U.S. patent application number 15/506622 was filed with the patent office on 2017-10-05 for magnesium alloy, magnesium alloy sheet, magnesium alloy structural member, and method for producing magnesium alloy.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Eisuke HIRO, Nozomu KAWABE, Michimasa MIYANAGA, Yukihiro OISHI.
Application Number | 20170283915 15/506622 |
Document ID | / |
Family ID | 55746498 |
Filed Date | 2017-10-05 |
United States Patent
Application |
20170283915 |
Kind Code |
A1 |
OISHI; Yukihiro ; et
al. |
October 5, 2017 |
MAGNESIUM ALLOY, MAGNESIUM ALLOY SHEET, MAGNESIUM ALLOY STRUCTURAL
MEMBER, AND METHOD FOR PRODUCING MAGNESIUM ALLOY
Abstract
A magnesium alloy contains, in mass %, from 1% to 12% inclusive
of Al and from 0.1% to 5% inclusive of Mn and has a structure in
which particles of compounds containing Al and Mn are dispersed.
The average diameter of the particles of the compounds is from 0.3
.mu.m to 1 .mu.m inclusive, and the area ratio of the particles of
the compounds is from 3.5% to 25% inclusive.
Inventors: |
OISHI; Yukihiro; (Itami-shi,
Hyogo, JP) ; KAWABE; Nozomu; (Itami-shi, Hyogo,
JP) ; MIYANAGA; Michimasa; (Itami-shi, Hyogo, JP)
; HIRO; Eisuke; (Itami-shi, Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD. |
Osak-shi, Osaka |
|
JP |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka-shi, Osaka
JP
|
Family ID: |
55746498 |
Appl. No.: |
15/506622 |
Filed: |
September 24, 2015 |
PCT Filed: |
September 24, 2015 |
PCT NO: |
PCT/JP2015/076885 |
371 Date: |
February 24, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 23/00 20130101;
B22D 21/04 20130101; B22D 11/06 20130101; B22D 11/001 20130101;
C22C 23/02 20130101; B21B 3/00 20130101; C22C 1/02 20130101; C22F
1/00 20130101; C22F 1/06 20130101 |
International
Class: |
C22C 23/02 20060101
C22C023/02; C22C 1/02 20060101 C22C001/02; B22D 11/00 20060101
B22D011/00; C22F 1/06 20060101 C22F001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2014 |
JP |
2014-211259 |
Claims
1. A magnesium alloy comprising, from 1 mass % to 12 mass %
inclusive of Al and from 0.1 mass % to 5 mass % inclusive of Mn,
wherein the magnesium alloy has a structure in which particles of a
compound containing Al and Mn are dispersed, an average diameter of
the particles of the compound is from 0.3 .mu.M to 1 .mu.m
inclusive, and an area ratio of the particles of the compound is
from 3.5% to 25% inclusive.
2. The magnesium alloy according to claim 1, wherein a maximum
diameter of the particles of the compound is less than 2.5
.mu.m.
3. The magnesium alloy according to claim 1, wherein an average
crystal grain size of the magnesium alloy is 10 .mu.m or less.
4. A magnesium alloy sheet formed from the magnesium alloy
according to claim 1.
5. A magnesium alloy structural member formed from the magnesium
alloy according to claim 1, the magnesium alloy structural member
having, in at least a part thereof, a plastically formed portion
subjected to plastic forming.
6. A method for producing a magnesium alloy, the method comprising
the step of subjecting a melt of a magnesium alloy to continuous
casting, the magnesium alloy containing, from 1 mass % to 12 mass %
inclusive of Al and from 0.1 mass % to 5 mass % inclusive of Mn,
wherein the temperature of the melt immediately before the melt
comes into contact with a mold is from 630.degree. C. to
690.degree. C. inclusive, and a cooling rate of the melt is
560.degree. C./second or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy suitable
for a constituent material of housings, various parts, etc., to a
magnesium alloy sheet suitable for raw materials (primary-processed
materials) for secondary-processed materials such as housings and
various parts, to a magnesium alloy structural member suitable for
secondary-processed materials such as housings and various parts,
and to a method for producing the magnesium alloy. Particularly,
the present invention relates to a magnesium alloy, a magnesium
alloy sheet, and a magnesium alloy structural member that are
excellent in impact resistance, mechanical properties, and plastic
formability and also excellent in productivity.
BACKGROUND ART
[0002] Magnesium alloys, which are lightweight and excellent in
specific strength and specific rigidity, have been increasingly
used as constituent materials of various parts such as parts of
automobiles and housings of mobile electronic-electric devices such
as cellular phones and laptop computers.
[0003] Magnesium alloys are lighter than many other metals, have
high specific strength, and also have excellent impact absorption
ability. Since various elements are added to active Mg (magnesium),
these magnesium alloys also have excellent corrosion resistance and
are preferable for the constituent materials of the above-described
various parts. Among these magnesium alloys, Mg--Al-based alloys
containing Al (aluminum), in particular, are excellent in strength
and corrosion resistance and are preferable for the constituent
materials described above.
[0004] PTL 1 discloses a magnesium alloy sheet that is composed of
a magnesium alloy containing Al and Mn (manganese) and is excellent
in impact resistance and mechanical properties not only at ordinary
temperature but also at low temperature.
[0005] This magnesium alloy sheet contains compounds containing Al
and Mn (mainly precipitates in crystal. These may be hereinafter
referred to as Al--Mn crystallized phases). The Al--Mn crystallized
phases are very fine and are very small in amount and preferably
are not substantially present. Therefore, in this magnesium alloy
sheet, cracking etc. caused by coarse Al--Mn crystallized phases
are unlikely to occur. Therefore, the magnesium alloy sheet is
excellent in impact resistance and mechanical properties and also
excellent in plastic formability such as press formability.
CITATION LIST
Patent Literature
[0006] PTL 1: Japanese Unexamined Patent Application Publication
No. 2011-006754
SUMMARY OF INVENTION
Technical Problem
[0007] It is desired to develop a magnesium alloy excellent in
impact resistance, mechanical properties such as strength, proof
stress, and elongation, and plastic formability such as rolling
formability and press formability and also excellent in
productivity.
[0008] The magnesium alloy sheet disclosed in PTL 1 is excellent in
impact resistance etc. as described above. However, to reduce the
amount of the Al--Mn crystallized phases, the temperature of a melt
of the alloy is set to be relatively high, i.e., 700.degree. C.
Theoretically, the Al--Mn crystallized phases are formed and grow
most easily when the temperature of the melt of the magnesium alloy
containing Al and Mn is around 630.degree. C., particularly lower
than 630.degree. C. Therefore, when the temperature of the melt of
the alloy is sufficiently higher than 630.degree. C., preferably
higher than 690.degree. C., the formation and growth of the Al--Mn
crystallized phases can be effectively prevented. However, when the
temperature of the melt of the alloy is increased, the following
occurs. (a) The melt of the alloy is easily oxidized, and this
causes a reduction in yield due to formation and mixing of the
oxide. (3) When, for example, a high-vacuum atmosphere is used in
order to prevent oxidation, the melt of the alloy becomes difficult
to handle because of high vapor pressure of Mg, so that workability
deteriorates. (.gamma.) A large amount of energy is needed to
maintain the melt of the alloy at high temperature. (.delta.) Since
the melt of the alloy is at high temperature, thermal degradation
of the facility is accelerated. From the above points of view, it
is difficult to improve the productivity. The above (.alpha.) to
(.delta.) can also cause an increase in cost.
[0009] One object of the present invention is to provide a
magnesium alloy excellent in impact resistance, mechanical
properties, and plastic formability and also excellent in
productivity.
[0010] Another object of the present invention is to provide a
magnesium alloy sheet excellent in impact resistance, mechanical
properties, and plastic formability and also excellent in
productivity.
[0011] Still another object of the present invention is to provide
a magnesium alloy structural member excellent in impact resistance
and mechanical properties and also excellent in productivity.
[0012] Yet another object of the present invention is to provide a
magnesium alloy production method that can produce a magnesium
alloy excellent in impact resistance, mechanical properties, and
plastic formability with high productivity.
Solution to Problem
[0013] A magnesium alloy according to one aspect of the present
invention contains, in mass %, from 1% to 12% inclusive of Al and
from 0.1% to 5% inclusive of Mn and has a structure in which
particles of a compound containing Al and Mn are dispersed. The
average diameter of the particles of the compound is from 0.3 .mu.m
to 1 .mu.m inclusive, and the area ratio of the particles of the
compound is from 3.5% to 25% inclusive.
[0014] A magnesium alloy production method according to one aspect
of the present invention includes the step of subjecting a melt of
a magnesium alloy containing, in mass %, from 1% to 12% inclusive
of Al and from 0.1% to 5% inclusive of Mn to continuous casting. In
this production method, the temperature of the melt immediately
before it comes into contact with a mold is from 630.degree. C. to
690.degree. C. inclusive, and the cooling rate of the melt is
560.degree. C./second or higher.
