U.S. patent application number 14/407260 was filed with the patent office on 2015-06-04 for magnesium alloy sheet and magnesium alloy structural member.
The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Hiroyuki Fujioka, Ryuichi Inoue, Nozomu Kawabe, Takahiko Kitamura, Nobuyuki Mori, Motonori Nakamura, Yukihiro Oishi, Mari Sogabe, Mitsutaka Tsubokura.
Application Number | 20150152527 14/407260 |
Document ID | / |
Family ID | 49758241 |
Filed Date | 2015-06-04 |
United States Patent
Application |
20150152527 |
Kind Code |
A1 |
Inoue; Ryuichi ; et
al. |
June 4, 2015 |
MAGNESIUM ALLOY SHEET AND MAGNESIUM ALLOY STRUCTURAL MEMBER
Abstract
Provided are a magnesium alloy sheet having excellent
formability in plastic forming, such as press forming, and a
magnesium alloy structural member. The magnesium alloy sheet is
obtained by subjecting a magnesium alloy to rolling and has a cross
section parallel to the thickness direction of the magnesium alloy
sheet, in which, when the length of the major axis and the length
of the minor axis of each of crystal grains in the cross section
are determined, an aspect ratio is defined as the ratio of the
length of the major axis to the length of the minor axis (length of
major axis/length of minor axis), and crystal grains having an
aspect ratio of 3.85 or more are defined as elongated grains, the
area fraction of the elongated grains in the cross section is 3% to
20%.
Inventors: |
Inoue; Ryuichi; (Itami-shi,
JP) ; Kitamura; Takahiko; (Itami-shi, JP) ;
Mori; Nobuyuki; (Itami-shi, JP) ; Oishi;
Yukihiro; (Osaka-shi, JP) ; Kawabe; Nozomu;
(Osaka-shi, JP) ; Nakamura; Motonori; (Itami-shi,
JP) ; Tsubokura; Mitsutaka; (Itami-shi, JP) ;
Fujioka; Hiroyuki; (Itami-shi, JP) ; Sogabe;
Mari; (Itami-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD. |
Osaka-shi, Osaka |
|
JP |
|
|
Family ID: |
49758241 |
Appl. No.: |
14/407260 |
Filed: |
June 11, 2013 |
PCT Filed: |
June 11, 2013 |
PCT NO: |
PCT/JP2013/066120 |
371 Date: |
December 11, 2014 |
Current U.S.
Class: |
420/407 ;
420/402 |
Current CPC
Class: |
C22C 23/02 20130101;
C22F 1/06 20130101; C22C 23/00 20130101; B21B 1/22 20130101 |
International
Class: |
C22C 23/02 20060101
C22C023/02; B21B 1/22 20060101 B21B001/22; C22C 23/00 20060101
C22C023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2012 |
JP |
2012-134228 |
Claims
1. A magnesium alloy sheet obtained by subjecting a magnesium alloy
to rolling, and having a cross section parallel to the thickness
direction of the magnesium alloy sheet in which the area fraction
of elongated grains in the cross section is 3% to 20%, where the
elongated grains are defined as crystal grains having an aspect
ratio of 3.85 or more in the cross section when the length of the
major axis and the length of the minor axis of each of crystal
grains in the cross section are determined, and the aspect ratio is
defined as the ratio of the length of the major axis to the length
of the minor axis.
2. The magnesium alloy sheet according to claim 1, wherein, when a
pole figure of (0001) planes of the elongated grains is taken,
crystal grains with the angle .theta..sub.TD in the sheet width
direction in the (0001) planes of the elongated grains within
5.degree. are selected, and the angle .theta..sub.RD in the rolling
direction in the (0001) planes of the selected crystal grains is
checked, the peak of the angle .theta..sub.RD in the rolling
direction is present at 9.degree. or more from the normal
direction.
3. The magnesium alloy sheet according to claim 1, wherein, when a
pole figure of (0001) planes of the elongated grains is taken,
crystal grains with the angle .theta..sub.RD in the rolling
direction in the (0001) planes of the elongated grains within
20.degree. are selected, and the angle .theta..sub.TO in the sheet
width direction in the (0001) planes of the selected crystal grains
is checked, the total area fraction of crystal grains with the
angle .theta..sub.TD in the sheet width direction from the normal
direction being -20.degree. or less and crystal grains with the
angle .theta..sub.TD in the sheet width direction from the normal
direction being +20.degree. or more is 20% to 70% relative to all
the elongated grains.
4. The magnesium alloy sheet according to claim 1, wherein the
average cross-sectional area of the elongated grains is 600
.mu.m.sup.2 or less.
5. The magnesium alloy sheet according to claim 1, wherein the
magnesium alloy contains 8.3% to 9.5% by mass of Al.
6. The magnesium alloy sheet according to claim 1, wherein the
cross-sectional area of each of the elongated grains is more than
25 .mu.m.sup.2 and 5,000 .mu.m.sup.2 or less.
7. A magnesium alloy structural member obtained by subjecting at
least part of the magnesium alloy sheet according to claim 1 to
press forming.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy sheet
obtained by rolling, and a magnesium alloy structural member made
from the magnesium alloy sheet. More particularly, the invention
relates to a magnesium alloy sheet having excellent plastic
formability.
BACKGROUND ART
[0002] Magnesium alloys, which are lightweight and have excellent
specific strength and specific rigidity, have been used as
materials constituting various structural members, such as housings
of mobile electric/electronic devices, e.g., cellular phones and
laptop computers, and parts of automobiles.
[0003] Magnesium alloys typically have a hexagonal close-packed
crystalline structure, in which slip planes at low temperatures,
such as at room temperature, are basal planes only. Therefore,
existing magnesium alloy structural members are typically cast
materials produced by a die casting process or thixomolding
process.
[0004] In recent years, as described in Patent Literatures 1 and 2,
a method has been studied in which a magnesium alloy is subjected
to rolling, and the resulting rolled sheet is subjected to plastic
forming, such as press forming. When rolling and plastic forming,
such as press forming, are performed on a magnesium alloy, plastic
formability can be enhanced by performing warm working in which the
material is heated and a processing jig, such as reduction rolls or
a press mold, is heated as described in Patent Literature 1.
CITATION LIST
Patent Literature
[0005] PTL 1: Japanese Unexamined Patent Application Publication
No. 2011-131274
[0006] PTL 2: Japanese Unexamined Patent Application Publication
No. 2005-298885
SUMMARY OF INVENTION
Technical Problem
[0007] In producing a plastic-formed structural member, such as a
press-formed body, composed of a magnesium alloy, there is a demand
for development of a magnesium alloy sheet having excellent plastic
formability. Furthermore, there is a demand for development of a
magnesium alloy sheet which enables production of a plastic-formed
structural member with excellent mechanical properties, such as
strength and impact resistance.
[0008] Patent Literature 1 discloses that by performing warm
rolling while controlling the temperatures of the material and
reduction rolls to specific values, it is possible to obtain a
rolled sheet having excellent plastic formability. In this rolled
sheet, a sufficient amount of processing strain is introduced by
rolling and coarsening of crystal grains is suppressed by the
temperature control described above. As a result, dynamic
recrystallization is caused during press forming, and excellent
plastic formability is exhibited. In general, mechanical properties
of a magnesium alloy depend on the size of crystal grains. As
crystal grains become finer, strength and elongation improve. In
the rolled sheet described above, coarsening of crystal grains is
suppressed, i.e., crystal grains are fine. Therefore, the rolled
sheet has excellent strength and elongation, and the press-formed
body obtained using the rolled sheet as a material also has
excellent strength and impact resistance.
