U.S. patent application number 12/396710 was filed with the patent office on 2009-10-01 for extruded member of aluminum alloy excelling in flexural crushing performance and corrosion resistance and method for production thereof.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd.). Invention is credited to Keiji Morita, Manabu Nakai, Shigenobu Yasunaga, Shinji Yoshihara.
Application Number | 20090242087 12/396710 |
Document ID | / |
Family ID | 41115322 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090242087 |
Kind Code |
A1 |
Morita; Keiji ; et
al. |
October 1, 2009 |
EXTRUDED MEMBER OF ALUMINUM ALLOY EXCELLING IN FLEXURAL CRUSHING
PERFORMANCE AND CORROSION RESISTANCE AND METHOD FOR PRODUCTION
THEREOF
Abstract
An extruded member of Al--Mg--Si aluminum alloy specially
composed of Mg, Si, Fe, Cu, Zn, Ti, etc. which has the equiaxed
re-crystallized grain structure in which intergranular precipitates
1 .mu.m or lager are separate from one another at large average
intervals and there are many cube orientations over the entire
thickness region thereof so that it excels in both flexural
crushing performance and corrosion resistance. The extruded member
is suitable for use as automotive body reinforcement members which
need outstanding lateral crushing performance under severe
collision conditions as well as good corrosion resistance.
Inventors: |
Morita; Keiji;
(Shimonoseki-shi, JP) ; Yoshihara; Shinji;
(Shimonoseki-shi, JP) ; Nakai; Manabu; (Kobe-shi,
JP) ; Yasunaga; Shigenobu; (Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel Ltd.)
Kobe-shi
JP
|
Family ID: |
41115322 |
Appl. No.: |
12/396710 |
Filed: |
March 3, 2009 |
Current U.S.
Class: |
148/690 ;
148/417 |
Current CPC
Class: |
C22F 1/047 20130101;
C22C 21/06 20130101 |
Class at
Publication: |
148/690 ;
148/417 |
International
Class: |
C22F 1/047 20060101
C22F001/047; C22C 21/16 20060101 C22C021/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2008 |
JP |
2008-078430 |
Claims
1. An extruded member of aluminum alloy which contains (in mass %)
Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%,
Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the
remainder being aluminum and inevitable impurities, and has the
metallographic structure whose cross section perpendicular to the
direction of extrusion shows the equiaxed recrystallized grain
structure in which intergranular precipitates 1 .mu.m or lager in
terms of the diameter of an equivalent circle are 3 .mu.m or more
separate from one another in the observation under a TEM of 5000
magnifications and also the average areal ratio of cube orientation
is 15% or larger over the entire thickness region including the
grain growth layer in the outermost surface in the cross section
perpendicular to the direction of extrusion.
2. The extruded member of aluminum alloy as defined in claim 1,
which contains Mg and Si such that
Mg(%).gtoreq.1.73.times.Si(%)-0.4. where Mg(%) and Si(%) denote the
content of Mg and Si in mass %, respectively.
3. The extruded member of aluminum alloy as defined in claim 1,
which has the equiaxed recrystallized grain structure such that the
average areal ratio of cube orientation is 20% or larger.
4. The extruded member of aluminum alloy as defined in claim 1,
which contains at least either of Cr: 0.001-0.18% or Zr:
0.001-0.18% in a total amount of 0.30% or less.
5. The extruded member of aluminum alloy as defined in claim 1,
which has flexural crushing performance such that the critical
bending radius (R) is 3.0 mm or smaller which does not cause
cracking in the 180.degree. bending test according to JIS Z2248 in
which the platy specimen is bent in the direction of extrusion, and
which has corrosion resistance such that the specimen does not
suffer intergranular corrosion in the alternating immersion
corrosion test according to ISO/DIS 11846B.
6. An energy absorbing member formed from the extruded member of
aluminum alloy defined in claim 1, which crushes under load in the
direction perpendicular to the direction of extrusion.
7. A method for producing an extruded member of aluminum alloy,
said method comprising a step of soaking a cast billet of aluminum
alloy at 500-590.degree. C., said billet containing (in mass %) Mg:
0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn: 0.001-0.35%, Cu:
0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%, with the
remainder being aluminum and inevitable impurities, a step of
subjecting the soaked billet to forced cooling to 400.degree. C. or
below at an average cooling rate of 100.degree. C./hr or above, a
step of reheating the cooled billet and subjecting the reheated
billet to hot extrusion such that the extrudate reaches the solid
solution temperature which is 500.degree. C. or higher at the
extruder exit, a step of immediately subjecting the extrudate to
forced cooling at an average cooling rate of 100.degree. C./hr or
above, and a step of subjecting the cooled extrudate to aging, so
that the resulting extruded member has a 0.2% proof stress of 240
MPa or greater and also has the metallographic structure whose
cross section perpendicular to the direction of extrusion shows the
equiaxed recrystallized grain structure in which intergranular
precipitates 1 .mu.m or lager in terms of diameter of an equivalent
circle are 3 .mu.m or more separate from one another in the
observation under a TEM of 5000 magnifications and also the average
areal ratio of the cube orientation is 15% or larger over the
entire thickness region including the grain growth layer in the
outermost surface in the cross section perpendicular to the
direction of extrusion.
8. The method for producing an extruded member of aluminum alloy as
defined in claim 7, wherein the cast billet of said Al--Mg--Si
aluminum alloy further contains at least either of Cr: 0.001-0.18%
or Zr: 0.001-0.18% in a total amount of 0.30% or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an extruded member of
Al--Mg--Si aluminum alloy excelling in flexural crushing
performance and corrosion resistance and a method for production
thereof. ("Aluminum" may be referred to as "Al" for short
hereinafter.) The term "extruded member of aluminum alloy" used in
the present invention denotes not only any members produced by hot
extrusion but also any parts incorporated into automotive bodies as
their reinforcement members (or energy absorbing members) mentioned
later.
[0003] 2. Description of the Related Art
[0004] Extruded members of aluminum alloy of 6000 series have been
used as reinforcement members. For their improvement in lateral
crushing performance (deformation due to crushing in the direction
of cross section) and bending formability, much has been suggested
about their metallographic structure.
[0005] One of such suggestions is about the method for producing
extruded members of aluminum alloy by soaking billets of aluminum
alloy of 6000 series (such as 6063), extruding the billets, and
cooling and ageing the extrudates. It is suggested that ageing
should be so performed as to provide mechanical properties
specified by 0.2% proof stress of 120-140 MPa and elongation of 12%
or more. This method is intended to produce extruded members of
aluminum alloy which have adequate 0.2% proof stress and elongation
for bending, limited variation in bending accuracy and yield
strength, and high resistance to buckling that occurs during
bending by "push through". (See Japanese Patent Laid-open No.
2001-316788.)
[0006] There is another suggestion about improvement in bending
performance by causing the extruded member of aluminum alloy of
6000 series to have the equiaxed granular structure, as disclosed
in Japanese Patent Laid-open No. 2002-241880. According to this
literature, the object is achieved when the aluminum alloy contains
Mg and Si in stoichiometrically equal amount and also contains such
transition metal elements as Mn, Cr, and Zr (that promote the
formation of fibrous structure) in a total amount of 0.1% or less
and the extrusion temperature is 500.degree. C. or above and
extrusion is immediately followed by water quenching (forced
cooling). The resulting equiaxed granular structure is such that
the average grain size is 100 .mu.m or smaller and the aspect ratio
of crystal grain is no larger than 2. The aspect ratio is a
length-to-thickness ratio of a crystal grain, with the length
measured in the direction of extrusion.
[0007] There is further another suggestion disclosed in Japanese
Patent Laid-open No. Hei-5-171328. This literature suggests that
hollow extruded members improve in bending formability if they have
the fibrous structure (with crystal grains elongated in the
direction of extrusion) in place of the equiaxed granular structure
mentioned above. According to this literature, the extruded member
is produced from an aluminum alloy containing such transition metal
elements as Mn, Cr, and Zr in a comparatively large total amount of
0.45-0.53% by extrusion at 500.degree. C. or above, which is
immediately followed by water quenching (forced cooling) in a water
bath.
[0008] It is known that the fibrous structure mentioned above is
effective for such extruded members as side member and bumper stay
to be used as energy absorbing members which need good longitudinal
crushing performance in their axial (or lengthwise) direction, so
that they resist Euler buckling (bending in a dogleg shape) but
undergo deformation in a bellow shape. See Japanese Patent
Laid-open Nos. Hei-9-256096 and 2003-183757. The former literature
proposes an extruded member of aluminum alloy which contains Mg and
Si in a stoichiometrically equal amount so that it has the fibrous
structure mentioned above. It also suggests that the tendency
toward transformation into recrystallization structure due to Mg
and Si contained in a stoichiometrically equal amount is avoided if
the extruded member contains such transition metal elements as Mn,
Cr, and Zr in a comparatively large total amount of 0.5% and
extrusion is performed at 500.degree. C. or above and immediately
followed by water quenching.
