U.S. patent application number 13/125023 was filed with the patent office on 2011-08-25 for formed product of magnesium alloy and magnesium alloy sheet.
Invention is credited to Ryuichi Inoue, Nozomu Kawabe, Takahiko Kitamura, Koji Mori, Nobuyuki Mori, Masatada Numano, Yukihiro Oishi, Nobuyuki Okuda.
Application Number | 20110203706 13/125023 |
Document ID | / |
Family ID | 42119101 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110203706 |
Kind Code |
A1 |
Oishi; Yukihiro ; et
al. |
August 25, 2011 |
FORMED PRODUCT OF MAGNESIUM ALLOY AND MAGNESIUM ALLOY SHEET
Abstract
A formed product of a magnesium alloy having excellent impact
resistance and a magnesium alloy sheet suitable as a material for
the formed product are provided. The formed product is produced by
press-forming a magnesium alloy sheet having an Al content of 7% by
mass to 12% by mass and has a flat portion that is not subjected to
drawing deformation. In a metal texture in a cross section of the
flat portion in the thickness direction, the number of coarse
intermetallic compound (Mg.sub.17Al.sub.12) particles having a
particle size of 5 .mu.m or more present in a surface area region
extending from a surface of the flat portion to a position
one-third of the thickness from the surface in the thickness
direction is five or less. The formed product has a texture in
which the number of coarse precipitations d.sub.1 is small and in
which fine precipitations d.sub.0 are dispersed. The formed product
is less likely to be dented even when impacted because of
dispersion strengthening owing to the fine precipitations and
solid-solution strengthening owing to Al that sufficiently forms a
solid solution.
Inventors: |
Oishi; Yukihiro; (Osaka,
JP) ; Kawabe; Nozomu; (Osaka, JP) ; Okuda;
Nobuyuki; (Osaka, JP) ; Mori; Nobuyuki;
(Osaka, JP) ; Numano; Masatada; (Osaka, JP)
; Mori; Koji; (Hyogo, JP) ; Kitamura;
Takahiko; (Osaka, JP) ; Inoue; Ryuichi;
(Osaka, JP) |
Family ID: |
42119101 |
Appl. No.: |
13/125023 |
Filed: |
September 29, 2009 |
PCT Filed: |
September 29, 2009 |
PCT NO: |
PCT/JP2009/005004 |
371 Date: |
April 19, 2011 |
Current U.S.
Class: |
148/557 ;
148/420 |
Current CPC
Class: |
C22F 1/06 20130101; C22C
23/02 20130101 |
Class at
Publication: |
148/557 ;
148/420 |
International
Class: |
C22C 23/02 20060101
C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2008 |
JP |
2008-272241 |
Claims
1-9. (canceled)
10. A formed product of a magnesium alloy produced by press-forming
a sheet composed of a magnesium alloy, wherein the magnesium alloy
contains 7% by mass to 12% by mass Al, the formed product has a
flat portion that is not subjected to drawing deformation, and
wherein in the case where in a metal texture in a cross section of
the flat portion in the thickness direction, a region extending
from a surface of the flat portion to a position one-third of the
thickness from the surface in the thickness direction is defined as
a surface area region, two 100 .mu.m.times.100 .mu.m areas in the
surface area region are set to fields of observation, and where
particles composed of an intermetallic compound containing Al and
Mg and each having a particle size of 5 .mu.m or more are defined
as coarse particles, the number of the coarse particles present in
each of the fields of observation is five or less.
11. The formed product of a magnesium alloy according to claim 10,
wherein when the following dent test of a 30 mm.times.30 mm
specimen with a thickness of t.sub.p cut from the flat portion is
performed, the depth x.sub.p of the dent of the specimen meets the
expression x.sub.p.ltoreq.0.47.times.t.sub.p.sup.-1.25: (Dent Test)
The specimen is arranged on a support having an opening with a
diameter of 20 mm so as to close the hole; in this state, a
cylindrical bar having a weight of 100 g and a tip radius r of 5 mm
is allowed to free fall from a position 200 mm above the specimen;
and wherein the depth x.sub.p of the dent is defined as a distance
between a straight line that connects both sides of the specimen
and the most dented point after the dent test.
12. The formed product of a magnesium alloy according to claim 10,
wherein the magnesium alloy contains at least one element selected
from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare-earth elements (except
Y).
13. The formed product of a magnesium alloy according to claim 11,
wherein the magnesium alloy contains at least one element selected
from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare-earth elements (except
Y).
14. The formed product of a magnesium alloy according to claim 12,
wherein the magnesium alloy contains, on a mass percent basis,
8.3%-9.5% Al and 0.5%-1.5% Zn.
15. The formed product of a magnesium alloy according to claim 13,
wherein the magnesium alloy contains, on a mass percent basis,
8.3%-9.5% Al and 0.5%-1.5% Zn.
16. The formed product of a magnesium alloy according to claim 14,
wherein a corrosion prevention layer is formed by
chemical-conversion treatment on a surface of the sheet of the
magnesium alloy.
17. The formed product of a magnesium alloy according to claim 15,
wherein a corrosion prevention layer is formed by
chemical-conversion treatment on a surface of the sheet of the
magnesium alloy.
18. A magnesium alloy sheet used for press forming, wherein the
magnesium alloy contains 7% by mass to 12% by mass Al, and wherein
in the case where in a metal texture in a cross section of the
sheet in the thickness direction, a region extending from a surface
of the sheet to a position one-third of the thickness from the
surface in the thickness direction is defined as a surface area
region, two 100 .mu.m.times.100 .mu.m areas in the surface area
region are set to fields of observation, and where particles
composed of an intermetallic compound containing Al and Mg and each
having a particle size of 5 .mu.M or more are defined as coarse
particles, the number of the coarse particles present in each of
the fields of observation is five or less.
19. The magnesium alloy sheet according to claim 18, wherein when
the following dent test of a 30 mm.times.30 mm specimen with a
thickness of t.sub.b cut from the sheet is performed, the depth
x.sub.b of the dent of the specimen meets the expression
x.sub.b.ltoreq.0.47.times.t.sub.b.sup.-1.25: (Dent Test) The
specimen is arranged on a support having an opening with a diameter
of 20 mm so as to close the hole; in this state, a cylindrical bar
having a weight of 100 g and a tip radius r of 5 mm is allowed to
free fall from a position 200 mm above the specimen; and wherein
the depth x.sub.b of the dent is defined as a distance between a
straight line that connects both sides of the specimen and the most
dented point after the dent test.
