TECHNICAL FIELD
[0001] The present invention relates to an extruded shape produced using an Al-Mg-Si-based
aluminum alloy.
BACKGROUND ART
[0002] In recent years, a reduction in weight of automobiles aimed to improve driving performance
and reduce fuel consumption has been desired from the viewpoint of environment protection.
[0003] Use of an aluminum alloy extruded shape as an automotive structural material has
been studied in order to meet the requirements for reducing fuel consumption by way
of a reduction in weight.
[0004] An automotive structural material is required to exhibit high strength, high bendability,
and high corrosion resistance, and a JIS 7000 series aluminum alloy (Al-Zn-Mg-based
aluminum alloy) and a JIS 6000 series aluminum alloy (Al-Mg-Si-based aluminum alloy)
have attracted attention. However, a 7000 series aluminum alloy (natural age hardening
alloy) has a drawback in that processing becomes difficult due to hardening when the
time elapsed from extrusion to bending is long. Moreover, a 7000 series aluminum alloy
shows a decrease in corrosion resistance under a stress environment.
[0005] Therefore, a 6000 series aluminum alloy has been considered to be a promising heat-treatable
alloy that does not undergo natural age hardening, and exhibits excellent corrosion
resistance.
[0006] An extruded shape formed of a known high-strength 6000 series aluminum alloy exhibits
high tensile strength, but exhibits insufficient elongation, and easily produces cracks
during bending.
[0007] In order to obtain high strength, water-cooling press quenching is performed immediately
after extrusion.
[0008] The water-cooling press quenching treatment has an advantage in that properties similar
to those obtained by solution/quenching treatment that reheats the extruded alloy
after extrusion can be obtained. However, since a difference in cooling rate occurs
between each cross-sectional area due to the cross-sectional shape of the extruded
shape, the difference in thickness, and the like, the extruded shape shows a non-uniform
temperature distribution during cooling, and strain occurs. Therefore, the dimensional
accuracy deteriorates, and it is difficult to reduce the thickness of the cross-sectional
profile. The degree of freedom of the cross-sectional shape decreases as a result
of preventing occurrence of such strain.
[0009] The water-cooling press quenching treatment has another disadvantage in that an increase
in cost occurs as compared with an air-cooling quenching treatment.
[0010] On the other hand, the air-cooling quenching treatment has an advantage in that cost
can be reduced as compared with the water-cooling press quenching treatment. However,
since the cooling rate is limited, high strength may not be obtained depending on
the alloy composition, and a deterioration in ductility may occur although high strength
can be obtained.
[0011] Patent Document 1 discloses an aluminum alloy extruded shape that exhibits excellent
axial crush properties and corrosion resistance, and includes 0.4 to 0.8% of Mg, 0.3
to 0.9% of Si, 0.05% or less of Cu, and 0.095% or less of Mn, Cr, Zr in total, wherein
the number of Mg
2Si moieties having a length of 3 µm in the extrusion direction is 50 or more per mm
2. However, it is considered that the alloy composition disclosed in Patent Document
1 provides excellent corrosion resistance, but achieves a proof stress of only about
220 MPa (i.e., cannot sufficiently contribute to a reduction in weight of the product).
Since a water-cooling press quenching treatment is normally used in Patent Document
1, it is considered that the extrusion productivity is low.
[0012] Since Cu, Mn, Cr, and Zr are considered to be impurities, and the content thereof
is limited, it is considered that an improvement in ductility cannot be achieved.
[0013] Patent Document 2 discloses an aluminum alloy extruded shape that exhibits excellent
hardenability and axial crush properties, and includes 0.45 to 0.75% of Mg, 0.45 to
0.80 of Si, 0.1 to 0.4% of excess Si, 0.15 to 0.40% of Mn, and 0 to 0.1 % of Cr, wherein
Mn and Cr compounds are finely dispersed. Patent Document 2 achieves good productivity
by utilizing an air-cooling press quenching treatment. However, the aluminum alloy
extruded shape disclosed in Patent Document 2 has a proof stress of only about 220
MPa.
[0014] Since it is necessary to add Cr that achieves sharp quench sensitivity, it is difficult
to improve the proof stress using an air-cooling means.
Patent Document 1:
JP-A-2002-285272
Patent Document 2:
JP-A-2004-225124
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0015] An object of the invention is to provide an Al-Mg-Si-based high-strength aluminum
alloy extruded shape that exhibits excellent corrosion resistance and ductility, and
exhibits excellent hardenability during extrusion (i.e., ensures high productivity),
and a method for producing the same.