[0009] Japanese Patent Laid-open No. 2003-183757 mentioned above
proposes an extruded member of aluminum alloy of 6000 series which
contains excess Si and also contains such transition metal elements
as Mn, Cr, and Zr in a comparatively large total amount of
0.25-0.48%. According to this 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
that the extruded member exhibits not only good longitudinal
crushing performance but also good lateral crushing
performance.
[0010] It is suggested in Japanese Patent Laid-open No. 2005-105317
that the extruded member of aluminum alloy of 6000 series to be
used as reinforcement members should have not only fibrous
structure but also anisotropically elongating structure so that it
possesses both good bending formability and good crush-cracking
resistance. According to this literature, the extruded member of
aluminum alloy contains excess Si and also contains such transition
metal elements as Mn, Cr, and Zr in a comparatively large total
amount of 0.15-0.30%. Moreover, it mentions that extrusion should
be performed at a comparative low temperature under 500.degree. C.
with a high extrusion ratio over 10, so that the extrudate has the
fibrous structure composed of crystal grains elongating in the
direction of extrusion, with the aspect ratio exceeding 5. The
resulting extruded member has an anisotropic structure such that
the elongation (.delta.1) in the direction deviating by 45 degrees
from the direction of extrusion is larger than elongation (82) and
(83) in the direction parallel and perpendicular, respectively, to
the direction of extrusion.
[0011] It is also suggested in Japanese Patent Laid-open No.
Hei-6-25783 that the extruded member of aluminum alloy of 6000
series to be used as side members and bumper reinforcement members
should have the equiaxed grain structure (with the aspect ratio of
crystal grains being no larger than 3) instead of the fibrous
structure so that it has both good bending formability and good
impact absorbing performance. [Aspect ratio is a ratio in length of
the long axis to the short axis of a crystal grain.] According to
this literature, the fine equiaxed grain structure contributes to
improved elongation and bending formability and also restricts the
amount and size of intergranular precipitate, thereby preventing
fragmentation of crystal grains from occurring at intergranular
precipitates at the time of impact.
[0012] In a practical situation where the extruded member of
aluminum alloy of 6000 series is used as automotive reinforcement
members, such as bumper reinforcement members and door guard bars,
they usually receive a concentrated collision force in the
approximately horizontal direction. In such a situation, the
extruded member of aluminum alloy of 6000 series is poor in
flexural crushing performance, which is important for improvement
in lateral crushing performance, even though it has the fibrous
structure or anisotropic structure (as suggested in Japanese Patent
Laid-open Nos. Hei-5-171328, Hei-9-256096, 2003-183757, and
2005-15317) or it has the equiaxed grain structure (as suggested in
Japanese Patent Laid-open Nos. 2003-241880 and Hei-6-25783).
[0013] Collision in the horizontal direction is typically pole
collision and offset collision. In the case of such collision, the
collision force in the horizontal direction locally concentrates on
the automotive reinforcement member, such as bumper reinforcement
member, thereby bending it in its lengthwise direction at the part
of collision (which receives the load of collision) and causing
damage to the automotive body.
[0014] To cope with collision under critical conditions, it is
necessary to improve the extruded member of aluminum alloy of 6000
series in flexural crushing performance. Meeting this requirement
is limited even with the comparatively strong extruded member
having the fibrous structure mentioned above, as well as the
extruded member having the equiaxed grain structure disclosed in
the two prior art technologies mentioned above.
[0015] The present invention was completed in view of the
foregoing. It is an object of the present invention to provide an
extruded member of aluminum alloy of 6000 series and a method for
production thereof, said extruded member having both good flexural
crushing performance and good corrosion resistance which are
required of reinforcement members of automotive body subject to
collision under more critical conditions.
OBJECT AND SUMMARY OF THE INVENTION
[0016] The present invention to achieve the above-mentioned object
is directed to an extruded member of aluminum alloy which contains
(in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe: 0.01-0.40%, Mn:
0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and Ti: 0.001-0.10%,
with the remainder being aluminum and inevitable impurities, and
has the metallographic structure whose cross section perpendicular
to the direction of extrusion shows the equiaxed recrystallized
grain structure in which intergranular precipitates 1 .mu.m or
lager in terms of the diameter of an equivalent circle are 3 .mu.m
or more separate from one another in the observation under a TEM of
5000 magnifications and also the average areal ratio of cube
orientation is 15% or larger over the entire thickness region
including the grain growth layer in the outermost surface in the
cross section perpendicular to the direction of extrusion.
[0017] The extruded member of aluminum alloy should preferably
contain Mg and Si such that
Mg(%).gtoreq.1.73.times.Si(%)-0.4
where Mg(%) and Si(%) denote the content of Mg and Si in mass %,
respectively.
[0018] The extruded member of aluminum alloy mentioned above should
preferably have the equiaxed recrystallized grain structure such
that the average areal ratio of cube orientation is 20% or larger.
Also, the extruded member of aluminum alloy mentioned above may
selectively contain at least either of Cr: 0.001-0.18% or Zr:
0.001-0.18% in a total amount of 0.30% or less. The extruded member
of aluminum alloy mentioned above should preferably have the
flexural crushing performance such that the critical bending radius
(R) is 3.0 mm or smaller which does not cause cracking in the
180.degree. bending test according to JIS Z2248 in which the platy
specimen is bent in the direction of extrusion, and the extruded
member of aluminum alloy mentioned above should preferably have the
corrosion resistance such that the specimen does not suffer
intergranular corrosion in the alternating immersion corrosion test
according to ISO/DIS 11846B.
[0019] The extruded member of aluminum alloy mentioned above will
find use as energy absorbing members which crush under load in the
direction perpendicular to the direction of extrusion.
[0020] The present invention to achieve the above-mentioned object
is directed to a method for producing an extruded member of
aluminum alloy, said method comprising a step of soaking a cast
billet of aluminum alloy at 500-590.degree. C., said billet
containing (in mass %) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe:
0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and
Ti: 0.001-0.10%, with the remainder being aluminum and inevitable
impurities, a step of subjecting the soaked billet to forced
cooling to 400.degree. C. or below at an average cooling rate of
100.degree. C./hr or above, a step of reheating the cooled billet
and subjecting the reheated billet to hot extrusion such that the
extrudate reaches the solid solution temperature which is
500.degree. C. or higher at the extruder exit, a step of
immediately subjecting the extrudate to forced cooling at an
average cooling rate of 100.degree. C./hr or above, and a step of
subjecting the cooled extrudate to ageing, so that the resulting
extruded member has a 0.2% proof stress of 240 MPa or greater and
also has the metallographic structure whose cross section
perpendicular to the direction of extrusion shows the equiaxed
recrystallized grain structure in which intergranular precipitates
1 .mu.m or lager in terms of the diameter of an equivalent circle
are 3 .mu.m or more separate from one another in the observation
under a TEM of 5000 magnifications and also the average areal ratio
of the cube orientation is 15% or larger over the entire thickness
region including the grain growth layer on the outermost surface in
the cross section perpendicular to the direction of extrusion.
[0021] The cast billet of Al--Mg--Si aluminum alloy mentioned above
may selectively contain at least either of Cr: 0.001-0.18% or Zr:
0.001-0.18% in a total amount of 0.30% or less.
[0022] The present inventors paid attention to the texture of the
extruded member of aluminum alloy of 6000 series which had not
attracted attention so much in the past, and they investigated anew
the effect of the texture on the flexural crushing performance and
corrosion resistance. As the result, they found that the texture,
particularly the equiaxed recrystallized grain structure having the
cube orientation, effectively improves flexural crushing
performance and corrosion resistance.
[0023] Much has been studied about the texture of aluminum alloy of
6000 series in the field of rolled plate to elucidate its effect on
the press formability and bending formability of automotive panels.
(Bending includes hemming, particularly flat hemming.) There are
numerous prior art technologies based on such studies. A typical
one of them teaches that the texture of aluminum alloy of 6000
series is more effective in improvement of flat hemming performance
according as the crystal grains having the cube orientation
increases in its ratio. As known well, the cube orientation is the
major orientation of the texture in the rolled sheet of aluminum
alloy of 6000 series. It is also one of the major crystal
orientations in Al--Mg--Si alloys.
[0024] Unlike extruded members (for use as reinforcement members),
the rolled sheets of aluminum alloy of 6000 series are used as
automotive body panels and hence they are very thin (about 1 mm or
below) for weight saving. Moreover, when they undergo press forming
and bending, they receive bending load which is different from
collision load the extruded members 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 comparatively low
strength (150 MPa or lower in terms of 0.2% proof stress), even in
the case of T4 material, in consideration of formability for
automotive body panel.