[0009] However, when a magnesium alloy having a hexagonal
close-packed structure is subjected to rolling, the c-axis of the
crystal (the axis perpendicular to the (0001) plane which is the
basal plane) is oriented perpendicular to the rolled surface
(surface of the material formed by being brought into contact with
a reduction roll among surfaces of the material). That is, the
rolled sheet has a structure in which the (0001) plane is oriented
parallel to the rolled surface. Therefore, the rolled sheet has
anisotropy in plastic forming, is hard to bend in a given
direction, and has poor plastic formability. Consequently, there is
a demand for development of a magnesium alloy sheet in which
anisotropy in plastic forming is reduced.
[0010] Patent Literature 2 discloses a method for producing a
magnesium alloy sheet in which after warm rolling, both treatment
with a roll leveler and recrystallization heat treatment are
repeated successively a plurality of times. In the rolled sheet
obtained by this production method, the c-axis (the {0002} plane)
is inclined with respect to the rolled surface, and therefore, bend
forming or the like can be performed even at low temperatures. On
the other hand, the rolled sheet has poor mechanical properties (in
particular, strength and rigidity) and is easily deformed even at
room temperature, and dent deformation can be caused by impact,
such as dropping.
[0011] In addition, a magnesium alloy incorporated with about 10.5%
to 16% by mass of Li has a cubic crystalline structure, and
therefore, it can be subjected to press forming even at room
temperature. However, this magnesium alloy is easily deformed at
room temperature and has poor strength and impact resistance.
Furthermore, since this magnesium alloy contains a large amount of
Li, it has poor corrosion resistance.
[0012] Accordingly, it is an object of the present invention to
provide a magnesium alloy sheet which can constitute a magnesium
alloy structural member having excellent strength and impact
resistance and which has excellent plastic formability. It is
another object of the present invention to provide a magnesium
alloy structural member having excellent strength and impact
resistance.
Solution to Problem
[0013] A magnesium alloy sheet according to the present invention
is obtained by subjecting a magnesium alloy to rolling and has a
cross section parallel to the thickness direction of the magnesium
alloy sheet, in which, when the length of the major axis and the
length of the minor axis of each of crystal grains in the cross
section are determined, an aspect ratio is defined as the ratio of
the length of the major axis to the length of the minor axis, and
crystal grains having an aspect ratio of 3.85 or more are defined
as elongated grains, the area fraction of the elongated grains in
the cross section is 3% to 20%.
Advantageous Effects of Invention
[0014] The magnesium alloy sheet according to the present invention
has excellent plastic formability.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1(A) is an inverse pole figure orientation map (IPF
Map) of elongated grains by an SEM-EBSD technique in Sample No. 2,
FIG. 1(B) is a graph showing the relationship between the aspect
ratio and the occurrence frequency of crystal grains in Sample No.
2, and FIG. 1(C) is a pole figure of (0001) planes of elongated
grains in Sample No. 2.
[0016] FIG. 2(A) is an angle graph in the rolling direction (RD
direction) of crystal grains with the angle in the sheet width
direction from the normal direction being within 5.degree.
regarding (0001) planes of elongated grains in Sample No. 2, and
FIG. 2(B) is an angle graph in the sheet width direction (TD
direction) of crystal grains with the angle in the rolling
direction from the normal direction being within 20.degree.
regarding (0001) planes of elongated grains in Sample No. 2.
[0017] FIG. 3(A) is a view used for explaining selection of
elongated grains, and FIG. 3(B) is a view used for explaining
selection of crystal grains with a specific angle from the pole
figure of (0001) planes.
[0018] FIG. 4(A) is an inverse pole figure orientation map (IPF
Map) of elongated grains by an SEM-EBSD technique in Sample No. 3,
FIG. 4(B) is a graph showing the relationship between the aspect
ratio and the occurrence frequency of crystal grains in Sample No.
3, and FIG. 4(C) is a pole figure of (0001) planes of elongated
grains in Sample No. 3.
[0019] FIG. 5(A) is an angle graph in the rolling direction (RD
direction) of crystal grains with the angle in the sheet width
direction from the normal direction being within 5.degree.
regarding (0001) planes of elongated grains in Sample No. 3, and
FIG. 5(B) is an angle graph in the sheet width direction (TD
direction) of crystal grains with the angle in the rolling
direction from the normal direction being within 20.degree.
regarding (0001) planes of elongated grains in Sample No. 3.
[0020] FIG. 6(A) is an inverse pole figure orientation map (IPF
Map) of elongated grains by an SEM-EBSD technique in Sample No. 4,
FIG. 6(B) is a graph showing the relationship between the aspect
ratio and the occurrence frequency of crystal grains in Sample No.
4, and FIG. 6(C) is a pole figure of (0001) planes of elongated
grains in Sample No. 4.
[0021] FIG. 7(A) is an angle graph in the rolling direction (RD
direction) of crystal grains with the angle in the sheet width
direction from the normal direction being within 5.degree.
regarding (0001) planes of elongated grains in Sample No. 4, and
FIG. 7(B) is an angle graph in the sheet width direction (TD
direction) of crystal grains with the angle in the rolling
direction from the normal direction being within 20.degree.
regarding (0001) planes of elongated grains in Sample No. 4.
DESCRIPTION OF EMBODIMENTS
Description of Embodiments of Present Invention
[0022] The present inventors produced rolled sheets composed of a
magnesium alloy under various conditions, and the rolled sheets, as
material sheets, were subjected to press forming to examine
formability. As a result, it has been found that a material sheet
in which breakage, surface roughening, and the like are unlikely to
occur even when subjected to high deformation and from which a
formed body having excellent surface texture is obtained has a
specific structure. Furthermore, it has been found that the
resulting formed body has excellent strength and impact resistance.
Moreover, it has been found that the material sheet can be produced
by using a material for rolling having a specific structure and by
subjecting the material for rolling to warm rolling under specific
conditions. The present invention has been achieved on the basis of
the findings described above. First, embodiments of the present
invention will be enumerated and described.
[0023] (1) A magnesium alloy sheet according to an embodiment of
the present invention is obtained by subjecting a magnesium alloy
to rolling and has a cross section parallel to the thickness
direction of the magnesium alloy sheet, in which the area fraction
of elongated grains in the cross section is 3% to 20%. The
elongated grains are defined as crystal grains having an aspect
ratio of 3.85 or more in the cross section when the length of the
major axis and the length of the minor axis of each of crystal
grains in the cross section are determined, and the aspect ratio is
defined as the ratio of the length of the major axis to the length
of the minor axis (length of the major axis/length of the minor
axis).
[0024] The structure in which the long crystal grains with a
specific size (elongated grains) are present in the specific range
can be considered as a structure in which the orientation is
disturbed to a certain extent. In the magnesium alloy sheet
according to the embodiment having such a specific structure,
anisotropy in plastic forming can be reduced and excellent
formability is exhibited compared with the structure in which all
crystal grains are oriented in a certain direction. Furthermore, in
the magnesium alloy sheet according to the embodiment, crystal
grains other than the elongated grains are fine because they have
been subjected to rolling and constitute a structure having certain
orientation (structure in which the c-axis is oriented
perpendicular to the rolled surface). Therefore, in the magnesium
alloy sheet according to the embodiment, it is possible to suppress
a decrease in strength due to the presence of the elongated grains,
and the fine, oriented structure allows high strength and
elongation. Thus, the magnesium alloy sheet has excellent strength,
elongation, and impact resistance. Note that a typical example of
the cross section in which the area fraction of elongated grains is
3% to 20% is a cross section parallel to the rolling direction.
[0025] (2) In an example of the magnesium alloy sheet according to
the embodiment, when a pole figure of (0001) planes of the
elongated grains is taken, crystal grains with the angle
.theta..sub.TD in the sheet width direction in the (0001) planes of
the elongated grains within 5.degree. are selected, and the angle
.theta..sub.RD in the rolling direction in the (0001) planes of the
selected crystal grains is checked, the peak of the angle
.theta..sub.RD in the rolling direction is present at 9.degree. or
more from the normal direction.