20. The magnesium alloy sheet according to claim 18, wherein the
magnesium alloy contains at least one element selected from Zn, Mn,
Si, Ca, Sr, Y, Cu, Ag, and rare-earth elements (except Y).
21. The magnesium alloy sheet according to claim 19, wherein the
magnesium alloy contains at least one element selected from Zn, Mn,
Si, Ca, Sr, Y, Cu, Ag, and rare-earth elements (except Y).
22. The magnesium alloy sheet according to claim 20, wherein the
magnesium alloy contains, on a mass percent basis, 8.3%-9.5% Al and
0.5%-1.5% Zn.
23. The magnesium alloy sheet according to claim 21, wherein the
magnesium alloy contains, on a mass percent basis, 8.3%-9.5% Al and
0.5%-1.5% Zn.
24. A method for producing a formed product of a magnesium alloy by
subjecting a sheet composed of a magnesium alloy to press forming,
the method comprising: a preparation step of preparing a cast sheet
composed of a magnesium alloy having an Al content of 7% to 12% by
mass and produced by a continuous casting process; a solution heat
treatment step of subjecting the cast sheet to solution heat
treatment at 350.degree. C. or higher; a rolling step of subjecting
the sheet material that has been subjected to the solution heat
treatment to rolling; and a press-forming step of subjecting the
rolled sheet obtained in the rolling step to press forming, wherein
in a cooling substep from the holding temperature of the solution
heat treatment in the solution heat treatment step, the cooling
rate is 0.1.degree. C./sec or more in a temperature range of
350.degree. C. to 250.degree. C., in the rolling step, the total
time that the sheet material, which is a workpiece, is held in a
temperature range of 250.degree. C. to 350.degree. C. is within 60
minutes, and in the press-forming step, the press forming is
performed in a temperature range of 200.degree. C. to 300.degree.
C.
25. A method for producing a magnesium alloy sheet composed of a
magnesium alloy, the magnesium alloy sheet being used for press
forming, the method comprising: a preparation step of preparing a
cast sheet composed of a magnesium alloy having an Al content of 7%
to 12% by mass and produced by a continuous casting process; a
solution heat treatment step of subjecting the cast sheet to
solution heat treatment at 350.degree. C. or higher; and a rolling
step of subjecting the sheet material that has been subjected to
the solution heat treatment to rolling, wherein in a cooling
substep from the holding temperature of the solution heat treatment
in the solution heat treatment step, the cooling rate is
0.1.degree. C./sec or more in a temperature range of 350.degree. C.
to 250.degree. C., and in the rolling step, the total time that the
sheet material, which is a workpiece, is held in a temperature
range of 250.degree. C. to 350.degree. C. is within 60 minutes.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy sheet
suitable as a material for, for example, housings of mobile
electronic devices and relates to a formed product of an magnesium
alloy, the formed product being produced by press forming. In
particular, the present invention relates to a formed product of a
magnesium alloy having excellent impact resistance.
BACKGROUND ART
[0002] Resins, such as acrylonitrile butadiene styrene (ABS)
copolymer resins and polycarbonate (PC) resins, and metals, such as
aluminium alloys and stainless steel (SUS), have been used as
housing materials for mobile electronic devices, such as cellular
phones and notebook personal computers.
[0003] Magnesium alloys, which are lightweight and excellent in
specific strength and specific rigidity, have recently been studied
as housing materials described above. Housings of magnesium alloys
are mainly formed of cast materials produced by die casting and
thixomolding. Press formed sheets of wrought magnesium alloys
typified by the AZ31 alloy according to American Society for
Testing and Materials (ASTM) standards are being used. In Patent
Literature 1, the press forming of an AZ91 alloy according to ASTM
standards is studied.
[0004] Thin, lightweight housings have recently been required.
Metals generally have higher impact resistance than resins and are
less likely to be broken. It is easy to reduce the thickness of
metals. Aluminium alloys, however, have poor plastic deformation
resistance and deform quite readily by an impact, such as falling.
Stainless steel is not easily broken or deformed but is heavy.
CITATION LIST
Patent Literature
[0005] PTL 1: Japanese Unexamined Patent Application Publication
No. 2007-098470
SUMMARY OF INVENTION
Technical Problem
[0006] Magnesium alloys has excellent plastic deformation
resistance compared with aluminium alloys, and are very light
compared with stainless steel. However, cast materials of magnesium
alloys have a strength inferior to those of press-formed bodies of
magnesium alloys. Furthermore, it is difficult to produce the cast
materials having thin walls. Press-formed bodies of the AZ31 alloy
also have an insufficient strength.
[0007] In the case where a rolled sheet of the AZ91 alloy as
described in Patent Literature 1 is subjected to press forming, the
resulting formed product has a higher strength than a press-formed
body of the AZ31 alloy. However, the inventors have investigated
and have found that an Al content as high as 7% by mass can cause
nonuniformity in the impact resistance of material sheets and
press-formed bodies obtained by forming the material sheets.
[0008] Accordingly, it is an object of the present invention to
provide a formed product of a magnesium alloy having excellent
impact resistance. It is another object of the present invention to
provide a magnesium alloy sheet suitable for the production of a
formed product of a magnesium alloy, the formed product having
excellent impact resistance.
Solution to Problem
[0009] The inventors have produced material sheets of a magnesium
alloys each having an Al content of 7% by mass or more by various
production methods. Press-formed bodies of the resulting sheets
were produced and examined for the impact resistance (dent
resistance). It was found that a press-formed body with good dent
resistance has small particles composed of an intermetallic
compound (precipitations), such as Mg.sub.17Al.sub.12, and a small
number of coarse particles. So, a production method for controlling
the maximum particle size and the number of the particles having
the maximum particle size, i.e., a production method for reducing
coarse precipitations, was studied. The total time that a sheet is
held in a specific temperature range mainly in a rolling step is
reduced compared with that in the related art. This resulted in a
magnesium alloy sheet having a small number of coarse
precipitations. Furthermore, a press-formed body produced by
press-forming the magnesium alloy sheet has excellent impact
resistance. These findings have led to the completion of the
present invention.
[0010] According to the present invention, a formed product of a
magnesium alloy is produced by press-forming a sheet composed of a
magnesium alloy having an Al content of 7% by mass to 12% by mass.
The formed product has a flat portion that is not subjected to
drawing deformation. In a metal texture in a cross section of the
flat portion in the thickness direction, when fields of observation
specified below are set, the number of coarse particles of an
intermetallic compound present in each of the fields of observation
is five or less.