[0025] The present invention, however, is intended for extruded
member for reinforcement which has a comparatively great wall
thickness of 2 mm or thicker and also has a rectangular hollow
cross section. This extruded member is a high-strength one having
0.2% proof stress of 240 MPa or higher. The above-mentioned rolled
sheet receives a bending load when it undergoes hemming, but the
bending load basically differs in deformation mechanism and pattern
from that which the extruded member according to the present
invention experiences at the time of 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 aluminum alloy
of 6000 series is useless for predicting how corrosion resistance
and flexural crushing performance are related with cube orientation
in the texture of the extruded member of aluminum alloy of 6000
series according to the present invention.
[0026] In the field of extruded member of aluminum alloy of 6000
series, it has been common practice to cause the extruded member to
have the fibrous structure elongating in the direction of extrusion
in order that the resulting hollow extruded member has good
crushing performance in the lengthwise (or axial) direction and
lateral (or crosswise) direction, as disclosed in Japanese Patent
Laid-open No. Hei-5-171328 mentioned above. Such fibrous structure
has the texture in which cube orientation does not develop and the
ratio of cube orientation (or crystal grains having cube
orientation) is limited to a very small value. The common knowledge
in the field of extruded member of aluminum alloy of 6000 series
does not permit one to predict how flexural bending performance and
corrosion resistance are related with cube orientation in the
texture of the extruded member of aluminum alloy of 6000 series
according to the present invention. The foregoing is the reason why
the texture of extruded members has not attracted attention so much
although there are some reports about whether the extruded member
of aluminum alloy of 6000 series should have the fibrous structure
or the equiaxed grain structure.
[0027] According to the present invention, the extruded member of
aluminum alloy of 6000 series is designed to have the texture in
the form of equiaxed recrystallized grain structure with increased
cube orientation so that it is improved in flexural crushing
performance and corrosion resistance. Thus, the extruded member of
aluminum alloy of 6000 series can be used as energy absorbing
members, such as bumper reinforcement and door guard bar, which
crush under loads in the lateral direction, in the same way as the
extruded member of aluminum alloy of 7000 series which has a
comparatively high strength, with the former outperforming the
latter in corrosion resistance.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The following is a detailed description of the extruded
member of aluminum alloy of 6000 series according to the present
invention.
[0029] (Texture)
[0030] As mentioned above, the extruded member of aluminum alloy of
6000 series as a reinforcement member improves in flexural crushing
performance according as the ratio of crystal grains having cube
orientation increases in its texture. It also improves in
resistance to such corrosion as intergranular corrosion in
corrosive environment like saline, according as the ratio of
crystal grains having cube orientation increases.
[0031] The present invention should meet the following requirements
in order that the extruded member used as reinforcement members has
improved flexural crushing performance and corrosion resistance.
The extruded member should have the metallographic structure whose
cross section in the thickness direction shows the equiaxed
recrystallized grain structure in which intergranular precipitates
1 .mu.m or lager in terms of the diameter of an equivalent circle
are 3 .mu.m or more separate from one another in the observation
under a TEM of 5000 magnifications and also the average areal ratio
of the cube orientation is 15% or larger, preferably 20% or lager,
over the entire thickness region including the grain growth layer
in the outermost surface in the cross section in the thickness
direction.
[0032] The present invention is designed such that the areal ratio
of cube orientation is made large regardless of the amount (or
areal ratio) of other orientations such as Goss orientation. If the
average areal ratio of cube orientation is too small, there are too
few crystal grains having cube orientation and hence the resulting
extruded member does not improve in flexural crushing performance
and corrosion resistance and hence it does not meet requirements
(specifications) for use as automotive reinforcement members.
[0033] (Relation of Cube Orientation to Flexural Crushing
Performance and Corrosion Resistance)
[0034] According as the ratio (or number) of crystal grains having
one orientation such as cube orientation increases, the difference
in orientation of the grain boundary of crystal grains decreases.
Thus, concentration of stress to the grain boundary is released
when flexural load is applied (or collision load is applied). As
the result, particularly in the case of energy absorbing members
such as bumper reinforcement and door guard bar, the flexural
crushing performance improves at the time of crushing (lateral
crushing) due to external load (flexural load) such as collision in
the direction perpendicular to the direction of extrusion of the
extruded member of aluminum alloy constituting it.
[0035] Also, crystal grains having cube orientation possess the
characteristics that crystals hardly rotate when they receive
deformation such as tensile and flexure due to the above-mentioned
load in the direction of extrusion (lengthwise direction) or the
widthwise direction (direction perpendicular to the direction of
extrusion). Consequently, if cube orientation develops, the ratio
of crystal grains having cube orientation becomes large and the
difference in orientation of grain boundary of crystal grains
becomes small, this small difference in crystal orientation is
maintained even after large deformation of tensile and flexure due
to said load is received. Owing to such an inherent effect of cube
orientation, stress concentration to grain boundary that occurs
when flexural load is applied (or collision load is applied) is
released and the flexural crushing performance improves.
[0036] On the other hand, if cube orientation does not improve,
even though other orientation, typically such as Goss orientation,
develops, particularly in the case of deformation in the direction
of extrusion (lengthwise direction) or widthwise direction
(perpendicular to the direction of extrusion), the above-mentioned
characteristics inherent to cube orientation do not exist (are not
exhibited). Therefore, in the case of deformation due to flexural
load, the flexural crushing performance decreases. The phenomenon
that stresses concentration to the grain boundary increases when
flexural load is applied and the flexural crushing performance
decreases is the same as that in the case of the above-mentioned
fibrous structure having a comparatively high strength. This is the
reason why the flexural crushing performance of the conventional
fibrous structure is largely limited.
[0037] And, according as cube orientation develops and the
difference in orientation of the grain boundary of crystal grains
becomes smaller, corrosion resistance such as grain boundary
corrosion resistance improves. On the other hand, if cube
orientation does not develop and the difference in orientation of
the grain boundary of crystal grains becomes larger, corrosion
resistance such as grain boundary corrosion resistance tends to
decrease.
[0038] (Reason for Defining All Areas in Thickness Direction of
Cross Section of Extruded Member)
[0039] In the present invention, cube orientation is defined as the
average areal ratio over the entire thickness region including the
outermost grain growth layer in the cross section in the thickness
direction (or the cross section in the direction of extrusion and
in the direction perpendicular to the direction of extrusion) of
the extruded member. In the cross section in the thickness
direction of the extruded member, there exist usually on both sides
of the outermost surface the grain growth layer (GG layer or layer
of coarse recrystallized grain structure) with a thickness of
several hundred microns which inevitably occurs as the outermost
surface comes into contact with the extrusion die. In the GG layer
in the outermost surface, random orientations predominate, cube
orientation does not develop, and crystal grains having cube
orientation are very few. Therefore, the thicker the GG layer in
the outermost surface, the thinner the equiaxed recrystallized
grain structure, which is inside in the thickness direction of the
extruded member and in which cube orientation develops, and the
effect of improving flexural crushing performance becomes smaller.
In other words, the degree of development of cube orientation over
the entire region in the thickness direction in the cross section
of the extruded member, or the ratio of crystal grains having cube
orientation, determines the flexural crushing performance of the
extruded member used as reinforcement member. Therefore, in the
present invention, particularly, in order to improve the flexural
crushing performance as reinforcement member, cube orientation is
prescribed in terms of the average areal ratio over the entire
region of the thickness of the extruded member, including the
outermost grain growth layer in the cross section in the thickness
direction.
[0040] Also, with the help of the crystal orientation analyzing
method (SEM/EBSP method) mentioned later, it is possible to measure
cube orientation over the entire thickness region including the
outermost grain growth layer, for example, over the region broader
than a thickness of 2 mm of the extruded member, and it is also
possible to obtain the average of the areal ratio. By contrast,
X-ray diffraction (or X-ray diffraction intensity), which is
commonly used for measurement of the texture, is designed to
measure the structure (or texture) in a comparatively micro region
for each crystal grain as compared with the crystal orientation
analyzing method that employs SEM/EBSP. Therefore, the X-ray
diffraction method needs a large number of measurements to cover
the area larger than 2 mm over the entire region in the thickness
direction of the extruded member, and it is practically incapable
of measuring the average areal ratio of cube orientation over the
entire region in the thickness of the extruded members as defined
in the present invention.