[0026] To put it plainly, in the example described above, there are
many crystal grains with the (0001) plane inclined to the rolling
direction (hereinafter, referred to as RD-inclined elongated
grains). That is, the example has a structure which includes
crystal grains oriented in different directions (RD-inclined
elongated grains) and which is random to some extent. Therefore,
anisotropy in plastic forming can be sufficiently reduced and
excellent plastic formability is exhibited compared with the
structure which is substantially composed of only crystal grains
with the c-axis oriented perpendicular to the rolled surface.
[0027] (3) In an example of the magnesium alloy sheet according to
the embodiment, when a pole figure of (0001) planes of the
elongated grains is taken, crystal grains with the angle
.theta..sub.RD in the rolling direction in the (0001) planes of the
elongated grains within 20.degree. are selected, and the angle
.theta..sub.TD in the sheet width direction in the (0001) planes of
the selected crystal grains is checked, the total area fraction of
crystal grains with the angle .theta..sub.TD in the sheet width
direction from the normal direction being -20.degree. or less and
crystal grains with the angle .theta..sub.TD in the sheet width
direction from the normal direction being +20.degree. or more is
20% to 70% relative to all the elongated grains.
[0028] To put it plainly, the example described above includes
crystal grains with the (0001) plane largely inclined to the sheet
width direction (hereinafter, referred to as TD-inclined elongated
grains) in a specific range. That is, the example has a structure
which includes crystal grains oriented in different directions
(TD-inclined elongated grains) and which is random to some extent.
Therefore, anisotropy in plastic forming can be sufficiently
reduced and excellent plastic formability is exhibited compared
with the structure which is substantially composed of only crystal
grains with the c-axis oriented perpendicular to the rolled
surface. Moreover, in the example, since the content of TD-inclined
elongated grains is in a specific range, degradation in mechanical
properties due to the presence of TD-inclined elongated grains is
suppressed, and excellent strength and impact resistance are
exhibited.
[0029] (4) In an example of the magnesium alloy sheet according to
the embodiment, the average cross-sectional area of the elongated
grains is 600 .mu.m.sup.2 or less.
[0030] In the example described above, the elongated grains are
small and unlikely to act as starting points for breakage during
plastic forming, and therefore, excellent plastic formability is
exhibited.
[0031] (5) In an example of the magnesium alloy sheet according to
the embodiment, the magnesium alloy contains 8.3% to 9.5% by mass
of aluminum (Al).
[0032] The magnesium alloy containing Al in the specific range
described above (hereinafter, referred to as the high Al-content
magnesium alloy) has excellent mechanical properties (in
particular, strength) and corrosion resistance. Accordingly, the
embodiment has excellent plastic formability because of the
specific structure including the elongated grains and also has
excellent mechanical properties (in particular, strength) and
corrosion resistance because of the specific composition.
[0033] (6) In an example of the magnesium alloy sheet according to
the embodiment, the cross-sectional area of each of the elongated
grains is more than 25 .mu.m.sup.2 and 5,000 .mu.m.sup.2 or
less.
[0034] In the example, since each of the elongated grains is small
and unlikely to act as a starting point for breakage, excellent
plastic formability is exhibited.
[0035] (7) A magnesium alloy structural member according to an
embodiment of the present invention is obtained by subjecting at
least part of the magnesium alloy sheet according to the embodiment
to press forming.
[0036] The magnesium alloy structural member according to the
embodiment is produced using, as a material, the magnesium alloy
sheet according to the embodiment having excellent plastic
formability, and therefore has high productivity and high shape
accuracy and dimensional accuracy. Furthermore, the magnesium alloy
structural member according to the embodiment is composed of the
magnesium alloy sheet having excellent mechanical properties, such
as strength and elongation, and therefore has excellent mechanical
properties, such as strength, rigidity, and impact resistance.
Details of Embodiments of Present Invention
[0037] The magnesium alloy sheet according to the embodiment and
the magnesium alloy structural member according to the embodiment
will be described in detail below.
[0038] [Magnesium Alloy Sheet]
[0039] (Composition)
[0040] The magnesium alloy sheet according to the embodiment and
the magnesium alloy structural member according to the embodiment
are each composed of any of magnesium alloys having various
compositions in which various additive elements are added to Mg
(balance: Mg and impurities, Mg: 50% by mass or more).
[0041] Examples of the additive element include at least one
element selected from the group consisting of Al, Zn, Mn, Si, Be,
Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au, and rare-earth elements
(excluding Y and Ce). In particular, a Mg--Al-based alloy
containing Al has excellent strength, rigidity, impact resistance,
and the like and also has excellent corrosion resistance. The Al
content is, for example, 0.1% by mass or more. As the Al content
increases, strength and corrosion resistance tend to become higher.
However, when the Al content exceeds 12% by mass, plastic
formability is degraded. Therefore, the Al content is preferably
12% by mass or less, and more preferably 11% by mass or less.
[0042] The content of the elements other than Al is, for example,
0.01% to 10% by mass, or 0.1% to 5% by mass. In particular, a
magnesium alloy containing 0.001% by mass or more in total,
preferably 0.1% to 5% by mass in total of at least one element
selected from the group consisting of Si, Sn, Y, Ce, Ca, and
rare-earth elements (excluding Y and Ce) has excellent heat
resistance and flame retardance. Examples of the impurities in the
magnesium alloy include Fe.
[0043] More specifically, 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), AS-based alloys (Mg--Al--Si-based alloys, Si: 0.2%
to 6.0% by mass), AX-based alloys (Mg--Al--Ca-based alloys, Ca:
0.2% to 6.0% by mass), and AJ-based alloys (Mg--Al--Sr-based
alloys, Sr: 0.2% to 7.0% by mass) specified in the standards of
American Society for Testing and Materials (ASTM). Other examples
include Mg--Al-RE-based alloys (RE: rare-earth element, RE: 0.001%
to 5% by mass, preferably 0.1% by mass or more).
[0044] Among Mg--Al-based alloys, alloys containing more than 7.2%
by mass of Al, in particular, alloys containing 8.3% to 9.5% by
mass of Al have more excellent mechanical properties, such as
strength and impact resistance and corrosion resistance, which is
preferable. Specific examples of the composition include AZ91
alloys and AZX911 alloys containing, in addition to Al, 0.5% to
1.5% by mass of Zn.
[0045] (Shape)
[0046] The magnesium alloy sheet according to the embodiment is
typically a rectangular sheet having a rectangular planar shape. By
appropriately cutting or punching, a sheet having a desired planar
shape, such as a circular, elliptic, or polygonal shape, can be
obtained. Furthermore, it is also possible to obtain a coil by
spirally winding a long rectangular sheet.
[0047] (Thickness, Width, and Length)
[0048] The magnesium alloy sheet according to the embodiment
typically has a uniform thickness overall. The thickness can be
selected appropriately. When the magnesium alloy sheet is used as a
material for a plastic-formed structural member, the plastic-formed
structural member has a thickness substantially equal to the
thickness of the material sheet. Therefore, as the thickness of the
magnesium alloy sheet is decreased, it is possible to achieve
reduction in thickness, size, and weight of the plastic-formed
structural member. Specifically, for example, the thickness is 0.1
mm or more and 2.5 mm or less, or 2 mm or less, in particular, 1.5
mm or less. Particularly, a thickness of 0.3 to 1.2 mm is easy to
use. Furthermore, the magnesium alloy sheet may have portions that
have partially different thicknesses, such as through-holes,
recesses, and protrusions.
