U.S. patent application number 14/001661 was filed with the patent office on 2013-12-19 for magnesium alloy material and method for producing the same.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. The applicant listed for this patent is Ryuichi Inoue, Nozomu Kawabe, Nobuyuki Mori, Yukihiro Oishi. Invention is credited to Ryuichi Inoue, Nozomu Kawabe, Nobuyuki Mori, Yukihiro Oishi.
Application Number | 20130333809 14/001661 |
Document ID | / |
Family ID | 46720965 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130333809 |
Kind Code |
A1 |
Oishi; Yukihiro ; et
al. |
December 19, 2013 |
MAGNESIUM ALLOY MATERIAL AND METHOD FOR PRODUCING THE SAME
Abstract
There are provided a magnesium alloy material and a method for
producing the magnesium alloy material. In a magnesium (Mg) alloy
material (e.g., Mg alloy sheet) having a sheet-shaped portion with
a thickness of 1.5 mm or more, when a region having 1/4 the
thickness of the sheet-shaped portion in a thickness direction from
a surface of the sheet-shaped portion is defined as a surface
region and a remaining region is defined as an internal region, the
ratio O.sub.F/O.sub.C of the basal plane peak ratio O.sub.F in the
surface region to the basal plane peak ratio O.sub.C (degree of
orientation of (002) planes) in the internal region satisfies
1.05<O.sub.F/O.sub.C. A sheet-shaped Mg alloy material is
obtained by performing rolling on a twin-roll continuous cast
material with multiple passes at a reduction ratio of each pass of
25% or less.
Inventors: |
Oishi; Yukihiro; (Osaka-shi,
JP) ; Mori; Nobuyuki; (Osaka-shi, JP) ; Inoue;
Ryuichi; (Osaka-shi, JP) ; Kawabe; Nozomu;
(Osaka-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Oishi; Yukihiro
Mori; Nobuyuki
Inoue; Ryuichi
Kawabe; Nozomu |
Osaka-shi
Osaka-shi
Osaka-shi
Osaka-shi |
|
JP
JP
JP
JP |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka-shi, Osaka
JP
|
Family ID: |
46720965 |
Appl. No.: |
14/001661 |
Filed: |
February 23, 2012 |
PCT Filed: |
February 23, 2012 |
PCT NO: |
PCT/JP2012/054419 |
371 Date: |
August 26, 2013 |
Current U.S.
Class: |
148/420 ;
164/476 |
Current CPC
Class: |
B22D 11/0622 20130101;
C22C 23/02 20130101; B22D 11/001 20130101; C22C 23/00 20130101;
C22F 1/06 20130101 |
Class at
Publication: |
148/420 ;
164/476 |
International
Class: |
C22C 23/02 20060101
C22C023/02; C22C 23/00 20060101 C22C023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2011 |
JP |
2011-038889 |
Claims
1. A magnesium alloy material composed of a magnesium alloy,
comprising a sheet-shaped portion, wherein the sheet-shaped portion
has a thickness of 1.5 mm or more, and the sheet-shaped portion
satisfies the following orientation, [Orientation] when a region
having 1/4 the thickness of the sheet-shaped portion in a thickness
direction from a surface of the sheet-shaped portion is defined as
a surface region and a remaining region is defined as an internal
region, X-ray diffraction peak intensities of a (002) plane, a
(100) plane, a (101) plane, a (102) plane, a (110) plane, and a
(103) plane in the surface region are respectively defined as
I.sub.F(002), I.sub.F(100), I.sub.F(101), I.sub.F(102),
I.sub.F(110), and I.sub.F(103), X-ray diffraction peak intensities
of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a
(110) plane, and a (103) plane in the internal region are
respectively defined as I.sub.C(002), I.sub.C(100), I.sub.C(101),
I.sub.C(102), I.sub.C(110), and I.sub.C(103), a degree of
orientation of the (002) plane in the surface region, which is
expressed as
I.sub.F(002)/{I.sub.F(100)+I.sub.F(002)+I.sub.F(101)+I.sub.F(102)+I.sub.F-
(110)+I.sub.F(103)}, is defined as a basal plane peak ratio
O.sub.F, and a degree of orientation of the (002) plane in the
internal region, which is expressed as
I.sub.C(002)/{I.sub.C(100)+I.sub.C(002)+I.sub.C(101)+I.sub.C(102)+I.sub.C-
(110)+I.sub.C(103)}, is defined as a basal plane peak ratio
O.sub.C, a ratio O.sub.F/O.sub.C of the basal plane peak ratio
O.sub.F in the surface region to the basal plane peak ratio O.sub.C
in the internal region satisfies 1.05<O.sub.F/O.sub.C.