[0041] (Equiaxed Recrystallized Grain Structure)
[0042] The reason why the present invention is intended for the
extruded member to have the equiaxed recrystallized grain structure
is that the fibrous structure as disclosed in Japanese Patent
Laid-open Nos. Hei-5-171328, Hei-9-256096, 2003-183757, and
2005-105317, in which the crystal grain has an aspect ratio
exceeding 5 and the crystal grain elongates in the direction of
extrusion, does not permit cube orientation to develop to such an
extent that the average areal ratio of cube orientation over the
entire region of the cross section of the extruded member in the
thickness direction is 15% or higher. The term "equiaxed
recrystallized grain structure" denotes the equiaxed grain
structure in which crystal grains have an average aspect ratio of 3
or smaller and the average aspect ratio is lower than 5 even though
crystal grains elongate in the direction of extrusion. The term
"aspect ratio of crystal grain" means the ratio of the long axis to
the short axis, with the long axis being measured in the direction
of extrusion and the short axis, in the thickness direction.
[0043] The prior art technologies disclosed in Japanese Patent
Laid-open Nos. 2002-241880 and Hei-6-25783 mentioned above are
intended for the extruded member of aluminum alloy of 6000 series
to have the equiaxed recrystallized grain structure. However, they
are able to provide the equiaxed recrystallized grain structure but
unable to make cube orientation develop to such an extent that the
average areal ratio of cube orientation over the entire region of
the cross section of the extruded member in the thickness direction
is 15% or higher. The prior art technology disclosed in Japanese
Patent Laid-open No. 2002-241880 is designed to produce the
extruded member in such a way that the content of Mg and Si is
stoichiometrically equivalent so that the equiaxed grain structure
develops and 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 above and extrusion is immediately followed by water
quenching for forced cooling. The extruded member produced in this
manner has the equiaxed grain structure in which the average
crystal grain size is 100 .mu.m or smaller and the aspect ratio of
the crystal grain (the ratio of the length of the crystal grain in
the direction of extrusion to the length of the crystal grain in
the thickness direction) is 2 or smaller. Japanese Patent Laid-open
No. Hei-6-25783 discloses in its Example an extruded member of
aluminum alloy of 6000 series which has the structure of excess Si
type and optionally contains transition metal elements, such as Mn,
Cr, and Zr, in a comparatively large total amount of 0.34%. The
extruded member does not undergo forced cooling such as water
quenching (that immediately follows extrusion) on-line but
undergoes separately solid solution treatment and quench hardening
off-line.
[0044] By contrast, in order for the extruded member to have the
grain structure such that cube orientation develops to such an
extent that its areal ratio exceeds 15% over the entire region of
the cross section in the thickness direction, as intended in the
present invention, it is necessary to positively increase the areal
ratio of cube orientation by controlling the manufacturing
condition, such as forced cooling that follows soaking treatment,
as mentioned later. Also, the composition of the extruded member
disclosed in Japanese Patent Laid-open No. Hei-6-25783, which is
not of extremely excess Si type and contains transition metal
elements, such as Mn, Cr, and Zr, in a reduced amount, is one
condition for cube orientation to develop, as mentioned later.
Therefore, the extruded members disclosed in Japanese Patent
Laid-open Nos. 2002-241880 and Hei-6-25783, which were produced by
the process merely involving ordinary soaking treatment without
positive control mentioned above, do not permit cube orientation to
develop even though they are extruded under the same condition. In
other words, the extruded members according to the prior art
technologies have the texture with random crystal orientations and
hence they have an average areal ratio of cube orientation which is
inevitably smaller than that in the extruded member according to
the present invention. That is, the ordinary manufacturing method
gives extruded members which have the equiaxed grain structure but
do not have the equiaxed grain structure in which cube orientation
develops as intended in the present invention.
[0045] (Measurement of Cube Orientation)
[0046] The areal ratio (or the existence ratio) of the orientation
of each crystal grain (orientation components of each crystal
grain), including cube orientation, is measured by means of the
crystal orientation analyzing method (SEM/EBSP method) that employs
EBSP (electron backscatter diffraction pattern) with help of an SEM
(scanning electron microscope).
[0047] The crystal orientation analyzing method that employs EBSP
is carried out in such a way 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 taken into a computer. In the
computer, the image is analyzed and compared with the pattern which
has been obtained by simulation with a known crystal, so that the
crystal orientation is identified.
[0048] The crystal orientation analysis with EBSP is not performed
on individual crystals but is performed on a specified region of
specimens by scanning at certain intervals. Therefore, the
above-mentioned process is performed on all the points of
measurement automatically, and hence there are obtained tens to
hundreds of thousands of data for crystal orientation at the end of
measurement. This method of measurement offers the advantage of
permitting observation over a broad field of view and providing
information about a large number of crystal grains, including
average crystal grain size, standard deviation of average crystal
grain size, and orientation analysis, within a few hours.
Therefore, it is most suitable for the extruded member according to
the present invention for which the texture is to be analyzed over
the entire region in the thickness direction or a broad area of 2
mm or thicker in thickness, including the GG layer in the outermost
surface.
[0049] The crystal orientation analysis with EBSP uses a specimen
for observation of structure which is taken from the cross section
of the extruded member. 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 by, for example,
JEOLJSM 5410 (SEM from Nippon Denshi) or EBSP measuring and
analyzing system "OIM" (Orientation Imaging Macrogroaph), bundled
with an analyzing program called OIMAnalysis, from TSL. The
analysis judges whether or not each crystal grain has the desired
orientation (or within 15.degree. from the ideal orientation) and
then determines the density of orientations in the field of view
for which measurement has been carried out. Measurement is carried
out at several points 3 .mu.m or less apart in the cross section of
the extruded member, and the values of measurements are
averaged.
[0050] The region of specimen for measurement is usually divided
into hexagonal sections and each section is irradiated with
electron beams so that reflected beams form the Kikuchi pattern.
Two-dimensional scanning with electron beams to measure the crystal
orientation at prescribed intervals gives the distribution of
orientations of the specimen. The thus obtained Kikuchi pattern is
analyzed to define the crystal orientation at the position for
incident beams. In other words, the resulting Kikuchi pattern is
compared with the data of a known crystal structure to identify the
crystal orientation at the measurement point.
[0051] (Texture)
[0052] Incidentally, as mentioned above, the texture including the
cube orientation of the extruded member is examined in the same way
as for rolled sheets, with the measurement point being regarded as
a plate.
[0053] Each orientation is represented as follows according to
"Texture" compiled by S. Nagashima (published by Maruzen)) and
"Light Metals" compiled by Institute of Light Metals, vol. 43
(1993), pp. 285-293. [0054] Cube orientation: {001}<100>
[0055] Goss orientation: {011}<100> [0056] CR orientation:
{001}<520> [0057] RW orientation: {001}<110>
[corresponding to Cube orientation turned with respect to the (100)
plane] [0058] Brass orientation: {011}<211> [0059] S
orientation: {123}<634> [0060] Cu orientation:
{112}<111> (or D orientation: {4411}<11118> [0061] SB
orientation: {681}<112>
[0062] (Intergranular Precipitates)
[0063] In order for the extruded member of aluminum alloy of 6000
series to exhibit good flexural crushing performance and corrosion
resistance when used as reinforcement members, the present
invention specifies that the extruded member has the texture in
which intergranular precipitates 1 .mu.m or larger in terms of the
diameter of an equivalent circle are 3 .mu.m or more separate from
one another on average in the observation under a TEM of 5000
magnifications. The average distance between intergranular
precipitates should preferably be 5 .mu.m or larger, more
preferably be larger than 10 .mu.m.
[0064] The term "intergranular precipitates" used in the present
invention denotes such compounds as MgSi or Si in the form of
simple substance, which are expected from the composition of the
aluminum alloy of 6000 series. MgSi forms the .beta.' phase to
increase the strength of the extruded member of aluminum alloy of
6000 series used as reinforcement members. Intergranular
precipitates, however, are harmful if they are excessively coarse
or they exist in an excessively large amount; they will start
rupture and propagate rupture even though the texture is controlled
as mentioned above, thereby deteriorating the flexural crushing
performance and corrosion resistance of the extruded member used as
automotive reinforcement members. The distance between
intergranular precipitates is specified as above in order that the
extruded member of aluminum alloy of 6000 series, which has the
texture with well-developed cube orientation as mentioned above,
exhibits good flexural crushing performance and corrosion
resistance.
[0065] In the case where intergranular precipitates 1 .mu.m or
lager in terms of the diameter of an equivalent circle are less
than 3 .mu.m separate from one another on average in the
observation under a TEM of 5000 magnifications, intergranular
precipitates are coarse or excessively close to one another and are
distributed densely. Therefore, they start intergranular rupture or
corrosion and propagate them when they receive flexural loads at
the time of collision. Therefore, the extruded member used as
automotive reinforcement members decreases in flexural crushing
performance and corrosion resistance even though the texture is
controlled as mentioned above. Incidentally, intergranular
precipitates smaller than 1 .mu.m in terms of the diameter of an
equivalent circle do not affect flexural crushing performance and
corrosion resistance so much. Therefore, the size of intergranular
precipitates is not specifically defined to avoid its confusion
with the distance between intergranular precipitates.
