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DESCRIPTION
Title of Invention: MAGNESIUM ALLOY MATERIAL
Technical Field
[0001]
The present invention relates to a magnesium alloy material suitable for
various
components, such as housings for electronic and electrical devices and parts
of
automobiles, and materials of these components. In particular, the present
invention
relates to a magnesium alloy material having high corrosion resistance.
Background Art
[0002]
Magnesium alloys containing various additive elements added to magnesium are
used in constituent materials for various components, such as housings for
mobile
electronic and electrical devices, for example, mobile phones and laptop
computers, and
parts of automobiles.
[0003]
Magnesium alloy components are mainly die-cast components and thixomold
components (AZ91 alloys as defined in the American Society for Testing and
Materials
standards). In recent years, components manufactured by press forming of a
sheet made
of a wrought magnesium alloy exemplified by an AZ31 alloy as defined in the
American
Society for Testing and Materials standards have been used. Patent Literature
1 proposes
a magnesium alloy sheet made of an alloy equivalent to an AZ91 alloy as
defined in the
American Society for Testing and Materials standards. The magnesium alloy
sheet has
excellent press workability.
[0004]
Since magnesium is an active metal, surfaces of the components and magnesium
alloy sheets serving as the materials of the components need corrosion
protection, such as
anodic oxidation treatment or chemical conversion treatment, to increase
corrosion
resistance.
Citation List
Patent Literature
[0005]
PTL 1: Japanese Unexamined Patent Application Publication No. 2007-098470
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Summary of Invention
Technical Problem
[0006]
Al-containing magnesium alloys, such as AZ31 alloys and AZ91 alloys, tend to
have higher corrosion resistance with increasing Al content. For example, AZ91
alloys
are considered to have high corrosion resistance among magnesium alloys. Even
components made of AZ91 alloys (mainly die-cast components and thixomold
components), however, require the corrosion protection as described above.
This is
because die-cast components of AZ91 alloys without corrosion protection may
suffer from
local corrosion in a corrosion test as described below. Thus, it is desirable
to increase the
corrosion resistance of magnesium alloy materials.
[0007]
Accordingly, it is an object of the present invention to provide a magnesium
alloy
material having high corrosion resistance.
Solution to Problem
[0008]
As described above, the corrosion resistance increases with increasing Al
content.
The present inventors studied corrosion resistance of various forms of
magnesium alloy
materials containing 7.3% by mass or more Al. Even at the same Al content, the
corrosion resistance depended on the form of a magnesium alloy material. In
order to
clear up the cause of this, the structure of each form was examined. Magnesium
alloy
materials having low corrosion resistance contained coarse precipitates (based
on an
additive element in an alloy; typically an intermetallic compound containing
at least one of
Al and Mg). Magnesium alloy materials having high corrosion resistance
contained
substantially uniformly dispersed fine precipitates or substantially no
precipitate.
[0009]
An additive element, such as Al, in a magnesium alloy is present principally
as at
least one of precipitates (typically intermetallic compounds), impurities in
crystal and
precipitated impurities, and solid solution. When Al is used in precipitates,
this decreases
the Al content of the main phase of a magnesium alloy in a region distant from
the
precipitates and their surroundings.
[0010]
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A structure containing the coarse precipitates has a higher Al concentration
than its
surroundings and is a structure in which a region having a relatively large
area of the high
Al concentration portion (a region mainly composed of the precipitates and
their
surroundings) is locally present. In other words, the structure includes many
local regions
having a relatively low Al concentration. The regions having a low Al
concentration
often suffer from corrosion, and local corrosion, such as pitting corrosion,
may occur and
advance.
[0011]
A structure containing substantially uniformly dispersed fine precipitates has
a
higher Al concentration than its surroundings and is a structure in which
micro regions
composed of high Al concentration portions are substantially uniformly
dispersed. In
other words, the structure includes substantially uniformly dispersed fine
precipitates, and
a large amount of Al remaining in the main phase is substantially uniformly
dispersed. A
structure containing substantially no precipitate is substantially composed of
the main
phase containing substantially uniformly dispersed Al and a small number of or
no very
fine precipitates. Substantially uniform dispersion of Al can prevent or
retard local
corrosion. A magnesium alloy material having such a structure can have high
corrosion
resistance.
[0012]
In order to measure the Al concentration over the coarse to micro regions, an
electron probe micro analyzer (EPMA) can be suitably utilized. The Al
concentration of
various forms of magnesium alloy materials was analyzed with an EPMA. It was
found
that, as described below in the examples, in a magnesium alloy material having
high
corrosion resistance a region satisfying x% by mass a wherein x% by mass
denotes the
Al content of the entire alloy material occupies half, and there are
substantially no portion
having a very low Al content and a relatively few portions having a very high
Al content.
Thus, the present inventors found that high corrosion resistance can be
quantitatively
defined with a parameter, such as the area percentage of Al concentration.
This
quantitative definition can be applied to any form of Al.
[0013]
The present invention is based on such findings and defines a magnesium alloy
material having high corrosion resistance using the Al concentration and the
area
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percentage thereof.
[0014]
The present invention relates to a magnesium alloy material made of a
magnesium
alloy containing 7.3% by mass or more and 16% by mass or less Al. This
magnesium
alloy material satisfies the following (1) to (3), wherein x% by mass denotes
the Al content
of the entire magnesium alloy material:
(1) a region having an Al content of (x x 0.8)% by mass or more and (x x 1.2)%
by
mass or less occupies 50% by area or more,
(2) a region having an Al content of (x x 1.4)% by mass or more occupies 17.5%
by
area or less, and
(3) there is substantially no region having an Al content of 4.2% by mass or
less.
[0015]
As described above, a magnesium alloy material according to the present
invention
includes substantially no region having low corrosion resistance, such as a
region having
an Al content of 4.2% by mass or less, and a region having a high Al
concentration (a
region having an Al concentration in the range of 0.8x% to 1.2x% by mass)
occupies 50%
or more, and a region having a very high Al concentration (a region having an
Al
concentration of 1.4x% by mass or more) is present in a small amount. Thus, a
magnesium alloy material according to the present invention includes
substantially no
region having a low Al concentration and can therefore effectively prevent
local corrosion.
Furthermore, a magnesium alloy material according to the present invention
includes few
or substantially no regions having a very high Al concentration (typically
includes a small
amount of fine precipitates containing Al (substantially no precipitates of
some form)), and
Al is sufficiently and widely dispersed in the magnesium alloy main phase.
Thus, a
magnesium alloy material according to the present invention has a
substantially uniformly
high Al concentration in at least the entire surface side region. A magnesium
alloy
material according to the present invention having such a structure has high
corrosion
resistance.
[0016]
In accordance with one aspect of the present invention, a region having an Al
content of (x x 0.8)% by mass or more and (x x 1.2)% by mass or less occupies
70% by
area or more, and a region having an Al content of (x x 1.4)% by mass or more
occupies
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5% by area or less.
[0017]
In accordance with this aspect, a region having a high Al concentration (a
region
having an Al concentration in the range of 0.8x% to 1.2x% by mass) occupies
70% or more,
and a region having a very high Al concentration (a region having an Al
concentration of
1.4x% by mass or more) occupies 5% by area or less or, in some instances, as
small as 3%
by area or less. Thus, Al is more uniformly dispersed, and the corrosion
resistance is
further improved.
[0018]
In accordance with one aspect of the present invention, a region having an Al
content of (x x 0.9)% by mass or more and (x x 1.2)% by mass or less occupies
50% by
area or more.
[0019]
In accordance with this aspect, a region having a higher Al concentration (a
region
having an Al concentration in the range of 0.9x% to 1.2x% by mass) occupies
half or more.
Thus, because of many regions having high corrosion resistance, the corrosion
resistance is
further improved.
[0020]
In accordance with one aspect of the present invention, the total area of an
intermetallic compound containing at least one of Al and Mg is 3% by area or
less in a
cross section in a surface side region of the magnesium alloy material.
[0021]
This aspect includes a small amount of or no very fine intermetallic compound
containing Al or Mg in at least the entire surface side region and has high
corrosion
resistance. Furthermore, in the presence of a very small amount of or no
intermetallic
compound containing Al, the main phase has a substantially uniformly high Al
concentration, resulting in high corrosion resistance.
[0022]
In accordance with one aspect of the present invention, the magnesium alloy
material is a sheet.
[0023]
This aspect can be suitably utilized in a material for a plastic worked
component
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subjected to plastic working, such as press forming, forging, or bending. In
particular,
under the specific plastic working conditions described below, the resulting
plastic worked
component can have an Al concentration distribution similar to that of the
sheet described
above and high corrosion resistance.
[0024]
In accordance with one aspect of the present invention, the magnesium alloy
material is a coiled long sheet.
[0025]
Since the coiled material is composed of a sheet having high corrosion
resistance,
the coiled material can be used as a material for a plastic worked component
subjected to
plastic working, such as press forming, forging, or bending, thereby
contributing to the
mass production of the plastic worked component. In particular, under the
specific plastic
working conditions described below, the resulting plastic worked component can
have an
Al concentration distribution similar to that of the coiled material and high
corrosion
resistance. The coiled material can be unwound and punched or cut to form a
sheet
having a desired shape, thereby contributing to the mass production of such a
sheet. The
resulting sheet substantially retains the Al concentration distribution of the
coiled material
and therefore has high corrosion resistance. The sheet can be used directly or
as a
material for the plastic worked component.
[0026]
In accordance with one aspect of the present invention, the magnesium alloy
material is a plastic worked component of a sheet.
[0027]
A magnesium alloy material according to the present invention may have various
forms as described below. In particular, a plastic worked component (formed
product)
having a desired shape as described above can be suitably used in various
components and
housings. When the sheet is subjected to solution heat treatment (in
particular, final
solution heat treatment) as described below, the sheet has high elongation.
Thus, when
such a sheet is subjected to plastic working, such as press forming or
forging, the sheet has
higher corrosion resistance and toughness than die-cast components and
thixomold
components. When the sheet is subjected to plastic working (primary
processing), such
as rolling, or the primary processed sheet is further subjected to final
solution heat
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treatment, this reduces or substantially eliminates internal defects, such as
voids (cavities),
in processing, such as rolling, thereby improving toughness, or the rolling
reduces the size
of crystal grains and thereby improves strength. When such a sheet is
subjected to plastic
working (secondary processing), such as press forming or forging, the sheet
has higher
corrosion resistance and better mechanical characteristics, such as toughness
and strength,
than die-cast components and thixomold components.
Advantageous Effects of Invention
[0028]
A magnesium alloy material according to the present invention has high
corrosion
resistance.
Brief Description of Drawings
[0029]
[Fig. 1] Figures 1(A) to 1(D) are FE-EPMA composition mapping images of
magnesium alloy materials. Figure 1(A) shows a sample No. 1: coiled material
(without
final solution heat treatment). Figure 1(B) shows a sample No. 2: coiled
material (with
final solution heat treatment). Figure 1(C) shows a sample No. 3: sheet
material (rapid
heated and quenched material). Figure l(D) shows a sample No. 100: die-cast
component.
[Fig. 2] Figures 2(A) to 2(D) are bar graphs of the relationship between Al
concentration and area percentage (%) in the magnesium alloy material. Figure
2(A)
shows the sample No. 1: coiled material (without final solution heat
treatment). Figure
2(B) shows the sample No. 2: coiled material (with final solution heat
treatment). Figure
2(C) shows the sample No. 3: sheet material (rapid heated and quenched
material).
Figure 2(D) shows the sample No. 100: die-cast component.