2. The magnesium alloy material according to claim 1, wherein, when
an average crystal grain size in the surface region is defined as
D.sub.F and an average crystal grain size in the internal region is
defined as D.sub.C, a ratio D.sub.C/D.sub.F of the average crystal
grain size D.sub.C in the internal region to the average crystal
grain size D.sub.F in the surface region satisfies
1.5<D.sub.C/D.sub.F.
3. The magnesium alloy material according to claim 1, wherein, when
a Vickers hardness (Hv) in the surface region is defined as H.sub.F
and a Vickers hardness (Hv) in the internal region is defined as
H.sub.C, a ratio H.sub.C/H.sub.F of the Vickers hardness H.sub.C in
the internal region to the Vickers hardness H.sub.F in the surface
region satisfies H.sub.C/H.sub.F<0.85.
4. The magnesium alloy material according to claim 1, wherein the
magnesium alloy contains Al as an additive element in a content of
5.0 mass % or more and 12 mass % or less.
5. A method for producing a magnesium alloy material by rolling a
raw material composed of a magnesium alloy, the method comprising:
a preparation step of preparing a sheet-shaped raw material
obtained by subjecting a molten magnesium alloy to continuous
casting by a twin-roll casting process; and a rolling step of
rolling the raw material with multiple passes to produce a
sheet-shaped magnesium alloy material having a thickness of 1.5 mm
or more, wherein a reduction ratio of each of the passes is 25% or
less.
6. The magnesium alloy material according to claim 2, wherein, when
a Vickers hardness (Hv) in the surface region is defined as H.sub.F
and a Vickers hardness (Hv) in the internal region is defined as
H.sub.C, a ratio H.sub.C/H.sub.F of the Vickers hardness H.sub.C in
the internal region to the Vickers hardness H.sub.F in the surface
region satisfies H.sub.C/H.sub.F<0.85.
7. The magnesium alloy material according to claim 2, wherein the
magnesium alloy contains Al as an additive element in a content of
5.0 mass % or more and 12 mass % or less.
8. The magnesium alloy material according to claim 3, wherein the
magnesium alloy contains Al as an additive element in a content of
5.0 mass % or more and 12 mass % or less.
9. The magnesium alloy material according to claim 4, wherein the
magnesium alloy contains Al as an additive element in a content of
5.0 mass % or more and 12 mass % or less.
10. The magnesium alloy material according to claim 6, wherein the
magnesium alloy contains Al as an additive element in a content of
5.0 mass % or more and 12 mass % or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy material
that is suitable for various members such as parts of transport
machines, e.g., automobiles, railway vehicles, and airplanes, parts
of bicycles, housings of electric and electronic devices, and other
structural members and that is also suitable for constituent
materials for the members, and to a method for producing the
magnesium alloy material. In particular, the present invention
relates to a thick magnesium alloy material having high corrosion
resistance and surface roughening resistance.