[0066] (Measurement of Average Distance Between Intergranular
Precipitates and Size of Intergranular Precipitates)
[0067] Measurement of the average distance between intergranular
precipitates and the size of intergranular precipitates is
performed on the equiaxed recrystallized grain structure in the
cross section of the extruded member. Unlike the texture to be
observed as mentioned above, this structure is at the center of the
cross section in the thickness direction of the extruded member,
with the GG layer in the outermost surface being excluded. The
specimen for equiaxed structure is made into a thin film, which is
subsequently observed under a TEM of 5000 magnifications.
[0068] Observation under a TEM is designed to examine the structure
in a much smaller (micro) region than the crystal orientation
analysis by means of SEM/EBSP mentioned above. It needs a huge
number of measurements over the entire region in the thickness
direction of the extruded member. Therefore, in actual practice,
observation is performed at one point in the center of the
thickness such that the total field of view is 40 .mu.m.sup.2 or
larger, and this procedure is repeated at ten points an adequate
distance apart in the lengthwise direction of the extruded member
and the resulting data are averaged. The size of each intergranular
precipitate is expressed in terms of the diameter of an equivalent
circle. All the intergranular precipitates in the field of view are
examined for the diameter of equivalent circle, and those which are
1 .mu.m or larger are selected. The average distance between the
adjacent intergranular precipitates thus selected are measured and
the resulting data are averaged.
[0069] (Chemical Composition)
[0070] According to the present invention, the extruded member of
aluminum alloy of 6000 series has the chemical composition as
follows. It needs good flexural crushing performance and corrosion
resistance so that it is used as automotive reinforcement members
as mentioned above.
[0071] To meet this requirement, the extruded member of aluminum
alloy of 6000 series covered by the present invention (or the cast
billet as a raw material thereof) should be an Al--Mg--Si aluminum
alloy containing (in mass %,) Mg: 0.60-1.20%, Si: 0.30-0.95%, Fe:
0.01-0.40%, Mn: 0.001-0.35%, Cu: 0.001-0.65%, Zn: 0.001-0.25%, and
Ti: 0.001-0.10%, with the remainder being aluminum and inevitable
impurities. It may further selectively contain at least either of
Cr: 0.001-0.18% or Zr: 0.001-0.18% in a total amount of 0.30% or
less. Percentage (%) for the content of each element is in terms of
mass %.
[0072] Any other elements than listed above are basically
impurities. The content of such impurities should be lower than the
level allowed by the AA and JIS standards. However, contamination
with impurities is liable to occur when the melt is prepared from
not only high-purity aluminum ground metal but also scraps of
6000-series alloy and other aluminum alloys in large amounts for
the purpose of recycling. Reducing these impurity elements below
the detection limit increases production cost, and a certain level
of their content should be allowed. Therefore, other elements than
listed above may be allowed according to the AA and JIS
standards.
[0073] The following is the base on which the content of each
element listed above is established for the aluminum alloy of 6000
series.
Si:
[0074] The content of Si should be 0.30-0.95%, which depends on the
content of Mg. The preferred content of Si should be 0.30-0.50% to
give the balance alloy mentioned above. Both Si and Mg are
essential elements which cause solid solution strengthening and
forms age precipitates (which contribute to strengthening) in
crystal grains at the time of artificial aging treatment, thereby
exhibiting the ability of age strengthening and producing strength
(proof stress) of 200 MPa or greater necessary for reinforcement
members. With too small a content, Si does not form the
above-mentioned compound phase at the time of artificial aging
treatment, with the age hardening and desired strength not
attained. With an excess content, Si does not give the balance
alloy which has the texture specified in the present invention. An
excessively low content of Si is detrimental to bending and
weldability.
Mg:
[0075] The content of Mg should be 0.60-1.20%, which depends on the
content of Si. The preferred content of Mg should be 0.61-1.0% to
give the balance alloy mentioned above. Mg together with Si is an
essential element which causes solid solution strengthening and
forms age precipitates (which contribute to strengthening) in
crystal grains at the time of artificial aging treatment, thereby
exhibiting the ability of age strengthening and producing strength
(proof stress) greater than 200 MPa necessary for reinforcement
members. With too small a content, Mg does not form the
above-mentioned compound phase at the time of artificial aging
treatment, with the age hardening and desired strength not
attained. With an excess content, Mg does not give the balance
alloy. An excessively low content of Mg is detrimental to
bending.
Content of Mg and Si:
[0076] In order that the extruded member of aluminum alloy of 6000
series has the equiaxed recrystallized grain structure, in which
the average areal ratio of cube orientation exceeds 15% and
intergranular precipitates 1 .mu.m or higher in terms of the
diameter of an equivalent circle are no less than 3 .mu.m apart on
average, the content of Mg and Si should be such that
Mg(%).gtoreq.1.73.times.Si(%)-0.4, preferably
Mg(%).gtoreq.1.73.times.Si(%)-0.2. This relationship was
established for the aluminum alloy of 6000 series specified in the
present invention to be a balance alloy which contains Mg and Si in
a stoichiometrically equivalent amount or an Si-excess aluminum
alloy with a comparatively small content of Si.
[0077] An aluminum alloy of 6000 series which contains Si in an
amount more than specified by Mg.gtoreq.1.73.times.Si, or an
aluminum alloy of Si-excess type which contains Si in a large
excess amount, increases in proof stress due to artificial age
hardening treatment at a comparatively low temperature and exhibits
good age hardening performance (BH performance) that imparts
necessary strength. Therefore, it is commonly used in the field of
aluminum alloy of 6000 series which needs good formability and high
strength after forming so that it is made into automotive panels by
press forming or bending.
[0078] However, if the extruded member of aluminum alloy of 6000
series according to the present invention has the Si-excess
composition, Si remains unmelted during extrusion and becomes
nuclei having various crystal orientations, resulting in the
texture with random orientations, with the development of cube
orientation suppressed and the ratio of cube orientation remarkably
decreased. It also tends to have the above-mentioned fibrous
structure elongating in the direction of extrusion.
[0079] For this reason, any extruded member produced from an
aluminum alloy of 6000 series containing excess Si would not have
the equiaxed recrystallized grain structure having cube
orientations such that their average areal ratio is 15% or higher
over the entire region in the thickness direction of the extruded
member, said thickness including the grain growth layer in the
outermost surface. (This depends on the manufacturing condition
such as extrusion.) Moreover, an excess Si content gives rise to a
large number of coarse intergranular precipitates arising from Si,
which would prevent the formation of the above-mentioned texture
and the structure in which intergranular precipitates 1 .mu.m or
lager in terms of the diameter of an equivalent circle are no less
than 3 .mu.m apart on average, which is necessary for the extruded
member to exhibit good flexural crushing performance and corrosion
resistance. Therefore, if the Si content exceeds an amount
specified by Mg(%) .gtoreq.1.73.times.Si(%)-0.4, or more
stringently Mg(%).gtoreq.1.73.times.Si(%)-0.2, the extruded member
would not exhibit good flexural crushing performance and corrosion
resistance when used as reinforcement members. (This depends on the
manufacturing conditions such as extrusion.)
Fe:
[0080] Fe functions in the same way as Mn, Cr, and Zr to form
dispersed particles (dispersion phase), hampers intergranular
movement after recrystallization, prevents crystal grains from
becoming coarse, and makes crystal grains fine. Fe is an element
which inevitably originates in a certain amount (substantial
amount) from scraps as a raw material for molten metal. The content
of Fe should be 0.01-0.40%. Fe does not produce its effect if its
content is excessively small. Fe in an excess content tends to give
rise to coarse crystals such as Al--Fe--Si crystals, which
deteriorate fracture toughness and fatigue characteristics.
Mn:
[0081] Mn is a transition metal element like Cr and Zr; it prevents
crystal grains from becoming coarse. It selectively combines with
other alloying elements to form dispersed particles (dispersion
phase) of intermetallic compound such as Al--Mn at the time of
soaking heat treatment and ensuing hot extrusion. These dispersed
particles are fine and dispersed densely and uniformly (to varied
degrees depending on the manufacturing conditions), so that they
effectively hinder intergranular movement after recrystallization
and prevent crystal grains from becoming coarse and make crystal
grains fine. Mn in an excessively small amount does not produce
these effects but makes crystal grains coarse (under certain
manufacturing conditions) to cause the extruded member to decrease
in strength and toughness. Mn also dissolves in the matrix to
increase strength.
[0082] Excess Mn, however, causes the extruded member to have the
fibrous structure elongating in the direction of extrusion. Thus it
prevents the formation of the equiaxed recrystallized grain
structure in which cube orientation has an average areal ratio
larger than 15% over the entire region in the thickness direction
of the extruded member. Moreover, excess Mn tends to form, at the
time of melting and casting, coarse intermetallic compounds and
crystals which start rupture and cause the extruded member (as
reinforcement members) to decrease in flexural crushing
performance, corrosion resistance, and bendability. Therefore, an
adequate content of Mn should be 0.001 to 0.35%, and a minimal
content is desirable.