[Fig. 3] Figures 3(A) to 3(D) are photomicrographs (x 5000) of magnesium alloy
materials. Figure 3(A) shows the sample No. 1: coiled material (without final
solution
heat treatment). Figure 3(B) shows the sample No. 2: coiled material (with
final solution
heat treatment). Figure 3(C) shows the sample No. 3: sheet material (rapid
heated and
quenched material). Figure 3(D) shows the sample No. 100: die-cast component.
[Fig. 4] Figure 4 is a graph of the relationship between Al concentration (%
by
mass) measured by ICP spectroscopy and EPMA X-ray intensity in magnesium alloy
materials having different Al contents.
Description of Embodiments
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[0030]
The present invention will be described in detail below.
[Magnesium Alloy Material]
(Composition)
A magnesium alloy constituting a magnesium alloy material according to the
present invention may have a composition in which Mg is combined with an
additive
element (the remainder: Mg and impurities, Mg: 50% by mass or more). In
particular, in
the present invention, the magnesium alloy is a high-concentration alloy in
which an
additive element constitutes 7.3% by mass or more, particularly a Mg-Al alloy
containing
at least Al as an additive element. A higher Al content tends to result in
higher corrosion
resistance as well as excellent mechanical characteristics, such as strength
and hardness.
Thus, an alloy having a high Al content, for example, of 7.3% by mass or more
as in the
present invention has higher corrosion resistance and better mechanical
characteristics than
alloys having a low Al content. An Al content of more than 16% by mass,
however,
results in poor plastic formability. Thus, the upper limit is 16% by mass. The
Al content
is preferably 12% by mass or less because of further improved plastic
formability,
particularly preferably 11% by mass or less, more preferably 8.3% to 9.5% by
mass.
[0031]
The additive element other than Al may be one or more elements selected from
the
group consisting of Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au,
and rare-earth
elements (except Y and Ce). Each of these elements may constitute 0.01% by
mass or
more and 10% by mass or less, preferably 0.1% by mass or more and 5% by mass
or less,
of the magnesium alloy. Among the additive elements, 0.001% by mass or more in
total,
preferably 0.1% by mass or more and 5% by mass or less in total, of at least
one element
selected from Si, Ca, Sn, Y, Ce, and rare-earth elements (except Y and Ce) can
improve
heat resistance and flame resistance. When a magnesium alloy contains a rare-
earth
element, the rare-earth element content is preferably 0.1% by mass or more in
total. In
particular, when a magnesium alloy contains Y, the Y content is preferably
0.5% by mass
or more. Examples of the impurities include Fe.
[0032]
Examples of more specific compositions of the Mg-Al alloy include AZ alloys
(Mg-
Al-Zn alloys, Zn: 0.2% to 1.5% by mass), AM alloys (Mg-Al-Mn alloys, Mn: 0.15%
to
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0.5% by mass), Mg-Al-rare-earth element (RE) alloys, AX alloys (Mg-Al-Ca
alloys, Ca:
0.2% to 6.0% by mass), AS alloys (Mg-Al-Si alloys, Si: 0.2% to 6.0% by mass),
and AJ
alloys (Mg-Al-Sr alloys, Sr: 0.2% to 7.0% by mass) as defined in the American
Society' for
Testing and Materials standards. In particular, Mg-Al alloys containing 8.3%
to 9.5% by
mass Al and 0.5% to 1.5% by mass Zn, typically AZ91 alloys, are preferred
because of
their high corrosion resistance and excellent mechanical characteristics.
[0033]
In the present invention, the Al content of the entire magnesium alloy
material
(hereinafter referred to as an Al overall average) x% by mass refers to the
total amount of
Al in the magnesium alloy material, irrespective of the state of Al in the
magnesium alloy
material (principally, at least one of precipitates, impurities in crystal and
precipitated
impurities, and solid solution). Typically, the total amount can be suitably
measured by
ICP spectroscopy (inductively coupled plasma atomic emission spectroscopy: ICP-
AES).
[0034]
(Al Concentration and Area Percentage (Area Ratio))
A magnesium alloy material according to the present invention is most
characterized by Al concentration distribution. More specifically, an analysis
of the Al
concentration on the surface of the alloy material shows that (1) a region
having an Al
content of Al overall average (x% by mass) 20% occupies 50% by area or more
(wherein
7.3 x 16). A region having an Al content of less than 0.8x% by mass (a
minimum of
5.84% by mass) has low corrosion resistance. A region having an Al content of
more than
1.2x% by mass (a maximum of 19.2% by mass) has high corrosion resistance, but
the
concentration of Al in this region makes it relatively easy for a region
having low corrosion
resistance to be present. In contrast, the region having an Al content in the
range of
0.8x% to 1.2x% by mass (hereinafter referred to as a central composition
region) has small
variations in Al concentration. Thus, when the region having such a uniform Al
concentration occupies 50% by area or more, regions having large variations in
Al
concentration, such as the region having an Al content of less than 0.8x% by
mass and the
region having an Al content of more than 1.2x% by mass, are difficult to be
present. Thus,
a magnesium alloy material according to the present invention includes few or
substantially no regions having low corrosion resistance, and at least the
surface side
region of the alloy material is composed of a region having a relatively high
Al
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concentration. This can prevent local corrosion and improve corrosion
resistance. A
higher area percentage of the central composition region tends to result in a
wider region
having a uniform Al concentration and a more uniform Al concentration. In
other words,
this tends to result in a decrease in Al concentration distribution width.
Thus, the area
percentage of the central composition region is preferably 55% by area or
more, more
preferably 70% by area or more, still more preferably 90% by area or more,
particularly
preferably 95% by area or more. When a region having a higher Al
concentration, more
specifically a region having an Al concentration in the range of 0.9x% to
1.2x% by mass
occupies 30% by area or more, more preferably 50% by area or more, the
corrosion
resistance is further improved because of the high Al concentration and
uniform presence
of the region having such a high concentration. A method for measuring the Al
concentration and a method for measuring the area percentage are described in
detail below.
[0035]
The Al concentration can be measured in any point in any cross section of a
magnesium alloy material. A region most closely involved in corrosion is a
surface of the
alloy material. Thus, at least a surface of a magnesium alloy material
according to the
present invention satisfies the Al concentration distribution specified above.
In
accordance with the present invention, the Al concentration distribution in
the inside of a
magnesium alloy material (for example, a region located at a depth of more
than a quarter
of the thickness from a surface) may be equivalent to the Al concentration
distribution on
the surface, or the Al concentration distribution in the inside may be
different from the Al
concentration distribution on the surface.
[0036]
A magnesium alloy material according to the present invention includes (2) few
regions having an Al content of Al overall average (x% by mass) x 140% or more
(wherein
7.3 x 16). A
region having an Al content of 1.4x% by mass (a maximum of 22.4% by
mass) or more has high corrosion resistance, but the concentration of Al in
this region
makes it easy for a region having a relatively low Al concentration and low
corrosion
resistance to be present. In contrast, in a magnesium alloy material according
to the
present invention, a region having an Al content of 1.4x% by mass or more
(hereinafter
referred to as an ultra-high concentration region) occupies as little as 17.5%
by area or less.
Thus, a region having low corrosion resistance is difficult to be present, and
the corrosion
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resistance is improved. A decrease in the area percentage of the ultra-high
concentration
region results in fewer regions having a relatively low Al concentration and
fewer regions
having low corrosion resistance. In other words, this tends to result in a
decrease in Al
concentration distribution width. Thus, the area percentage of the ultra-high
concentration region is preferably 15% by area or less, more preferably 14% by
area or less,
particularly preferably 5% by area or less, still more preferably 3% by area
or less, still
more preferably 1% by area or less. The area percentage of the ultra-high
concentration
region is still more preferably 0.15% by area or less, ideally zero.
[0037]
In accordance with a magnesium alloy material according to the present
invention,
(3) there is substantially no region having an Al content of 4.2% by mass or
less
(hereinafter referred to as a low-concentration region), that is,
substantially no region
having low corrosion resistance. In the presence of a portion having a
relatively high Al
content, corrosion may occur or advance preferentially in a portion having a
relatively low
Al content. In contrast, a magnesium alloy material according to the present
invention
includes substantially no portion having a very low Al concentration, that is,
substantially
no portion in which corrosion is likely to occur or advance, and therefore has
high
corrosion resistance. The term "substantially no", as used herein, means that
a point
having an Al content of 4.2% by mass or less is not observed by EPMA.
[0038]
(Structure)
It is desirable that a structure having high corrosion resistance include
smaller and
fewer, preferably substantially no, regions having a very high Al
concentration. Thus, a
structure containing substantially no Al-rich precipitates exemplified by Al-
rich
intermetallic compounds, such as A112Mg17, and those in combination with some
additive
element, such as Al2Ca, Al4Ca, and Al3Ni, is expected to have highest
corrosion resistance.
The corrosion resistance is excellent when the area percentage of the ultra-
high
concentration region satisfies a particular range and when there is
substantially no low-
concentration region. Thus, the present invention allows Al-rich precipitates,
such as the
intermetallic compounds described above, to be present, provided that the area
percentage
is satisfied. In particular, in the presence of an Al-rich intermetallic
compound, a
structure containing a substantially uniformly dispersed (the total area
percentage: 11% or
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less) small intermetallic compound (average particle size: 3.0 um or less,
preferably 0.5
pm or less) tends to have a substantially uniform Al concentration and is
therefore
preferred. Uniform presence of an Al-rich precipitate, such as the
intermetallic compound,
is expected to function as a barrier to corrosion.
[0039]
With some additive element in the alloy, in addition to the Al-rich
intermetallic
compound, another intermetallic compound containing Mg, such as Mg2Si, Mg2Ca,
Mg2Sn,
or Mgi7Sr2, may be formed. The intermetallic compound containing Mg or Al as
described above in at least the surface side region of a magnesium alloy
material preferably
constitutes 3% by area or less in terms of corrosion resistance. Thus, the
present
invention has no lower limit of the intermetallic compound. In the present
invention, the
surface side region in which the area percentage of the intermetallic compound
is to be
measured refers to a region located at a depth of 100 p.m or less from a
surface of the
magnesium alloy material. When the magnesium alloy material has a thickness of
less
than 100 m, the surface side region refers to a region located at a depth of
not more than a
quarter of the thickness from a surface of the magnesium alloy material. A
method for
measuring the area ratio of the intermetallic compound will be described
below.
[0040]
(Form)
From the perspective of manufacturing process, examples of the form of a
magnesium alloy material according to the present invention include wrought
materials
(with solution heat treatment during manufacture), such as rolled sheets and
extruded
materials; straightened materials manufactured by straightening of wrought
materials; heat-
treated materials manufactured by heat treatment of wrought materials in order
to remove
strain; solution-heat-treated materials manufactured by final solution heat
treatment of
various forms, including die-cast components, thixomold components, wrought
materials,
straightened materials, and cast materials (for example, materials
manufactured by a twin-
roll continuous casting process); wrought materials (after solution heat
treatment)
manufactured by flatting, such as rolling or extrusion, of solution-heat-
treated materials;
straightened materials (after solution heat treatment) manufactured by
straightening of
solution-heat-treated materials; heat-treated materials (after solution heat
treatment)
manufactured by flatting, such as rolling, and subsequently the heat treatment
of solution-
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heat-treated materials; and polished components manufactured by grinding
wrought
materials, straightened materials, heat-treated materials, and solution-heat-
treated materials.
Other examples include worked components manufactured by plastic working, such
as
drawing, bending, forging, or press forming, or machining, such as cutting or
punching, of
sheets in the form of a wrought material, a straightened material, a heat-
treated material, a
solution-heat-treated material, or a polished component.