SOLUTION TO PROBLEM
[0016] According to one aspect of the invention, there is provided a high-strength aluminum
alloy extruded shape that exhibits excellent corrosion resistance, ductility, and
hardenability, the aluminum alloy extruded shape including 0.65 to 0.90 mass% of Mg,
0.60 to 0.90 mass% of Si, 0.20 to 0.40 mass% of Cu, 0.20 to 0.40 mass% of Fe, 0.10
to 0.20 mass% of Mn, and 0.005 to 0.1 mass% of Ti, with the balance being Al and unavoidable
impurities, the aluminum alloy extruded shape having a stoichiometric Mg
2Si content of 1.0 to 1.3 mass%, an excess Si content relative to stoichiometric Mg
2Si of 0.10 to 0.30 mass%, and a total content of Fe and Mn of 0.35 mass% or more.
Note that the unit "mass%" may be hereinafter referred to as "%".
[0017] The extruded shape is obtained by extruding an aluminum alloy having the above composition,
cooling the extruded aluminum alloy at an average cooling rate of 100°C/min or less
immediately after the extrusion, and subjecting the cooled aluminum alloy to artificial
aging.
[0018] When the average cooling rate is 100°C/min or less, it suffices to air-cool the aluminum
alloy using a fan immediately after the extrusion instead of water-cooling the aluminum
alloy, and press quenching by air-cooling can be implemented.
[0019] For example, a cooling rate of 50 to 100°C/min can be achieved by cooling the extruded
shape extruded from an extrusion press using a fan.
[0020] The extruded shape thus produced has a structure in which crystal grains having an
aspect ratio of 4.0 or more have an average crystal grain size of 80 µm or less, and
has a 0.2% proof stress (σ) of 280 MPa or more.
[0021] The term "aspect ratio" used herein refers to the ratio (L
1/L
2) of the length L
1 of the crystal grains of the recrystallized structure in the extrusion direction
to the length L
2 of the crystal grains in the direction orthogonal to the extrusion direction.
[0022] The term "average crystal grain size" used herein refers to the average diameter
of circles respectively circumscribed to the crystal grains.
[0023] The extruded shape according to one aspect of the invention has an impact strength
determined by a Charpy impact test of 20 J/cm
2 or more.
[0024] The content range of each component is selected for the following reasons. Mg and
Si
[0025] Mg and Si contribute to an improvement in the strength of the extruded shape through
formation of Mg
2Si precipitates.
[0026] Since a decrease in extrudability occurs if the Mg content and/or the Si content
is too high, the upper limit of the Mg content is set to 0.90%, and the upper limit
of the Si content is set to 0.90%.
[0027] The Mg
2Si content is set to 1.0 to 1.3% in order to obtain a 0.2% proof stress of 280 MPa
or more while taking account of extrudability.
[0028] Excess Si relative to stoichiometric Mg
2Si can improve the 0.2% proof stress without significantly impairing extrudability.
[0029] However, a decrease in ductility may occur if the excess Si content relative to stoichiometric
Mg
2Si is too high. Therefore, the excess Si content relative to stoichiometric Mg
2Si is set to 0.10 to 0.30%.
[0030] It is preferable to control the excess Si content relative to stoichiometric Mg
2Si within the range of 0.10 to 0.20% from the viewpoint of ensuring excellent ductility.
Cu
[0031] Cu contributes to solid solution hardening, and ensures elongation when the Cu content
is within a given range.
[0032] Since a decrease in corrosion resistance and extrudability occurs if the Cu content
is too high, the Cu content is set to 0.2 to 0.4%.
Fe
[0033] One aspect of the invention is characterized in that the Fe content is set to 0.20
to 0.40%.
[0034] Fe refines the crystal grains of the extruded metal structure, and improves ductility.
[0035] Known refinement components such as Mn, Cr, and Zr increase quench sensitivity even
during air-cooling using a fan immediately after extrusion. In contrast, Fe does not
increase quench sensitivity, and makes it possible to perform quenching at a cooling
rate of 100°C/min or less.
Mn
[0036] It is known that Mn affects quench sensitivity during air-cooling using a fan immediately
after extrusion. The inventor of the invention conducted extensive studies, and found
that Mn does not significantly affect quench sensitivity during air-cooling using
a fan when the Mn content is 0.20% or less. The inventor also found that, when the
Mn content is 0.10 to 0.20%, a recrystallized structure that extends in the extrusion
direction is obtained in which propagation of cracks is suppressed as compared with
a spherical recrystallized structure, and the crystal grains have a small average
crystal grain size.