Cu and Zn:
[0083] Cu and Zn contribute to strength through solid solution
hardening and also remarkably promote age hardening while the final
product is undergoing aging treatment. The content of Cu and Zn
should be 0.001-0.65% and 0.001-0.25%, respectively. Cu and Zn in
an excessively small content do not produce the effects mentioned
above. On the other hand, excess Cu and Zn make the extruded member
highly sensitive to stress corrosion cracking and intergranular
corrosion, thereby deteriorating corrosion resistance and
durability. If Cu and Zn are to be contained, their content should
be as specified above.
Ti:
[0084] Ti makes crystal grains in an ingot fine and causes the
extruded member to have the structure composed of fine crystal
grains. The extruded member should be incorporated with Ti in an
amount of 0.001-0.10%. If the source of Ti contains B. the content
of B should be 1-300 ppm. Ti in an excessively small amount does
not produce the above-mentioned effect. Excess Ti, however, forms
coarse crystals and causes the extruded member (as reinforcement
members) to decrease in flexural crushing performance, corrosion
resistance, and bendability. Therefore, an adequate content of Ti
should be in the range specified above.
At least either of Cr and Zr:
[0085] Cr and Zr, which are transition metal elements, form
dispersed particles (dispersion phase) of intermetallic compound,
such as Al--Cr and Al--Zr, thereby preventing crystal grains from
becoming coarse, in the same way as Mn. However, excess Cr and Zr,
like excess Mn, cause the extruded member to have the fibrous
structure which elongates in the direction of extrusion. Therefore,
the content of Cr should be 0.001-0.18% and the content of Zr
should be 0.001-0.18%, and their total content should be no more
than 0.30%. Their content should be as low as possible.
[0086] (Sectional Form of Extruded Member)
[0087] The extruded member of aluminum alloy of 6000 series should
have a specific sectional form so that it exhibits good flexural
crushing performance when used as reinforcement members. The
sectional form should preferably be hollow so that the extruded
member has light weight and good flexural crushing performance
required of reinforcement members. The hollow sectional form should
typically (basically) be rectangular. The rectangular form consists
of two flanges (front and rear walls) and two webs (upper and lower
walls connecting both flanges). The rectangular cross section may
additionally have one or more inner ribs for reinforcement (or for
improvement in flexural crushing performance). Possible arrangement
of such inner ribs may be a single rib or double ribs parallel to
the upper and lower side walls or cross ribs connected to four
corners of the cross section.
[0088] The sectional form may be modified such that the flange is
wider than the distance between the webs (or the edges of the
flange extend beyond the webs) or the flange and web are curved
inward or outward. The hollow sectional form may be uniform over
the entire length of the extruded member or may vary from one place
to another along the length. The extruded member to be used as the
bumper reinforcement may have a hollow sectional form which is not
completely closed but is partly opened. This sectional form is less
strong than the completely closed one and disadvantageous for
weight saving and flexural crushing performance.
[0089] (Wall Thickness of Extruded Member)
[0090] The extruded member should have an adequate wall thickness
in relation to the sectional form so that it exhibits good flexural
crushing performance required of reinforcement members. Since the
present invention is intended for automotive reinforcement members
that absorb energy at the time of collision, the extruded member
should have a certain thickness unlike body panels of rolled thin
sheet, so that it exhibits good flexural crushing performance
required of reinforcement members. A greater thickness is desirable
for good flexural crushing performance but an excessively great
thickness increases weight, which is contrary to weight saving.
Therefore, an adequate wall thickness should be selected from a
range of 2-7 mm. It is not always necessary that the flanges, webs,
and inner ribs constituting the above-mentioned sectional form have
the same thickness, but they vary in thickness. For example, the
flange which receives loads at the time of collision may be thicker
than other parts.
[0091] (Manufacturing Method>
[0092] The following is a description of the method for producing
the extruded member of aluminum alloy of 6000 series. The extruded
member according to the present invention denotes one which
undergoes refining, such as quenching and artificial aging
treatment, after hot extrusion. The manufacturing process itself is
ordinary and known, except for the conditions of controlling the
texture. However, for the extruded member to have the texture with
cube orientations within the range specified in the present
invention, the manufacturing method should include the soaking step
which is controlled at a specific cooling rate.
[0093] The manufacturing method for the extruded member according
to the present invention starts with preparing a billet from the
aluminum alloy of 6000 series. The billet undergoes soaking, which
is 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 approximately to room temperature by water cooling (for
forced cooling) on-line. In this way there is obtained the extruded
member having the specific sectional form mentioned above. The
extruded member that has passed through a series of hot extrusion
steps also has undergone solution and quenching treatment.
Subsequently, the extruded member undergoes cutting and leveling
treatment and optional refining such as artificial age hardening.
Alternatively, the artificial age hardening may be performed
simultaneously with paint baking after the extruded member (as a
reinforcement member) has been built into the automotive body and
the automotive body has been painted, instead of being performed
preliminarily while the extruded member still remains as such.
Melting and Casting:
[0094] An aluminum alloy having the above-mentioned composition
conforming to 6000 series is melted, and the molten metal is cast
in the usual way, such as continuous casting and semicontinuous
casting (DC casting).
[0095] (Soaking Heat Treatment)
[0096] The billet of aluminum alloy which has been cast as
mentioned above subsequently undergoes soaking heat treatment.
Soaking is performed in the usual way at a temperature of
500.degree. C. or higher and lower than melting point, preferably
at 500-590.degree. C. Soaking is intended to homogenize the
structure, or 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 does not completely eliminate
segregation from crystal grains; residual segregation starts
rupture and deteriorates flexural crushing performance, mechanical
properties, and bendability.
[0097] After soaking, the billet undergoes forced cooling to
400.degree. C. (and down to room temperature) at an average cooling
rate of 100.degree. C./hr or above. Forced cooling should be
accomplished at as high a cooling rate as possible by air blowing
or with water. Once 400.degree. C. is reached, forced cooling is
continued or switched to self-cooling down to room temperature.
[0098] The cooling rate mentioned above is quite different from the
one employed in the case where ordinary 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 mentioned above. The
result of such slow cooling is that MgSi compounds dissolve
temporarily into solid solution during soaking treatment at a high
temperature but combine with FeAl compounds, which remain
undissolved because of their high melting point, during cooling, to
form another composite compounds (precipitates). Such precipitates
remain undissolved in the extrusion process and become nuclei
having various crystal orientations like excess Si mentioned above,
thereby altering the structure into the texture with random
orientations. This prevents the development of cube orientations
and remarkably decreases the ratio of cube orientations.
[0099] The billet undergoes reheating and hot extrusion in such a
way that the temperature of the extrudate (at the exit of the
extruder) is 500.degree. C. or above, which is high enough to keep
the extrudate in solution form. Immediately after extrusion, the
extrudate undergoes forced cooling at an average cooling rate of
100.degree. C./min or higher. This forced cooling is necessary to
achieve T5 refining, which may be combined with T6 refining (aging)
or T7 refining (over aging). For T5 refining, the extrudate at the
exit of the extruder is kept at 500.degree. C. or above, 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.
[0100] The temperature at the time of hot extrusion should be
rather low so that cube orientations develop easily and the texture
of the extruded member becomes the equiaxed recrystallized grain
structure in which the average areal ratio of cube orientations is
no less than 15% over the entire region in the thickness direction
of the extruded member. However, if the temperature of the
extrudate at the exit of the extruder is lower than 500.degree. C.
(which is solution temperature), coarse Mg--Si compounds
(precipitates) remain undissolved in the matrix and they start
rupture to deteriorate flexural crushing performance and corrosion
resistance. To meet the contradictory requirements, it is desirable
to select a lowest possible temperature of 500.degree. C. or above
for the extrudate at the exit of the extruder. However, it is not
always necessary to reheat the billet of 500.degree. C. or above
for extrusion, because even though the reheating temperature is
below 500.degree. C., the temperature of the extrudate is
500.degree. C. or higher on account of heat generation by hot
extrusion.
[0101] Forced cooling for quenching with water that immediately
follows extrusion is intended for the extruded member as
reinforcement members to improve in flexural crushing performance
and corrosion resistance. Forced cooling alters the texture of the
extruded member into the equiaxed recrystallized grain structure in
which the average areal ratio of cube orientations is 15% or higher
over the entire region in the thickness direction of the extruded
member. In addition, forced cooling also gives rise to
intergranular precipitates which are 1 .mu.m or larger in terms of
the diameter of an equivalent circle and are separated from one
another at an average interval of 3 .mu.m and above. Forced cooling
that immediately follows extrusion should be accomplished on-line
with any cooling means arranged near the exit of the extruder, such
as shower for water mist or spray, water bath, and air blower, or a
combination thereof. The cooling rate for forced cooling is
100.degree. C./min or above, which is much higher than that (about
50.degree. C./min) for the extrudate which is allowed to cool.