Advantageous Effects of Invention
[0015] The above magnesium alloy is excellent in impact resistance,
mechanical properties, and plastic formability and also excellent
in productivity. The above magnesium alloy production method can
produce a magnesium alloy excellent in impact resistance,
mechanical properties, and plastic formability with high
productivity.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 shows, in its upper part, a microphotograph
(secondary electron image) of a cross section of a magnesium alloy
sheet in an embodiment (sample No. 1-1) observed under a scanning
electron microscope (SEM) and also shows, in the lower part, a
binary image obtained by changing the contrast of the secondary
electron image.
[0017] FIG. 2 is a microphotograph (backscattered electron image)
of the cross section of the magnesium alloy sheet in the embodiment
(sample No. 1-1) observed under the SEM.
[0018] FIG. 3 shows a compositional mapping of the cross section of
the magnesium alloy sheet in the embodiment (sample No. 1-1), the
compositional mapping showing the concentration distribution of Mn
and being obtained by a field-emission electron probe microanalyzer
(FE-EPMA) at an electron gun acceleration voltage of 15 kV.
[0019] FIG. 4 is a histogram showing the concentration of Mn (the
number of Mn counts) versus the frequency of the concentration and
the cumulative frequency, the histogram being produced using the
compositional mapping shown in FIG. 3 and obtained by the FE-EPMA
(15 kV).
[0020] FIG. 5 shows, in its left part, a compositional mapping of
the cross section of the magnesium alloy sheet in the embodiment
(sample No. 1-1), the compositional mapping showing the
concentration distribution of Mn and being obtained by the
field-emission electron probe microanalyzer (FE-EPMA) at an
electron gun acceleration voltage of 5 kV. FIG. 5 also shows, in
the right part, a microphotograph (backscattered electron image) of
the same region as the region of the compositional mapping, the
microphotograph being observed under the SEM.
[0021] FIG. 6 is a histogram showing the concentration of Mn (the
number of Mn counts) versus the frequency of the concentration and
the cumulative frequency, the histogram being produced using the
compositional mapping shown in FIG. 5 and obtained by the FE-EPMA
(5 kV).
[0022] FIG. 7 is an illustration illustrating a test method of a
shock resistance test.
DESCRIPTION OF EMBODIMENTS
[0023] The present inventors have produced magnesium alloys having
compositions containing Al and Mn and capable of providing, in
particular, excellent strength and corrosion resistance under
various production conditions to examine the structure of a
magnesium alloy excellent in impact resistance, mechanical
properties, and plastic formability. Then the inventors have found
that a magnesium alloy having a structure in which compounds
containing Al and Mn (Al--Mn crystallized phases) and having a size
within a specific range are contained in an amount within a
specific range can have impact resistance, mechanical properties,
and plastic formability substantially comparable to those of a
magnesium alloy containing only a very small amount of the
above-described compounds or substantially no such compounds.
Specifically, when the magnesium alloy has a structure containing a
certain amount of the above-described compounds and these compounds
are relatively fine and distributed uniformly, the magnesium alloy
can have impact resistance, mechanical properties, and plastic
formability substantially comparable to those of a magnesium alloy
containing only a very small amount of the above-described
compounds or substantially no such compounds. The inventors have
also found that the magnesium alloy having the above-described
specific structure can be produced by a specific casting process
including performing continuous casting such that the temperature
of the melt of the alloy before it comes into contact with the mold
is set to be as low as possible and that the rate of cooling is
very high. In this production method, the melt of the alloy is
maintained at a relatively low temperature. This can mitigate the
above-described problems (.alpha.) to (.delta.) and other problems
that can occur when the melt of the alloy is at high temperature,
so that the productivity of the magnesium alloy excellent in impact
resistance, mechanical properties, and plastic formability can be
improved. The present invention is based on the above findings.
First, the details of the embodiments of the present invention will
be enumerated and described.
[0024] (1) A magnesium alloy according to one aspect of the present
invention comprises, in mass %, from 1% to 12% inclusive of Al and
from 0.1% to 5% inclusive of Mn and has a structure in which
particles of a compound containing Al and Mn are dispersed. In this
magnesium alloy, the average diameter of the particles of the
compound is from 0.3 .mu.m to 1 .mu.m inclusive, and the area ratio
of the particles of the compound is from 3.5% to 25% inclusive.
[0025] The average diameter of the particles of the compound is
measured using an image observed under an optical microscope.
[0026] The area ratio of the particles of the compound is measured
by a compositional mapping of a cross section of the magnesium
alloy by an FE-EPMA at an electron gun acceleration voltage of 5 kV
or 15 kV. The details of the measurement method will be described
later.
[0027] The above magnesium alloy contains Al and Mn in amounts
within the specific ranges and is therefore excellent in strength
and also excellent in corrosion resistance. Particularly, in the
magnesium alloy, although the particles of the compound containing
Al and Mn are present in a certain amount within the specific
range, the particles are fine. Therefore, even when the magnesium
alloy receives an impact of, for example, dropping or is subjected
to plastic forming such as rolling or press forming, the particles
are unlikely to serve as starting points of cracking etc., so that
the magnesium alloy is excellent in impact resistance, plastic
formability, and mechanical properties such as strength proof
stress, and elongation. The magnesium alloy has a dispersion
strengthened structure in which the above-described fine particles
of the compound are dispersed. This dispersion strengthened
structure increases the proof stress, and therefore the magnesium
alloy is resistant to denting and is excellent in impact
resistance. The magnesium alloy having the above-described specific
composition and structure can be produced by, for example, a
specific casting step described later and is therefore also
excellent in productivity.
[0028] (2) In one exemplary form of the magnesium alloy, the
maximum diameter of the particles of the compound is less than 2.5
.mu.m.
[0029] In the above-described form, although a certain amount of
the particles of the compound containing Al and Mn is present,
these particles are sufficiently small. Therefore, in the
above-described form, cracking originating from coarse compound
particles is unlikely to occur, and the magnesium alloy is more
excellent in impact resistance, mechanical properties such as
strength, proof stress, and elongation, and plastic
formability.
[0030] (3) In one exemplary form of the magnesium alloy, the
average crystal grain size of the magnesium alloy is 10 .mu.m or
less.
[0031] In the above form, the crystals themselves are fine, and
therefore cracking originating from coarse crystal grains is
unlikely to occur, so that the magnesium alloy is more excellent in
impact resistance, mechanical properties such as strength, proof
stress, and elongation, and plastic formability.
[0032] (4) A magnesium alloy sheet according to one aspect of the
present invention is formed from the magnesium alloy according to
any one of (1) to (3) above.
[0033] This magnesium alloy sheet, which is an example of the
magnesium alloy, is composed of the magnesium alloy having the
above-described specific structure and is therefore excellent in
impact resistance, mechanical properties such as strength, proof
stress, and elongation, and plastic formability such as press
formability and also excellent in productivity. The above magnesium
alloy sheet can be preferably used for raw materials for
secondary-processed materials (e.g., a magnesium alloy structural
member described later) subjected to plastic forming such as press
forming.
[0034] (5) A magnesium alloy structural member according to one
aspect of the present invention is formed from the magnesium alloy
according to any one of (1) to (3) above. The magnesium alloy
structural member has, in at least a part thereof, a plastically
formed portion subjected to plastic forming.
[0035] This magnesium alloy structural member, which is an example
of the magnesium alloy, is composed of the magnesium alloy having
the above-described specific structure and is therefore excellent
in impact resistance and mechanical properties such as strength,
proof stress, and elongation and also excellent in productivity. In
the magnesium alloy structural member, cracking is unlikely to
occur during plastic forming such as press forming. Therefore, the
magnesium alloy structural member is excellent in productivity.
[0036] (6) A magnesium alloy production method according to one
aspect of the present invention comprises the step of subjecting a
melt of a magnesium alloy to continuous casting, the magnesium
alloy containing, in mass %, from 1% to 12% inclusive of Al and
from 0.1% to 5% inclusive of Mn. In this production method, the
temperature of the melt immediately before the melt comes into
contact with a mold is from 630.degree. C. to 690.degree. C.
inclusive, and the cooling rate of the melt is 560.degree.
C./second or higher.
[0037] In the above magnesium alloy production method, the melt of
the magnesium alloy containing Al and Mn in amounts within the
specific ranges is used, and this allows the magnesium alloy
produced to have excellent strength and corrosion resistance.
Particularly, in the above magnesium alloy production method, the
temperature of the melt is set to be lower than the temperature
conventionally used. At the set temperature, compounds containing
Al and Mn (Al--Mn crystallized phases) are easily formed. However,
the cooling rate is set to be very high, so that the time during
which the material is held at around 630.degree. C. in the course
of solidification can be shortened. Therefore, with the above
magnesium alloy production method, only an appropriate amount of
the Al--Mn crystallized phases can be formed in the alloy. In
addition, the growth of the particles of the Al--Mn crystallized
phases is suppressed, so that relatively fine particles of the
Al--Mn crystallized phases, typically particles of the Al--Mn
crystallized phases having an average diameter of 1 .mu.m or less,
are present. With the above magnesium alloy production method, such
fine particles of the Al--Mn crystallized phases can be uniformly
dispersed.