[0037] Therefore, the total content of Fe and Mn is set to 0.35% or more.
Ti
[0038] Ti refines the crystal grains when casting a billet subjected to extrusion. The Ti
content is preferably 0.005 to 0.10%.
[0039] If the Ti content exceeds 0.10%, coarse intermetallic compounds may be easily produced,
and may not disappear during extrusion. As a result, the strength of the extruded
shape may decrease.
Additional components
[0040] Additional components (e.g., Cr, Zr, and Zn) other than the above components may
be included in the extruded shape as unavoidable impurities as long as the content
of each additional component is 0.05% or less, and the total content of additional
components is 0.15% or less.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0041] According to one aspect of the invention, the proof stress can be improved while
ensuring extrudability by setting the stoichiometric Mg
2Si content to 1.00 to 1.30%, and setting the excess Si content relative to stoichiometric
Mg
2Si to 0.10 to 0.30%. It is possible to achieve high strength and high ductility by
press quenching via air-cooling in case that the Fe content is set to 0.20 to 0.40%,
and the Mn content is set to 0.10 to 0.20% so that "Fe+Mn≥0.35 mass%" is satisfied.
[0042] It is also possible to improve the impact strength.
BRIEF DESCRIPTION OF DRAWINGS
[0043]
FIG. 1 shows the composition of each billet used for experiments and evaluations.
FIG. 2 shows the production conditions used for experiments and evaluations.
FIG. 3 shows evaluation results.
FIG. 4 shows an example of a comparison of the metal structure of an extruded shape.
DESCRIPTION OF EMBODIMENTS
[0044] Billets that differ in chemical composition were cast, extruded, and evaluated as
described below.
[0045] A molten metal including the alloy components shown in FIG. 1 was prepared, and cast
at a casting speed 60 mm/min or more to obtain a cylindrical billet having a diameter
of 8 inches.
[0046] FIG. 2 shows the subsequent production conditions.
[0047] The cast billet was homogenized at 565 to 595°C for 2 to 6 hours (see "HOMO conditions").
[0048] The billet was preheated to 480 to 520°C, and extruded to obtain an extruded shape
having a hollow cross-sectional shape (single-hollow cross-sectional shape) (W=50
mm, H=40 mm, t (thickness)=3 mm).
[0049] FIG. 2 shows the extrusion speed and the cooling rate.
[0050] The cooling rate was set to 50 to 100°C/min in order to achieve press quenching by
air-cooling using a fan. Note that the cooling rate was set to 200°C/min in Comparative
Example 5.
[0051] The extruded shape was cooled to room temperature, and subjected to artificial aging
at 185 to 200°C for 3 to 3.5 hours (see "Heat treatment conditions").
[0052] FIG. 3 shows the property evaluation results for the extruded shape thus produced.
Evaluation items and evaluation methods
[0053]
- (1) Tensile strength, 0.2% proof stress, and elongation: A JIS No. 4 tensile test
specimen was prepared from the extruded shape in accordance with JIS Z 2241. The specimen
was subjected to a tensile test using a tensile tester compliant to the JIS standard.
- (2) Microstructure: A specimen was cut from the extruded shape, mirror-finished, and
etched at 40°C for 3 minutes using a 3% NaOH aqueous solution. The surface of the
specimen was observed using an optical microscope.
[0054] FIG. 4 shows a photograph of the metal structure of Comparative Example 1 (see "RELATED-ART
ALLOY"), and a photograph of the metal structure of Example 1 (see "INVENTIVE ALLOY").
[0055] The aspect ratio was determined by calculating the average value (n=5 to 10) of the
ratios (L
1/L
2) of the length L
1 of the crystal grains in the extrusion direction to the length L
2 of the crystal grains in the direction orthogonal to the extrusion direction.
[0056] The crystal grain size was determined by calculating the average value (n=5 to 10)
of the diameters of circles respectively circumscribed to the crystal grains. (3)
Corrosion resistance: The stress corrosion cracking resistance (SCC resistance) was
evaluated.
[0057] A No. 1 specimen was prepared in accordance with JIS H 8711, and subjected to the
following cycle test in a state in which a stress equal to 100% of the 0.2% proof
stress was applied.
[0058] A cycle (3.5% NaCl aqueous solution, 25°C, 10 min → air-drying (25°C, 40% (humidity),
50 min)) is repeated 720 times, and a case where no cracks were observed was evaluated
as acceptable.