BACKGROUND ART
[0002] Lightweight magnesium alloys having high specific strength
and specific rigidity have been investigated as constituent
materials for various members such as housings of mobile electric
and electronic devices, e.g., cellular phones and laptop computers,
parts of automobiles, e.g., wheel covers and paddle shifts, parts
of railway vehicles, and parts of bicycles, e.g., frames. Members
composed of a magnesium alloy are mainly formed of cast materials
(AZ91 alloy of the American Society for Testing and Materials
(ASTM) standard) by a die casting process or a thixomolding
process. In recent years, press-formed materials obtained by
performing press forming on a sheet composed of a wrought magnesium
alloy such as AZ31 alloy of the ASTM standard have been used.
Patent Literature 1 discloses that a continuous cast material
composed of a magnesium alloy such as AZ91 alloy is produced by a
twin-roll casting process, and a rolled sheet obtained by rolling
the continuous cast material is subjected to press forming.
CITATION LIST
Patent Literature
[0003] Patent Literature 1: International Publication No.
2006/003899
SUMMARY OF INVENTION
Technical Problem
[0004] Focusing on the lightweight of magnesium alloys, a
relatively thin sheet having a thickness of 1 mm or less has been
investigated as a raw material for plastic formed materials such as
press formed materials. However, with the expansion of usage of
magnesium alloys, the development of not only the above-described
thin sheet but also a thick sheet, specifically, a thick sheet
having a thickness of 1.5 mm or more has been demanded focusing on
the specific strength and specific rigidity. Such a thick magnesium
alloy sheet, a method for producing the thick magnesium alloy
sheet, and a plastic formed material such as a press formed
material produced using the thick magnesium alloy sheet have not
been sufficiently studied.
[0005] A thick magnesium alloy sheet is obtained by employing a die
casting process or a thixomolding process. However, in cast
materials such as die cast materials, internal defects such as
cavities are easily formed, and furthermore the composition and
structure tend to become uneven. For example, additive element
components are locally highly concentrated and crystal grains are
randomly oriented. Therefore, cast materials such as die cast
materials have low corrosion resistance compared with plastic
formed materials such as rolled materials. In addition, cast
materials such as die cast materials have poor plastic formability
due to their internal defects and thus are not suitable as raw
materials for plastic forming.
[0006] Accordingly, it is an object of the present invention to
provide a thick magnesium alloy material having high corrosion
resistance and surface roughening resistance and a thick magnesium
alloy material subjected to plastic forming. It is another object
of the present invention to provide a method for producing a thick
magnesium alloy material having high corrosion resistance and
surface roughening resistance.
Solution to Problem
[0007] A magnesium alloy material subjected to plastic forming
(primary forming) such as rolling is excellent in terms of
mechanical properties such as strength, hardness, and toughness,
corrosion resistance, and plastic formability compared with die
cast materials and thixomolded materials even if they have the same
composition. This is because, in such a magnesium alloy material,
formation of defects during casting is reduced and the crystal is
made finer. A magnesium alloy material obtained by performing
plastic forming (secondary forming) such as press forming on the
magnesium alloy material subjected to the primary forming is also
excellent in terms of mechanical properties and corrosion
resistance. In particular, when a continuous cast material produced
by a continuous casting process such as a twin-roll casting process
is used as a raw material for primary formed materials, the
continuous cast material is excellent in terms of plastic
formability because segregation and generation of coarse impurities
in crystal and precipitated impurities are reduced compared with
die cast materials. Therefore, the inventors of the present
invention have produced a thick magnesium alloy sheet having a
thickness of 1.5 mm or more by rolling a continuous cast material
under various conditions. As a result, the inventors have found the
following. A magnesium alloy sheet produced under certain
conditions is thick and has high corrosion resistance. Furthermore,
when plastic forming such as press forming or bending is performed,
the resultant plastic formed material has a small number of small
projections and depressions on its surface, that is, has a smooth
surface (e.g., a beautiful surface with gloss), which means that
such a plastic formed material has high surface roughening
resistance. The present invention is based on the above
findings.