[0049] The width and length (maximum distance between two points on
the outline in the case of an irregular-shaped sheet, such as a
circular sheet, elliptical sheet, or polygonal sheet) of the
magnesium alloy sheet can be appropriately selected. For example,
in the case of a rectangular sheet, when the rectangular sheet is a
wide sheet with a width of 100 mm or more, or 200 mm or more, in
particular, 250 mm or more, and is used as a material for a
plastic-formed structural member, it is possible to produce
plastic-formed structural members having various sizes from
small-sized structural members, such as parts of mobile devices, to
large-sized structural members, such as parts of transportation
apparatuses. Furthermore, for example, in the case of a rectangular
sheet, when the rectangular sheet is a long sheet with a length of
50 m or more, 100 m or more, 200 m or more, or 400 m or more, and
is used as a material for a plastic-formed structural member, the
material can be continuously supplied to a plastic forming device,
and the plastic-formed structural member can be mass-produced. When
a coil is formed by spirally winding such a long sheet, transport
and supply to a plastic forming device are facilitated.
[0050] (Form)
[0051] The magnesium alloy sheet according to the embodiment has at
least been subjected to rolling. Specific examples thereof include
a rolled sheet (as rolled) and a treated sheet which has been
subjected to treatment described below after rolling. Examples of
the treatment include heat treatment (annealing) for removing
strain introduced during rolling, polishing, straightening,
anticorrosion treatment, such as chemical conversion treatment or
anodic oxidation treatment, coating, hairline finish, and
decorative treatment, such as diamond cutting and etching. By
carrying out any of these treatments in a temperature range lower
than the recrystallization temperature of the alloy constituting
the magnesium alloy sheet, the treated sheet substantially
maintains the structure immediately after rolling (specific
structure including elongated grains).
[0052] (Mechanical Properties)
[0053] The magnesium alloy sheet according to the embodiment has
the specific structure, which will be described later, and has been
subjected to rolling, and therefore has excellent mechanical
properties compared with the cast sheet composed of a magnesium
alloy having the same composition. Although depending on the
composition, for example, in the case where a magnesium alloy sheet
is composed of a high Al-content magnesium alloy, such as an AZ91
alloy, it is possible to obtain a magnesium alloy sheet having a
tensile strength of 270 to 450 MPa and a 0.2% proof stress of 220
to 350 MPa, and a magnesium alloy sheet having an elongation at
break of 1% to 15% (at room temperature for each). Note that being
composed of a high Al-content magnesium alloy and having a tensile
strength and a 0.2% proof stress satisfying the ranges described
above support that the sheet has been subjected to rolling.
[0054] (Structure)
[0055] The magnesium alloy sheet according to the embodiment
basically has a hexagonal close-packed crystalline structure and at
least one long crystal grain, which is referred to as the elongated
grain, is present therein. The magnesium alloy sheet has a
structure containing the elongated grains in a specific range
(specific area fraction).
[0056] The elongated grains are defined as crystal grains having an
aspect ratio of 3.85 or more when a cross section is taken parallel
to the thickness direction of the magnesium alloy sheet, the length
of the major axis and the length of the minor axis of each of
crystal grains in the cross section are determined, and the aspect
ratio is defined as the ratio, length of the major axis/length of
the minor axis. Detailed description will be made later on how to
take the cross section, a method for measuring the length of the
major axis and the length of the minor axis, and a method for
selecting elongated grains. The present inventors have studied and
found that the rolled sheet subjected to rolling under the specific
conditions described below has very many crystal grains having an
aspect ratio of about 1.4 to 3.4 and some crystal grains having a
high aspect ratio (20% or less in terms of area fraction). Since it
is believed that crystal grains that affect plastic formability are
long to some extent, in the magnesium alloy sheet according to the
embodiment, crystal grains having an aspect ratio of 3.85 or more
are defined as elongated grains. If crystal grains other than
elongated grains are small to some extend (preferably about 10
.mu.m or less in terms of average crystal grain size), the
elongated grains may have a high aspect ratio of, for example, 10
or more.
[0057] Elongated grains tend to be stretched in the rolling
direction (travelling direction of the material) as rolling
proceeds. Therefore, in order to properly select elongated grains,
it is considered to be appropriate to take a cross section of the
magnesium alloy sheet cut along a plane parallel to both the
thickness direction and the rolling direction (so-called
longitudinal section) and to measure the length of the minor axis
and the length of the major axis of each of crystal grains in the
cross section. In the case where it is possible to determine the
rolling direction of the magnesium alloy sheet, a cross section
parallel to both the thickness direction and the rolling direction
(i.e., longitudinal section) may be defined as the measurement
cross section. In the case where the magnesium alloy sheet is
wound, for example, in a coil shape, since the longitudinal
direction usually corresponds to the rolling direction, a cross
section parallel to the longitudinal direction may be defined as
the measurement cross section. In the case where the magnesium
alloy sheet is a rectangular sheet, a circular sheet, or the like
and it is not possible to determine the rolling direction, any
cross section parallel to the thickness direction is defined as the
measurement cross section, and presence or absence of a cross
section in which the area fraction of elongated grains having an
aspect ratio of 3.85 or more is 3% to 20% (which is, hereinafter,
referred to as the relevant cross section) is determined. In the
case where there is a relevant cross section, a direction parallel
to the relevant cross section is defined as the rolling direction
of the magnesium alloy sheet and a direction perpendicular to both
the rolling direction and the thickness direction is defined as the
sheet width direction.
[0058] The area fraction of elongated grains is obtained by summing
up the area of at least one elongated grain present in a given
field of view in the cross section and calculating the ratio of the
total area of elongated grains to the area of the field of view.
When the area fraction of elongated grains is 3% or more,
anisotropy in plastic forming is reduced by the elongated grains,
plastic formability can be enhanced, and for example, the limiting
drawing ratio can be increased. As the area fraction of elongated
grains increases, plastic formability tends to become better, and
it is possible to achieve an increase in the limiting drawing ratio
and suppression of occurrence of cracks. However, when there are
too many elongated grains, the elongated grains themselves may act
as starting points for breakage to cause cracks, and surface
roughening due to irregularities of the elongated grains may occur,
resulting in deterioration in the surface texture or reduction in
productivity. Therefore, the area fraction of elongated gains is
set at 20% or less. More preferably, the area fraction of elongated
gains is 5% to 15%.
[0059] When the elongated grains are excessively large, as
described above, breakage and surface roughening are likely to be
caused. Therefore, the average cross-sectional area of the
elongated grains is preferably 600 .mu.m.sup.2 or less. When the
elongated grains are excessively small, it becomes difficult to
obtain the effect of reducing anisotropy in plastic forming.
Therefore, it is believed to be preferable that the average
cross-sectional area of elongated grains be about 100 .mu.m.sup.2
or more. Furthermore, the cross-sectional area of each of the
elongated grain is preferably more than 25 .mu.m.sup.2 and 5,000
.mu.m.sup.2 or less. As the area per elongated grain becomes
smaller, breakage and surface roughening are more likely to be
suppressed. Thus, 5,000 .mu.m.sup.2 or less, or 4,800 .mu.m.sup.2
or less, in particular, 4,500 .mu.m.sup.2 or less is believed to be
preferable. When the area per elongated grain is excessively small,
it becomes difficult to obtain the effect of reducing anisotropy in
plastic forming. Therefore, more than 25 .mu.m.sup.2 or 30
.mu.m.sup.2 or more is believed to be preferable.