[0011] Furthermore, according to the present invention, a magnesium
alloy sheet is used for press forming and is composed of a
magnesium alloy having an Al content of 7% by mass to 12% by mass,
in which the number of coarse particles of an intermetallic
compound present in each of fields of observation specified below
is five or less.
[0012] In the metal texture of a cross section of the flat portion
or the magnesium alloy sheet in the thickness direction, when a
region extending from a surface of the flat portion or a surface of
the sheet to a position one-third of the thickness from the surface
in the thickness direction is defined as a surface area region, any
two 100 .mu.m.times.100 .mu.m areas in the surface area region are
set to the fields of observation.
[0013] The term "coarse particles" indicates particles composed of
an intermetallic compound containing Al and Mg and each having a
particle size of 5 .mu.m or more.
[0014] The term "particle size" indicates the diameter of a circle
having an area equivalent to the area of the cross section of the
particle.
[0015] Note that the intermetallic compound present in the cross
section may be identified by measuring the composition and the
structure of the particles using an energy dispersive x-ray
spectrometer (EDS), X-ray diffraction, and so forth.
[0016] The alloy sheet having the specific texture according to the
present invention may be produced by, for example, a production
method including steps described below.
[0017] A preparation step: A cast sheet composed of a magnesium
alloy having an Al content of 7% to 12% by mass and produced by a
continuous casting process is prepared.
[0018] A solution heat treatment step: The cast sheet is subjected
to solution heat treatment at 350.degree. C. or higher.
[0019] A rolling step: The resulting sheet material that has been
subjected to the solution heat treatment is subjected to
rolling.
[0020] In particular, in a cooling substep from the holding
temperature of the solution heat treatment, the cooling rate is
0.1.degree. C./sec or more in a temperature range of 350.degree. C.
to 250.degree. C. In the rolling step, the total time that the
sheet material, which is a workpiece, is held in a temperature
range of 250.degree. C. to 350.degree. C. is within 60 minutes.
[0021] As described above, in the cooling process in the solution
heat treatment (that is, immediately before rolling) and the
rolling step, the minimization of the length of the time that the
sheet is held at a specific temperature range (250.degree. C. to
350.degree. C.) in which precipitations are precipitated and liable
to grow to form coarse particles reduces the number of coarse
particles, thereby yielding a texture in which fine precipitations
d.sub.0 are dispersed as illustrated in part (1) of FIG. 1.
[0022] Conventionally, rolling is performed multiple times
(multipass) at an appropriate working ratio (rolling reduction) in
such a manner that a desired thickness is achieved, as illustrated
in part (2) of FIG. 2 (each pass is represented by "rolling n"
(n=1, 2, . . . ) in FIG. 2). Here, heating a workpiece (a cast
sheet or a rolled sheet before subjecting the final rolling) to
250.degree. C. or higher results in higher plastic formability. So,
in the rolling step, preferably, the workpiece is heated and
subjected to warm rolling or hot rolling in at least the early
stage (rough rolling) of the rolling. However, in particular, for a
magnesium alloy having an Al content as high as 7% by mass or more,
heating the magnesium alloy to 250.degree. C. or higher is liable
to cause the growth of precipitations of, for example, an
intermetallic compound to form coarse particles. Furthermore, in
the cooling process in the solution heat treatment step, when the
magnesium alloy passes through the temperature range of 250.degree.
C. to 350.degree. C., precipitations are liable to coarsen.
[0023] Conventionally, the total time that a workpiece is held in
the temperature range of 250.degree. C. to 350.degree. C.
immediately before and during a rolling step has not been well
studied. The inventors have studied on the total time and have
found as follows: For a magnesium alloy having an Al content of 7%
to 12% by mass, in the case where the total holding time in the
foregoing temperature range exceeds 1 hour in at least the rolling
step, a texture containing coarse precipitations d.sub.1 each
having a particle size of 5 .mu.m or more is formed, as illustrated
in part (2) of FIG. 1. In contrast, in the case where the total
holding time in the foregoing temperature range is within 1 hour in
the rolling step, it is possible to reduce the coarse
precipitations. Furthermore, only in the rolling step, in the case
where the cooling rate in the solution heat treatment is increased
in addition to the fact that the total holding time in the
foregoing temperature range is within 1 hour, the formation of
coarse precipitations is more effectively inhibited. In particular,
the sum of the total holding time in the foregoing temperature
range in the rolling step and the holding time in the foregoing
temperature range in the cooling process in the solution heat
treatment step is preferably within 1 hour.
[0024] The alloy sheet of the present invention has a small number
of coarse precipitations in the surface area region and has a
texture in which very fine precipitations are dispersed (part (1)
of FIG. 1). Since the alloy sheet of the present invention has a
small number of coarse precipitations, it is believed that a
reduction in the amount of Al that forms a solid solution in a
matrix (Mg) due to the presence of a large number of coarse
precipitations is small and that a reduction in solid-solution
strengthening due to the reduction in the Al content is small.
Thus, the alloy sheet of the present invention is less likely to be
dented even when impacted and has excellent impact resistance
because of improvement in the rigidity of the sheet itself as a
result of dispersion strengthening owing to the dispersion of
precipitations and because of maintaining the strength owing to the
prevention of the reduction in the amount of Al that forms a solid
solution. Furthermore, the alloy sheet of the present invention
having a small number of coarse precipitations also has excellent
plastic formability and can be easily subjected to press
forming.
[0025] The alloy sheet of the present invention obtained by the
control of the holding time in the specific temperature range
mainly in the rolling step as described above is subjected to press
forming to produce a formed product of the present invention. In
the case of using the alloy sheet of the present invention, the
texture constituting the alloy sheet of the present invention and
having a small number of coarse precipitations is generally
maintained in a portion (flat portion) of the formed product of the
present invention where the degree of deformation due to press
forming is low.
[0026] That is, the formed product of the present invention also
has a texture which has a small number of coarse precipitations in
a surface area region and in which very fine precipitations are
dispersed. Thus, the formed product of the present invention has
excellent impact resistance and is less likely to be dented because
of dispersion strengthening owing to the dispersion of fine
precipitations and because of solid-solution strengthening owing to
Al that sufficiently forms a solid solution, as described
above.
[0027] The present invention will be described in more detail
below.