[0008] A magnesium alloy material of the present invention is
composed of a magnesium alloy and includes a sheet-shaped portion
having a thickness of 1.5 mm or more. The sheet-shaped portion
satisfies the following orientation,
[Orientation]
[0009] when a region having 1/4 the thickness of the sheet-shaped
portion in a thickness direction from a surface of the sheet-shaped
portion is defined as a surface region and a remaining region is
defined as an internal region,
[0010] X-ray diffraction peak intensities of a (002) plane, a (100)
plane, a (101) plane, a (102) plane, a (110) plane, and a (103)
plane in the surface region are respectively defined as
I.sub.F(002), I.sub.F(100), I.sub.F(101), I.sub.F(102),
I.sub.F(110), and I.sub.F(103),
[0011] X-ray diffraction peak intensities of a (002) plane, a (100)
plane, a (101) plane, a (102) plane, a (110) plane, and a (103)
plane in the internal region are respectively defined as
I.sub.C(002), I.sub.C(100), I.sub.C(101), I.sub.C(102),
I.sub.C(110), and I.sub.C(103),
[0012] a degree of orientation of the (002) plane in the surface
region, which is expressed as
I.sub.F(002)/{I.sub.zf(100)+I.sub.F(002)+I.sub.F(101)+I.sub.F(102)+I.sub.-
F(110)+I.sub.F(103)}, is defined as a basal plane peak ratio
O.sub.F, and
[0013] a degree of orientation of the (002) plane in the internal
region, which is expressed as
I.sub.C(002)/{I.sub.C(100)+I.sub.C(002)+I.sub.C(101)+I.sub.C(102)+I.sub.C-
(110)+I.sub.C(103)}, is defined as a basal plane peak ratio
O.sub.C,
[0014] a ratio O.sub.F/O.sub.C of the basal plane peak ratio
O.sub.F in the surface region to the basal plane peak ratio O.sub.C
in the internal region satisfies 1.05<O.sub.F/O.sub.C.
[0015] The magnesium alloy material of the present invention can be
produced, for example, by the following production method of the
present invention. A method for producing a magnesium alloy
material according to the present invention is a method for
producing a magnesium alloy material by rolling a raw material
composed of a magnesium alloy, the method including a preparation
step and a rolling step below.
[0016] Preparation step: a step of preparing a sheet-shaped raw
material obtained by subjecting a molten magnesium alloy to
continuous casting by a twin-roll casting process
[0017] Rolling step: a step of rolling the raw material with
multiple passes to produce a sheet-shaped magnesium alloy material
having a thickness of 1.5 mm or more
[0018] In the rolling step, the reduction ratio of each of the
passes is 25% or less.
[0019] The reduction ratio (%) herein is defined as {(thickness
t.sub.b of raw material before Rolling-thickness t.sub.a of raw
material after rolling)/thickness t.sub.b of raw material before
rolling}.times.100.
[0020] According to the production method of the present invention,
rolling with multiple passes can be favorably performed by
employing, as a raw material, a continuous cast material in which
defects and impurities in crystal and precipitated impurities, from
which cracking or the like is caused, and segregation are present
in a small amount or are substantially not present. Furthermore, by
performing rolling with multiple passes while relatively decreasing
the reduction ratio of each pass, plastic forming by rolling is
sufficiently imparted to a surface portion of a rolled material
compared with an internal portion of the rolled material. In other
words, the surface structure and internal structure of the rolled
material can be differentiated from each other by repeatedly
performing rolling at a relatively low reduction ratio. Therefore,
the production method of the present invention provides a magnesium
alloy material (typically a rolled sheet (one form of the magnesium
alloy material of the present invention)) constituted by a
structure in the surface region and a structure in the internal
region which are different from each other. More specifically, the
structure in the surface region is a texture in which basal planes
of magnesium alloy crystals are mainly oriented so as to be
parallel to the rolling direction (a direction in which a raw
material to be rolled travels) by sufficiently providing plastic
forming by rolling (a texture in which c axes of the magnesium
alloy crystals are oriented so as to be orthogonal to the rolling
direction). The structure in the internal region is a structure in
which the basal planes of the magnesium alloy crystals are oriented
more randomly than those of the surface region.