[0060] In the elongated grains, preferably, the (0001) plane, which
is the slip plane in the hexagonal close-packed magnesium alloy, is
not parallel, but is inclined with respect to the surface of the
magnesium alloy sheet (typically, the rolled surface formed by
being brought into contact with a reduction roll). Typically, the
(0001) plane is preferably inclined to at least one of the rolling
direction and the sheet width direction. In one example, when a
pole figure of (0001) planes of elongated grains is taken, and
regarding crystal grains with a small angle .theta..sub.TD in the
sheet width direction in the (0001) planes (crystal grains inclined
within 5.degree. from the normal direction), the peak of the angle
.theta..sub.RD in the rolling direction in the (0001) planes is
checked, the peak is present at a position deviated from the normal
direction, and specifically, the position of the peak is at
9.degree. or more. In this example, since there are many crystal
grains in which the (0001) planes of elongated grains are inclined
to the rolling direction, the effect of reducing anisotropy in
plastic forming due to the presence of the inclined crystal grains
can also be obtained, and more excellent plastic formability is
exhibited. The angle (absolute value) of the position of the peak
is preferably as large as possible within a range of 90.degree. or
less.
[0061] In another example, when a pole figure of (0001) planes of
elongated grains is taken, and regarding crystal grains with the
angle .theta..sub.RD in the rolling direction in the (0001) planes
in a specific range (crystal grains inclined within 20.degree. from
the normal direction), the angle .theta..sub.TD in the sheet width
direction in the (0001) planes is checked, the area fraction of
elongated grains with a large angle .theta..sub.TD in the sheet
width direction (elongated grains at 20.degree. or more from the
normal direction: TD-inclined elongated grains) is 20% to 70%.
TD-inclined elongated grains are crystal grains in which the (0001)
plane is particularly largely inclined to the sheet width
direction. Since such TD-inclined elongated grains are present in
an amount of 20% or more (in terms of area%) relative to all the
elongated grains, the effect of reducing anisotropy in plastic
forming due to the presence of the TD-inclined elongated grain can
also be obtained, and more excellent plastic formability is
exhibited. As the area percentage of the TD-inclined elongated
grains becomes larger, the effect of reducing anisotropy is more
easily obtained. However, strength, impact resistance, and the like
are decreased, and the surface texture is degraded. Therefore, 70%
or less (in terms of area%) is preferable. More preferably, the
area fraction of the TD-inclined elongated grains is 25% to
50%.
[0062] Crystal grains other than the elongated grains are each fine
and have a structure in which the (0001) plane is oriented parallel
to the rolled surface (structure in which the c-axis is oriented
perpendicular to the rolled surface). The average crystal grain
size of the crystal grains other than the elongated grains is, for
example, 1 to 10 .mu.m.
[0063] [Magnesium Alloy Structural Member]
[0064] The magnesium alloy structural member according to the
embodiment is a formed body obtained by subjecting at least part of
the magnesium alloy sheet according to the embodiment to plastic
forming (in particular, press forming). Examples thereof include a
structural member obtained by subjecting the entire magnesium alloy
sheet to plastic forming, e.g., a tubular structural member, and a
structural member obtained by subjecting only part of the magnesium
alloy sheet to plastic forming, e.g., an L-shaped structural member
or a structural member having a recessed cross section. A typical
example of plastic forming is warm working. The material
temperature during plastic forming is 350.degree. C. or lower,
preferably 300.degree. C. or lower, and in particular, 150.degree.
C. to 280.degree. C., or 150.degree. C. to 220.degree. C. In
plastic forming (secondary forming), such as press forming, the
period of time in which the material is held at the material
temperature described above is relatively short (typically, about
several seconds to several minutes, although depending on the
forming). Therefore, the magnesium alloy structural member
according to the embodiment after plastic forming substantially
maintains the composition and structure of the magnesium alloy
sheet according to the embodiment, and has excellent strength,
rigidity, and impact resistance as in the magnesium alloy sheet
according to the embodiment.
[0065] The magnesium alloy structural member according to the
embodiment may be at least partially subjected to polishing, anti
corrosion treatment, such as chemical conversion treatment or
anodic oxidation treatment, coating, hairline finish, or decorative
treatment, such as diamond cutting or etching. The magnesium alloy
structural member may have through-holes, recesses, protrusions, or
the like. Furthermore, the magnesium alloy structural member may be
joined with a resin formed body.
[0066] [Method For Producing Magnesium Alloy Sheet]
[0067] The magnesium alloy sheet according to the embodiment having
the specific structure described above can be produced, for
example, by a production method including the steps described
below.
[0068] Casting step: a step in which a magnesium alloy is subjected
to continuous casting to prepare a cast sheet.
[0069] Solution treatment step: a step in which the cast sheet is
subjected to solution treatment to produce a solution-treated
sheet.
[0070] Rolling step: a step in which the solution-treated sheet is
subjected to one or more passes of warm rolling.
[0071] In particular, the solution treatment is performed such that
the average crystal grain size is more than 15 .mu.m and less than
60 .mu.m after the solution treatment. The warm rolling is
performed at a preheating temperature of the material of
220.degree. C. to 280.degree. C., at a reduction roll temperature
of 200.degree. C. to 300.degree. C., and at a rolling reduction per
pass of 30% or less.
[0072] (Casting Step)
[0073] In the known art, as the cast material, which is a material
for casting, an ingot or a material obtained by cutting a thick
slab has been used (ingot in Patent Literature 2). In a continuous
casting process, since rapid solidification is possible, occurrence
of oxides, segregation, and the like can be reduced, and generation
of coarse impurities in crystal and precipitated impurities
exceeding 10 .mu.m can be suppressed. That is, it is possible to
reduce pieces of foreign matter which can be starting points for
breakage during rolling. It is also possible to reduce the average
crystal grain size to some extent. Therefore, formation of very
coarse elongated grains (for example, in which the area per grain
is more than 600 .mu.m.sup.2) and formation of excessive number of
elongated grains are likely to be suppressed. The average crystal
grain size of the cast sheet is preferably 15 to 50 .mu.m. The
cooling rate (casting velocity) is controlled such that the average
crystal grain size of the cast sheet is in the range described
above while taking into consideration the composition of the
magnesium alloy and the thickness of the cast sheet. In particular,
a twin-roll continuous casting process, by which a cast sheet
having excellent rigidity and thermal conductivity, little
segregation, and an excellent rolling property are likely to be
produced, is preferable. Furthermore, in the continuous casting
process, a long cast sheet can be easily produced. By using a long
cast sheet as the material for rolling, a long rolled sheet can be
produced, and productivity of the magnesium alloy sheet according
to the embodiment can be improved.
[0074] The thickness, width, and length of the cast sheet can be
appropriately selected. For example, when the thickness is 10 mm or
less, or 7 mm or less, in particular, 5 mm or less, grain
refinement by rapid cooling and suppression of segregation can be
achieved, and a cast sheet having high strength is likely to be
obtained. For example, by producing a long cast sheet with a length
of 30 m or more, or 50 m or more, in particular 100 m or more, or a
wide cast sheet with a width of 100 mm or more, or 200 mm or more,
in particular 250 mm or more, and using it as the material for
rolling, a long rolled sheet, or a wide rolled sheet can be
produced.
[0075] (Solution Treatment Step)
[0076] By subjecting the cast sheet to solution treatment, a
homogeneous composition can be obtained, improvement in mechanical
properties and rolling property due to solid solution of
precipitates can be achieved, and the size of crystal grains can be
controlled. The solution treatment is performed, for example, under
conditions of a heating temperature of 350.degree. C. to
420.degree. C. and a holding time of 1 to 15 hours. Since the
solution treatment is performed at a relatively high temperature as
described above, as the holding time is increased, crystal grains
become more likely to grow. Consequently, elongated grains are
likely to be formed, resulting in excessive formation of elongated
grains or formation of coarse elongated grains. Therefore, the
holding time in the solution treatment is set to be short. Although
depending on the composition and thickness of the cast sheet and
rolling conditions in the subsequent step, the holding time is more
preferably 2 to 12 hours.