[0041]
Among the wrought materials, the rolled sheets may have small crystal grains
because of rolling and have a fine structure having an average grain size of
10 pm or less
or even 5 lam or less or have a structure containing few, small, or
substantially no internal
defects, such as voids (cavities) (a structure having an actually measured
density of 99% or
more of the density of a theoretical density material as determined from the
material
composition). The presence of such a fine structure or a structure having a
high actually
measured density can be indicative of a rolled sheet. A rolled sheet having
few, small, or
preferably substantially no internal defects as described above has excellent
mechanical
characteristics, such as tensile strength, elongation, and rigidity and can be
suitably used in
a sturctural component or as a material for a sturctural component.
[0042]
Straightening may be performed by leveler roller processing. Depending on the
degree of straightening, a straightened material manufactured by leveler
roller processing
may have a shear band and may have a structure having a grain boundary that is
difficult to
clearly observe even in microscopic observation. In this case, the structure
is not even a
non-amorphous structure because a monochromatic X-ray diffraction peak can be
obtained.
Thus, the fact that a monochromatic X-ray diffraction peak can be obtained and
that no
grain boundary can be observed can be indicative of a straightened material
manufactured
by leveler roller processing. A straightened material, particularly a
straightened material
manufactured by leveler roller processing, tends to cause recrystallization
during plastic
working, such as press forming, thereby having excellent plastic formability.
At a low
degree of straightening, the appearance, structure, and mechanical properties
may be
similar to those of the rolled sheets.
[0043]
Since solution-heat-treated materials have been subjected to solution heat
treatment,
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which produces a supersaturated solid solution, an additive element, such as
Al, is mainly
present as a solid solution, and a precipitate, such as an Al-containing
intermetallic
compound, for example, Al 12Mg17, Al(MnFe), Al2Ca, A14Ca, or A1:31\li, is
rarely present and,
if present at all, is present in a small amount. Thus, a low abundance of an
Al-containing
intermetallic compound can be indicative of a solution-heat-treated material.
More
specifically, when the percentage of the total area of an intermetallic
compound containing
at least one of Al and Mg in a cross section in the surface side region of a
magnesium alloy
material (typically a region located at a depth of 100 p.m or less from the
surface) is 3% or
less or 1% or less, the magnesium alloy material is a solution-heat-treated
material.
Solution-heat-treated materials tend to substantially retain their hardness or
elongation
after heat treatment at 400 C for 30 hours or more (the surface of a test
specimen has been
ground). Thus, a variation in mechanical characteristics due to heat treatment
can be
utilized as an indicator of a solution-heat-treated material.
[0044]
As described above, in solution-heat-treated materials, a supersaturated solid
solution is entirely formed, and Al can be substantially uniformly dispersed.
More
specifically, a solution-heat-treated material that has been subjected to
final solution heat
treatment has an Al concentration distribution such that the central
composition region
constitutes 90% by area or more and the ultra-high concentration region
constitutes 3% by
area or less or even 1% by area or less. Thus, the solution-heat-treated
material has still
higher corrosion resistance. Solution-heat-treated materials contain
substantially no
defect that can be a starting point of cracking, such as a coarse precipitate
(typically an
intermetallic compound), in plastic working and therefore have excellent
plastic
formability. Thus, such solution-heat-treated materials can be suitably used
as materials
for plastic worked components.
[0045]
Polished components have a smooth surface and have excellent surface texture.
Thus, surface smoothness (for example, a maximum height Rz of 20 [tm or less)
or
polishing marks are indicative of polished components.
[0046]
In the case of heat-treated materials, for example, the fact that (1) no shear
band is
observed in the inside of a magnesium alloy material or (2) particles having a
crystal grain
CA 02823292 2013-06-27
size of 0.1 gm or less constitute 5% by area or less in a cross section is
indicative of heat-
treated materials, although it depends on the heat treatment conditions.
[0047]
In the case that the worked components are plastic worked components, when a
sheet of a magnesium alloy material according to the present invention is used
as a
material and is subjected to plastic working under particular conditions, the
plastic worked
component has an Al concentration distribution such that the central
composition region
constitutes 50% by area or more, the ultra-high concentration region
constitutes 17.5% by
area or less, and there is substantially no low-concentration region, and the
plastic worked
component has high corrosion resistance. Thus, the plastic worked component
can
substantially retain the Al concentration distribution of the material. The
machined
materials can also substantially retain the Al concentration distribution of
the material.
[0048]
From the perspective of shape, examples of the form of a magnesium alloy
material
according to the present invention include sheets (composed of substantially
flat front and
back sides parallel to each other and side surfaces between the front and back
sides; the
distance (= thickness) between the front and back sides is substantially
constant on the
whole; as viewed from the top, the sheets can assume various planar shapes,
such as
rectangular, circular, elliptical, and polygonal), coiled materials of a long
sheet, and various
special-shaped materials other than the sheets. Sheets can assume various
shapes by
cutting or punching, for example, various planar shapes, such as circular,
elliptical,
polygonal, and shapes having a through-hole (including a large through-hole,
such as a
window). Sheets may have a portion having a different thickness by using a
profile roller
as described below; for example, a sheet partly has a recessed portion or a
rib (a raised
portion). Examples of the special-shaped materials include three-dimensionally
shaped
materials formable by a die-casting process or a thixomold process and three-
dimensionally shaped materials formable by plastic working, such as press
forming.
Examples of the three-dimensionally shaped materials include sheets having a
projection,
such as a rib, or a recessed portion and partly having a different thickness;
boxes having
a ]-shaped cross section or ]-shaped frames for use in housings for various
apparatuses;
closed-end tubes; and those having a relatively simple shape, for example,
spheres,
ellipsoids, and polygonal prisms, such as triangular prisms. Special-shaped
materials may
CA 02823292 2013-06-27
16
have a through-hole (including a large through-hole, such as a window). Such
forms
having recessed and raised portions or a through-hole can be easily formed by
a die-casting
process. A material having a desired shape manufactured by cutting or
polishing of an
ingot may also be used. Special-shaped materials may have a plastic worked
portion that
has been subjected to plastic working, such as press forming.
[0049]
A magnesium alloy material according to the present invention may have any of
various shapes as described above. In particular, a coiled material can be
suitably used as
a material for a plastic worked component that has been subjected to plastic
working, such
as press forming, and contribute to the mass production of the plastic worked
component.
In the case that a magnesium alloy material according to the present invention
is a coiled
material, a more specific form may be a coiled rolled sheet, a coiled cast
material, or a
coiled solution-heat-treated material manufactured by final solution heat
treatment of a
coiled rolled sheet. The coiled material may have a thickness of 10 mm or less
or 5 mm
or less, a width of 100 mm or more, 200 mm or more, or particularly 250 mm or
more, and
a length of 30 m or more, 50 m or more, or particularly 100 m or more. Such
long
materials and wide materials are suitable as materials for plastic worked
components. In
particular, rolled sheets or rolled sheets that have been subjected to final
solution heat
treatment may be thinner or longer than the cast materials. For example, thin
materials
may have a thickness of 2 mm or less, particularly 1.5 mm or less, more
particularly 1 mm
or less, and long materials may have a length of 50 m or more, 100 m or more,
particularly
200 m or more. Thin materials having a thickness of 2 mm or less can be
suitably used as
materials for low-profile light-weight plastic worked components. The
thickness is
preferably 0.1 mm or more, and thin materials having a thickness in the range
of 0.3 to 1.2
mm are easy to use.
[0050]
Because of its high corrosion resistance, a magnesium alloy material according
to
the present invention can be used without corrosion protection, such as
chemical
conversion treatment or anodic oxidation treatment, in some corrosive
environment. This
can obviate the necessity for a corrosion protection process, increase the
productivity of the
magnesium alloy material, and reduce wastes, thereby reducing environmental
load. As a
matter of course, a magnesium alloy material according to the present
invention may be
CA 02823292 2013-06-27
17
subjected to corrosion protection, such as chemical conversion treatment or
anodic
oxidation treatment, that is, may have an anticorrosive layer. In this case,
the Al
concentration can be measured by removing the anticorrosive layer from the
surface by
polishing or cutting to expose a substrate surface made of a magnesium alloy
even without
performing precise cross-sectional observation. In addition to the
anticorrosive layer, a
coating layer can further improve corrosion resistance or impart a color or a
pattern to the
magnesium alloy material, thereby improving commercial value. An anticorrosive
layer
or a coating layer may be formed on a desired portion.
[0051]
[Manufacturing Method]
Manufacture of a magnesium alloy material according to the present invention
involves at least one solution heat treatment before the completion of an end
product. In
particular, in a manufacturing method including a hot rolling process, in
addition to a
solution heat treatment process, the cooling rate of a material to a
particular temperature
can be controlled within a particular range in a cooling process after final
hot rolling.
Whether the hot roiling process is performed or not, the cooling rate in a
particular
temperature range can be controlled in a particular range in the final
solution heat
treatment process. For example, a method for manufacturing a magnesium alloy
material
according to the present invention may include a rolling process
(manufacturing methods
1-1 to 1-5) or no rolling process (a manufacturing method 2). In the following
manufacturing methods, at least one process may be omitted or the order of
processes may
be changed, provided that a magnesium alloy material according to the present
invention
has a particular Al concentration distribution that satisfies the conditions
(1) to (3).
[0052]
(Manufacturing Method 1-1)
In the case that a magnesium alloy material according to the present invention
is a
rolled sheet, for example, the magnesium alloy material can be manufactured by
a method
including the following preparation process, intermediate solution heat
treatment process,
and rolling process.
Preparation process: a process of preparing a cast material made of a
magnesium
alloy that contains 7.3% by mass or more and 16% by mass or less Al and
manufactured by
a continuous casting process.
CA 02823292 2013-06-27
18
Intermediate solution heat treatment process: a process of performing solution
heat
treatment of the cast material at a holding temperature of at least the
following minimum
holding temperature for a holding time of one hour or more and 25 hours or
less to
manufacture an intermediate-solution-heat-treated material.
Minimum holding temperature: a temperature 10 C lower than a temperature
(solidus temperature) at which Al is dissolved in Mg in a Mg-Al binary phase
diagram (%
by mass)
Rolling process: a process of performing at least one pass of hot rolling to
the
intermediate-solution-heat-treated material to manufacture a rolled sheet.
In particular, in manufacturing processes from the intermediate solution heat
treatment process, the thermal history of a material to be processed
(typically a rolled
sheet) is controlled such that the total time of holding the material at a
temperature of
150 C or more and 300 C or less is 12 hours or less and that the material is
not heated to a
temperature of more than 300 C.
In the rolling process, after final hot rolling, the average cooling rate of
the material
from the starting temperature of cooling to a material temperature of 100 C or
less is
0.8 C/min or more.
[0053]
The minimum holding temperature, that is, the temperature 10 C lower than the
solidus temperature in the Mg-Al binary phase diagram (% by mass) is typically
represented by the following formula (the same applies to a manufacturing
method 1-2
described below). When the Al overall average x% by mass of a magnesium alloy
is 5%
by mass or more and 13% by mass or less, the solidus temperature is in the
range of 283 C
to 437 C and increases with increasing Al overall average. The minimum holding
temperature is expressed by the following linear expression.
(Formula) (Minimum holding temperature) = 20 x x + (182 - 10) = 20x + 172
When the Al overall average is more than 13% by mass and 16% by mass or less,
the minimum holding temperature is (437-10) C = 427 C.
[0054]
In the manufacturing method 1-1, in particular, the total time of holding a
material
at a temperature in the range of 150 C to 300 C in the processes from the
intermediate
solution heat treatment to preferably the completion of an end product is
reduced to 12
CA 02823292 2013-06-27
19
hours or less. At a temperature in the range of 150 C to 300 C, an Al-rich
intermetallic
compound, such as A112Mg17, is easy to grow. A relatively short holding time
at this
temperature range can suppress the growth of the intermetallic compound,
suppress an
increase in the area of an ultra-high concentration region or a low-
concentration region, or
reduce the area percentage of the intermetallic compound. After the final hot
rolling,
cooling in the cooling process is controlled at the particular cooling rate
until the
temperature reaches at least 100 C so as to substantially prevent Al
diffusion. This can
suppress the growth of the intermetallic compound and an increase in the area
of an ultra-
high concentration region or a low-concentration region. The cooling rate is
preferably
increased to suppress an increase in the area of an ultra-high concentration
region or a low-
concentration region. The growth of the intermetallic compound is also
suppressed by
heating the material to not more than 300 C.