<<Composition>>
[0028] Magnesium alloys include ones having various compositions
and each containing Mg and an additive element (remainder: Mg and
impurities). The sheet and the formed product of the present
invention are composed of a Mg--Al-based alloy containing at least
7% by mass to 12% by mass Al serving as an additive element. The
additive element other than Al is at least one element selected
from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, and rare-earth elements (except
Y). In the case where the element is contained, the proportion
thereof is in the range of 0.01% by mass to 10% by mass and
preferably 0.1% by mass to 5% by mass. More specific examples of
the Mg--Al-based alloy include AZ-based alloys (Mg--Al--Zn-based
alloys, Zn: 0.2% to 1.5% by mass), AM-based alloys
(Mg--Al--Mn-based alloys, Mn: 0.15% to 0.5% by mass), and Mg--Al-RE
(rare-earth element)-based alloys according to ASTM standards. In
particular, Mg--Al-based alloys containing 8.3% to 9.5% by mass Al
and 0.5% to 1.5% by mass Zn, typically, an AZ91 alloy, have
excellent mechanical properties, such as corrosion resistance,
strength, and plastic deformation resistance, compared with other
Mg--Al-based alloys, such as the AZ31 alloy.
<<Thickness of Magnesium Alloy Sheet>>
[0029] The alloy sheet of the present invention is subjected to
press forming, such as bending and drawing, and is used as a
material for a thin, lightweight component, such as a housing. For
a housing produced by press forming, in order to achieve a small
thickness of a portion of the housing where the thickness is not
changed substantially by deformation during plastic forming (a flat
portion of the formed product of the present invention), the alloy
sheet of the present invention preferably has a thickness of 2.0 mm
or less, particularly preferably 1.5 mm or less, more preferably 1
mm or less. Within the foregoing range, the magnesium alloy sheet
having a larger thickness has higher strength, and the magnesium
alloy sheet having a smaller thickness is more suitable for a thin,
lightweight housing. The thickness may be selected, depending on an
intended use.
<<Mechanical Properties>>
[0030] The alloy sheet of the present invention is less likely to
be dented when subjected to an impact, such as falling.
Specifically, in the case where a dent test of a 30 mm.times.30 mm
specimen with a thickness of t.sub.b cut from the alloy sheet of
the present invention is performed as described below, the depth
x.sub.b of the dent of the specimen meets the expression
x.sub.b.ltoreq.0.47.times.t.sub.b.sup.-1.25. Furthermore, in the
formed product of the present invention, a flat portion that is not
subjected to drawing deformation has a small number of coarse
precipitations as described above. The properties of the alloy
sheet of the present invention are substantially maintained as
described above. Thus, after a specimen (thickness: t.sub.p) the
same as that of the alloy sheet of the present invention as
described above is cut from the flat portion, the dent test
described below is performed. The depth x.sub.p of the dent of the
specimen meets the expression
x.sub.p.ltoreq.0.47.times.t.sub.p.sup.-1.25. Note that the
thickness t.sub.p of the specimen cut from the flat portion of the
formed product of the present invention is substantially equal to
the thickness t.sub.b (i.e., t.sub.p=t.sub.b) of the specimen cut
from the magnesium alloy sheet serving as a material for press
forming, for example, the alloy sheet of the present invention.
(Dent Test)
[0031] A specimen is arranged on a support having an opening with a
diameter of 20 mm so as to close the hole. In this state, a
cylindrical bar having a weight of 100 g and a tip radius r of 5 mm
is allowed to free fall from a position 200 mm above the
specimen.
[0032] The depth x.sub.b of the dent or the depth x.sub.p of the
dent are each defined as a distance between a straight line that
connects both sides of the specimen and the most dented point after
the dent test.
<<Shape of Formed Product>>
[0033] The formed product of the present invention typically has a
shape including a top plate (bottom face) and side walls each
extending upright from the outer edge of the top plate. More
specific examples thereof include a bracket shape consisting of a
rectangular plate-like top plate and a pair of opposite side walls;
a box shape including two pairs of opposite side walls and having a
bracket-shaped cross section; and a closed-end cylinder including a
disk-like top plate and a cylindrical side wall.
[0034] The shape of each of the top plate and the side walls is
typically a flat plane. The shape and size thereof are not limited.
Each of the top plate and the side walls may include a boss and so
forth integrally formed or joined, a through hole and a recess
formed in the thickness direction, a groove formed in the thickness
direction, a bump, and a portion having a locally varying thickness
formed by plastic forming, surface cutting, or the like. In the
formed product of the present invention, the flat portion that is
not subjected to drawing is defined as follows: When a piece cut
from a region excluding a portion that includes the boss and so
forth is placed on a horizontal plane, a portion of the piece where
the degree of warpage is low is referred to as the flat portion.
More specifically, with respect to a surface of the piece placed on
the horizontal plane, the surface facing the horizontal plane, a
portion where a distance between the horizontal plane and a point
of the surface most remote from the horizontal plane is within 1 mm
in the vertical direction is defined as the flat portion. A dent is
commonly likely to be made in a flat portion. So, for the alloy
sheet of the present invention and the formed product of the
present invention, dent resistance is evaluated in the flat portion
described above.
<<Surface of Formed Product>>
[0035] The formed product of the present invention may include a
covering layer for corrosion prevention, protection, an ornament,
or the like on a surface of the magnesium alloy sheet. The
magnesium alloy mainly contained in the formed product of the
present invention has an Al content of 7% by mass or more and thus
has excellent corrosion resistance compared with alloys having a
low Al content, for example, the AZ31 alloy. Furthermore, the
magnesium alloy sheet is subjected to anticorrosion treatment,
e.g., chemical-conversion treatment or anodic-oxidation treatment,
to form a corrosion prevention layer, thereby further enhancing the
corrosion resistance of the formed product of the present
invention. Note that a step of forming the covering layer for
corrosion prevention, coating, or the like, does not substantially
affect the size and deposition of precipitations. Thus, even when
the formed product of the present invention includes the covering
layer for corrosion prevention or the like, the number of the
coarse particles is five or less. Furthermore, in the case where
the dent test is performed,
x.sub.p.ltoreq.0.47.times.t.sub.p.sup.-1.25 is met.
<<Production Method>>
[Preparation Step]
[0036] A cast sheet produced by a continuous casting process, such
as a twin-roll casting process, in particular, a casting process
described in WO/2006/003899 is preferably used. In the continuous
casting process, rapid solidification can be performed, thereby
reducing oxide and segregation and providing a cast sheet having
excellent rollability. The size of the cast sheet is not
particularly limited. An excessively thick cast sheet is liable to
cause segregation. So, the thickness is preferably 10 mm or less
and particularly preferably 5 mm or less.