[0038] If, although the temperature of the melt is high, the
cooling rate is low, the particles of the Al--Mn crystallized
phases grow. In this case, for example, a structure in which coarse
particles having a maximum diameter of 2.5 .mu.m or more are
unevenly distributed may be obtained. These coarse particles can
serve as starting points of cracking etc. Since Al and Mn are
contained in the coarse particles, the amounts of Al and Mn
necessary for fine particles may not remain, so that the amount of
the fine particles present may not be sufficient. In this case, the
dispersion strengthening effect of the fine Al--Mn crystallized
phases may not be obtained sufficiently. Therefore, in a magnesium
alloy in which coarse Al--Mn crystallized phases are present
locally, impact resistance, mechanical properties, and plastic
formability may deteriorate. However, with the above magnesium
alloy production method, the magnesium alloy produced can be
excellent in impact resistance and mechanical properties such as
strength and proof stress because of dispersion strengthening by
the particles of the Al--Mn crystallized phases that are generally
harder than the magnesium alloy serving as the matrix phase. In
addition, with the above magnesium alloy production method, the
particles of the Al--Mn crystallized phases are fine, and these
fine particles are unlikely to serve as starting points of cracking
etc. Therefore, the magnesium alloy produced can also be excellent
in toughness such as elongation, impact resistance, and plastic
formability.
[0039] In the above magnesium alloy production method, since the
temperature of the melt is relatively low, the following can be
achieved. (.alpha.') The oxidation of the melt can be easily
suppressed, so that a reduction in yield due to oxide can be
reduced. (.beta.') Workability is excellent. (.gamma.') The energy
required to maintain the temperature of the melt can be reduced.
(.delta.') Thermal degradation of the production facility can be
reduced. In addition, in the above magnesium alloy production
method, since continuous casting is performed, the magnesium alloy
can be mass-produced. Since the cooling rate is high, a fine
crystalline structure can be easily obtained. Therefore, a
magnesium alloy excellent in impact resistance, mechanical
properties, and plastic formability can be easily produced. In view
of the above, with the above magnesium alloy production method, a
magnesium alloy excellent in impact resistance, mechanical
properties, and plastic formability can be produced with high
productivity.
DETAILS OF EMBODIMENT OF THE PRESENT INVENTION
[0040] A magnesium alloy, a magnesium alloy sheet, a magnesium
alloy structural member, and a magnesium alloy production method
according to an embodiment of the present invention will be
described one by one. The unit of the content of each element is
percent by mass.
(Magnesium Alloy, Magnesium Alloy Sheet, and Magnesium Alloy
Structural Member)
[0041] Composition
[0042] One feature of the magnesium alloy in the embodiment is that
the magnesium alloy has a composition containing at least both Al
and Mn as additive elements. The magnesium alloy may contain, in
addition to the composition containing Al and Mn, a second additive
element described later, so long as a specific amount of compounds
containing Al and Mn (Al--Mn crystallized phases) having a specific
size can be formed in the production process. In any of these
compositions, the balance is Mg and inevitable impurities, and the
content of Mg is more than 50%.
[0043] The content of Al is from 1% to 12% inclusive. When Al is
contained within the above range, excellent mechanical properties
such as strength and excellent corrosion resistance, in particular,
are obtained. The larger the content of Al within the above range,
the higher the strength and the corrosion resistance. Therefore,
the content of Al may be 3% or more, 5% or more, 5.5% or more, and
7% or more. A magnesium alloy containing Al in an amount of from
8.3% to 9.5% inclusive, e.g., an ASTM standard AZ91 alloy, has
mechanical properties and corrosion resistance superior to those of
a magnesium alloy containing Al in an amount of about 3%, e.g., an
ASTM standard AZ31 alloy. The lower the content of Al within the
above range, the more easily plastic forming such as bending can be
performed. Therefore, the content of Al may be 7% or less and
particularly 4% or less. The content of Al that provides a good
balance between strength and workability may be from 5.5% to 12%
inclusive. Part of Al in the alloy is present as compounds such as
intermetallic compounds typified by compounds containing Al and Mn
and compounds containing Al and Mg, and the other part is dissolved
in Mg to form a solid solution.
[0044] The content of Mn is from 0.1% to 5% inclusive. When Mn is
contained within the above range, excellent corrosion resistance is
obtained. The larger the content of Mn within the above range, the
higher the corrosion resistance. Therefore, the content of Mn may
be 0.15% or more. The larger the content of Mn, the more easily the
compounds containing Al and Mn are formed and grow. In this case,
the amount of solute Al tends to decrease, and coarse compound
particles tend to be present. Therefore, the content of Mn may be
2% or less, 1.5% or less, and particularly 1% or less. A Mn content
of from 0.2% to 0.5% inclusive is expected to effectively suppress
excessive formation and growth of the above compounds.
[0045] The second additive element may be at least one element
selected from Zn (zinc), Ca (calcium), Si (silicon), Be
(beryllium), Sr (strontium), Y(yttrium), Ag (silver), Sn (tin), Zr
(zirconium), Ce (cerium), Au (gold), and rare-earth elements
(excluding Y and Ce). The specific content of Zn may be from 0.2%
to 7.0% inclusive, and the specific content of Ca may be from 0.2%
to 6.0% inclusive. The specific content of Si may be from 0.2% to
1.0% inclusive, and the specific content of Be may be from 0.0001%
to 0.002% inclusive. The specific content of Sr may be from 0.2% to
7.0% inclusive, and the specific content of Y may be from 1.0% to
6.0% inclusive. The specific content of Ag may be from 0.5% to 3.0%
inclusive, and the specific content of Sn may be from 0.01% to 2.0%
inclusive. The specific content of Zr may be from 0.1% to 1.0%
inclusive, and the specific content of Ce may be from 0.05% to 1.0%
inclusive. The specific contents of rare-earth elements (excluding
Y and Ce) may be from 1.0% to 3.5% inclusive.
[0046] When the second additive element is contained, only one of
the listed elements may be contained, or a combination of two or
more elements may be contained. The second additive element
contained can provide the following effects: various excellent
properties including mechanical properties such as strength and
elongation (e.g., Zn, Zr, etc.), high-temperature strength and
creep resistance (e.g., Si, rare-earth elements, Ag, etc.), and
flame resistance (e.g., Ca etc.); a reduction in crystal size; and
suppression of hot cracking (e.g., Zr etc.). Specific production
conditions described later may be used to produce a magnesium alloy
having a composition including Al and Mn in amounts within the
above specific ranges and further including the second additive
element. Also in this magnesium alloy, the compounds containing Al
and Mn and having a specific size are contained in a specific
amount, and the particles of these compounds are uniformly
distributed.
[0047] More specific examples of the composition of the magnesium
alloy containing Al and Mn include the following. [0048] ASTM
standard AM-based alloys (AM60 alloy, AM100 alloy, etc.) [0049]
ASTM standard AZ-based alloys (AZ61 alloy, AZ80 alloy, AZ81 alloy,
AZ91 alloy, etc.)
[0050] The AZ-based alloys contain, in addition to Al and Mn, from
0.2% to 1.5% inclusive of Zn as the second additive element. As the
Al content in the AZ-based alloys increases, the mechanical
properties such as strength and proof stress and corrosion
resistance tend to be improved. As the Al content decreases, the
plastic formability tends to be improved.
[0051] Structure
[0052] One feature of the magnesium alloy in the embodiment is that
the magnesium alloy has a structure in which relatively fine
particles formed of compounds containing Al and Mn are uniformly
dispersed. The compounds containing Al and Mn are crystallized
phases that are formed mainly during casting. These crystallized
phases have high hardness. When the crystallized phases are once
formed, it is difficult to change their size and content during a
production process after the casting. Therefore, in the magnesium
alloy in the embodiment, specific casting conditions described
later, for example, are used to control the size and content of the
above-described compounds (crystallized phases).
[0053] Compositions of Compounds
[0054] Examples of the compounds containing Al and Mn include
intermetallic compounds containing only Al and Mn and intermetallic
compounds containing iron (Fe) etc. in addition to Al and Mn. Fe
contained in the latter intermetallic compounds is an inevitable
impurity. The compositions of these compounds can be examined by
component analysis using energy dispersive X-ray analysis (EDX) or
Auger electron spectroscopy (AES).
[0055] Size of Compounds
[0056] The compounds containing Al and Mn are present as particles
in the matrix of the magnesium alloy in the embodiment. The average
diameter of the particles of the compounds is from 0.3 .mu.m to 1
.mu.m inclusive. When the average particle diameter is within the
above range, the particles of the compounds can well function as a
dispersion strengthening material for the structure and are less
likely to serve as starting points of cracking, so that the
magnesium alloy is excellent in impact resistance, mechanical
properties, and plastic formability. The average particle diameter
can be from 0.3 .mu.m to 0.9 .mu.m inclusive and particularly from
0.35 .mu.m to 0.8 .mu.m inclusive.