[0055]
(Manufacturing Method 1-2)
In the case that a magnesium alloy material according to the present invention
is a
solution-heat-treated material that has been subjected to a rolling process,
the magnesium
alloy material can be manufactured by a method including the following
preparation
process, intermediate solution heat treatment process, rolling process, and
final solution
heat treatment process, for example.
Preparation process: a process of preparing a cast material made of a
magnesium
alloy that contains 7.3% by mass or more and 16% by mass or less Al and
manufactured by
a continuous casting process.
Intermediate solution heat treatment process: a process of performing solution
heat
treatment of the cast material at a holding temperature of at least the
following minimum
holding temperature for a holding time of one hour or more and 25 hours or
less to
manufacture an intermediate-solution-heat-treated material.
Minimum holding temperature: a temperature 10 C lower than a temperature
(solidus temperature) at which Al is dissolved in Mg in a Mg-Al binary phase
diagram (%
by mass)
Rolling process: a process of performing at least one pass of hot rolling to
the
intermediate-solution-heat-treated material to manufacture a rolled sheet.
Final solution heat treatment process: a process of performing final solution
heat
CA 02823292 2013-06-27
treatment of the rolled sheet at a holding temperature of at least the minimum
holding
temperature for a holding time of one hour or more and 40 hours or less.
In particular, in the final solution heat treatment process, the cooling rate
at a
temperature in the range of 330 C to 380 C satisfies the following.
A surface layer region, which is located at a depth of 10 pm or less from a
surface
of the rolled sheet, is cooled at 1 C/min or more.
[0056]
The final solution heat treatment performed after rolling as described above
can
form a solid solution of a precipitate, such as an Al-rich intermetallic
compound, produced
in the processes up to the rolling process and thereby effectively suppress an
increase in the
area of an ultra-high concentration region or a low-concentration region or
reduce the area
percentage of the intermetallic compound. The cooling rate after the final hot
rolling in
the rolling process of the manufacturing method 1-2 may be in a particular
range in the
same manner as in the manufacturing method 1-1. Also in the manufacturing
method 1-2,
from the final solution heat treatment process to the completion of an end
product, the
thermal history of the material is preferably controlled such that the total
time of holding
the material at a temperature of 150 C or more and 300 C or less is minimized
and that the
material is not heated to a temperature of more than 300 C. This can maintain
the Al
concentration distribution of the solution-heat-treated material manufactured
through the
final solution heat treatment process.
[0057]
(Manufacturing Method 1-3)
A rolled sheet manufactured by the manufacturing method 1-1 may be subjected
to
final heat treatment in order to remove strain. More specifically, in the case
that a
magnesium alloy material according to the present invention is a heat-treated
material that
has been subjected to a rolling process, the magnesium alloy material can be
manufactured
by a method including a final heat treatment process described below, in
addition to the
preparation process, the intermediate solution heat treatment process, and the
rolling
process performed in the manufacturing method 1-1.
[0058]
(Manufacturing Method 1-4)
The rolled sheet or the solution-heat-treated material manufactured by the
CA 02823292 2013-06-27
21
manufacturing method 1-1 or 1-2 may be subjected to straightening (typically
hot
straightening) in order to improve its straightness or may be subjected to
washing or
polishing in order to improve its surface texture (remove an oxidized layer,
surface defects,
or a lubricant used in the rolling). In particular, in the case that a
magnesium alloy
material according to the present invention is a straightened material or a
polished
component, the magnesium alloy material can be manufactured by a method
including at
least one of a straightening process and a polishing process described below,
in addition to
the preparation process, the intermediate solution heat treatment process, the
rolling
process (specified in the manufacturing method 1-1 or 1-2), and the final
solution heat
treatment process (only specified in the manufacturing method 1-2).
[0059]
(Form of Products of Manufacturing Methods 1-1 to 1-4)
A manufacturing method including a preparation process, an intermediate
solution
heat treatment process, and a rolling process and a manufacturing method
further including
at least one process selected from a final solution heat treatment process, a
final heat
treatment process, a straightening process, a polishing process, and a washing
process can
be used to manufacture a sheet having a predetermined length (a short sheet
that is
probably difficult to wind (for example, a length of 5 m or less, particularly
1 m or less);
hereinafter referred to as a sheet material) or a long sheet.
[0060]
For example, the sheet material can be manufactured by cutting a cast material
into
a cast material having a predetermined length (a cast sheet) in the
preparation process and
performing the subsequent processes using the cast material. Alternatively,
the sheet
material can also be manufactured by winding a long cast material to prepare a
coiled cast
material in the preparation process, manufacturing a coiled material also in
each process,
and finally cutting the coiled material into a predetermined length. Without
the cutting, a
long sheet is manufactured, and the long sheet can be wound to manufacture a
magnesium
alloy material according to the present invention in the form of a coiled
material. In the
manufacture of a coiled material, each process from the preparation process
generally
includes feeding and winding of the coiled material. In this case, a material
subjected to
each process is a coiled material of a long material or a wide material. Thus,
a large
amount of material can be transferred or heated and can be continuously
treated in each
CA 02823292 2013-06-27
22
process, thereby improving the productivity of a magnesium alloy material. A
magnesium alloy material according to the present invention can be
manufactured by using
a sheet material or a coiled material as a material in each process.
[0061]
(Manufacturing Method 1-5)
In the case that a magnesium alloy material according to the present invention
is a
plastic worked component manufactured by plastic working of a sheet of any one
of the
rolled sheet, the solution-heat-treated material, the heat-treated material,
the straightened
material, the polished component, and the washed material described above, the
magnesium alloy material can be manufactured by any of the methods described
above
further including the following plastic working process.
Plastic working process: a process of preheating a sheet at a holding
temperature of
350 C or less (preferably 300 C or less) for a holding time of 8 hours or less
(preferably
0.5 hours or less) and performing plastic working of the sheet during heating.
[0062]
Each of the processes of the manufacturing methods 1-1 to 1-5 will be further
described below.
<<Preparation Process>>
The cast material is preferably manufactured by a continuous casting process.
The
continuous casting process can consistently yield cast materials having
substantially
uniform quality in the longitudinal direction. The continuous casting process
allows rapid
solidification and can thereby reduce oxides and segregation. The continuous
casting
process can also reduce the amount of coarse crystallized precipitate having a
size of more
than 10 !um, which can act as a starting point of cracking in rolling, and
yield cast materials
having excellent plastic formability, for example, in rolling or extrusion. In
particular, a
twin-roll continuous casting process can easily yield flat cast materials with
little
segregation. Although, the cast material may have any cross-sectional area,
thickness,
width, or length, an excessively large thickness may result in segregation.
Thus, the
thickness is preferably 10 mm or less, more preferably 7 mm or less,
particularly
preferably 5 mm or less. A long cast material having a length of 30 m or more,
50 m or
more, particularly 100 m or more or a wide cast material having a width of 100
mm or
more, 250 mm or more, particularly 600 mm or more can be used as a rolled
sheet to
CA 02823292 2013-06-27
23
manufacture a long rolled sheet or a wide rolled sheet. Depending on the
desired form,
the cast material may be wound to manufacture a coiled cast material or may be
cut into a
casted sheet material having a predetermined length. In the case that the cast
material is
wound to manufacture a coiled cast material having a small inner diameter,
heating to
150 C or more immediately before winding the cast material can prevent
cracking and
allows the coiled cast material to be easily manufactured.
[0063]
<<Intermediate Solution Heat Treatment Process>>
The cast material is subjected to intermediate solution heat treatment to make
its
composition uniform and produce a solid solution of an element, such as Al,
thereby
reducing the amount of coarse precipitates and yield a material having
excellent plastic
formability, for example, in rolling or extrusion. The holding temperature of
the
intermediate solution heat treatment is typically 350 C or more and 450 C or
less,
particularly 380 C or more, more particularly 390 C or more and 420 C or less.
The
holding time is one hour or more and 25 hours or less, particularly 10 hours
or more and 25
hours or less. The holding time is preferably increased as the Al content
increases. In a
cooling process started from the holding temperature, the cooling rate is
preferably
increased, for example, by forced cooling, such as water cooling or air blast,
in the same
manner as in a final solution heat treatment described below (preferably 1
C/min or more,
more preferably 50 C/min or more) to suppress the growth or precipitation of
precipitates.
In particular, it is easy to control the cooling rate of a casted sheet
material.
[0064]
The cast material may be directly subjected to intermediate solution heat
treatment
or, before intermediate solution heat treatment, may be subjected to rolling
at a low rolling
reduction (rolling reduction: approximately 1% to 15%/pass) or surface
grinding.
[0065]
<<Rolling Process>>
In rolling of a magnesium alloy, when the material temperature is room
temperature,
it is difficult to increase rolling reduction, thus resulting in low
production efficiency. In
terms of productivity, therefore, at least one pass of hot rolling is
preferably performed.
Heating a material (an intermediate-solution-heat-treated material or a rolled
sheet during
rolling) can improve plastic formability, for example, in rolling. The plastic
formability is
CA 02823292 2013-06-27
24
further improved as the material temperature is increased. An increase in
material
temperature, however, results in coarsening of a precipitate, such as an Al-
containing
intermetallic compound, which results in an increase in the area of an ultra-
high
concentration region or a low-concentration region. Coarse precipitates cause
a
deterioration in plastic formability. Thus, the material temperature is
preferably 300 C or
less, particularly preferably 150 C or more and 280 C or less. A material may
be heated
with heating means, such as an atmospheric furnace, in a preheating process.
Any
furnace that can house a material (a sheet material or a coiled material) can
be used.
[0066]
In particular, in the case that a rolled material (sheet) manufactured by
rolling a
casted sheet material is a magnesium alloy material according to the present
invention
having the particular Al concentration distribution, the holding time at the
holding
temperature in the preheating process is preferably reduced. As described
above, the
holding time of a material at the particular temperature range of 150 C to 300
C can be
minimized (preferably 12 hours or less) principally in rolling to effectively
suppress the
growth of precipitates (an Al-rich intermetallic compound in particular) and
prevent an
increase in the area of an ultra-high concentration region or a low-
concentration region or
the area percentage of an intermetallic compound. When a coiled material of a
long
material or a wide material is entirely heated to a substantially uniform
temperature, at
least part of the coiled material tends to be held at the particular
temperature range for a
longer time in preheating. In particular, when a material is densely wound to
reduce a
clearance between turns so as to manufacture a small coiled material even from
a long
material or a wide material, it takes a somewhat long time to substantially
uniformly heat
the entire coiled material. Thus, there may be a region that is held at the
particular
temperature range for a longer time. In order to suppress the growth of
precipitates over
the entire coiled material, it is desirable to control the size of the coiled
material such that
the preheating time is included in the total time or reduce the preheating
time. The
preheating time may be reduced by installing an in-line heater (typically a
heater utilizing
radiant heat, electric heating, or induction heating) immediately before a
rolling apparatus
to perform rapid heating. Use of the in-line configuration can also reduce the
time
between heating and rolling. The holding time of a material at a temperature
in the range
of 150 C to 300 C may be reduced by quenching a rolled sheet passing through a
rolling
CA 02823292 2013-06-27
apparatus (typically a rolling roller) with a refrigerant or a lubricant
(preferably at a
cooling rate of 1 C/sec or more). Performing the rapid heating and quenching
can
effectively reduce the holding time of a material at a temperature in the
range of 150 C to
300 C in the rolling process. In particular, the rapid heating and quenching
can be easily
performed when the material to be rolled is a short material, such as a casted
sheet material.