[Solution Heat Treatment Step]
[0037] The cast sheet is subjected to solution heat treatment to
homogenize the composition. In the solution heat treatment, the
holding temperature is set to 350.degree. C. or higher. In
particular, preferably, the holding temperature is in the range of
380.degree. C. to 420.degree. C. for a holding time of 60 to 2400
minutes. In the case of a higher Al content, the holding time is
preferably increased. Furthermore, to produce the alloy sheet of
the present invention, in the cooling substep from the holding
temperature, the holding time in the temperature range of
350.degree. C. to 250.degree. C. is controlled. Specifically, to
reduce the holding time in the foregoing temperature range as
illustrated in part (1) of FIG. 2, the cooling rate in this
temperature range is set to 0.1.degree. C./sec or more (holding
time: about 16.6 minutes or less) and preferably 0.5.degree. C./sec
or more (holding time: 3.3 minutes or less). Such a cooling rate
can be achieved by forced cooling, e.g., water cooling or an air
blast. The minimization of the holding time in the foregoing
temperature range prevents the precipitation of an intermetallic
compound containing Al and Mg and, in particular, effectively
inhibits the formation of coarse particles even in the case of a
magnesium alloy having a high Al content.
[Rolling Step]
[0038] To increase the plastic formability (rollability) of the
sheet that has been subjected to the solution heat treatment, in at
least rough rolling, a sheet material heated to 200.degree. C. or
higher and, in particular, 250.degree. C. or higher is preferably
subjected to rolling, as described above. A higher heating
temperature enhances the plastic formability of the sheet material.
However, a heating temperature exceeding 350.degree. C. causes
problems of the occurrence of seizure and the coarsening of crystal
grains to reduce the mechanical properties of the sheet material
after rolling. Thus, the heating temperature is preferably
350.degree. C. or lower and more preferably 270.degree. C. to
330.degree. C. Rolling is performed multiple times (multipass),
thereby achieving an intended thickness, reducing the average
crystal grain size of the magnesium alloy, and enhancing the press
formability. Rolling may be performed under known conditions. For
example, rollers may be heated in addition to the sheet material.
Controlled rolling disclosed in Patent Literature 1 may be
combined. Furthermore, in the final pass and passes near the final
pass, in order to increase dimensional accuracy and so forth, the
heating temperature of the sheet material may be set to a low
temperature (for example, room temperature).
[0039] In the rolling step described above, the holding time in the
temperature range of 250.degree. C. to 350.degree. C. is
controlled. Specifically, as illustrated in part (1) of FIG. 2, in
order to reduce the holding time in the foregoing temperature range
in each pass in the rolling step, for example, the heating time to
heat a workpiece is reduced, the rolling speed (circumferential
speed of the roll) is increased, or the cooling rate is increased.
The rolling conditions are controlled in such a manner that the
total holding time in the temperature range of 250.degree. C. to
350.degree. C. in the rolling step is within 60 minutes or less. A
higher Al content facilitates the deposition of precipitations. So,
the total holding time is preferably adjusted, depending on the Al
content. Furthermore, the total holding time is preferably
minimized. The total holding time is preferably 45 minutes or less
and particularly preferably 30 minutes. This specific rolling
results in the alloy sheet of the present invention having a small
number of coarse precipitations in the surface area region and
excellent impact resistance, as described above.
[0040] Intermediate heat treatment is performed between the passes
of the rolling to eliminate or reduce strain, residual stress,
texture, and so forth, which are introduced into the sheet
material, which is a workpiece, by processing before the
intermediate heat treatment, thereby preventing inadvertent
cracking, strain, and deformation in the subsequent rolling and
achieving smoother rolling. The intermediate heat treatment is
preferably performed at a holding temperature of 250.degree. C. to
350.degree. C. This temperature range is liable to cause the growth
of precipitations to form coarse particles as described above.
Thus, in the case where the intermediate heat treatment is
performed, preferably, the total holding time includes the
treatment time of the intermediate heat treatment and is
controlled.
<<Treatment after Rolling>>
(Final Heat Treatment (Annealing))
[0041] The resulting rolled sheet may be subjected to final heat
treatment at, for example, 300.degree. C. or higher, thereby
eliminating processing strain and performing complete
recrystallization. In this final heat treatment, precipitations are
liable to grow in the temperature range of 250.degree. C. to
350.degree. C. So, in the case where the final heat treatment is
performed after rolling, preferably, the total holding time
includes the treatment time of the final heat treatment and is
controlled. The time of the final heat treatment is controlled as
described above, so that the magnesium alloy sheet of the present
invention has a small number of coarse precipitations.
(Warm Flattening Treatment)
[0042] Alternatively, the final heat treatment is not performed
after rolling, and warm flattening treatment may be performed in
which strain is imparted to the resulting rolled sheet using a
roller leveler or the like with the rolled sheet heated to
100.degree. C. to 250.degree. C. In the case where the resulting
sheet that has been subjected to the warm flattening treatment is
subjected to press forming, the sheet is recrystallized during the
press forming, thereby resulting in a formed product having a fine
crystal texture. Fine crystal grains are likely to be formed, and a
texture in which fine precipitations are more evenly dispersed is
likely to be formed, as compared with the case where the final heat
treatment is performed. Thus, in the case where the warm flattening
treatment is performed, the magnesium alloy sheet of the present
invention has higher impact resistance because of a small number of
coarse precipitations and the foregoing fine texture. Note that in
the warm flattening treatment, the heating temperature of the
rolled sheet is set to at most 250.degree. C., so that
precipitations may be less likely to coarsen.
[Press Forming]
[0043] The formed product of the present invention may be produced
by press-forming a rolled sheet obtained by the foregoing rolling
step or press-forming a treated sheet obtained by subjecting the
rolled sheet to the final heat treatment or the warm flattening
treatment described above. The press forming is preferably
performed in the temperature range of 200.degree. C. to 300.degree.
C. in order to increase the plastic formability of the rolled sheet
or the treated sheet, which is a workpiece. It is believed that
even if the press forming is performed at a temperature in the
temperature range of 250.degree. C. to 350.degree. C., the
problems, such as the coarsening of precipitations as described
above are less likely to occur because the holding time in the
temperature range of 250.degree. C. to 350.degree. C. in the press
forming is very short.