[0021] When the magnesium alloy material of the present invention
is the above-described particular rolled sheet (i.e., when the
entire magnesium alloy material of the present invention is
constituted by the sheet-shaped portion), the structure in the
surface region is a texture having a particular orientation, more
specifically, a texture in which (002) planes, which are basal
planes of magnesium alloy crystals, are strongly oriented and the
structure in the internal region is a structure in which (002)
planes are oriented more weakly than those in the surface region.
The texture in which (002) planes are strongly oriented is one of
indices indicating that deformation due to plastic forming is
sufficiently applied during plastic forming such as rolling. As the
plastic forming such as rolling is sufficiently performed, the
crystal grain size of a magnesium alloy tends to decrease, which
increases the total area of crystal grain boundaries. As a result,
the ratio of impurity elements present relative to the crystal
grain boundaries is relatively decreased and thus the magnesium
alloy material of the present invention having the above-described
particular structure has high corrosion resistance. In particular,
when the surface region exposed to an outside atmosphere has a
structure finer than that of the internal region, higher corrosion
resistance is achieved. Therefore, the magnesium alloy material of
the present invention is thick and has high corrosion resistance.
Furthermore, the magnesium alloy material is constituted by
different structures between the surface region and the internal
region as described above, whereby the magnesium alloy material has
different properties (e.g., mechanical properties such as hardness,
strength, impact resistance, and toughness, corrosion resistance,
and vibration resistance) between the surface region and the
internal region. By using such a difference in properties, the
magnesium alloy material of the present invention can be expected
to be used for various members and as a raw material for the
various members. In addition, the magnesium alloy material of the
present invention has good plastic formability such as press
formability or bendability because the degree of orientation of the
basal planes ((002) planes) in the internal region is small (the
degree of orientation density in a texture is small). Thus, the
magnesium alloy material can be suitably used as a raw material to
be subjected to plastic forming such as press forming or bending.
The surface region is constituted by a fine crystalline structure.
Therefore, even if plastic forming such as press forming is
performed, large projections and depressions are not easily formed
on the surface of a raw material and a plastic formed material (one
form of the magnesium alloy material of the present invention)
having a smooth surface is provided. Accordingly, the magnesium
alloy material of the present invention has high surface roughening
resistance. The resultant plastic formed material also has good
surface texture.
[0022] According to an embodiment of the magnesium alloy material
of the present invention, when an average crystal grain size in the
surface region is defined as D.sub.F and an average crystal grain
size in the internal region is defined as D.sub.C, a ratio
D.sub.C/D.sub.F of the average crystal grain size D.sub.C in the
internal region to the average crystal grain size D.sub.F in the
surface region satisfies 1.5<D.sub.C/D.sub.F.
[0023] According to the above embodiment, the crystal grain size in
the internal region is larger than that in the surface region. In
other words, the crystal grain size in the surface region is
sufficiently smaller than that in the internal region, which
increases the length of the crystal grain boundaries as described
above and thus high corrosion resistance is achieved. According to
the above embodiment, since the surface region is constituted by a
fine crystalline structure, good plastic formability and high
surface roughening resistance are achieved. Furthermore, since the
crystal grain size in the internal region is larger than that in
the surface region, and thus high heat resistance is achieved.
[0024] According to an embodiment of the magnesium alloy material
of the present invention, when a Vickers hardness (Hv) in the
surface region is defined as H.sub.F and a Vickers hardness (Hv) in
the internal region is defined as H.sub.C, a ratio H.sub.C/H.sub.F
of the Vickers hardness H.sub.C in the internal region to the
Vickers hardness H.sub.F in the surface region satisfies
H.sub.C/H.sub.F<0.85.