Alternatively, the time for heating a material to a substantially uniform
temperature can be
relatively reduced by stacking a plurality of materials and heating them at
the same time
while the materials are placed at appropriate intervals. This method can also
be easily
performed when the material to be rolled is a short material, such as a casted
sheet material.
For example, when a rolled material (sheet) having a predetermined length and
a particular
Al concentration distribution that satisfies the conditions (1) to (3) is
subjected to at least
one pass of hot rolling, the total holding time of preheating before the
rolling is preferably
0.01 hours or more and 8 hours or less, particularly preferably 0.01 hours or
more and 0.3
hours or less. Thus, such preheating conditions can be controlled to
manufacture a
magnesium alloy material containing substantially no precipitate and having a
narrow Al
concentration distribution width, that is, a magnesium alloy material having
improved
corrosion resistance.
[0067]
One or more passes of rolling including the hot rolling may be performed. A
plurality of passes of rolling can yield a rolled sheet having a small
thickness, reduce the
average grain size of the structure of the rolled sheet (for example, 101AM or
less,
preferably 5 ttm or less), or improve plastic formability, for example, in
press forming.
The number of passes, the rolling reduction in each pass, and the total
rolling reduction can
be appropriately determined so as to manufacture a rolled sheet having a
desired thickness.
Known rolling conditions, for example, heating a reduction roll as well as a
material may
be employed.
[0068]
In particular, in a cooling process after the final hot rolling in the
manufacture of a
rolled material (sheet) or a coiled rolled sheet having a particular Al
concentration
distribution that satisfies the conditions (1) to (3), the average cooling
rate from the start of
cooling to a point at which the material temperature reaches 100 C or less is
preferably
0.8 C/min or more. After the final hot rolling, a material can be rapidly
cooled to
CA 02823292 2013-06-27
26
effectively prevent the growth of precipitates during cooling and an increase
in the area of
an ultra-high concentration region or a low-concentration region or the area
percentage of
an intermetallic compound. In particular, since a coiled material tends to be
heated for a
long period of time as described above, rapid cooling after the final hot
rolling is effective
in suppressing the formation or an increase in the area of an ultra-high
concentration region
or a low-concentration region. When final solution heat treatment is performed
after
rolling, it is not necessarily to satisfy the cooling rate. It is, however,
expected that a
decrease in the area of an ultra-high concentration region or a low-
concentration region or
the area percentage of an intermetallic compound before the final solution
heat treatment
allows the area of the ultra-high concentration region or the low-
concentration region or
the area percentage of the intermetallic compound to be easily reduced also in
a finally
obtained solution-heat-treated material. The average cooling rate may be
determined by
measuring the material temperature at the beginning of cooling after the final
hot rolling
and calculating the average cooling rate using (Tmp - 100)/t ( C/min), wherein
Tmp ( C)
denotes the measured temperature and t (min) denotes the time to reach 100 C.
The
cooling may be controlled to satisfy (Tmp - 100)/t ( C/min) 0.8 ( C/min). The
material
temperature may be measured with a contact-type sensor, such as a
thermocouple, or a
non-contact sensor, such as in thermography. An ultrathin thermocouple may be
attached
to a material surface.
[0069]
The cooling rate is preferably as high as PC/sec or more, more preferably 5
C/sec
or more. In the cooling process, any cooling means that can achieve the
cooling rate may
be used. In particular, the cooling rate may be increased utilizing forced
cooling. The
forced cooling means may be means that utilizes a gas medium, such as a fan
(air cooling)
or an air blast (jet air cooling), means that utilizes a liquid medium, such
as water cooling,
or means that utilizes a solid medium, such as a chill roll. In particular,
use of air cooling,
such as an air blast, can obviate the necessity of a process of removing a
liquid refrigerant
deposited on a material or prevent deterioration in surface texture caused by
the deposition
of a liquid refrigerant. A liquid refrigerant can be used to easily increase
the cooling rate.
A liquid refrigerant containing a detergent (for example, a surfactant) that
can remove a
lubricant used in rolling is preferred because cooling and washing can be
performed
simultaneously. Although forced cooling means may be installed off line, in-
line
CA 02823292 2013-06-27
27
installation can ensure a large contact area between a material surface and a
coolant,
thereby improving cooling efficiency. In the case of a coiled material, after
the final hot
rolling, a material may be wound before the cooling. The coiled material may
be directly
cooled or may be unwound to easily increase the cooling rate. Natural cooling
instead of
the forced cooling means may be performed, provided that the cooling rate can
be achieved.
[0070]
Rolling at a low rolling reduction, such as finish rolling, may be cold
working.
Variations in Al concentration rarely occur in cold working, and the Al
concentration
distribution before the cold working is substantially maintained.
[0071]
In multi-pass rolling, an intermediate heat treatment between passes may be
performed provided that the holding time at a temperature in the range of 150
C to 300 C
is included in the total time described above. Strain, residual stress, and
texture
introduced into a material in plastic working up to the heat treatment
(principally rolling)
can be removed or reduced by intermediate heat treatment. This can prevent
accidental
cracking, strain, or deformation in rolling after the heat treatment, thereby
allowing rolling
to be performed without any interruption. The holding temperature of a
material in this
intermediate heat treatment is also preferably 300 C or less. The holding
temperature is
preferably 150 C or more, particularly preferably 250 C or more and 280 C or
less. The
holding time may be approximately 0.5 to 3 hours. Also in a cooling process
after the
intermediate heat treatment, the cooling rate is preferably increased
(preferably 1 C/min or
more, more preferably 50 C/min or more) to suppress the growth of
precipitates.
[0072]
The rolled sheet may have any thickness, width, and length. Use of a lubricant
in
the rolling can decrease frictional resistance during the rolling and prevent
seizure of a
material, thereby facilitating the rolling. A reduction roll having a recessed
portion on its
outer periphery (a profile roller) may be used to manufacture a rolled sheet
having a rib.
A reduction roll having a raised portion on its outer periphery (a profile
roller) may be used
to manufacture a rolled sheet having a recessed portion. A rolled sheet thus
manufactured
may be cut or ground in a desired recessed or raised form or a stepped form or
may be
provided with a boss or a through-hole.
[0073]
CA 02823292 2013-06-27
28
<<Final Solution Heat Treatment>>
Final solution heat treatment after the rolling allows precipitates to be
redissolved,
thereby sufficiently reducing the area of an ultra-high concentration region
or the amount
of intermetallic compound and substantially eliminating a low-concentration
region. A
holding temperature lower than the minimum holding temperature or a holding
time of less
than one hour results in insufficient supersaturated solid solution and an
insufficient
reduction in the area of the ultra-high concentration region or the amount of
intermetallic
compound. An excessively high holding temperature (typically more than 450 C)
or a
holding time of more than 40 hours may result in seizure of a main phase or a
decrease in
productivity because heating after solid solution is sufficiently formed
causes energy loss.
Thus, the holding temperature is preferably reduced. For example, the holding
temperature is 390 C or more and 420 C or less, and the holding time is 10
hours or more
and 25 hours or less.
[0074]
In the final solution heat treatment, the cooling rate at a temperature in the
range of
330 C to 380 C in a cooling process started from the holding temperature is
adjusted to
1 C/min or more. In a magnesium alloy having a high Al content of 7.3% by mass
or
more, it is believed that a precipitate, for example, an Al-rich intermetallic
compound, such
as Ali2Mg17, is easily formed at a temperature in the range of 330 C to 380 C.
Thus, it is
desirable to pass through this temperature range as fast as possible. In the
manufacture of
a magnesium alloy material according to the present invention having an Al
content of
7.3% by mass or more, the cooling rate at a temperature in the range of 330 C
to 380 C is
increased to reduce the time to pass through the temperature range in which
precipitates are
easily formed, thereby suppressing the precipitation of the intermetallic
compound and
suppressing an increase in the area of an ultra-high concentration region or a
low-
concentration region associated with the formation of the precipitate. The
cooling rate is
preferably as high as 1 C/min or more, more preferably 50 C/min or more.
[0075]
At least a surface layer region of a rolled sheet to be treated satisfies the
cooling rate.
As described above, corrosion occurs and advances from a surface of a
magnesium alloy
material. Thus, at least the surface layer region to be treated in the
magnesium alloy
material is cooled at the cooling rate such that the surface layer region may
have a high
CA 02823292 2013-06-27
29
corrosion resistance state, that is, have a particular Al concentration
distribution that
satisfies the conditions (1) to (3). More specifically, forced cooling as
described above
can be suitably utilized. In particular, use of air cooling, for example, a
fan or an air blast
using air blast means, such as a cool air jet mechanism, has advantages,
including
resistance to oxidation and small variations in cooling, as well as removal of
a liquid
refrigerant and prevention of deterioration in surface texture caused by the
deposition of a
liquid refrigerant. When a liquid refrigerant is used, a cooling method, for
example, mist
spray for spraying a liquid refrigerant, such as water or a reducing liquid,
water sprinkling,
or immersion in a liquid refrigerant may be utilized. When the final solution
heat
treatment is followed by straightening or plastic working, such as press
forming, a
lubricant may be used as a liquid refrigerant, and a solution-heat-treated
material may be
cooled by coating with the lubricant or immersion in the lubricant. When it is
desirable to
remove a lubricant used in rolling, a liquid refrigerant containing a
detergent may be used
as forced cooling means as described above. Water cooling using a liquid
refrigerant has
a higher cooling rate than air cooling. A coiled material may be directly
cooled or may be
unwound to easily increase the cooling rate. Depending on the thickness of a
sheet of a
coiled material, the cooling rate of an unwound coiled material can be
approximately
50 C/min for a jet mechanism and approximately 400 C/min for water cooling.
After
cooling to approximately room temperature, the unwound coiled material may be
wound.
A sheet (including a cast material) that has been subjected to the final
solution heat
treatment process also has excellent plastic formability and therefore can be
wound at an
industrially used roll diameter even at approximately room temperature.
[0076]
The cooling rate may be controlled by measuring the material surface
temperature
after the final solution heat treatment, setting the time (min) such that the
cooling rate at a
temperature in the range of 330 C to 380 C is a desired rate, and controlling
the cooling
state to achieve a desired rate. Since a magnesium alloy has high thermal
conductivity,
the temperature of a region located at a depth of 10 gm or less from the
surface (a surface
layer region) is equivalent to the temperature of the outermost surface. Thus,
the cooling
rate of the surface region can be determined by measuring the temperature of
the outermost
surface of the material. The temperature of the outermost surface of the
material may be
measured with a contact-type sensor, such as a thermocouple, or a non-contact
sensor, such
CA 02823292 2013-06-27
as in thermography, as described above.
[0077]
<<Final Heat Treatment>>
The holding temperature in the final heat treatment after rolling is
preferably 300 C
or less. More specific conditions include a holding temperature of 100 C or
more and
300 C or less and a holding time of 5 minutes or more and 60 minutes or less.
The
holding time of a material (rolled sheet) at a temperature of 150 C or more
and 300 C or
less in the final heat treatment process is preferably included in the total
time. The
holding time is preferably less than 30 minutes. Under such particular
conditions, a rolled
sheet having the particular Al concentration distribution that satisfies the
conditions (1) to
(3) and having reduced or no strain resulting from rolling can be
manufactured.