[0057] Preferably, the maximum diameter of the compounds containing
Al and Mn is less than 2.5 When coarse particles of 2.5 .mu.m or
more are not present, cracking originating from such coarse
particles is unlikely to occur, and deterioration of impact
resistance, mechanical properties, and plastic formability caused
by such coarse particles can be suppressed. In addition, a
reduction in the amount of fine particles due to the presence of
these coarse particles can be suppressed, so that an appropriate
amount of fine particles can be contained. Therefore, the magnesium
alloy can be excellent in impact resistance, mechanical properties,
and plastic formability. The smaller the compounds, the smaller the
number of coarse particles serving as the starting points of
cracking, and the more easily a structure containing an appropriate
amount of fine particles is obtained. Therefore, the maximum
diameter is preferably 2 .mu.m or less, more preferably 1.5 .mu.m
or less, still more preferably 1.2 .mu.m or less, and yet more
preferably 1 .mu.m or less. When the average particle diameter of
the above compounds is within the above range and the maximum
diameter of the above compounds is less than 2.5 .mu.m and
preferably 2 .mu.m or less, the variations in size of the above
compounds are small, and the size is uniform. Therefore, in this
form, variations in characteristics due to the variations in size
of the above compounds can also be suppressed, and good
characteristics can be achieved.
[0058] Content of Compounds
[0059] The content of the compounds containing Al and Mn is
determined by the area ratio of the compounds in a cross section of
the magnesium alloy, and the area ratio is from 3.5% to 25%
inclusive. When the area ratio is 3.5% or more, a sufficient amount
of the above compounds is present in the magnesium alloy, and the
dispersion strengthening effect of the particles of the compounds
can be preferably obtained. When the area ratio is 25% or less, an
appropriate amount of the above compounds is present, and the
embrittlement of the alloy due to the presence of an excessive
amount of the above compounds, a reduction in corrosion resistance
due to a reduction in the amount of solute Al, etc. are suppressed,
so that the magnesium alloy is excellent in impact resistance,
mechanical properties, and plastic formability.
[0060] The area ratio is measured as follows. A cross section of
the magnesium alloy is taken, and an observation field described
below (e.g., a square region of 195 .mu.m.times.195 .mu.m) is
selected in the cross section. A compositional mapping by FE-EPMA
is performed on the observation field to determine the distribution
of the concentration of Mn. Assume that substantially all the Mn in
the observation field is present as the compounds containing Al and
Mn. Then the area ratio of Mn in the observation field is regarded
as the area ratio of the compounds containing Al and Mn.
Specifically, the distribution of the Mn concentration determined
by the compositional mapping is used to determine the area ratio of
the above compounds. A specific computation method will be
described later.
[0061] A region of the magnesium alloy that extends inwardly from
its surface to a depth of 30% of the thickness of the magnesium
alloy is referred to as a surface layer region, and the
above-described observation field is selected in this surface layer
region. The reason that the observation field is selected in the
surface layer region is that a region that undergoes cracking and
directly receives an impact of, for example, dropping may generally
be the surface layer region.
[0062] The distribution of the Mn concentration varies depending on
the acceleration voltage of an electron gun used in the FE-EPMA. As
the acceleration voltage increases, the amount of information
obtained tends to increase, and the concentration (level) of Mn
tends to increase. Specifically, the value of the area ratio may
vary depending on the magnitude of the acceleration voltage. To
measure the area ratio, the acceleration voltage of the electron
gun is 15 kV or less.
[0063] For example, the area ratio when the compositional mapping
by the FE-EPMA is performed on the observation field in the cross
section using an electron gun acceleration voltage of 15 kV is 9.5%
or more, from 10% to 25% inclusive, and particularly from 15% to
24% inclusive.
[0064] For example, the area ratio when the compositional mapping
by the FE-EPMA is performed on the observation field in the cross
section using an electron gun acceleration voltage of 5 kV is from
3.5% to 15% inclusive, from 4.0% to 12% inclusive, and particularly
from 5.0% to 10% inclusive.
[0065] Crystal Grain Size
[0066] In one exemplary form of the magnesium alloy in the
embodiment, the magnesium alloy has a fine crystalline structure.
One example of such a structure is a structure in which the average
crystal grain size satisfies 10 .mu.m or less. When the average
crystal grain size is 10 .mu.m or less, substantially no coarse
crystal grains are present, and the occurrence of cracking due to
coarse crystal grains can be reduced. Therefore, in this form, the
magnesium alloy is more excellent in impact resistance, mechanical
properties such as strength and elongation, and plastic
formability. The smaller the crystal grains, the more effectively
the occurrence of cracking due to coarse crystal grains can be
reduced. The average crystal grain size may be, for example, 6
.mu.m or less and particularly 4 .mu.m or less. The lower limit of
the average crystal grain size may be, for example, 2 .mu.m and
particularly 1 .mu.m. To reduce the crystal grain size, it is
effective to perform plastic forming such as rolling after casting.
Representative examples of the magnesium alloy having a fine
crystalline structure include rolled sheets and press-formed sheets
obtained by subjecting the rolled sheets to press forming. It is
expected that the crystal grain size can be further reduced easily
by increasing the cooling rate in the casting step (560.degree.
C./second or higher and particularly 600.degree. C./second or
higher) or adding the second additive element described above.
[0067] Forms Classified by Production Process
[0068] Specific forms of the magnesium alloy in the embodiment are
classified by their production process as follows. (1) Cast
material. (2) Primary-processed material (such as a rolled
material) prepared by subjecting the cast material to plastic
forming (primary processing) such as rolling. (3) Treated material
prepared by subjecting the primary-processed material to various
types of treatment such as polishing, leveling, heat treatment
performed for the purpose of, for example, removal of strain,
anticorrosive treatment (chemical conversion treatment, anodic
oxidation treatment), treatment for decoration purposes (cutting
such as diamond cut finishing and hairline finishing, etching, shot
blasting, etc.), and coating treatment. (4) Secondary-processed
material (a magnesium alloy structural member in the embodiment)
prepared by subjecting the primary-processed material or the above
treated material to plastic forming (secondary processing) such as
press forming. (5) Surface-treated material (a magnesium alloy
structural member in the embodiment) prepared by subjecting the
secondary-processed material to surface treatment such as
anticorrosive treatment, coating, or processing for decoration. In
the primary-processed material such as the rolled material and the
above treated material, their average crystal grain size is smaller
than that of the cast material as described above, and cracking
etc. are less likely to occur. Therefore, they can be used as raw
materials for secondary-processed materials such as press-formed
materials. Typically, the entire portion of the primary-processed
material is subjected to plastic forming and is therefore a
plastically formed portion. The secondary-processed material may be
in such a form that only part of the raw material is subjected to
plastic forming to form a plastically formed portion (e.g., a
press-formed material having a bent portion) or may be in such a
form that the entire portion of the raw material is subjected to
plastic forming (e.g., a processed material bent into a cylindrical
shape).
[0069] Shape
[0070] Specific examples of the shape of the magnesium alloy in the
embodiment include a sheet having first and second surfaces
parallel to each other (a magnesium alloy sheet in the embodiment).
The first and second surfaces are typically flat surfaces but may
be subjected to, for example, bending to form bent surfaces. The
planar shape of the sheet is typically rectangular, but the sheet
may be punched into a circular shape or any other shape. The sheet
may have any of the following forms classified by their production
process described above: (1) a cast material, (2) a
primary-processed material (such as a rolled sheet), (3) a treated
material, (4) a secondary-processed material, and (5) a
surface-treated material. Specific examples of the shape of the
secondary-processed material include a member including a bottom
portion and side wall portions extending from the bottom portion
and having a rectangular U-shaped cross section (a member having a
sheet portion).
[0071] Size
[0072] When the magnesium alloy in the embodiment is in the form of
a sheet (a magnesium alloy sheet in the embodiment) or a member
prepared by subjecting at least part of the sheet to plastic
forming such as press forming (a magnesium alloy structural member
in the embodiment), the thickness of the sheet may be 5 mm or less.
The thickness of the sheet is the average distance between the
first and second surfaces. When the sheet has been subjected to
plastic forming such as rolling, i.e., is a primary-processed
material or a secondary-processed material, the thickness can be
easily made uniform over the entire sheet and can be further
reduced easily. In one exemplary form, the thickness may be about 3
mm or less and particularly 2.5 mm or less. The larger the
thickness of the sheet, the higher the strength and stiffness. When
the thickness of the sheet is small (preferably 2 mm or less, more
preferably 1.5 mm or less, and still more preferably 1.2 mm or
less), a thin and lightweight primary-processed material and
secondary-processed material can be formed. The lower limit of the
thickness of the sheet may be 0.1 mm or more and particularly 0.3
mm or more. The thickness of the final sheet may be selected by
controlling the casting conditions, the rolling conditions, etc.
according to its desired application purpose. In forms other than
the forms in which the thickness is uniform over the entire sheet
and the entire member, portions with different thicknesses may be
present (e.g., a form in which a through hole is provided and a
form in which a groove or a protrusion is provided).
[0073] Characteristics
[0074] The magnesium alloy in the embodiment is excellent in
mechanical properties such as strength, proof stress, and
elongation. In one exemplary form of the magnesium alloy in the
embodiment, at least one of a tensile strength (room temperature)
of 270 MPa or more, a 0.2% proof stress (room temperature) of 200
MPa or more, and a rupture elongation (room temperature) of 5% or
more is satisfied. Preferably, all the three are satisfied.
Examples of such a form include the magnesium alloy subjected to
the above-described plastic forming such as rolling, i.e., the
primary-processed material and the secondary-processed material.