[0044] After the press forming, heat treatment may be performed to
eliminate strain and residual stress introduced by press forming
and to improve the mechanical properties. With respect to heat
treatment conditions, the heating temperature is in the range of
100.degree. C. to 400.degree. C., and the heating time is in the
range of about 5 minutes to about 60 minutes. Also in this heat
treatment, it is preferred that the holding time in the temperature
range of 250.degree. C. to 350.degree. C. is not long. Furthermore,
a formed product obtained by pressing may not be treated. However,
as described above, if treatment to form the covering layer for
corrosion prevention, protection, an ornament, or the like is
performed, the corrosion resistance, the commodity value, and so
forth are further enhanced.
Advantageous Effects of Invention
[0045] A formed product of an magnesium alloy of the present
invention and a magnesium alloy sheet of the present invention have
excellent impact resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0046] FIG. 1 illustrates schematic views of the textures of
magnesium alloy sheets, part (1) illustrating a sample in which the
total holding time in a temperature range of 250.degree. C. to
350.degree. C. in a rolling step is within 60 minutes, and part (2)
illustrating a sample in which the total holding time exceeds 60
minutes.
[0047] FIG. 2 illustrates graphs each showing the relationship
between a temperature mainly in a rolling step and a holding time
at the temperature in the production process of a magnesium alloy
sheet, part (1) illustrating the case where the total holding time
(total time) in the temperature range of 250.degree. C. to
350.degree. C. in the rolling step is within 60 minutes, and part
(2) illustrating the case where the total holding time (total time)
exceeds 60 minutes.
[0048] FIG. 3 is a schematic explanatory drawing illustrating a
dent test.
[0049] FIG. 4 illustrates graphs of the degree of the dent of a
magnesium alloy sheet after the dent test, part (1) illustrating
sample a, and part (2) illustrating sample d.
[0050] FIG. 5 is a graph showing the relationship between the
thickness of a sheet constituting a formed product of a magnesium
alloy and the depth of the dent.
DESCRIPTION OF EMBODIMENTS
[0051] Embodiments of the present invention will be described
below.
Test Example 1
[0052] A plurality of sheets composed of a magnesium alloy and
press-formed bodies obtained by press-forming these magnesium alloy
sheets were produced and examined for metal textures and impact
resistance.
[0053] A plurality of cast sheets (thickness: 4 mm) composed of a
magnesium alloy having a composition equivalent to that of the AZ91
alloy (Mg-9.0% Al-1.0% Zn (all units are percent by mass)) were
prepared by a twin-roll casting process. Each of the resulting cast
sheets were subjected to solution heat treatment at 400.degree. C.
for 24 hours. Cooling in the solution heat treatment was performed
by an air blast in such a manner that the cooling rate in the
temperature range of 350.degree. C. to 250.degree. C. was
0.1.degree. C./sec or more. The sheet material that had been
subjected to the solution heat treatment was rolled multiple times
under rolling conditions described below so as to have a thickness
of 0.6 mm. The resulting rolled sheets were subjected to final heat
treatment at 300.degree. C. for 10 minutes, thereby resulting in
magnesium alloy sheets.
(Rolling Conditions)
[0054] Working ratio (rolling reduction): 5% per pass to 40% per
pass Heating temperature of sheet: 200.degree. C. to 400.degree. C.
Roll temperature: 100.degree. C. to 250.degree. C.
[0055] In this test, for each pass in the rolling step, the heating
temperature of the sheets and the rolling speed (circumferential
speed of the roll) were adjusted to change the total holding time
that the sheet materials, which were workpieces subjected to
rolling, were held in the temperature range of 250.degree. C. to
350.degree. C., thereby preparing four types of samples in which
the total holding times were 20 minutes (sample a), 35 minutes
(sample b), 50 minutes (sample c), and 80 minutes (sample d).
[0056] The magnesium alloy sheets that had been subjected to the
final heat treatment were subjected to square cup deep-drawing
processing at a heating temperature of 250.degree. C., thereby
providing press-formed bodies. Each of the press-formed bodies had
a box shape including a rectangular top plate having dimensions of
48 mm.times.98 mm and side walls each extending upright from the
top plate.
[0057] For comparison, a commercially available AZ31 alloy material
(thickness: 0.6 mm) and aluminium alloy material (A5052 material,
thickness: 0.6 mm) were prepared. The AZ31 alloy material was
subjected to square cup deep-drawing processing under conditions
the same as those of the rolled sheets composed of the AZ91 alloy
described above. Similarly, the A5052 material was subjected to
square cup deep-drawing processing at room temperature.
[0058] The metal texture of each of the resulting magnesium alloy
sheets and the press-formed bodies was observed as described below,
and precipitations were studied. Furthermore, a dent test of each
of the resulting magnesium alloy sheets and the resulting
press-formed bodies was performed, and impact resistance was
evaluated.
<Magnesium Alloy Sheet>
<<Precipitations>>
[0059] Each of the resulting magnesium alloy sheets composed of the
AZ91 alloy was cut in the thickness direction. The resulting cross
section was observed with an optical microscope (1000.times.). In
the cross section, any two 100 .mu.m.times.100 .mu.m areas in the
surface area region were selected from a surface area region
extending from a surface of the sheet to a position one-third of
the thickness from the surface. These areas were defined as the
fields of observation. In each of the fields of observation, the
particle size of particles composed of an observed intermetallic
compound containing Al and Mg was measured. The number of particles
having a particle size of 5 .mu.m or more was counted.
<<Impact Resistance>>
[0060] The resulting magnesium alloy sheets composed of the AZ91
alloy and the prepared AZ31 alloy material and A5052 material
(aluminium alloy material) were cut into 30 mm.times.30 mm
specimens. In this test, as illustrated in FIG. 3, a support 20
having a horizontal surface with a round hole 21 having a diameter
d of 20 mm was prepared. The depth of the round hole 21 was set in
such a manner that a cylindrical bar 10 described below was able to
be sufficiently inserted therein. A specimen 1 was placed so as to
close the round hole 21. In this state, the cylindrical ceramic bar
10 having a weight of 100 g and a tip radius r of 5 mm was arranged
at a position 200 mm above the specimen 1 in such a manner that the
central axis of the bar was arranged coaxially with the central
axis of the round hole 21. After the cylindrical bar 10 was allowed
to free fall from the position toward the specimen 1, the depth of
the dent of the specimen 1 was measured. With respect to the depth
(mm) of the dent, a distance between a straight line that connected
both opposite sides of the specimen 1 and the most dented point was
measured with a point micrometer. Regarding each of samples a and
d, in a 30 mm.times.30 mm specimen, a straight line which was
parallel to a side having a length of 30 mm and which passed
through the most dented point was selected. The depth of the dent
was measured at each of a plurality of points on the straight line
as described above. FIG. 4 illustrates the results.