[0078]
<<Straightening>>
Straightening after rolling or after final solution heat treatment can improve
the
flatness of a sheet. Although straightening can be performed at room
temperature or
lower, hot straightening can further improve the flatness. The holding
temperature for hot
straightening is preferably 300 C or less. More specifically, the holding
temperature is
100 C or more and 300 C or less, preferably 150 C to 280 C. The holding time
of a
material (for example, a rolled sheet) at a temperature of 150 C or more and
300 C or less
in the straightening process is preferably included in the total time. In hot
straightening,
for example, a roll leveler can be suitably used. The roll leveler includes a
furnace for
heating a material and a roll unit that includes a plurality of rolls for
continuously bending
(straining) the heated material. The rolls are disposed at upper and lower
positions in a
staggered arrangement. The roll leveler can be used to continuously straighten
a long
material. A hot press machine can also be used in hot straightening. Also
after hot
straightening, when the average cooling rate from the start of cooling to a
point at which
the material temperature reaches 100 C or less is 0.8 C/min or more, this can
effectively
suppress an increase in the area of an ultra-high concentration region or a
low-
concentration region caused by the growth of precipitates or an increase in
the amount of
precipitates, such as an intermetallic compound. This cooling rate may be
achieved using
forced cooling means as described above or natural cooling. In particular,
when rolling is
continuously followed by hot straightening, the cooling rate is preferably
controlled as
CA 02823292 2013-06-27
31
described above so as to realize the particular Al concentration distribution
and reduce
curling in a coiled material, thereby yielding a flat sheet.
[0079]
<<Plastic Working>>
When a sheet material or a coiled material thus manufactured is subjected to
plastic
working, such as press forming, the sheet material or the coiled material may
be heated to
improve plastic formability. The material temperature is preferably 350 C or
less, more
preferably 300 C or less, particularly preferably 280 C or less. In
particular, 150 C or
more and 280 C or less and 150 C or more and 220 C or less are suitable. When
a
material is preheated at this temperature for a holding time of 8 hours or
less as described
above, this can suppress the growth of precipitates and effectively prevent an
increase in
the area of an ultra-high concentration region or a low-concentration region
or the area
percentage of an intermetallic compound. Provided that a material is heated
such that
desired plastic working is possible, a shorter holding time is preferred. The
holding time
is preferably 0.5 hours or less (30 minutes or less), more preferably 0.3
hours or less. In
particular, as described above, a coiled material sometimes requires a longer
time to
entirely have a substantially uniform temperature than a sheet material. In
order to reduce
the holding time, therefore, for example, a heater for rapid heating may be
used, or a fan or
a baffle plate may be placed in a furnace. Although the time for plastic
working, such as
press forming, depends on the shape of a material, press forming takes a short
time in the
range of several seconds to several minutes and is substantially free from
failures, such as
coarsening of precipitates. Plastic working under such particular conditions
can yield a
plastic worked component having the particular Al concentration distribution
that satisfies
the conditions (1) to (3).
[0080]
Heat treatment after the plastic working can remove strain or residual stress
caused
by the plastic working and improve the mechanical characteristics of the
material. The
heat treatment conditions include a holding temperature in the range of 100 C
to 300 C
and a holding time in the range of approximately 5 to 60 minutes. It is
desirable that the
holding time at a temperature in the range of 150 C to 300 C in the heat
treatment be also
included in the total time described above.
[0081]
CA 02823292 2013-06-27
32
<<Total Time of Holding Material in Particular Temperature Range>>
As described above, the total time for holding a material at a temperature in
the
range of 150 C to 300 C in the processes from the intermediate solution heat
treatment to
preferably the completion of an end product (such as rolling (including
intermediate heat
treatment), final heat treatment, straightening, and preheating before plastic
working
processes) is preferably as relatively short as 12 hours or less. In the case
that the final
solution heat treatment is performed, the total time of holding a material at
a temperature in
the range of 150 C to 300 C in the processes from the final solution heat
treatment to the
completion of an end product is preferably 12 hours or less.
[0082]
In order to ensure a sufficient heating time for plastic working, such as
rolling, the
total time for holding a temperature in the range of 150 C to 300 C is
preferably 0.01
hours or more. More preferably, manufacturing conditions, for example, the
working
ratio or the total working ratio in each pass of the rolling process,
preheating conditions
(such as preheating means and time), cooling process conditions (such as
cooling means
and time), and the line speed are controlled such that the temperature range
is 150 C or
more and 280 C or less, still more preferably 150 C or more and 220 C or less
and the
total time is 8 hours or less, particularly 0.3 hours or less. Since the
precipitation of the
Al-rich intermetallic compound increases with increasing Al content, the total
time is
preferably controlled also in a manner that depends on the Al content.
[0083]
As described above, when the final solution heat treatment is performed in and
after
the intermediate solution heat treatment, it is preferable not to heat a
material to more than
300 C in and after the final solution heat treatment. However, heating for a
short period
of time (preferably 8 hours or less, more preferably one hour or less) that
causes no
coarsening of precipitates may be allowable.
[0084]
A specific manufacturing method as a manufacturing method that includes a
rolling
process as described above may include processes of casting ¨> intermediate
solution heat
treatment (preferably the cooling rate is controlled in a cooling process) -->
rolling ¨>
intermediate heat treatment (depending on the holding temperature, the cooling
rate may
be controlled in the cooling process) ¨> rolling ---> straightening,
polishing, and washing.
CA 02823292 2013-06-27
33
In accordance with this manufacturing method, intermediate solution heat
treatment before
rolling can decrease and minimize the size of precipitates, and the subsequent
rolling can
decrease the size of structure or improve mechanical characteristics.
[0085]
(Manufacturing Method 2)
In the case that a magnesium alloy material according to the present invention
is
formed by a manufacturing method including no rolling process, typically a
formed
product (including a special-shaped material) formed by die casting, the
magnesium alloy
material can be manufactured by a method that includes the following
preparation process
and final solution heat treatment process.
Preparation process: a process of preparing one material selected from die-
cast
components, thixomold components, and extruded materials each made of a
magnesium
alloy containing 7.3% by mass or more and 16% by mass or less Al.
Final solution heat treatment process: a process of performing final solution
heat
treatment of the material at a holding temperature not less than a temperature
(minimum
holding temperature) that is 10 C lower than a temperature at which Al is
dissolved in Mg
in a Mg-Al binary phase diagram (% by mass) for a holding time of one hour or
more and
40 hours or less.
In particular, the cooling rate at a temperature in the range of 330 C to 380
C
satisfies the following.
A surface layer region, which is located at a depth of 10 pm or less from a
surface
of the material, is cooled at 1 C/min or more.
[0086]
In accordance with the manufacturing method 2, a material prepared in the
preparation process can be subjected to final solution heat treatment in the
same manner as
in the manufacturing method 1-2 to manufacture a solution-heat-treated
material having a
particular Al concentration distribution that satisfies the conditions (1) to
(3). In
particular, the manufacturing method 2 can be suitably used to manufacture a
complicated
three-dimensionally shaped magnesium alloy material as described above.
[0087]
The die-casting conditions and the thixomold conditions may be known
conditions.
An extruded material can be manufactured by preparing an ingot made of a
magnesium
CA 02823292 2013-06-27
34
alloy containing the particular amount of Al and extruding the ingot under
known
conditions.
[0088]
<<Other Processes>>
A polishing process of polishing (preferably wet polishing) a rolled sheet, a
heat-
treated material, a straightened material, or a solution-heat-treated material
manufactured
by the manufacturing method 1 or 2 can yield a polished component having the
particular
Al concentration distribution that satisfies the conditions (1) to (3) (a
magnesium alloy
material according to one embodiment of the present invention). The
manufacturing
method 1 or 2 further including a process of performing corrosion protection,
such as
chemical conversion treatment or anodic oxidation treatment, or a process of
forming a
coating layer can yield a magnesium alloy material according to the present
invention that
includes a substrate having the particular Al concentration distribution that
satisfies the
conditions (1) to (3) and an anticorrosive layer or a coating layer disposed
on the substrate.
The materials and forming conditions for the anticorrosive layer or the
coating layer may
be known materials and conditions. The corrosion protection preferably
includes
pretreatment, such as degreasing, acid etching, desmutting, and/or surface
conditioning.
When plastic working is performed, the anticorrosive layer or the coating
layer can be
formed after the plastic working to avoid damage caused by the plastic
working.
EXAMPLES
[0089]
More specific embodiments of the present invention will be described below.
[Test Example 1]
Al-containing magnesium alloy materials were manufactured under various
conditions. Their Al concentration distributions and corrosion resistance were
examined.
[0090]
In this test, magnesium alloy material samples No. 1 to No. 5 manufactured as
described below and a commercially available die-cast component (an AZ91 alloy
sheet
having a thickness of 3 mm, a width of 75 mm, and a length of 150 mm) for a
comparison
purpose were prepared. The die-cast component was subjected to wet belt
polishing
described below under the same conditions as polishing for the samples No. 1
to No. 5 to
manufacture a polished sheet, which was referred to as a sample No. 100.
CA 02823292 2013-06-27
[0091]
The following are processes of manufacturing the samples No. 1 to No. 5.
Sample No. 1: coiled material (without final solution heat treatment after
rolling)
Casting --> intermediate solution heat treatment ---> rolling --->
straightening ---->
polishing
Sample No. 2: coiled solution-heat-treated material (with final solution heat
treatment after rolling)
Casting ---> intermediate solution heat treatment --> rolling ¨> final
solution
heat treatment ¨> straightening ---> polishing
Sample No. 3: sheet material (rapid heated and quenched material) x without
intermediate winding
Casting (cutting after casting) ¨> intermediate solution heat treatment -->
rolling --> straightening --> polishing
Sample No. 4: solution-heat-treated material (die casting)
Preparation of die-cast component ¨> final solution heat treatment
Sample No. 5: solution-heat-treated material (extrusion)
Preparation of extruded material ¨> final solution heat treatment
[0092]
<<Samples No. 1 and No. 2>>
A long cast sheet (having a thickness of 4 mm and a width of 300 mm) made of a
magnesium alloy having a composition corresponding to an AZ91 alloy (Mg-
8.75%A1-
0.65%Zn (on a mass basis)) and manufactured by a twin-roll continuous casting
process
was temporarily wound to manufacture a coiled cast material. The coiled cast
material
was subjected to solution heat treatment (intermediate solution heat
treatment) in a batch
furnace at 400 C 20 x 8.75 + 172 = 347 C) for 24 hours. The resulting coiled
intermediate-solution-heat-treated material was unwound, was subjected to a
plurality of
passes of rolling under the following conditions, and was wound to manufacture
a coiled
rolled sheet having a thickness of 0.6 mm, a width of 250 mm, and a length of
800 m.
[0093]
(Rolling Conditions)
Rolling reduction: 5% to 40%/pass
Material temperature: 200 C to 280 C
CA 02823292 2013-06-27
36
Roll temperature: 100 C to 250 C
[0094]
An unwound material was fed between a feed drum in a furnace and a winding
drum in another furnace and was transferred by the rotation of the drums. The
material
was rolled with a reduction roll disposed between the drums. The rotation of
the feed
drum and the winding drum was reversed for every pass to continuously perform
a
plurality of passes of rolling. A material wound around a furnace was heated
to the
temperature described above for every pass, and the heated material was fed
between the
drums.
[0095]
After a final pass of hot rolling was applied to the material, the material
was wound,
and the material temperature was controlled with the furnace. The material was
unwound
and was blown by a flow of air maintained at constant temperature to control
the cooling
rate. The temperature of the air was controlled such that the average cooling
rate up to a
point at which the material temperature (200 C to 280 C) reached 100 C was 2.0
C/min
for the sample No. 1 or 1.7 C/min for the sample No. 2, and the average
cooling rate from
100 C to room temperature (approximately 20 C) was 1.0 C/min for the sample
No. 1 or
0.9 C/min for the sample No. 2. The rolled sheet cooled to room temperature
was wound
to manufacture a coiled rolled sheet.