When Al is contained in an amount of 5% or more or plastic forming
such as rolling is performed, at least one of a tensile strength of
from 280 MPa to 450 MPa inclusive, a 0.2% proof stress of from 230
MPa to 350 MPa inclusive, and a rupture elongation of from 5% to
15% inclusive can be satisfied, and preferably all the three can be
satisfied. However, this depends on the composition, the production
process, etc.
[0075] The magnesium alloy in the embodiment is less likely to be
dented when the alloy receives an impact of, for example, dropping.
For example, when a shock resistance test described later is
performed, the amount of dent is small and is less than 0.63 mm.
When the magnesium alloy in the embodiment has been subjected to
the above-described plastic forming such as rolling, i.e., is a
primary-processed material or a secondary-processed material, the
amount of dent is further small and is 0.6 mm or less and
particularly 0.55 mm or less.
(Magnesium Alloy Production Method)
[0076] One feature of the magnesium alloy production method in the
embodiment is that the method includes a specific casting step in
order to form a structure in which compounds having specific
compositions, i.e., compounds containing Al and Mn, are contained
in a specific amount and have a specific size. Specifically, this
casting step includes the following three conditions: (1)
Continuous casting is performed. (2) The temperature of the melt is
set to be relatively low. (3) The cooling rate of the melt is set
to be very high. The casting step will next be described in detail,
and then steps after the casting will be described.
[0077] Casting Step
[0078] Continuous Casting
[0079] In the magnesium alloy production method in the embodiment,
a melt of a magnesium alloy having a specific composition
containing Al and Mn within the specific ranges described above is
prepared, and then continuous casting is performed. In the
continuous casting, rapid solidification can be performed.
Therefore, the amount of oxide and the amount of segregation can be
reduced, and the formation of coarse crystallized phases can be
easily reduced. In addition, the size of the compounds containing
Al and Mn can be easily controlled to the above-described specific
value. Specific examples of the continuous casting process include
a twin-roll process. The twin-roll process is suitable for
production of a cast sheet. In the twin-roll process, the cooling
rate can be increased by reducing the thickness of the cast sheet
(to preferably 5 mm or less), reducing the temperature of the rolls
(to preferably 100.degree. C. or lower), changing the material of
the rolls, etc.
[0080] Temperature of Melt
[0081] The temperature of the melt before it comes into contact
with a mold is from 630.degree. C. to 690.degree. C. inclusive. The
reason that the lower limit is defined as above is that, when the
temperature of the melt is lower than 630.degree. C., the compounds
containing Al and Mn are very easily formed. The reason that the
upper limit is defined as above is that, when the temperature is
higher than 690.degree. C., productivity becomes low because the
temperature of the melt is excessively high. By setting the
temperature of the melt within the above range, the compounds
containing Al and Mn can be preferably formed during the
solidification process and contained in an appropriate amount (the
specific amount described above). To form the above compounds
sufficiently, it is preferable that the temperature of the melt is
as low as possible. The temperature of the melt is preferably
685.degree. C. or lower, more preferably 680.degree. C. or lower,
and still more preferably 675.degree. C. or lower. When the
temperature of the melt is 635.degree. C. or higher, 640.degree. C.
or higher, and particularly 645.degree. C. or higher, excessive
formation and coarsening of the above compounds can be easily
suppressed, and the amount and size of the above compounds can be
easily controlled. Therefore, it is expected that the productivity
can be improved. As the content of Al decreases, the melting
temperature tends to increase. The temperature of the melt is
controlled within the above range according to the composition.
[0082] Before the melt comes into contact with the mold, the melt
is held in facilities such as a melting furnace, a conveying
launder, and a holding furnace. When the temperature of the melt in
these facilities for holding the melt is set to be uniform, i.e.,
to a temperature selected within the range of from 630.degree. C.
to 690.degree. C. inclusive, the temperature can be easily
controlled. Since this temperature range is relatively lower than
the conventional range, thermal damage to the facilities can be
easily reduced, and the service life of the facilities can be
extended. From this point of view, it is expected to improve the
productivity and reduce the cost.
[0083] Cooling Rate
[0084] The above melt having a relatively low temperature is
rapidly cooled at a cooling rate of 560.degree. C./second or
higher. In this rapid cooling, retention time at around 630.degree.
C. that is within a temperature range in which the compounds
containing Al and Mn are easily formed in the solidification
process is sufficiently short. In this case, excessive formation
and coarsening of the above compounds can be effectively
suppressed, and a structure in which the above compounds are
present in a certain amount and are relatively fine can be
preferably formed. The higher the cooling rate, the more
preferable. The cooling rate may be 600.degree. C./second or
higher, 620.degree. C./second or higher, and particularly
650.degree. C./second or higher. The cast material obtained by the
above-described rapid solidification has a dispersion strengthened
structure in which the above compounds having the above-described
specific size are uniformly dispersed in at least the surface layer
region of the cast material. In this structure, the crystals are
also fine.
[0085] The cooling rate is computed using a DAS (dendrite arm
spacing). Let .alpha. and .beta. be constants based on the
composition of the magnesium alloy, d (.mu.m) be the DAS, and V
(.degree. C./second) be the cooling rate. Then the following
relation (1) can be used:
d=.alpha..times.V.sup.-.beta.. relation (1)
[0086] For example, in an ASTM standard AZ-based alloy, .alpha. in
relation (1) above is 35.5, and .beta.=0.31. The DAS is denoted by
d.sub.AZ, and the cooling rate V.sub.AZ is represented as
follows:
d.sub.AZ=35.5.times.V.sub.AZ.sup.-0.31.
[0087] Test pieces with different compositions and sizes
(thicknesses, widths, etc.) are used to determine the relation
between the DAS and the cooling rate in advance, and correlation
data is produced. Good workability is obtained when the cooling
conditions are controlled using this correlation data such that a
desired cooling rate is achieved.
[0088] Examples of the method for achieving a cooling rate of
560.degree. C./second or higher include the following. (1) The
surface temperature of the mold is reduced (e.g., to 100.degree. C.
or lower and particularly 80.degree. C. or lower). For example, a
forced cooled mold such as a water-cooled mold is used. In this
case, the surface temperature of the mold can be maintained low.
(2) The size of the cast material is reduced. For example, when the
cast material is a cast sheet, its thickness is 5 mm or less, 4.5
mm or less, and particularly 4 mm or less. (3) A mold formed of a
material having high cooling ability is used. For example, when a
mold formed of a material with high thermal conductivity is used,
the cooling rate can be increased because of the high heat
radiation performance of the mold.
[0089] Preferably, the casting step (including the cooling step) is
performed in an inert gas atmosphere in order to prevent, for
example, oxidation of the magnesium alloy.
[0090] Steps after Casting
[0091] Rolling Step
[0092] When the magnesium alloy in the embodiment is formed into a
rolled material (typically a rolled sheet), the above-described
cast material (typically a cast sheet) is subjected to at least one
rolling pass. Specifically, one exemplary form of the magnesium
alloy production method in the embodiment includes the casting step
described above and the step of subjecting the cast material
obtained by the continuous casting to at least one rolling pass
(this step may be hereinafter referred to as a rolling step).
Preferably, the at least one rolling pass is warm rolling at a
rolling temperature of from 200.degree. C. to 400.degree. C.
inclusive. The number of passes in the rolling step, a rolling
reduction per pass, the total rolling reduction, etc. may be
appropriately selected such that a rolled sheet with a desired
thickness is obtained. By subjecting the cast material to rolling,
a rolled structure (typically a recrystallized structure) can be
obtained instead of a cast structure. As a result of the rolling,
the following effects are expected. (1) A fine structure having an
average crystal grain size of 20 .mu.m or less and particularly 10
.mu.m or less is easily obtained. (2) The occurrence of internal
defects and surface defects such as segregation, shrinkage
cavities, and pores during casing can be reduced, and a good
surface texture can be obtained. (3) The formation of a fine
recrystallized structure easily allows the strength and corrosion
resistance to be further improved. The rolled sheet obtained
through the above-described rolling step has a dispersion
strengthened structure in which at least its surface layer region
has a finer crystalline structure and the compounds containing Al
and Mn having the above-described specific size are uniformly
dispersed. The production method may further include, after the
rolling step, the step of performing at least one additional
process such as the above-described polishing, leveling,
anticorrosive treatment, coating, processing for decoration
purposes, and heat treatment for the purpose of, for example,
removal of strain.
[0093] Secondary Processing Step
[0094] When the magnesium alloy in the embodiment is processed into
a plastically formed member, at least part of the above rolled
sheet (which may have been subjected to an additional process such
as polishing or leveling) is subjected to plastic forming.
Specifically, one exemplary form of the magnesium alloy production
method in the embodiment includes the above-described casting step,
the above-described rolling step, and the step of subjecting at
least part of the raw material subjected to the rolling step to
plastic forming (secondary processing).
[0095] Specific examples of the plastic forming (secondary
processing) include press forming (deep drawing, punching,
upsetting, etc.), forging, and bending. Preferably, the plastic
forming is performed as warm working at a processing temperature of
from 200.degree. C. to 280.degree. C. inclusive. This is because
since the plastic formability of the raw material (typically the
above rolled sheet) is improved, the plastic forming (secondary
processing) can be performed with high precision. With the warm
working, the amount of a coarse recrystallized structure in the
structure of the raw material can be reduced, so that the
deterioration of the mechanical properties and corrosion resistance
can be reduced. The plastic forming (secondary processing) may be
performed on only part of the raw material or on the entire raw
material. The production method may further include, after the
secondary processing step, the step of performing at least one
additional process such as the above-described polishing,
anticorrosive treatment, coating, processing for decoration
purposes, and heat treatment for the purpose of, for example,
removal of strain.