<Press-Formed Body>
<<Precipitations>>
[0061] In each of the resulting box-shaped press-formed bodies
composed of the AZ91 alloy, a flat portion that was not subjected
to drawing deformation, specifically, the top plate, was cut in the
thickness direction. The resulting cross section was observed in
the same way as the magnesium alloy sheet described above, and
fields of observation were set. In two fields of observation, the
number of particles which was composed of an intermetallic compound
containing Al and Mg and which had a particle size of 5 .mu.m or
more was counted.
<<Impact Resistance>>
[0062] In each of the resulting box-shaped press-formed bodies
composed of the AZ91 alloy and a press-formed body composed of the
AZ31 alloy and a press-formed body composed of A5052, which were
separately produced, a 30 mm.times.30 mm specimen was cut from a
flat portion that was not subjected to drawing deformation,
specifically, the top plate. As with the magnesium alloy sheet
described above, the depth (mm) of the dent was measured with the
jig illustrated in FIG. 3.
<<Thickness>>
[0063] In each of the resulting box-shaped press-formed bodies
composed of the AZ91 alloy, the thickness was measured at any four
points in the 30 mm.times.30 mm specimen cut from the top plate.
The results demonstrated that the thickness at any point was equal
to the thickness of the magnesium alloy sheet described above
(thickness of the specimen: 0.6 mm).
[0064] Table I shows the number of precipitations (number) and the
depth (mm) of the dent. Table I also shows the value x of the
expression x=0.47.times.t.sup.-1.25 for samples a to d. With
respect to the number of precipitations, Table I shows a smaller
number of precipitations in the two fields of observation.
TABLE-US-00001 TABLE I Alloy sheet Press-formed body Number of
Depth of Number of Depth of Value of intermetallic dent
intermetallic dent expression Sample compound (mm) compound (mm) x
= 0.47 .times. t.sup.-1.25 a(20 min) 0 0.68 0 0.68 0.890 b(35 min)
0 0.70 0 0.70 c(50 min) 0 0.73 0 0.73 d(80 min) 6 0.91 7 0.92 AZ31
-- 0.99 -- 1.00 -- A5052 -- 1.45 -- 1.48 --
[0065] It is found that the sheets and the press-formed bodies
composed of the magnesium alloy having an Al content of 7% by mass
or more has excellent impact resistance compared with the sheet and
the press-formed body composed of the AZ31 alloy having a low Al
content and the sheet and the press-formed body composed of the
aluminum alloy.
[0066] Observation of the metal texture demonstrated that in
samples a to d composed of the magnesium alloy having an Al content
of 7% by mass or more, a large number of precipitations of an
intermetallic compound (Mg.sub.17Al.sub.12) containing Al and Mg
were deposited. However, as shown in Table I, for each of samples a
to c in which the total holding time in the temperature range of
250.degree. C. to 350.degree. C. was within 1 hour (60 minutes) in
the rolling step, each of the magnesium alloy sheet and the
press-formed body did not have a coarse intermetallic compound but
had a texture in which a fine intermetallic compound was dispersed
as illustrated in part (1) of FIG. 1. It is found that in each of
samples a to c having a small number of coarse precipitations, the
depth of the dent is small; hence, samples a to c have excellent
impact resistance. It is also found that even in the case where the
final heat treatment is performed after the rolling, the holding
time is controlled in such a manner that the total of the holding
time in the temperature range of 250.degree. C. to 350.degree. C.
during the rolling step and the holding time in the temperature
range of 250.degree. C. to 350.degree. C. during the final heat
treatment after the rolling is within 1 hour, thereby resulting in
excellent impact resistance.
Test Example 2
[0067] Magnesium alloy sheets having different thicknesses and
press-formed bodies obtained by press-forming these magnesium alloy
sheets were produced and examined for metal textures and impact
resistance.
[0068] A plurality of cast sheets (each having a composition
equivalent to that of the AZ91 alloy and a thickness of 4 mm)
similar to those in Test Example 1 were prepared. Under the same
conditions as those in Test Example 1, the solution heat treatment
(400.degree. C. for 24 hours, the cooling rate from 350.degree. C.
to 250.degree. C.: 0.1.degree. C./sec or more) and multipass
rolling (rolling reduction: 5% per pass to 40% per pass, heating
temperature of the sheets: 200.degree. C. to 400.degree. C., and
roll temperature: 100.degree. C. to 250.degree. C.) were performed
to provide rolled sheets. As with Test Example 1, also in this
test, the total holding time that the sheet materials were held in
the temperature range of 250.degree. C. to 350.degree. C. in the
rolling step, was changed. Furthermore, in this test, the rolled
sheets having different thicknesses were produced by adjusting the
rolling reduction. The total time was set to 35 minutes or 80
minutes by adjusting the heating time of the sheets and the rolling
speed. Moreover, in this test, samples in which the total holding
times, including the time of the final heat treatment after the
rolling, in the foregoing temperature range were 45 minutes (sample
.alpha.) and 90 minutes (sample .beta.) were prepared.
[0069] The resulting rolled sheets were subjected to the final heat
treatment at 300.degree. C. for 10 minutes and then were subjected
to square cup deep-drawing processing at a heating temperature of
250.degree. C., thereby providing box-shaped press-formed bodies
similar to those in Test Example 1.
[0070] In each of the resulting magnesium alloy sheets and the
press-formed bodies that had been subjected to the final heat
treatment, the number of precipitations was measured by the
observation of the texture of a cross section as in Test Example 1.
Furthermore, similarly to Test Example 1, a specimen was formed,
and a dent test was performed to measure the depth of the dent.
Table II shows the results. In Table II, the results of samples
each having a thickness of 0.6 mm (0.6 mm) are those of Test
Example 1.
TABLE-US-00002 TABLE II Alloy sheet Press-formed body Number of
Depth of Number of Depth of Value of intermetallic dent
intermetallic dent expression Sample compound (number) (mm)
compound (number) (mm) x = 0.47 .times. t.sup.-1.25 0.5 mmt-.alpha.