[0096]
The coiled rolled sheet was unwound and was subjected to hot straightening
with a
roll leveler to manufacture a straightened coiled material (material
temperature: 250 C).
The straightened coiled material was unwound and was subjected to wet belt
polishing
with a #600 polishing belt. The resulting polished sheet was wound to
manufacture a
coiled polished sheet. This coiled polished sheet is hereinafter referred to
as a sample No.
1. In the manufacturing processes from the intermediate solution heat
treatment to the
completion of the final coiled polished sheet, the sample No. 1 was held at a
temperature in
the range of 150 C to 300 C for a total time of 12 hours or less and was not
heated to a
temperature of more than 300 C.
[0097]
The coiled rolled sheet was subjected to final solution heat treatment in a
batch
furnace at 350 C (20 x 8.75 + 172) = 347) for one hour and was cooled to room
CA 02823292 2013-06-27
37
temperature (approximately 20 C) by forced cooling. The cooling was performed
by
blowing cool air with a jet mechanism against the cylindrical surface of the
coiled material
removed from the batch furnace. In particular, the temperature, the volume,
and the
velocity of the cool air were controlled such that the average cooling rate in
a surface layer
region (a region located at a depth of 10 lam or less from the surface) of the
sheet of the
coiled material was 3 C/min 1 C/min) at a temperature in the range of 380 C to
330 C.
A thermocouple was attached to an appropriate position of the coiled material,
and the cool
air was controlled such that the cooling rate at a portion having the lowest
cooling rate was
3 C/min. Although the coiled material was cooled herein, an unwound coiled
material
may be cooled.
[0098]
The coiled solution-heat-treated material was unwound and was subjected to hot
straightening and wet belt polishing under the same conditions as in the
sample No. 1.
The resulting coiled polished sheet is hereinafter referred to as a sample No.
2. In the
manufacturing processes from the final solution heat treatment to the
completion of the
final coiled polished sheet, the sample No. 2 was held at a temperature in the
range of
150 C to 300 C for a total time of 12 hours or less and was not heated to a
temperature of
more than 300 C. In and after the final solution heat treatment, a process in
which the
sample No. 2 was held at a temperature in the range of 150 C to 300 C was
substantially
only hot straightening. Since the material having a reduced thickness owing to
rolling
was subjected to hot straightening, the holding time at the temperature range
could be
several minutes.
[0099]
<<Sample No. 3>>
A plurality of cast sheets (casted sheet materials each having a thickness of
5 mm, a
width of 300 mm, and a length of 500 mm) made of a magnesium alloy having the
same
composition as the sample No. 1 or 2 and manufactured by a twin-roll
continuous casting
process were prepared. The cast sheets were subjected to solution heat
treatment
(intermediate solution heat treatment) at 400 C for 24 hours. While being cut
to adjust
the length, the cast sheets were subjected to a plurality of passes of rolling
in the same
manner as in the samples No. 1 and No. 2 to manufacture a rolled material
(sheet) having a
thickness of 0.6 mm, a width of 300 mm, and a length of 2000 mm. Before each
pass of
CA 02823292 2013-06-27
38
rolling, the material was preheated to a predetermined temperature with
heating means for
rapid heating. The total holding time for preheating was 3 hours. After a
final pass of
hot rolling was applied to the material, the material was placed on a steel
sheet for cooling
(which can control temperature by circulating a heating medium) to control the
cooling
rate. The temperature of the steel sheet for cooling and the residence time on
the steel
sheet were controlled such that the average cooling rate up to a point at
which the material
temperature (200 C to 280 C) reached 100 C was 60 C/min, and the average
cooling rate
to room temperature (approximately 20 C) was 40 C/min. The rolled sheet thus
manufactured was subjected to hot straightening and wet belt polishing under
the same
conditions as in the samples No. 1 and No. 2. The resulting polished sheet
(sheet
material; hereinafter also referred to as a rapid heated and quenched
material) is referred to
as a sample No. 3. In the same manner as in the sample No. 1, in the
manufacturing
processes from the intermediate solution heat treatment to the completion of
the final
polished sheet, the sample No. 3 was held at a temperature in the range of 150
C to 300 C
for a total time of 12 hours or less and was not heated to a temperature of
more than 300 C.
[0100]
<<Sample No. 4>>
A commercially available die-cast component like the sample No. 100 (an AZ91
alloy sheet (Al: 8.75% by mass) having a thickness of 3 mm, a width of 75 mm,
and a
length of 150 mm) was prepared. The die-cast component was subjected to final
solution
heat treatment at 380 C (_>_ (20 x 8.75 + 172) = 347) for 20 hours and was
cooled to room
temperature (approximately 20 C) by forced cooling. The cooling was performed
by
blowing cool air with a jet mechanism against the surface of the sheet in the
same manner
as in the sample No. 2. The temperature, the volume, and the velocity of the
cool air were
controlled such that the cooling rate in a surface layer region was 50 C/min 1
C/min) at
a temperature in the range of 380 C to 330 C. After the forced cooling,
planarization
(straightening) was performed by hot press forming at 250 C or less. Wet
polishing was
then performed in the same manner as in samples No. I and No. 2. The resulting
polished
sheet is referred to as a sample No. 4.
[0101]
<<Sample No. 5>>
A commercially available die-cast component like the sample No. 100 was
CA 02823292 2013-06-27
39
redissolved, casted, and extruded to prepare a material (an AZ91 alloy sheet
having a
thickness of 3 mm, a width of 50 mm, and a length of 150 mm). The extruded
material
was subjected to final solution heat treatment at 380 C for 20 hours and was
cooled to
room temperature (approximately 20 C) by forced cooling with a jet mechanism
in the
same manner as in the sample No. 4. After the forced cooling, the material was
subjected
to straightening by hot press forming and wet polishing under the same
conditions as in the
sample No. 4. The resulting polished sheet is referred to as a sample No. 5.
Also in the
case of the sample No. 5, the temperature, the volume, and the velocity of the
cool air were
controlled such that the cooling rate in a surface layer region was 50 C/min
1 C/min) at
a temperature in the range of 380 C to 330 C.
[0102]
The cooling rate in the cooling process after the final hot rolling and the
cooling
rate in the final solution heat treatment process can be easily controlled by
preparing
correlation data as described below in advance and referring to the
correlation data.
Correlation data between the cooling rate and parameters of forced cooling
means, such as
the temperature, the volume, and the velocity of cool air, are prepared by
making it
possible to measure the temperature of the outermost surface of a target to be
cooled, such
as a plurality of coiled materials or sheet materials having different
thicknesses, lengths,
and numbers of turns, or a point at a depth of 10 vtm from the surface of the
target to be
cooled with a temperature sensor (for example, a groove is formed at the
point, and a
temperature sensor is buried in the groove) and measuring the time required
for the
material temperature at the beginning of cooling in the cooling process to
reach 100 C and
the time for the material temperature to change from 380 C to 330 C to
determine the
cooling rate while the parameters are changed. When the target to be cooled is
a coiled
material, a temperature sensor is placed at an appropriate position on the
coiled material,
and correlation data with respect to the cooling rate of a portion having the
lowest cooling
rate may be prepared in advance.
[0103]
In order to measure the Al content x% by mass of the entire sample (Al overall
average), a test specimen for the total amount was cut from the samples No. 1
to No. 5 and
the comparative sample No. 100. The Al overall average was determined by ICP
spectroscopy using the test specimen. All the samples had x = 8.75% by mass.
CA 02823292 2013-06-27
[0104]
A test specimen for mapping was cut from the samples No. 1 to No. 5 and the
comparative sample No. 100. A surface element Al of the test specimen was
analyzed
with a field emission (FE)-EPMA apparatus (JXA-8530F manufactured by JEOL
Ltd.).
The measurement conditions are described below.
[0105]
(Measurement Conditions)
Accelerating voltage: 15 kV
Irradiation current: 100 nA
Sampling time: 50 ms
[0106]
The Al content (% by mass) in the elementary analysis was determined by
preparing
the following calibration curve and converting the X-ray intensity of EPMA
into the Al
content (% by mass) using the calibration curve.
[0107]
[Preparation of Calibration Curve]
A commercially available AZ31 alloy material, AZ61 alloy material, and
material
equivalent to an AZ91 alloy having different Al contents were subjected to
solution heat
treatment (400 C x 120 hours) and were homogenized to make samples. The
material
equivalent to an AZ91 alloy was utilized by cutting the coiled solution-heat-
treated
material of the sample No. 2. The Al content was measured on a surface of the
sample by
ICP spectroscopy. The X-ray intensity of Al (cps/ A) was measured by
elementary
analysis with FE-EPMA under the measurement conditions described above.
[0108]
As illustrated in Fig. 4, the X-ray intensity y was expressed as a linear
function of
the Al content x. An approximate expression of the linear function y = 11977x
+ 1542.5
is used as a calibration curve. The approximate expression has a correlation
coefficient
R2 of 0.9998 and is reliable.
[0109]
Figures 1(A) to 1(D) are mapping images (observation field: 24 fun x 18 p,m)
with
respect to the Al content measured on the surface of the sample with FE-EPMA.
Figure
1(A) shows the coiled material of the sample No. 1. Figure 1(B) shows the
coiled
CA 02823292 2013-06-27
41
solution-heat-treated material of the sample No. 2. Figure 1(C) shows the
sheet material
(rapid heated and quenched material) of the sample No. 3. Figure l(D) shows
the die-cast
component of the sample No. 100. Although shown in a gray scale in Fig. 1,
depending
on the Al concentration, Fig. 1 is practically colored in black (Al
concentration: 0% by
mass), dark blue, blue, light blue, green, yellow, orange, red, pink, and
white (Al
concentration: 8.75 x 1.4 = 12.25% by mass or more) in increasing order of the
Al content.
White granules in Fig. 1(A), Fig. 1(B), and Fig. 1(C) and white substances of
indefinite
shapes in Fig. l(D) are Al-rich intermetallic compounds.
[0110]
Figure l(D) shows that the die-cast component of the sample No. 100 has many
regions having a very high Al concentration. It is also shown that there is a
region having
a very low Al concentration. In contrast, Figs. 1(A) to 1(C) show that the
coiled material
of the sample No. 1, the coiled solution-heat-treated material of the sample
No. 2, and the
sheet material (rapid heated and quenched material) of the sample No. 3 have
no local
large region having a very high Al concentration. In particular, it is shown
that the coiled
solution-heat-treated material of the sample No. 2 has a very few small
regions having a
very high Al concentration. It is also shown that the coiled material, the
coiled solution-
heat-treated material, and the sheet material (rapid heated and quenched
material) have
substantially no region having a very low Al concentration. In the same manner
as in the
sample No. 2, the samples No. 4 and No. 5 of the die-cast component and the
extruded
material that have been subjected to final solution heat treatment had a very
few small
region having a very high Al concentration and substantially no region having
a very low
Al concentration.
[0111]
Using these mapping images, the area percentage of a low-concentration region
having an Al concentration of 4.2% by mass or less, the area percentage of a
central
composition region having an Al concentration of 0.8x (= 8.75 x 0.8 = 7)% by
mass or
more and 1.2x (= 8.75 x 1.2 = 10.5)% by mass or less, the area percentage of a
region
having an Al concentration of 0.9x (= 8.75 x 0.9 = 7.875)% by mass or more and
1.2x% by
mass or less, the area percentage of an ultra-high concentration region having
an Al
concentration of 1.4x (= 8.75 x 1.4 = 12.25)% by mass or more, and the maximum
value
and the minimum value of the Al concentration were determined in the
observation fields
CA 02823292 2013-06-27
42
of the samples. Table shows the results. Figures 2(A) to 2(D) are graphs of
the Al
concentration distribution.