(4) Impact strength: A JIS V-notch No. 4 tensile test specimen was prepared from the
extruded shape in accordance with JIS Z 2242. The impact strength was measured using
a Charpy impact tester compliant to the JIS standard.
[0059] The target impact strength was set to 20 J/cm
2 or more.
Evaluation results
[0060] The extruded shapes of Examples 1 to 10 had a flat recrystallized metal structure
(microstructure) in which crystal grains having an aspect ratio of 4.0 or more had
an average crystal grain size of 80 µm or less.
[0061] The extruded shapes of Examples 1 to 10 had a proof stress of 280 MPa or more (i.e.,
exhibited high strength), and had an elongation (ductility) of 8% or more.
[0062] The extruded shapes of Examples 1 to 10 had a Charpy impact strength of 20 J/cm
2 or more.
[0063] The extruded shapes of Comparative Examples 1 to 5 showed high elongation, but had
low proof stress.
[0064] The extruded shapes of Comparative Examples 1 to 3 had low proof stress since the
Cu content and the excess Si content were low.
[0065] The extruded shape of Comparative Example 4 had low proof stress since the Mg
2Si content was low, and the extruded shape of Comparative Example 5 had low proof
stress since the excess Si and the total content of Mn and Fe were low.
[0066] The extruded shapes of Comparative Examples 6 to 8 are poor in both of proof stress
and elongation.
[0067] This is because the Fe content, the Cu content, and the Mg content were low.
[0068] The extruded shapes of Comparative Examples 9 to 13 achieved the target proof stress,
but had low elongation and low impact strength.
[0069] This is because the total content of Fe and Mn was low.
[0070] The extruded shape of Comparative Example 14 had low proof stress, low elongation,
and low impact strength since the excess Si content and the total content of Fe and
Mn were low.
[0071] The extruded shape of Comparative Example 15 had low proof stress since the excess
Si content was low although the Si content and the Mg content were sufficient.
INDUSTRIAL APPLICABILITY
[0072] Since the aluminum alloy extruded shape according to the embodiments of the invention
exhibits excellent corrosion resistance, ductility, and hardenability, the aluminum
alloy extruded shape may be widely used as structural materials for vehicles, machines,
and the like.
1. A high-strength aluminum alloy extruded shape that exhibits excellent corrosion resistance,
ductility, and hardenability, the aluminum alloy extruded shape comprising 0.65 to
0.90 mass% of Mg, 0.60 to 0.90 mass% of Si, 0.20 to 0.40 mass% of Cu, 0.20 to 0.40
mass% of Fe, 0.10 to 0.20 mass% of Mn, and 0.005 to 0.1 mass% of Ti, with the balance
being Al and unavoidable impurities, the aluminum alloy extruded shape having a stoichiometric
Mg2Si content of 1.0 to 1.3 mass%, an excess Si content relative to stoichiometric Mg2Si of 0.10 to 0.30 mass%, and a total content of Fe and Mn of 0.35 mass% or more.
2. The high-strength aluminum alloy extruded shape as defined in claim 1, wherein crystal
grains of the aluminum alloy extruded shape having an aspect ratio of 4.0 or more
have an average crystal grain size of 80 µm or less.
3. The high-strength aluminum alloy extruded shape as defined in claim 1 or 2, wherein
the aluminum alloy extruded shape has a proof stress of 280 MPa or more.
4. The high-strength aluminum alloy extruded shape as defined in claim 1 or 2, wherein
the aluminum alloy extruded shape has an impact strength determined by a Charpy impact
test of 20 J/cm2 or more.
5. A method for producing a high-strength aluminum alloy extruded shape that exhibits
excellent corrosion resistance, ductility, and hardenability, the method comprising
extruding an aluminum alloy, cooling the extruded aluminum alloy at an average cooling
rate of 100°C/min or less immediately after the extrusion, and subjecting the cooled
aluminum alloy to artificial aging, the aluminum alloy comprising 0.65 to 0.90 mass%
of Mg, 0.60 to 0.90 mass% of Si, 0.20 to 0.40 mass% of Cu, 0.20 to 0.40 mass% of Fe,
0.10 to 0.20 mass% of Mn, and 0.005 to 0.1 mass% of Ti, with the balance being Al
and unavoidable impurities, the aluminum alloy having a stoichiometric Mg2Si content of 1.0 to 1.3 mass%, an excess Si content relative to stoichiometric Mg2Si of 0.10 to 0.30 mass%, and a total content of Fe and Mn of 0.35 mass% or more.