[0096] The magnesium alloy in the embodiment and its production
method will be described more specifically by way of a Test
Example.
Test Example 1
[0097] Magnesium alloys having different compositions shown in
Table 1 were used to produce magnesium alloy sheets under various
conditions, and these magnesium alloy sheets were subjected to
press forming to produce press-formed materials. For each of the
magnesium alloy sheets obtained, structural observation, a tensile
test (ordinary temperature), an impact resistance test (ordinary
temperature), a pass/fail check of press formability, and a
pass/fail determination of productivity were performed.
[0098] The content of each element is represented by percent by
mass (mass %). [0099] AZ91 shown in Table 1 is a magnesium alloy
containing Al, Mn, and Zn in amounts corresponding to those in an
ASTM standard AZ91 alloy. The alloy contains 9.1% of Al, 0.16% of
Mn, and 0.72% of Zn. [0100] AZX911 shown in Table 1 is a magnesium
alloy containing Al, Mn, and Zn in amounts corresponding to those
in the ASTM standard AZ91 alloy and further containing Ca. The
alloy contains 9.0% of Al, 0.16% of Mn, 0.74% of Zn, and 1.0% of
Ca. [0101] AZ61 shown in Table 1 is a magnesium alloy containing
Al, Mn, and Zn in amounts corresponding to those in an ASTM
standard AZ61 alloy. The alloy contains 6.1% of Al, 0.22% of Mn,
and 0.70% of Zn. [0102] AM60 shown in Table 1 is a magnesium alloy
containing Al and Mn in amounts corresponding to those in an ASTM
standard AM60 alloy. The alloy contains 6.2% of Al and 0.20% of
Mn.
[0103] In this Example, cast sheets (magnesium alloy sheets),
rolled sheets (magnesium alloy sheets), and press-formed materials
(magnesium alloy structural members) were produced using a
production process including twin-roll continuous casting, rolling,
and press forming in this order. Specifically, an ingot of a
magnesium alloy having one of the compositions shown in Table 1 was
melted in an inert atmosphere to prepare a melt of the alloy. The
temperature of the melt immediately before it comes into contact
with the mold (hereinafter referred to as molten alloy temperature,
.degree. C.) is shown in Table 1. In this case, a facility
including a melting furnace, a holding furnace for holding the
melt, and a conveying unit for conveying the melt from the holding
furnace to the mold (a pair of rolls) was used, and the temperature
of the melt in the conveying unit is used as the "molten alloy
temperature." The temperature of the melt in the conveying unit is
the temperature setting of the facility. The melt was brought into
contact with the mold (rolls) and thereby solidified to produce a
cast sheet having a thickness of 5.0 mm.
[0104] The cooling rate (.degree. C./second) in the casting step is
shown in Table 1. In samples Nos. 1-1 to 1-5, 1-101, and 1-201, the
cooling rate was changed by controlling the temperature of the
rolls, the peripheral speed of the rolls, the rate of casting, etc.
In samples Nos. 1-1 to 1-5, and 1-101, a water-cooled mold was
used, and the casting was performed while the rolls were cooled
such that the roll temperature was 100.degree. C. or lower.
[0105] Each of the cast sheets obtained was subjected to a
plurality of warm rolling passes to produce a rolled sheet with a
thickness of 0.7 mm. The conditions of the warm rolling were a
rolling temperature of from 200.degree. C. to 400.degree. C.
inclusive, a rolling reduction per pass of from 5% to 20%
inclusive, and a total rolling reduction of 86%.
[0106] Each of the rolled sheets obtained was cut into a 200
mm.times.30 mm piece, and the cut piece was used as a raw material
for pressing. The raw material was subjected to press forming
(square cup drawing) to produce a press-formed material having a
rectangular U-shaped cross section and including a top portion and
leg portions extending from the top portion. The press conditions
were a heating temperature of 250.degree. C., and a corner R
connecting the top portion and a leg portion was 2 mm.
[0107] After the continuous casting, each of the cast sheets may be
subjected to heat treatment (solution treatment) for homogenization
of the structure or aging treatment, subjected to intermediate heat
treatment during rolling, or subjected to final heat treatment
after final rolling. The rolled sheet may be subjected to leveling
to improve flatness or may be subjected to polishing to further
smoothen the surface.
[0108] Structural Observation
[0109] Metallographic observation was performed on each of the
rolled sheets of the obtained samples as follows. Each rolled sheet
was cut along a plane parallel to its thickness direction to obtain
a cross section (a vertical cross section). This cross section is a
CP cross section prepared using a commercial cross section polisher
(CP). In the CP cross section, a region extending in a thickness
direction from the surface of the sheet to a depth of 30% of the
thickness of the sheet was used as a surface layer region (0.7
mm.times.0.3=0.21 mm in this case), and an observation field was
arbitrarily selected in the surface layer region. An upper part of
FIG. 1 shows a secondary electron image obtained by SEM observation
of the selected observation field in the rolled sheet of sample No.
1-1, and the lower part shows a binary image obtained by binarizing
the secondary electron image. FIG. 2 is a backscattered electron
image obtained by SEM observation of the selected observation
field.
[0110] As can be seen from the upper part of FIG. 1, many small and
large particles are dispersed in a fine crystalline structure.
Specifically, relatively large particles shown in light grey
(maximum length: about 0.1 .mu.m to about 1 .mu.m) and relatively
small particles shown in white were found to be present. Component
analysis was performed on these particles. The relatively large
particles (light grey) were found to be a compound containing Al
and Mg (a .beta. phase, mainly precipitates), and particles (white)
smaller than the .beta. phase were found to be compounds containing
Al and Mn (Al--Mn crystallized phases). To facilitate the
clarification of the presence of the white particles, the contrast
was changed as shown in the lower part of FIG. 1. As can be seen,
the white particles are uniformly dispersed in the crystalline
structure. As also can be seen, a certain amount of the white
particles is present, although the white particles are smaller than
the .beta. phase and the amount of the white particles is smaller
than the amount of the .beta. phase. This can also be seen from
FIG. 2. As can also be seen in the SEM backscattered electron image
shown in FIG. 2, light grey particles and white particles are
present. A certain amount of the white particles is present,
although the white particles are smaller than the light grey
particles and the amount of the white particles is smaller than the
amount of the light grey particles. It was therefore found that, in
the rolled sheet of sample No. 1-1, a certain amount of the
compounds containing Al and Mn (Al--Mn crystallized phases) was
present although they were relatively small and their amount was
relatively small. It was also found that these compounds were
uniformly dispersed. The rolled sheets of samples Nos. 1-2 to 1-5
have the same structure as the rolled sheet of sample No. 1-1,
i.e., have a fine crystalline structure in which the relatively
small Al--Mn crystallized phases are present in a certain amount
and are uniformly distributed.
[0111] In the rolled sheet of sample No. 1-101, the amount of the
compounds containing Al and Mn (Al--Mn crystallized phases) was
very small. In the rolled sheet of sample No. 1-201, although the
amount of the compounds containing Al and Mn was very small, coarse
particles were present.
[0112] For each of the rolled sheets of the samples, an image
observed under an optical microscope was used to measure an average
crystal grain size. The results are shown in Table 1. The average
crystal grain size was measured according to "Steels-Micrographic
determination of the apparent grain size, JIS G 0551 (2005), an
intercept method using linear test lines." Straight lines parallel
to the thickness direction of the rolled sheet were drawn in the
image observed, and line segments intersecting crystal grains were
used as grain sizes. The average crystal grain sizes of the rolled
sheets of samples Nos. 1-1 to 1-5 were 10 .mu.m or less. This shows
that the crystal grains in each of the rolled sheets of samples
Nos. 1-1 to 1-5 are fine.
[0113] For each of the rolled sheets of the samples obtained, the
above-described white particles in the SEM image (the binary image
converted from the secondary electron image) were extracted as the
compounds containing Al and Mn (Al--Mn crystallized phases), and
the average diameter (.mu.m) and the maximum diameter (.mu.m) of
the extracted particles of the Al--Mn crystallized phases were
examined. The results are shown in Table 1. The diameters of the
particles of the Al--Mn crystallized phases were determined as
follows. The diameters of circles having the same areas as the
extracted particles were determined. The average of the diameters
of all the particles present in the observation field (a 195
.mu.m.times.195 .mu.m square region selected in the above-described
surface layer region) was used as the average particle diameter of
the Al--Mn crystallized phases. The largest value among the
diameters of all the particles was used as the maximum diameter of
the Al--Mn crystallized phases.
[0114] For each of the rolled sheets of the samples obtained, a
compositional mapping by FE-EPMA was produced in the observation
field selected in the CP cross section to examine the distribution
of the Mn concentration. The concentration of Mn was analyzed under
two different conditions with different electron gun acceleration
voltages. These conditions are shown below.
