a low temperature and a low rate, has a low
temperature at the extruder exit, thereby has a fibrous structure
(having an area ratio of recrystallized grains of less than 65%)
and is poor in yield strength.
[0103] The specimen in Comparative Example 10, as having a chemical
composition containing excess Si, suffers from the development of
Goss orientation grains, is poor in bending crush resistance, and
has insufficient corrosion resistance due to increase in coarse
intergranular precipitates, although having an equiaxed
recrystallized structure.
[0104] The specimen in Comparative Example 11, as undergoing forced
cooling immediately after extrusion at a low cooling rate and thus
suffering from quench delay suffers from increase in coarse
intergranular precipitates and insufficient corrosion
resistance.
[0105] The specimens in Comparative Examples 12 to 14, as having a
low temperature at the extruder exit and thereby suffering from
increase in coarse intergranular precipitates, fail to have an
equiaxed recrystallized structure having an area ratio of
recrystallized grains of 65% or more. Specifically, the specimen in
Comparative Example 12 has a poor strength, whereas the specimen in
Comparative Example 13 having a fibrous structure and the specimen
in Comparative Example 14 including an equiaxed recrystallized
structure (having an area ratio of recrystallized grains of 65% or
more) have poor bending crush resistance.
[0106] The specimen in Comparative Example 15 has an excessively
high Si content, the specimen in Comparative Example 16 has an
excessively high Fe content, the specimen in Comparative Example 18
has an excessively high Mn content, the specimen in Comparative
Example 19 has an excessively high Mg content, the specimen in
Comparative Example 20 has excessively high Cr and Zr contents, and
the specimen in Comparative Example 22 has an excessively high Ti
content, each being inferior in bending crush resistance. The
specimen in Comparative Example 17 has an excessively high Cu
content, and the specimen in Comparative Example 21 has an
excessively high Zn content, each being inferior in corrosion
resistance. The specimen in Comparative Example 23 has insufficient
Si and Mg contents and thereby has insufficient strength (yield
strength). The specimen in Comparative Example 24, as undergoing
post-soaking cooling at an excessively low rate and thereby
suffering from the generation of coarse Mg--Si compounds, has poor
bending crush resistance.
[0107] The foregoing results of Examples demonstrate that the
chemical composition, structure, or preferred manufacturing
conditions specified in the present invention are critical or
effective for the extrudate to have good bending crush resistance,
mechanical properties, and other properties.
INDUSTRIAL APPLICABILITY
[0108] The present invention provides the 6000-series aluminum
alloy extrudate and the manufacturing method thereof, which
extrudate has both good bending crush resistance and good corrosion
resistance required of reinforcements for automotive bodies. The
extrudate is suitable for use as automotive body reinforcements,
such as bumper reinforcements and door guard bars, which need
outstanding transverse crushing performance.
* * * * *