[0025] According to the above embodiment, the Vickers hardness in
the internal region is lower than that in the surface region. In
other words, the Vickers hardness in the surface region is
sufficiently higher than that in the internal region and thus high
wear resistance is achieved.
[0026] The magnesium alloy material of the present invention can be
composed of a magnesium alloy containing various additive elements
(balance: Mg and impurities). In particular, an alloy containing
additive elements in a high concentration, specifically, a
magnesium alloy containing additive elements in a total content of
5.0 mass % or more has good mechanical properties such as strength
and hardness, high corrosion resistance, flame resistance, heat
resistance, and the like, though depending on the types of additive
elements.
[0027] Specific examples of the additive elements include at least
one element selected from Al, Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag,
Sn, Li, Zr, Ce, Ni, Au, and rare-earth elements (except for Y and
Ce). An example of the impurities is Fe.
[0028] A Mg--Al series alloy containing Al has high corrosion
resistance and also has good mechanical properties such as strength
and hardness. Therefore, according to an embodiment of the
magnesium alloy material of the present invention, the magnesium
alloy contains Al as an additive element in a content of 5.0 mass %
or more and 12 mass % or less. As the Al content increases, the
above effects tend to increase. The Al content is preferably 7 mass
% or more and more preferably 7.3 mass % or more. The upper limit
of the Al content is 12 mass % and preferably 11 mass % because the
plastic formability degrades when the Al content exceeds 12 mass %.
In particular, a magnesium alloy containing 8.3 mass % to 9.5 mass
% of Al provides high strength and corrosion resistance. The total
content of elements other than Al is 0.01 mass % or more and 10
mass % or less and preferably 0.1 mass % or more and 5 mass % or
less.
[0029] Specific examples of the Mg--Al series alloy include AZ
series alloys (Mg--Al--Zn series alloys, Zn: 0.2 mass % to 1.5 mass
%, such as AZ 31 alloy, AZ61 alloy, and AZ91 alloy) in the ASTM
standard, AM series alloys (Mg--Al--Mn series alloys, Mn: 0.15 mass
% to 0.5 mass %), AS series alloys (Mg--Al--Si series alloys, Si:
0.01 mass % to 20 mass %), Mg--Al--RE (rare-earth element) series
alloys, AX series alloys (Mg--Al--Ca series alloys, Ca: 0.2 mass %
to 6.0 mass %), and AJ series alloys (Mg--Al--Sr series alloys, Sr:
0.2 mass % to 7.0 mass %). Examples of the alloy containing 8.3
mass % to 9.5 mass % of Al include Mg--Al--Zn series alloys further
containing 0.5 mass % to 1.5 mass % of Zn, e.g., AZ91 alloy.
[0030] A magnesium alloy containing at least one element selected
from Y, Ce, Ca, Si, Sn, and rare-earth elements (except for Y and
Cc) in a total content of 0.001 mass % or more and preferably 0.1
mass % or more and 5 mass % or less, the balance being Mg and
impurities, provides high heat resistance and flame resistance.
When rare-earth elements are contained, the total content is
preferably 0.1 mass % or more. In particular, when Y is contained,
the content is preferably 0.5 mass % or more.
Advantageous Effects of Invention
[0031] A magnesium alloy material according to the present
invention is a thick magnesium alloy material having high corrosion
resistance and surface roughening resistance. In a method for
producing a magnesium alloy material according to the present
invention, a thick magnesium alloy material having high corrosion
resistance and surface roughening resistance can be produced.
DESCRIPTION OF EMBODIMENTS
[0032] The present invention will now be further described in
detail.
[Magnesium Alloy Material]
(Composition)
[0033] The magnesium alloy material of the present invention is
composed of a magnesium alloy containing 50 mass % or more of Mg
and, typically, the additive elements described above.