[0077] The holding time is adjusted within the range described
above such that the average crystal grain size of the heat-treated
sheet after the solution treatment (solution-treated sheet) is more
than 15 .mu.m and less than 60 .mu.m. When the average crystal
grain size of the solution-treated sheet is 15 .mu.m or less,
crystal grains before rolling are excessively small, and elongated
grains are not sufficiently formed after rolling. When strain is
introduced by rolling, recrystallization can be caused by the
strain. When crystals before recrystallization are excessively
small, they do not grow sufficiently even if recrystallization is
performed, and it is believed that elongated grains are unlikely to
be formed. On the other hand, when the average crystal grain size
of the solution-treated sheet is more than 60 .mu.m, crystal grains
before rolling are excessively large, resulting in excessive
formation of elongated grains and formation of coarse elongated
grains. The reason for this is believed to be that, since crystals
before rolling are excessively large, strain due to rolling is
unlikely to be accumulated, recrystallization due to strain energy
does not sufficiently occur, and therefore, coarse crystals remain
as they are or coarse crystals are further stretched by rolling.
The average crystal grain size of the solution-treated sheet is
more preferably 20 to 50 .mu.m.
[0078] (Rolling Step)
[0079] By subjecting the solution-treated sheet to one or more
passes of rolling, it is possible to achieve improvement in
mechanical properties due to work hardening, improvement in
formability of secondary forming (plastic forming, such as press
forming) due to control of the crystalline structure, reduction in
the thickness of the sheet, and the like. In particular, at least
one pass of rolling is performed by warm rolling. The warm rolling
is performed under conditions of a preheating temperature of the
material of 220.degree. C. to 280.degree. C., a reduction roll
temperature of 200.degree. C. to 300.degree. C., and a rolling
reduction per pass of 30% or less. By performing warm rolling under
the specific conditions described above on the solution-treated
sheet with an average crystal grain size in the specific range, it
is possible to obtain the magnesium alloy sheet according to the
embodiment having a structure in which elongated grains are present
in the specific range in the structure composed of fine crystal
grains whose c-axis is oriented perpendicular to the rolled
surface. In addition, when warm rolling is performed under the
specific conditions described above, the following advantages can
be obtained: (1) plastic formability of the material is enhanced,
and edge cracking can be reduced; (2) the rolling reduction per
pass can be increased (e.g., 10% or more), and productivity can be
increased; (3) degradation in the surface texture due to burning or
the like can be suppressed; and (4) thermal degradation of
reduction rolls can be suppressed.
[0080] When heating (preheating) of the material is performed using
a heating furnace separately provided, the entire material is
likely to be heated uniformly. However, the temperature of the
material can be decreased while the material is transported and
brought into contact with reduction rolls. Therefore, preferably,
the transport distance or transport time may be adjusted, a
heat-insulating cover may be provided on the transport path, or the
temperature of the atmosphere may be controlled so that the
material temperature can be 180.degree. C. or higher immediately
before contact with reduction rolls.
[0081] Although warm rolling may be performed in all passes of
rolling, in the case where rolling with a small rolling reduction
is performed, for example, in finish rolling, cold rolling may be
performed.
[0082] A lubricant is preferably used in the rolling because
friction between the material and the reduction rolls is reduced
and rolling can be performed satisfactorily.
[0083] In the case where rolling is performed with multiple passes,
as described in Patent Literature 1, by performing reverse rolling,
a long rolled sheet can be produced with high productivity. In the
case where reverse rolling is performed, as described in Patent
Literature 1, a rolling system may be constructed, which includes a
reel for uncoiling the material, a reel for coiling the material,
and reduction rolls disposed between the two reels. By using this
system and reversing the two reels, reverse rolling with multiple
passes can be performed. When each reel is configured to be placed
in a heating furnace which preheats the material, a large amount of
the material can be preheated at one time, it is possible to
shorten the time until the preheated material is introduced between
a pair of reduction rolls arranged opposite each other, and a
decrease in the temperature of the material can be suppressed. In
the case where reverse rolling is performed, a coil spirally wound
is used as the material. Furthermore, in the case where reverse
rolling is performed, the resulting material is a coil obtained by
spirally winding a rolled sheet.
Experimental Example
[0084] Magnesium alloy sheets were produced under various
conditions, and the cross-sectional structures thereof were
examined. Furthermore, the resulting magnesium alloy sheets were
subjected to press forming, and plastic formability was
evaluated.
[0085] In this experiment, a molten metal of a magnesium alloy with
a composition corresponding to an AZ91 alloy (Mg-8.7%Al-0.65%Zn, in
terms of mass %) was prepared, a cast sheet with a thickness of 4
mm was continuously formed and coiled by a twin-roll casting
machine, and thereby, cast coils were produced. In this example,
the casting velocity was adjusted so that the average crystal grain
size was about 15 to 50 .mu.m. The resulting cast coils were placed
in a heating furnace (batch furnace) and subjected to solution
treatment to produce solution-treated sheets (solution-treated
coils). By varying the conditions for solution treatment, the
crystal gain size after the solution treatment was varied. The
heating temperature in the solution treatment was selected from the
range of 350.degree. C. to 420.degree. C., and the holding time was
varied. The holding time of Sample No. 100 was set to be shortest
(0.5 hours), and the holding time of Sample No. 200 was set to be
longest (100 hours). The holding time of Sample Nos. 1 to 4 was
selected from the range to 1 to 15 hours, and as the sample number
decreased, the holding time was shortened.
[0086] Regarding each of the solution-treated sheets obtained after
the solution treatment, the average crystal grain size was measured
by the method described below. The results thereof are shown in
Table. A specimen for embedding is cut out of each solution-treated
sheet so that a cross section parallel to the casting direction and
a cross section parallel to the sheet width direction can be
observed. The cut-out specimen for embedding is embedded in a resin
and subjected to mirror polishing and etching in that order. Then,
each cross section is observed with an optical microscope, and the
crystal grain size is measured by a line method. A micrograph is
taken, at an observation magnification of 100 times, of each of the
cross section in the casting direction and the cross section in the
sheet width direction. Three line segments corresponding to 1,500
.mu.m are drawn on the micrograph, and the number of crystal grains
present on each line segment is counted. The value obtained by
dividing the line segment length by the number of crystal grains
(line segment length/number of crystal grains) is defined as the
crystal grain size on the line segment. The average value of
crystal grain sizes of the three line segments in the cross section
along the casting direction and crystal grain sizes of the three
line segments in the cross section along the sheet width direction
is defined as the average crystal grain size.
[0087] The resulting solution-treated coils were uncoiled and
subjected to warm rolling with multiple passes, and thereby, rolled
sheets (rolled coils) were produced. Each of the rolled coils was
formed of a rolled sheet with a thickness of 0.8 mm, a width of 250
mm, and a length of 760 m (total rolling reduction: 80%). In this
example, a reverse rolling system was used, which included two
heating furnaces, each having a built-in reel, and reduction rolls
disposed between the two heating furnaces. The material was
preheated in the heating furnace for each pass, the material in a
heated state was supplied to the reduction rolls, and the
travelling direction of the material was changed by reversing the
reels. Thus, reverse rolling with multiple passes was performed.
For each sample, rolling was performed under conditions of a
rolling reduction per pass of 20% to 25%, a preheating temperature
of the material of 260.degree. C., and a reduction roll temperature
of 250.degree. C.