0 0.98 0 0.99 1.118 0.5 mmt-.beta. 7 1.22 6 1.21 0.6 mmt-.alpha. 0
0.70 0 0.70 0.890 0.6 mmt-.beta. 6 0.91 7 0.92 0.8 mmt-.alpha. 0
0.51 0 0.52 0.621 0.8 mmt-.beta. 8 0.68 7 0.71
[0071] Table II shows that although the depth of the dent varies
depending on the thickness of the magnesium alloy sheet or the
press-formed body (top plate), sample .alpha. in which the total
holding time in the temperature range of 250.degree. C. to
350.degree. C. is within 60 minutes in the rolling step does not
have a coarse intermetallic compound having a particle size of 5
.mu.m or more in the surface area region (the number of the coarse
intermetallic compound is zero), regardless of the thickness, and
has a smaller depth of the dent than that of sample .beta. having
the same thickness.
[0072] For such a press-formed body having excellent impact
resistance, the relationship between the thickness t.sub.p of the
press-formed body (top plate) and the depth x of the dent was
studied. FIG. 5 illustrates the results. From a graph illustrated
in FIG. 5, the relationship between the thickness t.sub.p and the
depth x of the dent in sample .alpha. can be most simply
represented by x=k.times.t.sub.p.sup.-1 (where k represents a
coefficient). With respect to the coefficient k that distinguishes
between sample .alpha. and sample .beta., when values from 0 to 1
in 0.01 steps are substituted for k at the thicknesses t.sub.p
ranging from 0.5 to 0.8, the expression was evaluated. In this
case, it is believed that k is preferably 0.5 and the neighborhood
thereof. However, the coefficient k tends to vary slightly,
depending on the thickness. So, in view of the case of a thickness
t.sub.p of less than 0.5 mm or more than 0.8 mm, the relational
expression to distinguish between sample .alpha. and sample .beta.
was reviewed to the extent that sample .alpha. did not deviate from
the relational expression x=0.5.times.t.sup.-1 (i.e., k=0.5) as
much as possible. Specifically, with the coefficient k fixed to
0.5, values from -1 in 0.01 steps were substituted for the exponent
of the thickness t.sub.p to determine a preferred curve. Then the
coefficient k was again determined in the same way as above. As a
result, the expression was found to be x=0.47.times.t.sup.-1.25.
Thus, x.ltoreq.0.47.times.t.sub.p.sup.-1.25 is used as an index of
the formed product of the present invention. Furthermore, a
magnesium alloy sheet was similarly examined. For the magnesium
alloy sheet, x.ltoreq.0.47.times.t.sub.b.sup.-1.25 (where t.sub.b:
thickness) was also applicable. Thus,
x.ltoreq.0.47.times.t.sub.b.sup.-1.25 is used as an index of the
magnesium alloy sheet of the present invention.
Test Example 3
[0073] Magnesium alloy sheets produced by performing another
treatment after the rolling were prepared. The magnesium alloy
sheets were subjected to press forming to produce press-formed
bodies. They were examined for metal textures and impact
resistance.
[0074] In this test, a plurality of cast sheets (each having a
composition equivalent to that of the AZ91 alloy and a thickness of
4 mm) similar to those in Test Example 1 were prepared. Under the
same conditions as those in Test Example 1, the solution heat
treatment (400.degree. C. for 24 hours, the cooling rate from
350.degree. C. to 250.degree. C.: 0.1.degree. C./sec or more) was
performed. The sheet materials that had been subjected to the
solution heat treatment were subjected to multipass rolling
(rolling reduction: 5% per pass to 40% per pass, heating
temperature of the sheets: 200.degree. C. to 280.degree. C., and
roll temperature: 100.degree. C. to 250.degree. C.), thereby
providing rolled sheets. In this test, the total time that each of
the sheet materials was held in the temperature range of
250.degree. C. to 350.degree. C. in the rolling step was set to 45
minutes.
[0075] The resulting rolled sheets were subjected to warm
flattening treatment. Here, the warm flattening treatment is
performed with a roller leveler including a furnace capable of
heating a rolled sheet and a roller section that includes a
plurality of rollers configured to continuously impart a bend
(strain) to a heated rolled sheet. The roller section includes the
plural rollers which face each other and which are located above
and below in a staggered configuration.
[0076] In the roller leveler, each of the rolled sheets is
transferred to the roller section while being heated in the
furnace. Each time the sheet is passed between the upper rollers
and the lower rollers in the roller section, these rollers impart a
series of bends to the sheet. Here, the warm flattening was
performed in the temperature range of 220.degree. C. to 250.degree.
C. The transfer speed and so forth during the flattening was
adjusted in such a manner that the total time that the rolled sheet
was held in the temperature range of 250.degree. C. to 350.degree.
C. was within 60 minutes.
[0077] The magnesium alloy sheets that had been subjected to the
warm flattening treatment were subjected to square cup deep-drawing
processing at a heating temperature of 250.degree. C., thereby
providing box-shaped press-formed bodies similar to those in Test
Example 1.
[0078] In each of the resulting magnesium alloy sheets and the
press-formed bodies, the number of precipitations was measured by
the observation of the texture of a cross section as in Test
Example 1. Furthermore, similarly to Test Example 1, a specimen was
formed, and a dent test was performed to measure the depth of the
dent. Table III shows the results.
TABLE-US-00003 TABLE III Alloy sheet Press-formed body Number of
Depth of Number of Depth of Value of intermetallic dent
intermetallic dent expression Sample compound (number) (mm)
compound (number) (mm) x = 0.47 .times. t.sup.-1.25 3-1 0 0.68 0
0.68 0.890 2-1 0 0.70 0 0.70 (0.6 mmt-.alpha.)
[0079] Table III shows that any sample has a small depth of the
dent and excellent impact resistance. In particular, it is found
that sample 3-1, in which the magnesium alloy sheet that had been
subjected to the warm flattening treatment after the rolling was
used, has a small depth of the dent and excellent impact
resistance, compared with sample 2-1 (0.6 mmt-.alpha. in Test
Example 2), in which the final heat treatment was performed after
the rolling.
[0080] The foregoing embodiments may be appropriately changed
without departing from the scope of the present invention. The
present invention is not restricted to the foregoing
configurations. For example, the composition of the magnesium
alloy, the thickness of the magnesium alloy sheet, the shape of the
press-formed body, and so forth may be appropriately changed.
INDUSTRIAL APPLICABILITY
[0081] A formed product of a magnesium alloy of the present
invention can be suitably used for components of various electronic
devices, in particular, housings for mobile electronic devices. A
magnesium alloy sheet of the present invention can be suitably used
as a material for the formed product of the magnesium alloy of the
present invention.
REFERENCE SIGNS LIST
[0082] 1 specimen, 10 cylindrical bar, 20 support, 21 round hole,
d.sub.0, d.sub.1 intermetallic compound (precipitation)
* * * * *