[0115] (1) The acceleration voltage of the electron gun: 15 kV,
irradiation current: 100 nA, measurement time: 50 ms, measurement
element: Mn (LiFH), measurement area: 195 .mu.m.times.195 .mu.m
square region
[0116] (2) The acceleration voltage of the electron gun: 5 kV,
irradiation current: 100 nA, measurement time: 500 ms, measurement
element: Mn (TAPH), measurement area: 24 .mu.m.times.24 .mu.m
square region
[0117] In conditions (2) with a smaller acceleration voltage, the
measurement area was smaller than that in conditions (1). However,
it had been checked that, even when the measurement area in
conditions (2) was the same as that in conditions (1), no
significant difference was found in the measurement results (the
distribution of the Mn concentration).
[0118] FIG. 3 shows an FE-EPMA compositional mapping of Mn in the
rolled sheet of sample No. 1-1 under conditions (1) with an
acceleration voltage of 15 kV. A color scale is shown on the right
side of FIG. 3. In this compositional mapping, the concentration of
Mn is represented by a color shade changing from white, pink, red,
orange, yellow, green, light blue, blue, to black. A color close to
white means a high Mn concentration, and a color close to black
means a low Mn concentration. In FIG. 3, the level of Mn at a point
with the highest Mn concentration is set to 135, and the level of
Mn at a point with no Mn is set to 0. Then the concentration of Mn
at each point is represented by a value relative to the level 135.
The percentage of each level is represented as an area ratio (Area
%). As shown in the compositional mapping in FIG. 3, a large number
of regions composed of red-to-blue particle-like clusters are
present in a black background. By comparing the positions of the
red-to-blue particle-like regions present in the compositional
mapping in FIG. 3 with the positions of the white particles present
in the SEM image (backscattered electron image) in the same
observation field (see FIG. 2), the red-to-blue particle-like
regions in the compositional mapping were found to be included in
the compounds containing Al and Mn (Al--Mn crystallized phases).
This may show that Mn in the rolled sheet of sample No. 1-1 is
present as the Al--Mn crystallized phases. Therefore, in this case,
all the Mn present is treated as compounds with Al.
[0119] FIG. 4 is a graph of the frequency of the level of Mn (the
number of Mn counts) and the cumulative frequency produced using
the compositional mapping (15 kV) of Mn shown in FIG. 3. In the
graph in FIG. 4, the horizontal axis represents the level of Mn (0
to 135, the level is shown up to 110 in FIG. 4). The left vertical
axis represents the frequency of the level of Mn, and the right
vertical axis represents the cumulative frequency (%) of the level
of Mn.
[0120] The cumulative frequency at a level of Mn is equivalent to
the area ratio (Area %) at this level. The average S.sub.Level of
the levels of Mn was computed and found to be
S.sub.Level.apprxeq.10, and the overall concentration of Mn was
found to be very small. Therefore, when regions in which the level
of Mn is about the average S.sub.Level are treated as noise, Mn may
be more suitably extracted. Specifically, the compounds containing
Al and Mn (Al--Mn crystallized phases) may be more suitably
extracted. The standard deviation .sigma..sub.Level of the levels
of Mn was determined. Then the average
S.sub.Level+3.sigma..sub.Level was used as a threshold value, and
regions in which the level of Mn was equal to or larger than the
average S.sub.Level+3.sigma..sub.Level were treated as the Al--Mn
crystallized phases. The cumulative frequency (%) in a portion in
which the level was equal to or larger than the average
S.sub.Level+3.sigma..sub.Level (11.3 in this case) was treated as
the area ratio (%, 15 kV) of the Al--Mn crystallized phases. The
results are shown in Table 1.
[0121] A left part of FIG. 5 shows an FE-EPMA compositional mapping
of Mn in the rolled sheet of sample No. 1-1 when conditions (2)
with an acceleration voltage of 5 kV were used, and the right part
shows an SEM image (backscattered electron image) in the same
observation field. Also in the compositional mapping in FIG. 5, the
concentration of Mn is represented by a color shade, as in FIG. 3.
In the compositional mapping in FIG. 5, the level of Mn at a point
with the highest Mn concentration is set to 55, and the level of Mn
at a point with no Mn is set to 0. The percentage of each level is
represented by an area ratio (Area %). In the compositional mapping
in the left part of FIG. 5, the irradiation energy of the electron
gun is smaller than that of conditions (1), and the amount of
information about Mn is smaller than that under conditions (1), so
that the maximum level of Mn is small, i.e., 55. However, as shown
in the left part of FIG. 5, the color shade of Mn can be observed,
and red-to-blue particle-like clusters are present, as in the
compositional mapping in FIG. 3. By comparing the positions of the
red-to-blue particle-like regions present in the compositional
mapping in the left part of FIG. 5 with the positions of the white
particles in the SEM image (backscattered electron image) in the
right part of FIG. 5, the red-to-blue particle-like regions in the
compositional mapping were found to be the compounds containing Al
and Mn (Al--Mn crystallized phases).
[0122] FIG. 6 is a graph of the frequency of the level of Mn (the
number of Mn counts) and the cumulative frequency produced using
the compositional mapping (5 kV) of Mn shown the left part of FIG.
5. In the graph in FIG. 6, as in the graph in FIG. 4, the
horizontal axis represents the level of Mn (0 to 55, the level is
shown up to 50 in FIG. 6). The left vertical axis represents the
frequency of the level of Mn, and the right vertical axis
represents the cumulative frequency (%) of the level of Mn. Also in
this case, the average S.sub.Level of the levels of Mn and the
standard deviation .sigma..sub.Level were determined. Then the
average S.sub.Level+3.sigma..sub.Level was used as a threshold
value, and the cumulative frequency (%) in a portion in which the
level was equal to or larger than the average
S.sub.Level+3.sigma..sub.Level (12 in this case) was determined as
the area ratio (%, 5 kV) of the compounds containing Al and Mn
(Al--Mn crystallized phases). The results are shown in Table 1.
[0123] For each of the press-formed materials of samples Nos. 1-1
to 1-5, the metallographic structure of the top portion
substantially free from bending etc. was observed in the same
manner as in the rolled sheet. The top portion was found to have a
fine crystalline structure as fine as that of the rolled sheet, and
the compounds containing Al and Mn (Al--Mn crystallized phases)
were dispersed in the structure. In the top portion, the average
crystal grain size, the average diameter of the above compounds,
their maximum diameter, and their area ratio were equivalent to
those of the rolled sheet. Therefore, the top portion is considered
to have substantially the same structure as that of the rolled
sheet.
[0124] For each of the cast sheets of samples Nos. 1-1 to 1-5, the
metallographic structure was observed in the same manner as in the
rolled sheet. The cast sheet was found to have a fine crystalline
structure although the crystal grains were larger than those in the
rolled sheet. Also in the cast sheets of samples Nos. 1-1 to 1-5,
at least their surface layer region was found to have a structure
in which the compounds containing Al and Mn were uniformly
dispersed. The average diameter of the above compounds, their
maximum diameter, and their area ratios (15 kV and 5 kV) were
examined and found to be equivalent to those of the rolled sheet.
This shows that the above compounds present in the cast sheets are
substantially maintained in the rolled sheets of samples Nos. 1-1
to 1-5.
[0125] Tensile Test (Ordinary Temperature, about 20.degree. C.)
[0126] JIS 13B plate-shaped test pieces (JIS Z 2201(1998)) were
produced using the rolled sheets (thickness: 0.7 mm) of the samples
obtained, and a tensile test (gage length GL=50 mm) was performed
according to a metallic material tensile test method in JIS Z 2241
(1998). The tensile strength (MPa), 0.2% proof stress (MPa), and
rupture elongation (%) of each test piece were measured (the number
of times of evaluation: n=1 for all the cases). The results are
shown in Table 2.
[0127] Shock Resistance Test (Room Temperature, about 20.degree.
C.)
[0128] Plate-shaped pieces of 30 mm.times.30 mm were cut from the
rolled sheets (thickness: 0.7 mm) of the samples obtained, and the
cut plate-shaped pieces were used as test pieces. In this test, as
shown in FIG. 7, a support 20 having a circular hole 21 with a
diameter d=20 mm was prepared on a horizontal plane. The depth of
the circular hole 21 was set such that a cylindrical rod 10
described layer could be inserted sufficiently into the circular
hole 21. A test piece 1 was places so as to close the circular hole
21. While this state was maintained, the ceramic-made cylindrical
rod 10 having a weight of 100 g and a forward end r=5 mm was placed
at a height of 200 mm from the test piece 1 such that the center
axis of the cylindrical rod 10 was coaxial with the center axis of
the circular hole 21. Then the cylindrical rod 10 was allowed to
free-fall from the placement point (at a height of 200 mm) toward
the test piece 1, and the amount of dent in the test piece 1 was
measured. The amount of dent (mm) was measured as follows. A
straight line connecting opposed sides of the test piece 1 was
drawn, and the distance between this straight line and the most
dented portion was measured using a point micrometer. The results
are shown in Table 2.
[0129] Formability
[0130] For each of the press-formed materials (materials subjected
to square-cup drawing) of the samples obtained, the presence or
absence of cracking in a rounded corner portion was visually
checked. A press-formed material with no cracking was rated "Good,"
and a press-formed material with cracking was rated "Bad." The
results are shown in Table 2.