[0088] By appropriately cutting the resulting rolled coils, sheets
for structure observation were obtained. Each sheet is cut along a
plane parallel to both the thickness direction and the rolling
direction to take a longitudinal section. The longitudinal section
is observed with a field emission scanning electron microscope
(FE-SEM), and the observed image is analyzed and measured using an
electron backscatter diffraction (EBSD) technique. Specifically, in
a given field of view in the longitudinal section (in this example,
one field of view of 3.16.times.10.sup.5 .mu.m.sup.2 (0.316
mm.sup.2)), grain particles are identified by crystal grain
orientation, and the area of the crystal grain is obtained
regarding all the crystal grains in the field of view. Furthermore,
elliptical approximation is performed on the outline of each
crystal grain, and the length of the major axis (length in the
major axis direction): a and the length of the minor axis (length
in the minor axis direction: b are obtained. The elliptical
approximation is performed by a known method using formulae
described below.
[0089] The distance d.sub.ij between points x.sub.j and y.sub.j on
the ellipse is obtained by Formula 1 below. The maximum value of
the distance d.sub.ij is equal to the length in the major axis
direction: a in the ellipse. The angle .gamma. between the major
axis and the horizontal axis is obtained by Formula 2 below. In
Formula 2, x.sub.j.sup.max, y.sub.j.sup.max, x.sub.i.sup.max, and
y.sub.i.sup.max represent two coordinate points, which have the
maximum distance. The central coordinates of the ellipse are
expressed by Formula 3 and Formula 4. In Formula 3 and Formula 4,
x.sub.k and y.sub.k represent coordinate points of all the data
contained in the crystal grains. In order to obtain the length in
the minor axis direction: b, x.sub.k and y.sub.k are converted to
the basic coordinate system of the ellipse using Formula 5 and
Formula 6. Then, the length of the minor axis: b is obtained using
the average formula shown in Formula 7.
d ij = ( x i - x j ) 2 + ( y i + y j ) 2 Formula 1 .gamma. = tan -
1 y j ma x - y i ma x x j ma x - x i ma x Formula 2 x _ = x k
Formula 3 y _ = y k Formula 4 x i ' = ( x i - x _ ) cos .gamma. + (
y i - y _ ) sin .gamma. Formula 5 y i ' = ( x i - x _ ) sin .gamma.
+ ( y i - y _ ) cos .gamma. Formula 6 b = 1 N i = 0 N y ' i 2 ( 1 -
x ' i 2 / a 2 ) Formula 7 ##EQU00001##
[0090] Using the length of the major axis: a and the length of the
minor axis: b for each crystal grain, the aspect ratio (length of
the major axis/length of the minor axis) is obtained. Elongated
grains are selected from the field of view in the longitudinal
section on the basis of the aspect ratio. In this example, the
elongated grains were selected taking into account the area of
crystal grains, in addition to the aspect ratio. Specifically, the
average: S.sub.ave of areas of all the crystal grains and the
standard deviation: .sigma..sub.S of areas of all the crystal
grains were obtained, and S.sub.ave+3.sigma..sub.S was obtained as
the threshold of the area. Then, crystal grains in which the aspect
ratio was 3.85 or more and the area was equal to or more than the
threshold S.sub.ave+3.sigma..sub.S (crystal grains present in a
region surrounded by the dashed rectangular frame in FIG. 3(A))
were considered as elongated grains. It is believed that long
crystal grains can be more appropriately selected by selecting
elongated grains taking into account the area of crystal grains.
Note that, without taking into account the area, crystal grains
with an aspect ratio of 3.85 or more may be selected as elongated
grains. Furthermore, regarding the selected elongated grains, using
the area of each elongated grain, the average (average
cross-sectional area) was obtained. Furthermore, among the selected
elongated grains, the cross-sectional area of the grain having the
smallest area (minimum cross-sectional area) and the
cross-sectional area of the grain having the largest area (maximum
cross-sectional area) were obtained. The results thereof are shown
in Table.
[0091] FIGS. 1(A), 4(A), and 6(A) are each an inverse pole figure
orientation map of elongated grains (FIG. 1(A): Sample No. 2, FIG.
4(A): Sample No. 3, and FIG. 6(A): Sample No. 4). The color key for
crystal orientation images is shown below each map. FIGS. 1(B),
4(B), and 6(B) are each a graph showing the relationship between
the aspect ratio and the occurrence frequency of crystal grains
(FIG. 1(B): Sample No. 2, FIG. 4(B): Sample No. 3, and FIG. 6(B):
Sample No. 4).
[0092] Regarding the selected elongated grains, a pole figure of
(0001) planes of elongated grains was formed, in which, in the
sheet for structure observation, the thickness direction was
defined as the ND direction (normal direction), the rolling
direction was defined as the RD direction, and the sheet width
direction was defined as the TD direction. FIGS. 1(C), 4(C), and
6(C) are each a pole figure of (0001) planes of elongated grains
(FIG. 1(C): Sample No. 2, FIG. 4(C): Sample No. 3, and FIG. 6(C):
Sample No. 4).
[0093] In each of the resulting pole figures, crystal grains with
the angle .theta..sub.TD in the sheet width direction (TD
direction) in the (0001) planes of the elongated grains within
5.degree. are selected. Specifically, as shown in FIG. 3(B),
crystal grains in which the angle .theta..sub.TD in the TD
direction is in the range of -5.degree. to +5.degree. are selected.
Then, regarding the angle .theta..sub.RD in the rolling direction
(RD direction) in the (0001) planes of the selected crystal grains,
a graph is formed. FIGS. 2(A), 5(A), and 7(A) are each a graph
showing the occurrence frequency of the angle .theta..sub.RD in the
RD direction (FIG. 2(A): Sample No. 2, FIG. 5(A): Sample No. 3, and
FIG. 7(A): Sample No. 4). Using the resulting graphs, presence or
absence of a peak of the angle .theta..sub.RD in the RD direction
was checked, and the angle (inclination angle .theta..sub.P) from
the normal direction at the peak was checked. The results thereof
are shown in Table. In this example, since a plurality of peaks are
present, the maximum value (large) and the minimum value (small) of
the inclination angle .theta..sub.P (absolute value) are shown in
Table.
[0094] Furthermore, in each of the resulting pole figures, crystal
grains with the angle .theta..sub.RD in the rolling direction (RD
direction) in the (0001) planes of the elongated grains within
20.degree. are selected. Specifically, as shown in FIG. 3(B),
crystal grains in which the angle .theta..sub.RD in the RD
direction is in the range of -20.degree. to +20.degree. are
selected. Then, regarding the angle .theta..sub.TD in the TD
direction in the (0001) planes of the selected crystal grains, a
graph is formed. FIGS. 2(B), 5(B), and 7(B) are each a graph
showing the occurrence frequency of the angle .theta..sub.RD in the
TD direction (FIG. 2(B): Sample No. 2, FIG. 5(B): Sample No. 3, and
FIG. 7(B): Sample No. 4). Using the resulting graphs, the total
area fraction .SIGMA.S.sub.20 of crystal grains with the angle
.theta..sub.TD in the TD direction from the normal direction being
-20.degree. or less and crystal grains with the angle
.theta..sub.TD in the TD direction from the normal direction being
+20.degree. or more was checked. The total area was obtained by
integrating the hatched regions in each of FIGS. 2(B), 5(B), and
7(B). The results thereof are shown in Table.
[0095] The areas of crystal grains, elliptical approximation,
calculation of the length of the major axis, the length of the
minor axis, and the aspect ratio, selection of elongated grains,
formation of pole figures, the inclination angle .theta..sub.P at
the peak of the angle .theta..sub.RD in the RD direction, and the
total area fraction .SIGMA.S.sub.N of crystal grains can be easily
and automatically obtained using commercially available
mathematical software accompanying a commercially available
SEM-EBSD system. In this example, SUPRA35VP manufactured by Carl
Zeiss was used as the SEM, and OIM Analysis 5.31 manufactured by
EDAX-TSL was used as the software of EBSD.