[0112]
Figures 3(A) to 3(D) are scanning electron microscope (SEM) photomicrographs
(x
5000) of the samples No. 1 to No. 3 and No. 100. Light gray granules in Figs.
3(A) to
3(C) and light gray substances of indefinite shapes in Fig. 3(D) are
precipitates. Figure
3(D) shows that the die-cast component of the sample No. 100 contains large
precipitates
of indefinite shapes. This agrees with the fact that ultra-high concentration
regions
having a very high Al concentration have large indefinite shapes in the
mapping image.
In contrast, it is shown that the coiled material of the sample No. 1 in Fig.
3(A), the coiled
solution-heat-treated material of the sample No. 2 in Fig. 3(B), and the sheet
material
(rapid heated and quenched material) of the sample No. 3 in Fig. 3(C) contain
small
precipitates and substantially uniformly distributed circular granules. In
particular, it is
shown that the coiled material of the sample No. 1 contains substantially
uniformly
dispersed circular granules having a substantially uniform size, and the
coiled solution-
heat-treated material of the sample No. 2 contains a very few very small
precipitates.
This agrees with the fact that small ultra-high concentration regions are
dispersed in the
mapping image. The solution-heat-treated materials of the samples No. 4 and
No. 5
contained a very few very small precipitates as in the coiled solution-heat-
treated material
of the sample No. 2. The compositions of the light gray granules and the
substances of
indefinite shapes were examined with an energy dispersive X-ray spectrometer
(EDS).
They were found to be intermetallic compounds containing Al and Mg, such as
Mg17A112
and Al(MnFe). The presence of the intermetallic compounds can also be detected
by
analyzing the composition and structure by X-ray diffraction.
[0113]
The average particle size (pm) and the total area ratio (% by area) of the
intermetallic compounds in the samples No. 1 to No. 5 and No. 100 were
measured.
Table also shows the results. The average particle size and the area ratio can
be easily
determined by image processing of the photomicrographs using a commercially
available
image-processing apparatus.
[0114]
The average particle size of an intermetallic compound was measured as
described
CA 02823292 2013-06-27
43
below. For each sample, three fields (a 22.7 VI,M X 17 gm region per field)
are selected for
each observed image of five cross sections in the thickness direction. These
fields were
selected in the surface side region located at a depth of 100 pm or less from
the sample
surface. The equivalent circular diameter (the diameter of a circle having an
area
equivalent to the area of an intermetallic compound) of each intermetallic
compound in
one observation field is determined for each observation field. The average
particle size
in an observation field is calculated by dividing the sum total of the
equivalent circular
diameters by the number of intermetallic compounds in the observation field:
(the total of
the equivalent circular diameters)/(the total number). Table shows the average
of the
average particle sizes of 15 observation fields for each sample.
[0115]
The total area ratio of an intermetallic compound was measured as described
below.
As described above, observation fields were selected in the surface side
region. For each
observation field, the areas of all the intermetallic compounds in one
observation field are
measured to calculate the total area. The area ratio of an observation field
is calculated
by dividing the total area by the area of the observation field (385.9 gm2):
(the total
area)/(the area of the observation field). Table shows the average of the area
ratios of 15
observation fields for each sample.
[0116]
The corrosion loss (gg/cm2) and the amount of eluted Mg (gg/cm2) of the
samples
No. 1 to No. 5 and No. 100 were measured in a saltwater corrosion test. Table
shows the
results.
[0117]
The corrosion loss was measured in a salt spray test according to JIS H 8502
(1999)
performed as the saltwater corrosion test. A test specimen for corrosion is
prepared from
the samples No. 1 to No. 5 and No. 100. The mass (initial value) of the test
specimen for
corrosion is measured. An unnecessary portion of the test specimen for
corrosion is
masked such that a test surface having a predetermined size is exposed in the
test specimen
for corrosion. The masked test specimen for corrosion is placed in a corrosion
test
apparatus. The masked test specimen for corrosion is leaned against the
apparatus at a
predetermined angle with respect to the bottom of the apparatus (the angle
between the
bottom of the apparatus and the test specimen: 70 to 80 degrees). The test
specimen for
CA 02823292 2013-06-27
44
corrosion is sprayed with a test solution (5% by mass aqueous NaC1 solution,
temperature:
35 2 C) and is left to stand for a predetermined time (96 hours). After the
predetermined time, the test specimen for corrosion is removed from the
corrosion test
apparatus, and the mask is removed. A corrosion product on the test specimen
for
corrosion is removed by chromic acid dissolution in accordance with a method
described
in Reference Table 1 in JIS Z 2371 (2000). After the corrosion product is
removed, the
mass of the test specimen for corrosion is measured. A difference between the
mass and
the initial value is divided by the area of the test surface of the test
specimen for corrosion
to determine the corrosion loss (lig/cm2).
[0118]
The amount of eluted Mg was measured under the following conditions in a salt
immersion test performed as the saltwater corrosion test. A test specimen for
corrosion is
prepared from the samples No. 1 to No. 5 and No. 100. An unnecessary portion
of the
test specimen for corrosion is masked such that a test surface having a
predetermined size
is exposed in the test specimen for corrosion. The masked test specimen for
corrosion is
completely immersed in a test solution (5% by mass aqueous NaC1 solution,
fluid volume:
(A) x 20 ml, wherein (A) cm2 denotes the area (exposed area) of the test
surface of the test
specimen) for a predetermined time (96 hours, at room temperature (25 2 C)
with air
conditioning). After the predetermined time, the test solution is collected.
The amount
of Mg ions in the test solution is measured by ICP-AES. The amount of Mg ions
is
divided by the area of the test surface of the test specimen for corrosion to
determine the
amount of eluted Mg ( g/cm2).
[0119]
[Table I]
[Table]
_____________________________________________________________________ _
_______ _ _____________________
Sample No. . 1 2 3 4 5
100
Form Coiled material Coiled solution-heat- Sheet
material Solution-heat- Solution-heat- Die-cast
(without final treated material (rapid heated
treated treated material component
solution heat (with final solution and
quenched material (die (extrusion)
treatment) heat treatment) material)
casting)
_
Region having Al content of 4.2% 0 0 0
0 0 0.3
or less (area%)__
___________________________________________________________________
Region having Al content of 56.5 95.3 91.8
95.0 94.0 41.1
0.8x%-1.2x% (area%)
Region having Al content of 30.1 39.0 70.1
45.9 44.9 28.2
0.9x%-1.2x% (area%)
0
I.)
Region having Al content of 1.4x% 13.5 0.1 4.0 0.2
2.9 17.7 0
I.)
or more (area%)
u.)
I.)
Maximum Al (mass%) 29.8 15,7 25.2
21.4 21.4 34.8 ko
"
.
,
Minimum Al (mass%) 5.1 ________ 6.1 ________ 6.4 6.9
6.9 3.5
0
Average particle size of 0.3 <0.1 0.2
<0.1 <0.1 1.0 Ul H
CA
1
intermetallic compound (1..tm)
0
(5)
1
Area ratio of intermetallic 9.1 0.3 3.5 0.2
2.9 5.5 I.)
-A
compound (area%) _________________ .
_______________________________________________
Corrosion loss (pgicm7)- 625 242.1 173.4 ,
220.9 219.0 2200
Amount of eluted Mg (.1g/cm2) 625.6 401.0 330.4
445.6 443.1 818.6
CA 02823292 2013-06-27
46
[0120]
As shown by the table, in the samples No. 1 to No. 5, a central composition
region
having an Al concentration in the range of 0.8x% to 1.2x% by mass (x = 8.75)
in at least
the surface side region occupies 50% by area or more, there is no low-
concentration region
having an Al concentration of 4.2% by mass or less, and an ultra-high
concentration region
having an Al concentration of 1.4x% by mass or more occupies 17.5% by area or
less. In
particular, in the samples No. 1 to No. 5, the ultra-high concentration region
occupies 15%
by area or less, and a region having an Al concentration in the range of 0.9x%
to 1.2% by
mass occupies 30% by area or more. Thus, the samples No. 1 to No. 5 have small
variations in Al concentration. This is also shown by the graphs in Figs. 2(A)
to 2(D).
Figures 2(A) to 2(C) show that the Al concentration distributions of the
samples No. 1 to
No. 3 have peaks at an Al overall average of 8.75% by mass and its vicinity.
It is shown
that the samples No. 1 to No. 3 have no portion having a very low Al
concentration. The
samples No. 4 and No. 5 also had an Al concentration distribution similar to
that of the
sample No. 2. As shown by the table, the samples No. 1 to No. 5 having such
small
variations in Al concentration have small corrosion loss and a small amount of
eluted Mg,
thus having high corrosion resistance.
[0121]
In particular, the coiled solution-heat-treated material of the sample No. 2,
the sheet
material of the sample No. 3, and the solution-heat-treated materials of the
samples No. 4
and No. 5 have a very large central composition region having an Al
concentration in the
range of 0.8x% to 1.2x% by mass, which occupies 70% by area or more, and a
very small
ultra-high concentration region having an Al concentration of 1.4x% by mass or
more,
which occupies 5% by area or less. The central composition region occupies 90%
by area
or more in some samples, and the ultra-high concentration region occupies 3%
by area or
less in some samples. Thus, Al is more uniformly distributed in the samples
No. 2 to No.
5. In
particular, the sample No. 2 has a very small ultra-high concentration region
and has
a small difference between the maximum value and the minimum value of the Al
concentration, indicating that Al is still more uniformly distributed. The
samples No. 2 to
No. 5 having such a substantially uniform composition have high corrosion
resistance.
[0122]
In the coiled solution-heat-treated material of the sample No. 2 and the
solution-
CA 02823292 2015-05-12
47
heat-treated materials of the samples No. 4 and No. 5 that have been subjected
to final
solution heat treatment, the intermetallic compound is as little as 3% by area
or less, and
the maximum value of the Al concentration is relatively low. This also
indicates that the
samples No. 2, No. 4, and No. 5 have high corrosion resistance. In particular,
since the
sample No. 2 is a long material, the sample No. 2 can contribute to the mass
production of
a plastic worked component having high corrosion resistance and is expected to
be of
industrial importance.
[0123]
In contrast, the die-cast component of the sample No. 100 contains a little
central
composition region having an Al concentration in the range of 0.8x% to 1.2x%
by mass
and a low-concentration region having an Al concentration of 4.2% by mass or
less. In
particular, the minimum value of Al corresponds to that of an AZ31 alloy. The
sample
No. 100 has a high area percentage of the intermetallic compound. Thus, it is
supposed
that the sample No. 100 has a portion having relatively low corrosion
resistance and
therefore has low corrosion resistance.
[0124]
[Test Example 2]
The sheets of the samples No. 1 to No. 5 manufactured in the test example 1
was
subjected to press forming, and their Al concentrations were measured in the
same manner.
The coiled material of the sample No. 1 and the coiled solution-heat-treated
material of the
sample No. 2 were unwound and were cut into rectangular sheets having a
predetermined
length. The sheets were preheated to 250 C and were subjected to press forming
during
heating. The total time of the holding time for the preheating and press
forming is 2
minutes (0.1 hours or less).
[0125]
The press formed materials (plastic worked components) had an Al concentration
distribution similar to those of the samples No. 1 to No. 5. Thus, it is
supposed that the
press formed materials also have high corrosion resistance.
CA 02823292 2015-05-12
48
Industrial Applicability
[0127]
A magnesium alloy material according to the present invention can be suitably
utilized in components of various electronic and electrical devices,
particularly housings
for mobile or small electronic and electrical devices, components for various
fields
requiring high strength, for example, parts of automobiles and components for
transportation equipment, such as aircrafts, skeleton components, and bags. A
magnesium alloy material according to the present invention can be suitably
utilized as a
material for these components.