[0131] Productivity
[0132] When the temperature of the melt in the casting step was
690.degree. C. or lower, the productivity was rated "good."
TABLE-US-00001 TABLE 1 Casting conditions Rolled sheet Temperature
Compound of melt Average Average Area Area (molten alloy Cooling
crystal particle Maximum ratio ratio Sample temperature) rate grain
size diameter diameter (15 kV) (5 Kv) No. Composition .degree. C.
.degree. C./sec .mu.m .mu.m .mu.m % % 1-1 AZ91 650 700 4.9 0.42 1.0
23 8.2 1-2 A291 670 560 5.1 0.35 0.8 19 6.1 1-3 AZ61 660 650 5.2
0.49 0.7 16 5.5 1-4 AM60 660 650 6.0 0.45 0.9 17 5.8 1-5 AZX911 650
700 5.0 0.55 1.1 20 6.9 1-101 AZ91 695 750 5.3 0.35 0.6 8 2.4 1-201
AZ91 700 545 5.5 1.2 2.6 9.4 3.2
TABLE-US-00002 TABLE 2 Rolled sheet Compound Impact Area Average
0.2% resistance ratio particle Tensile Proof Rupture Amount Sample
(15 kV) diameter strength stress elongation of dent No. Composition
% .mu.m MPa MPa % mm Formability 1-1 AZ91 23 0.42 343 260 11 0.35
Good 1-2 A291 19 0.35 345 262 10 0.36 Good 1-3 AZ61 16 0.49 320 241
12 0.48 Good 1-4 AM60 17 0.45 312 234 14 0.49 Good 1-5 AZX911 20
0.55 335 255 12 0.38 Good 1-101 AZ91 8 0.35 321 243 11 0.66 Good
1-201 AZ91 9.4 1.2 320 239 6 0.63 Bad
[0133] As shown in Table 1, in each of samples Nos. 1-1 to 1-5, the
average diameter of the particles of the compounds containing Al
and Mn was from 0.3 .mu.m to 1 .mu.m inclusive, and the area ratio
of the particles of the compounds by FE-EPMA was from 3.5% to 25%
inclusive. In samples Nos. 1-1 to 1-5, their strength, proof
stress, elongation, and plastic formability were equivalent to
those of sample No. 1-101 containing a very small amount of the
above compounds. It was found that samples Nos. 1-1 to 1-5 had high
strength and high toughness and were excellent in plastic
formability irrespective of their composition. According to the
above tests, the tensile strength of each of samples Nos. 1-1 to
1-5 was 300 MPa or more, their 0.2% proof stress was 230 MPa or
more, and their rupture elongation was more than 6%. This shows
that, even when these samples Nos. 1-1 to 1-5 are subjected to
press forming such as square cup drawing, breakage and cracking are
likely to occur. In samples Nos. 1-1 to 1-5, the area ratio of the
particles of the compounds was larger than that in sample No.
1-101, and the amount of dent was 0.5 mm or less, so that the
impact resistance was better than that of sample No. 1-101. One
reason that samples Nos. 1-1 to 1-5 are excellent in impact
resistance, mechanical properties, and plastic formability as
described above may be as follows. Although a certain amount of the
above compounds is present, the compounds are relatively fine.
Therefore, 1. the dispersion strengthening effect is obtained
preferably, and 2. the above compounds are unlikely to serve as the
starting points of cracking etc.
[0134] The following was also found for samples Nos. 1-1 to
1-5.
[0135] The maximum diameter of the above compounds is 1.2 .mu.m or
less. This allows the occurrence of cracking etc. originating from
the above compounds to be effectively suppressed.
[0136] The crystals are also fine, and the average crystal grain
size is 10 .mu.m or less. This also allows the occurrence of
cracking etc. originating from coarse crystal grains to be
effectively suppressed. This may be the reason that excellent
impact resistance, mechanical properties, and plastic formability
are achieved.
[0137] Since the temperature of the melt during continuous casting
is relatively low, thermal damage to the facility can be
suppressed. Therefore, high productivity is achieved.
[0138] As can be seen from the above tests, the area ratio of the
compounds containing Al and Mn in each of samples Nos. 1-1 to 1-5
was higher than that in each of samples Nos. 1-101 and 1-201,
irrespective of whether the acceleration voltage of the electron
gun for the analysis by the FE-EPMA was 5 kV or 15 kV. A sufficient
amount of the compounds containing Al and Mn was found to be
present.
[0139] In this test, when the acceleration voltage was 5 kV, the
area ratio in samples Nos. 1-1 to 1-5 was 5% or more and was higher
than that in samples Nos. 1-101 and 1-201. The area ratio in
samples Nos. 1-101 and 1-201 was less than 3.5%.
[0140] In this test, when the acceleration voltage was 15 kV, the
area ratio in samples Nos. 1-1 to 1-5 was 10% or more and was
higher than that in samples Nos. 1-101 and 1-201. The area ratio in
samples Nos. 1-101 and 1-201 was 9.4% or less.
[0141] It was found that a magnesium alloy excellent in impact
resistance, mechanical properties, plastic formability, and also
productivity can be produced by setting the temperature of the melt
immediately before it comes into contact with the mold to be
relatively low, i.e., from 630.degree. C. to 690.degree. C.
inclusive, and then rapidly cooling the melt at a cooling rate of
560.degree. C./second or higher, as described above. As can be seen
from the above tests, even when the composition is changed, a
magnesium alloy having impact resistance, mechanical properties,
and plastic formability can be produced with high productivity by
controlling the temperature of the melt and its cooling rate within
the above ranges, so long as the compounds containing Al and Mn are
contained in the above-described specific amount and have the
specific average particle diameter.
[0142] In sample No. 1-201, although the temperature of the melt
was very high, the cooling rate was very slow, i.e., less than
550.degree. C./second. It was found that, in sample No. 1-201,
although the amount of the compounds containing Al and Mn was
smaller than that in sample No. 1-1, coarse particles (2.5 .mu.m or
more) were present. In sample No. 1-201, its impact resistance,
mechanical properties, and plastic formability are worse than those
of sample No. 1-1 having the same composition as sample No. 1-201.
The reason for these results may be that the retention time in the
temperature range of around 630.degree. C. in which the above
compounds are easily formed and grow during solidification is long.
As a result of growth of the particles the compounds, coarse
particles are formed and serve as the starting points of cracking,
and the dispersion strengthening effect by the fine compounds
becomes insufficient. This may be the reason for the deterioration
of the impact resistance, mechanical properties, and plastic
formability deteriorate.
[0143] In sample No. 1-101, the temperature of the melt was high,
and the cooling rate was also high. Therefore, the amount of the
compounds containing Al and Mn was very small. In sample No. 1-101,
the impact resistance, in particular, was worse than that of sample
No. 1-1 having the same composition. This may be because the
dispersion strengthening by the fine compounds was
insufficient.
[0144] For each of the press-formed materials of samples Nos. 1-1
to 1-5, a test piece similar to that in the rolled sheet was
produced from the top portion, and the same tensile test (room
temperature) and the same shock resistance test (room temperature)
as those for the rolled sheet were performed. The results were
substantially the same as those for the rolled sheet. Therefore,
the press-formed material was also excellent in impact resistance
and mechanical properties. One of the reasons for this may be that
at least part of the press-formed material substantially maintains
the structure of the rolled sheet, i.e., the at least part of the
press-formed material has a fine crystalline structure and the
compounds containing Al and Mn having the specific size are
uniformly dispersed in this structure.
[0145] The present invention is not limited to the above-described
examples. The present invention is defined by the scope of the
claims and is intended to include any modifications within the
scope of the claims and meaning equivalent to the scope of the
claims. For example, in the Test Example described above, the
composition (the types of the additive elements and their
contents), the shape and size (thickness, length, width, etc.) of
the magnesium alloy sheet, the production conditions (the
specifications of the mold, the temperature of the mold, the molten
alloy temperature, the cooling rate, the thickness of the cast
sheet, etc. in the casting conditions) may be appropriately
changed.
INDUSTRIAL APPLICABILITY
[0146] The magnesium alloy and magnesium alloy sheet of the present
invention can be preferably used as raw materials for plastically
formed members (magnesium alloy structural members) subjected to
various types of plastic forming such as press forming, bending,
and forging. Particularly, the magnesium alloy sheet can be
preferably used as raw materials for members desirably having
characteristics such as lightweight, small thickness, high
strength, and vibration damping ability, and examples of such
members include: exterior members such as housings and covers of
various electronic-electric devices (personal computers (PCs),
tablet PCs, cellular phones such as smartphones and folding
cellular phones, digital cameras, etc.); structural members and
constituent members of transportation apparatuses such as
automobiles and aircrafts; bags; and various protective cases. The
magnesium alloy and magnesium alloy structural member of the
present invention can be preferably used for, for example, the
above described exterior members such as housings, the structural
members and constituent members of the transportation apparatuses,
bags, and protective cases. The magnesium alloy production method
of the present invention can be preferably used to produce
magnesium alloys such as the above-described magnesium alloy sheet
and the above-described magnesium alloy structural member.
REFERENCE SIGNS LIST
[0147] 1 test piece, 10 cylindrical rod, 20 support, 21 circular
hole
* * * * *