[0096] The resulting rolled coils were subjected to straightening
and then surface polishing. The polished sheets were subjected to
press forming, and press formability was evaluated as plastic
formability. The straightening was performed, using a known roll
leveler (refer to Patent Literature 1), under a warm condition
(roll temperature: 250.degree. C.). Regarding the polishing, wet
polishing was performed using an abrasive belt (polishing amount:
about 30 .mu.m in total for both sides).
[0097] Press formability was evaluated in terms of (1) limiting
drawing ratio,(2) breakage caused by pressing, and (3) surface
roughness in press-formed portion. Press forming conditions are
shown below.
[0098] (1) Limiting drawing ratio: Using a cylindrical punch with a
diameter of 50 mm and a shoulder R of 2 mm, a cupping test is
conducted under a warm condition (250.degree. C.). As the materials
for drawing, by cutting the polished sheet described above into a
circular shape, circular sheets with various diameters D (mm) were
prepared. Then, the limiting drawing ratio (LRD) was checked. The
limiting drawing ratio is defined as the ratio, material diameter
Dmax/punch diameter d (50 mm in this example).
[0099] (2) Breakage caused by pressing: Using a prismatic punch
with a punch R of 0 mm, a right-angle bending test is conducted
under a warm condition (250.degree. C.). As the material for
bending, a rectangular sheet was prepared by cutting the polished
sheet to a predetermined length (length 200 mm). After the sheet
was bent at a right angle, presence or absence of cracks on the
outer peripheral surface of the bent portion was checked by visual
observation. The material with no breakage is evaluated to be good
(.largecircle.).
[0100] (3) Surface roughness in press-formed portion: In the
material subjected to the right-angle bending test described in
(2), surface roughness of the outer peripheral surface of the bent
portion was measured. The surface roughness, in terms of arithmetic
mean roughness Ra, was measured, using a commercially available
surface roughness tester, in accordance with JIS B 0601(2001)/ISO
4287(1997).
TABLE-US-00001 TABLE Solution- Rolled sheet Rolled sheet treated
Rolled Rolled TD: -20.degree. elongated grains Press Press- sheet
sheet area sheet peak or less Average Minimum Maximum formability
formed average fraction of inclination TD: +20.degree. cross-
cross- cross- evaluation Breakage portion Compre- crystal elongated
angle or more sectional sectional sectional limiting caused surface
hensive Sample grain size grains .theta. .sub.P(.degree. ) area
fraction area area area drawing by roughness evalu- No. (.mu.m) (%)
Small Large .SIGMA. S.sub.20 (%) (.mu.m.sup.2) (.mu.m.sup.2)
(.mu.m.sup.2) ratio pressing Ra (.mu.m) ation 100 15 1.5 10.5 14.7
16.8 59 25 245 2 .largecircle. 0.5 .DELTA. 1 20 3.1 10.1 15.8 20.5
101 30 504 2.3 .largecircle. 0.7 .largecircle. 2 30 8.2 9.9 15.9
26.7 167 55 724 2.5 .largecircle. 0.9 .circleincircle. 3 40 12.2
12.9 13.9 48.6 280 75 1370 2.3 .largecircle. 1.6 .largecircle. 4 50
19.7 10.9 14.9 64.0 575 139 4140 2.3 .largecircle. 1.9
.largecircle. 200 60 25.6 11.0 16.1 71.0 740 185 5430 2 Cracks 2.5
X occurred Obvious surface roughening
[0101] As shown in Table, in Sample Nos. 1 to 4 having a cross
section (cross section parallel to both the thickness direction and
the rolling direction in this example) in which a plurality of
elongated grains with an aspect ratio of 3.85 or more are present
and the area fraction of the elongated grains is 3% to 20%,
excellent formability of plastic forming, such as press forming, is
exhibited. In this example, in Sample Nos. 1 to 4, the limiting
drawing ratio is large at more than 2, high deformation is
possible, cracks are unlikely to occur even when subjected to
right-angle bending, the bent portion is smooth, and the surface
texture is excellent.
[0102] Such magnesium alloy sheets having excellent plastic
formability have a structure in which fine crystal grains with a
low aspect ratio and long crystal grains (elongated grains) are
mixed as shown in FIGS. 1(A), 1(B), 4(A), 4(B), 6(A), 6(B), and the
like. From this, it is believed that in Sample Nos. 1 to 4, since
irregular-shaped crystal grains are included in a specific range,
anisotropy in plastic forming is reduced and plastic formability is
enhanced compared with the case where a sheet is composed of fine
crystal grains with a uniform shape. Furthermore, in this example,
many elongated grains have the (0001) plane that is inclined to
both the rolling direction and the sheet width direction, that is,
are crystal grains whose c-axis is inclined to the rolled surface.
Specifically, the inclination angle .theta..sub.P is 9.degree. or
more (in this example, both the maximum value and the minimum value
of the inclination angle .theta..sub.P are 9.degree. or more), and
the total area fraction .SIGMA.S.sub.20 satisfies a range of 20% to
70%. It is believed that, in Sample Nos. 1 to 4, since such crystal
grains whose c-axis is inclined to the rolled surface are included
in a specific range, anisotropy in plastic forming can be further
reduced, and plastic formability can be further enhanced.
Furthermore, it is believed that, since the average cross-sectional
area of the elongated grains is 600 .mu.m.sup.2 or less, the
elongated grains are unlikely to act as starting points for
breakage, and plastic formability can be enhanced. Furthermore, it
is believed that, since the cross-sectional area of each of the
elongated grains is more than 25 .mu.m.sup.2 and 5,000 .mu.m.sup.2
or less, the elongated grains are unlikely to act as starting
points for breakage, and plastic formability can be enhanced.
[0103] Test pieces were made from the resulting polished sheets,
and using a commercially available tensile tester, tensile strength
(room temperature) and 0.2% proof stress (room temperature) were
measured. As a result, Sample Nos. 1 to 4 each had a tensile
strength of 270 MPa or more and a 0.2% proof stress of 220 MPa or
more, indicating high strength. The reason for such results is
believed to be that, since the content of elongated grains whose
c-axis is oriented non-parallel to the rolled surface is within the
specific range, and substantially all of crystal grains other than
the elongated grains are fine with the c-axis being oriented
perpendicular to the rolled surface, high strength can be
maintained. Furthermore, by using such a magnesium alloy sheet
having high strength as the material, it is anticipated that the
magnesium alloy structural member subjected to press forming will
have high strength and excellent impact resistance and will be
unlikely to be dented.
[0104] The magnesium alloy sheet having excellent plastic
formability as described above can be produced by subjecting a
continuously cast material to solution treatment, setting the
crystal grain size after the solution treatment within a specific
range, and controlling the preheating temperature of the material
and the reduction roll temperature during rolling to specific
ranges.
[0105] It is to be understood that the present invention is not
limited to the embodiments described above, but the embodiments can
be appropriately changed within a range not departing from the gist
of the present invention. For example, the composition of the
magnesium alloy, the thickness, width, and length of the sheet,
production conditions (solution treatment temperature/holding time,
the rolling reduction per pass, the material temperature and
reduction roll temperature during rolling, and the total rolling
reduction), and the like can be appropriately changed.
INDUSTRIAL APPLICABILITY
[0106] The magnesium alloy sheet according to the present invention
can be suitably used as the material for magnesium alloy structural
members which have been subjected to various types of plastic
forming, such as press forming, e.g., bending, drawing, and
shearing, forging, and upsetting. The magnesium alloy structural
member according to the present invention can be suitably used for
structural members constituting various electric/electronic devices
(more specifically, housings and reinforcing members of mobile and
small electric/electronic devices, and the like), structural
members constituting transportation apparatuses, such as
automobiles and aircraft, exterior structural members, such as
various housings and covers, framework parts, bags, and the
like.
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