(Form)
[0034] A sheet-shaped portion in the magnesium alloy material of
the present invention means a portion having a pair of surfaces
parallel to each other with a substantially uniform interval
(distance) between the surfaces, that is, a portion having a
uniform thickness. The magnesium alloy material of the present
invention has a sheet-shaped portion in at least part thereof. When
this is satisfied, the magnesium alloy material may have a form
having a portion with a thickness locally different from that of
the other portions through a process such as a cutting process,
e.g., a form including, for example, a boss joined thereto, a form
having a groove, and a form having a through-hole that connects the
top and the bottom.
[0035] A typical form of the magnesium alloy material of the
present invention having the sheet-shaped portion is a form
(magnesium alloy sheet) in which the entire magnesium alloy
material has a sheet-like shape. The shape (planar shape) of the
magnesium alloy sheet may be a rectangular shape, a circular shape,
or the like. The magnesium alloy sheet may be in the form of a coil
stock obtained by coiling a continuous long sheet or in the form of
a short sheet having a predetermined length and shape. The
magnesium alloy sheet may have various forms in accordance with the
production process. Examples of the forms include a rolled sheet, a
heat treated sheet and a leveled sheet respectively obtained by
performing a heat treatment and leveling described below on the
rolled sheet, and a polished sheet and a coated sheet respectively
obtained by performing polishing and coating on the rolled sheet,
heat-treated sheet, or leveled sheet.
[0036] The magnesium alloy material of the present invention may
also be a formed product obtained by performing plastic forming
(secondary forming) such as press forming, e.g., bending and
drawing on the magnesium alloy sheet or a partly formed material
having a plastic formed portion, which is obtained by partly
performing plastic forming on the magnesium alloy sheet (herein, at
least part of the formed product or partly formed material is the
sheet-shaped portion). The formed product may be a box or frame
having a U-shaped cross section that includes a top (bottom) and a
sidewall extending perpendicularly from the periphery of the top
(bottom) or a covered tube that includes a discoidal top and a
cylindrical sidewall. At least the top corresponds to the
sheet-shaped portion. The form of the magnesium alloy material can
be selected in accordance with desired applications.
(Thickness)
[0037] In the magnesium alloy material of the present invention,
the sheet-shaped portion has a thickness of 1.5 mm or more. Any
thickness can be selected from a thickness of 1.5 mm or more in
accordance with desired applications. Herein, to increase the
thickness of the sheet-shaped portion, the thickness of a cast
material serving as a raw material needs to be also increased. If
the thickness of the cast material is increased, the rollability is
degraded due to the above-described defects or the like. Therefore,
the thickness of the sheet-shaped portion is preferably 10 mm or
less and particularly preferably 5 mm or less for the purpose of
producing a thick rolled sheet (one form of the magnesium alloy
material of the present invention) with high productivity.
[0038] When the magnesium alloy material of the present invention
is the formed product or the partly formed material, the structure
and mechanical properties of the magnesium alloy sheet serving as a
raw material for plastic forming are substantially maintained in a
portion (e.g., the sheet-shaped portion) that undergoes less
deformation during the plastic forming
(Structure)
<Orientation>
[0039] In the magnesium alloy material of the present invention, at
least the surface region in the above sheet-shaped portion is
constituted by a structure having a texture of basal planes and the
internal region is constituted by a structure having a small degree
of orientation of basal planes. When the surface region exposed to
an outside atmosphere is constituted by a structure in which (002)
planes are strongly oriented, higher corrosion resistance is
achieved as described above. As the difference in the degree of
orientation between the surface region and the internal region
increases, the corrosion resistance, surface hardness, and surface
roughening resistance are expected to be improved. However, if the
difference in the degree of orientation is excessively increased,
it becomes difficult to uniformly perform plastic forming such as
press forming. Therefore, the above basal plane peak ratio
O.sub.F/O.sub.C preferably satisfies
O.sub.F/O.sub.C.ltoreq.1.2.