[0102] The T5 refining treatment omits post-extrusion steps such as
reheating, solution treatment, and quenching for the extruded
member. Under certain circumstances, the T5 refining treatment may
be replaced by the T6 refining treatment which consists of separate
reheating of the extruded member at 500.degree. C. and above, and
ensuing solution treatment, quenching, and artificial aging that
follow extrusion.
Ageing Treatment:
[0103] The extruded member undergoes artificial aging treatment
after cutting to length and leveling treatment. The artificial
aging treatment should be carried out at 150-250.degree. C. for a
prescribed period of time. Duration of aging treatment controls age
hardening; it should be properly selected to maximize strength or
extended for averaging that improves corrosion resistance.
EXAMPLES
[0104] The examples of the present invention will be described
below. Samples of extruded members were prepared from aluminum
alloy of 6000 series varying in composition as shown in Table 1 and
under different conditions as shown in Table 2. The extruded member
has a rectangular sectional form with a center rib. Each sample was
examined for structure and characteristics (such as mechanical
properties, flexural crushing performance, and corrosion
resistance). Each sample in Table 1, except for Comparative Example
5, contains Mg and Si in such an amount as to satisfy the following
relation.
Mg(%).gtoreq.1.73.times.Si(%)-0.4, or
Mg(%).gtoreq.1.73.times.Si(%)-0.2
[0105] To be concrete, each sample of the extruded member was
prepared as follows. First, the aluminum alloy whose composition is
shown in Table 1 was melted and cast into a billet. The billet
underwent soaking treatment at a temperature shown in Table 2, and
soaking was followed by cooling to room temperature at an average
cooling rate (.degree. C./hr) shown in Table 2. The average cooling
rate was 120.degree. C./hr in the case of forced cooling by a
blower and 40.degree. C./hr in the case of self-cooling. The cooled
billet was heated again and immediately subjected to hot extrusion
at an extrusion rate (m/min) and a temperature (.degree. C.),
measured at the exit of the extruder, which are shown in Table 2.
Immediately after extrusion, the extrudate underwent forced cooling
to the neighborhood of room temperature with the help of cooling
means shown in Table 2. Thus there was obtained an extruded member
having a square sectional form with a central rib. The forced
cooling was carried out with water or air at a cooling rate of
about 50.degree. C./s or about 20.degree. C./s, respectively. The
resulting extruded member underwent artificial age hardening
treatment for 3 hours at a temperature shown in Table 2.
[0106] The sectional form of the extruded member has the following
dimensions. The flanges (or the front and rear walls) are 40 mm
long and 2.3 mm thick. The webs (or the side walls) and the central
rib are 40 mm long and 2.0 mm thick. The extruded member was cut to
a length of 1300 mm.
[0107] After artificial age hardening treatment, the web of the
extruded member was cut into a test sample in sheet form. The test
sample was examined for structure and characteristic properties.
The results are shown in Table 2.
[0108] (Structure of Test Sample)
Average Areal Ratio of Cube Orientation:
[0109] After refining as mentioned above and aging at room
temperature for 15 days, the test sample was examined for texture
by means of SEM-EBSP. The texture was analyzed to obtain the
average areal ratio (%) of cube orientation over the entire region
of the cross section in the thickness direction, including the
grain growth layer in the outermost surface. If the average areal
ratio (%) of cube orientation is subtracted from 100%, the
remainder is the average total areal ratio of other orientations
than cube orientation, which include Goss, CR, RW, Brass, S, Cu,
and SB orientations.
[0110] Each test sample was also examined by SEM-EBSP for the
recrystallized grain structure in terms of the aspect ratio of
crystal grains. The structure in which crystal grains have an
average aspect ratio smaller than 5 or greater than 5 was
designated as the equiaxed granular structure or the fibrous
structure, respectively.
Average Distance Between Intergranular Precipitates:
[0111] After refining as mentioned above and aging at room
temperature for 30 days, the test sample was examined for the
structure in the thickness direction by observation under a TEM of
5000 magnifications as mentioned above so as to measure the average
distance (.mu.m) between intergranular precipitates 1 .mu.m or
lager in terms of the diameter of an equivalent circle. The results
are shown in FIG. 2.
[0112] (Characteristics of Test Sample)
[0113] After refining as mentioned above and aging at room
temperature for 30 days, the test sample was examined for
characteristic properties, such as 0.2% proof stress (As proof
stress in MPa), elongation (%), flexural crushing performance, and
corrosion resistance. The results are shown in Table 2.
Tensile Test:
[0114] The test sample was cut into a specimen for tensile test,
No. 5 conforming to JIS Z2201, which measures 25 mm wide, 50 mm
long, and 2.0 mm thick as extruded. The specimen length and tensile
force are parallel to the direction of extrusion. The tensile force
was applied at a rate of 5 mm/min up to 0.2% proof stress and 20
mm/min thereafter. Five measurements were averaged.
Test for Flexural Crushing Performance:
[0115] The test sample (in sheet form) was bent to 180.degree.
according to the press-bending method (JIS Z2248), in the direction
perpendicular to the direction of extrusion. The bending test was
repeated, with the bending radius (R mm) gradually reduced to the
limit at which cracking occurs in the outside of the bent corner
(or in the stretched side). Any test sample having the critical
bending radius greater than 3.0 mm is regarded as good in flexural
crushing performance and suitable for use as automotive
reinforcement members.
Corrosion Resistance:
[0116] The test sample mentioned above was tested for corrosion
resistance by dipping under the following conditions according to
ISO/DIS 11846B. The test method consists of dipping the sample 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 sample to see intergranular corrosion
cracking. The sample is rated by the following criterion. [0117]
.times.: Intergranular corrosion cracking occurred. [0118] .DELTA.:
Intergranular corrosion occurred but intergranular corrosion
cracking did not occur. [0119] .largecircle.: Neither intergranular
corrosion cracking nor intergranular corrosion occurred (even
though corrosion occurred all over the surface).
[0120] As shown in Tables 1 and 2, samples in Examples 1 to 18
contain Mg and Si in an amount specified by the present invention
and undergo soaking and hot extrusion under preferred conditions
with regard to soaking temperature, forced cooling that follows
soaking, temperature at the extruder exit, extrusion speed, and
forced cooling with water that follows immediately after extrusion.
Therefore, they have the equiaxed recrystallized grain structure
having the cube orientation and the average intervals of
intergranular precipitates, as specified in the present invention,
and hence they excel in flexural crushing performance and corrosion
resistance, and they also excel in mechanical properties such as
strength and elongation. These outstanding characteristic
properties suggest that the extruded member is suitable for use as
automotive reinforcement members which might encounter more serious
collisions such as pole collision and offset collision, and that
the extruded member has good flexural crushing performance and good
corrosion resistance required of reinforcement members.
[0121] By contrast, the samples in Comparative Examples 1 to 4 have
the composition (shown in Table 1) conforming to the present
invention but they are produced under conditions not conforming to
the present invention. Therefore, as shown in Table 2, they do not
have the specific equiaxed recrystallized grain structure having
cube orientations and/or the specific average intervals for
intergranular precipitates, as specified in the present invention.
Thus, the samples in these Comparative Examples are inferior in
flexural crushing performance and/or corrosion resistance to those
in Working Examples.
[0122] Comparative Example 1 shows the effect of an excessively low
cooling rate employed after soaking. Comparative Example 2 shows
the effect of an excessively low cooling rate (due to air cooling)
employed after immediately after extrusion. Comparative Example 3
shows the effect of an excessively low soaking temperature.
Comparative Example 4 shows the effect that is produced when the
temperature at the exit of the extruder is excessively lower than
the solid solution temperature.
[0123] The samples in Comparative Examples 5 to 13 are produced
under the desirable conditions shown in Table 2 but have the
composition shown in Table 1 which is outside the range specified
in the present invention. Therefore, as shown in Table 2, they do
not have the specific equiaxed recrystallized grain structure
having cube orientations and/or the specific average intervals for
intergranular precipitates, as specified in the present invention.
Thus, the samples in these Comparative Examples are inferior in
flexural crushing performance and/or corrosion resistance to those
in Working Examples.
[0124] The sample in Comparative Example 5 contains too much Si,
and hence the content of Mg and Si therein does not satisfy the
following relation.
Mg(%).gtoreq.1.73.times.Si(%)-0.4, or
Mg(%).gtoreq.1.73.times.Si(%)-0.2
[0125] The sample in Comparative Example 6 contains too much Mg.
The sample in Comparative Example 7 contains too much Cu. The
sample in Comparative Example 8 contains too much Mn. The sample in
Comparative Example 9 contains too much Zr. The sample in
Comparative Example 10 contains too less Fe. The sample in
Comparative Example 11 contains too less Si. The sample in
Comparative Example 12 contains too much Zn. The sample in
Comparative Example 13 contains too much Ti.
[0126] The foregoing results of Examples demonstrate that the
composition, structure, and manufacturing conditions specified in
the present invention are essential for the extruded member to have
good flexural crushing performance, corrosion resistance, and
mechanical properties.
TABLE-US-00001 TABLE 1 Composition of Al alloy (in mass %, in ppm
for B, remainder is Al) Division Number Si Fe Cu Mn Mg Cr Zn Ti Zr
Working 1 0.420 0.200 0.140 0.020 0.800 0.050 0.002 0.020 0.000
Examples 2 0.370 0.200 0.150 0.180 0.800 0.000 0.010 0.020 0.000 3
0.420 0.200 0.150 0.002 0.800 0.100 0.003 0.020 0.000 4 0.420 0.210
0.130 0.200 0.800 0.050 0.003 0.020 0.000 5 0.420 0.200 0.150 0.003
0.800 0.000 0.003 0.020 0.080 6 0.420 0.200 0.020 0.020 0.800 0.050
0.010 0.020 0.020 7 0.530 0.210 0.140 0.200 0.910 0.050 0.020 0.020
0.000 8 0.510 0.190 0.140 0.200 0.790 0.050 0.010 0.020 0.000 9
0.490 0.190 0.140 0.200 0.780 0.000 0.020 0.020 0.000 10 0.550
0.210 0.350 0.200 0.900 0.050 0.005 0.020 0.000 11 0.570 0.200
0.510 0.330 0.950 0.050 0.000 0.020 0.000 12 0.420 0.200 0.530
0.003 0.800 0.050 0.003 0.020 0.000 13 0.420 0.200 0.150 0.002
0.800 0.050 0.050 0.020 0.000 14 0.420 0.200 0.150 0.003 0.800
0.050 0.150 0.020 0.000 15 0.420 0.200 0.150 0.020 0.800 0.050
0.003 0.050 0.000 16 0.600 0.200 0.150 0.100 1.000 0.050 0.010
0.020 0.000 17 0.700 0.150 0.150 0.100 1.100 0.000 0.010 0.020
0.020 18 0.380 0.050 0.150 0.220 0.800 0.100 0.020 0.020 0.010
Comparative 1 0.420 0.200 0.140 0.020 0.800 0.050 0.000 0.020 0.000
examples 2 0.400 0.200 0.140 0.020 0.800 0.050 0.000 0.020 0.000 3
0.700 0.200 0.100 0.020 1.150 0.050 0.000 0.000 0.000 4 0.700 0.200
0.100 0.020 1.150 0.050 0.000 0.000 0.000 5 1.000 0.200 0.150 0.050
1.000 0.050 0.000 0.020 0.000 6 0.850 0.200 0.150 0.050 1.300 0.050
0.010 0.020 0.000 7 0.400 0.200 0.800 0.050 0.800 0.050 0.010 0.020
0.000 8 0.400 0.200 0.150 0.370 0.800 0.050 0.020 0.020 0.000 9
0.400 0.200 0.150 0.150 0.800 0.200 0.000 0.020 0.200 10 0.400
0.500 0.150 0.100 0.800 0.100 0.000 0.020 0.000 11 0.220 0.200
0.150 0.100 0.400 0.050 0.000 0.020 0.000 12 0.400 0.200 0.150
0.050 0.800 0.050 0.350 0.020 0.000 13 0.400 0.200 0.150 0.050
0.800 0.050 0.000 0.200 0.000
TABLE-US-00002 TABLE 2 (Continued from Table 1) Conditions of
extrusion Conditions of soaking Cooling treatment means Rate of
cooling from applied soaking temperature Extrusion immediately
Aging Soaking temperature to room temperature Extrusion temperature
speed after treatment Division Number (.degree. C.) (.degree.
C./hr) at exit (.degree. C.) (m/min) extrusion .degree. C. .times.
3 hr Working 1 520 120 530 4 Water spray 190 Examples 2 580 120 530
3 Water spray 190 3 550 120 530 10 Water spray 190 4 580 120 530 3
Water spray 190 5 550 120 530 3 Water spray 190 6 550 120 530 3
Water spray 190 7 550 120 550 10 Water spray 190 8 550 120 550 10
Water spray 190 9 550 120 550 10 Water spray 190 10 550 120 550 10
Water spray 190 11 550 120 550 10 Water spray 190 12 550 120 530 3
Water spray 190 13 550 120 530 3 Water spray 190 14 500 120 530 3
Water spray 190 15 550 120 530 3 Water spray 190 16 580 120 530 10
Water spray 190 17 580 120 530 3 Water spray 190 18 580 120 530 3
Water spray 190 Comparative 1 550 40 530 3 Water spray 190 examples
2 550 120 530 3 Air blow 190 3 480 120 530 3 Water spray 190 4 550
120 480 3 Water spray 190 5 550 120 530 3 Water spray 190 6 550 120
530 3 Water spray 190 7 550 120 530 3 Water spray 190 8 550 120 530
3 Water spray 190 9 550 120 530 3 Water spray 190 10 550 120 530 3
Water spray 190 11 550 120 530 3 Water spray 190 12 550 120 530 3
Water spray 190 13 550 120 530 3 Water spray 190 Characteristics of
extruded member Structure of extruded member Flexural Average
crushing Average intervals performance Corrosion areal ratio
between Critical resistance of cube intergranular Tensile 0.20%
bending Intergranular Recrystallized orientations precipitates
strength proof stress Elongation radius R corrosion Division Number
grain structure (%) (.mu.m) (MPa) (MPa) (%) (mm) susceptibility
Working 1 Equiaxed grain 25 >30 277 252 12 0.5 .largecircle.
Examples 2 Equiaxed grain 30 25 280 260 13 0.5 .largecircle. 3
Equiaxed grain 28 28 270 248 14 0.5 .largecircle. 4 Equiaxed grain
32 25 275 255 13 1.0 .largecircle. 5 Equiaxed grain 30 >30 265
242 15 0.5 .largecircle. 6 Equiaxed grain 18 >30 260 242 14 0.5
.largecircle. 7 Equiaxed grain 25 25 302 271 12 2.0 .largecircle. 8
Equiaxed grain 31 27 288 261 12 1.0 .largecircle. 9 Equiaxed grain
30 25 291 266 12 2.0 .largecircle. 10 Equiaxed grain 25 20 320 295
12 2.0 .largecircle. 11 Equiaxed grain 28 10 355 325 12 3.0
.largecircle. 12 Equiaxed grain 30 5 323 285 14 2.0 .largecircle.
13 Equiaxed grain 24 >30 275 253 13 0.5 .largecircle. 14
Equiaxed grain 28 20 285 265 11 2.0 .largecircle. 15 Equiaxed grain
24 >30 275 254 11 1.0 .largecircle. 16 Equiaxed grain 30 18 320
295 12 2.0 .largecircle. 17 Equiaxed grain 30 15 335 300 11 3.0
.largecircle. 18 Equiaxed grain 32 20 285 260 13 1.0 .largecircle.
Comparative 1 Equiaxed grain 12 10 260 235 10 3.5 .largecircle.
examples 2 Equiaxed grain 32 2.8 278 199 15 4.0 .quadrature. 3
Equiaxed grain 8 2 275 260 8 5.0 .largecircle. 4 Equiaxed grain 8 2
265 235 9 6.0 .largecircle. 5 Equiaxed grain 10 2.8 360 320 10 10.0
.largecircle. 6 Equiaxed grain 8 2.8 350 325 11 10.0 .largecircle.
7 Equiaxed grain 35 2 330 300 14 4.0 X 8 Fibrous grain 7 2.8 275
245 10 4.0 .largecircle. 9 Fibrous grain 6 2.8 265 240 11 4.0
.largecircle. 10 Equiaxed grain 8 2.8 255 230 8 10.0 .largecircle.
11 Equiaxed grain 25 >30 180 150 15 1.0 .largecircle. 12
Equiaxed grain 32 2 295 275 10 6.0 X 13 Equiaxed grain 10 >30
245 235 10 10.0 .largecircle.
[0127] The present invention provides the extruded member of
aluminum alloy of 6000 series and the manufacturing method
therefor, said extruded member having both good flexural crushing
performance and good corrosion resistance which are required of
reinforcement members for automotive bodies. The extruded member is
suitable for use as automotive body reinforcement members, such as
bumper reinforcement and door guard bar, which need outstanding
lateral crushing performance.
* * * * *