<Average Crystal Grain Size>
[0040] In a typical form of the magnesium alloy material of the
present invention, the crystal grain size in the internal region is
larger than that in the surface region. In this form, the internal
region has high heat resistance and the surface region having a
relatively small crystal grain size has high corrosion resistance
and hardness as described above. In particular, when the surface
region is constituted by a relatively fine structure, high hardness
is achieved and thus high wear resistance is achieved. This makes
it difficult to form scratches and provides good surface texture.
Therefore, the magnesium alloy material of the present invention is
expected to be suitably used for structural materials and the like
that require durability. As the difference in the average crystal
grain size between the surface region and the internal region
increases, the corrosion resistance, surface roughening resistance,
and surface hardness are expected to be improved. However, if the
difference in the average crystal grain size is excessively
increased, it becomes difficult to uniformly perform plastic
forming such as press forming. Therefore, the above ratio
D.sub.C/D.sub.F of the average crystal grain sizes preferably
satisfies D.sub.C/D.sub.F.ltoreq.2.0.
[0041] In the case where a thick sheet-shaped magnesium alloy
material having a thickness of 1.5 mm or more is produced by
performing rolling as described above, there are limitations to
achievement of a uniform and fine grain size across the entire
region in the thickness direction. In the magnesium alloy material
of the present invention, the average crystal grain sizes in the
surface region and internal region are 3.5 .mu.m or more. However,
since the plastic formability tends to improve as the crystal grain
size decreases, both the average crystal grain sizes in the surface
region and internal region of the sheet-shaped portion are
preferably 20 .mu.m or less and particularly preferably 10 .mu.m or
less. The average crystal grain size varies depending on the
reduction ratio and the heating temperature of a raw material in
the rolling step, and tends to decrease as the reduction ratio
increases and the heating temperature decreases.
(Mechanical Properties)
[0042] The magnesium alloy material of the present invention has
better mechanical properties such as strength, hardness, and
toughness than cast materials such as die cast materials because
rolling is performed. For example, the Vickers hardness in the
surface region is higher than that in the internal region as
described above. As the difference in the Vickers hardness between
the surface region and the internal region increases, the surface
hardness relatively increases. However, if the difference in the
Vickers hardness is excessively increased (the surface hardness is
excessively increased), press formability is degraded. Therefore,
the ratio H.sub.C/H.sub.F of the Vickers hardnesses (Hv) preferably
satisfies 0.7.ltoreq.H.sub.C/H.sub.F. The absolute value of the
Vickers hardness tends to increase as the content of additive
elements increases, though depending on the rolling conditions such
as the reduction ratio and the heating temperature of a raw
material. When the magnesium alloy material of the present
invention is a plastic formed material (formed product) or a partly
formed material, the hardness tends to be further increased by work
hardening.
(Other Structures)
[0043] By subjecting at least part of the surface of the magnesium
alloy material of the present invention to an anti-corrosion
treatment such as a chemical conversion treatment or an anodic
oxidation treatment to form an anti-corrosion layer, higher
corrosion resistance is achieved. Furthermore, by coating at least
part of the surface of the magnesium alloy material of the present
invention to form a coating layer, the design and commercial value
are improved.
[Production Method]
[0044] Each step of the above-described production method of the
present invention will now be further described in detail.
(Preparation Step)
<Casting>
[0045] In the production method of the present invention, a
continuous cast material is used as a starting material. In a
continuous casting process, rapid solidification can be performed.
Therefore, segregation and formation of oxides can be reduced even
when additive elements are contained in a large amount, which can
suppress the generation of coarse impurities in crystal and
precipitated impurities having a size of more than 10 .mu.m from
which cracking may be caused. Thus, a cast material having good
plastic formability such as rollability can be produced. In a
continuous casting process, a long cast material can also be
produced in a continuous manner. A long material produced by the
continuous casting process can be used as a raw material for
rolling. When a long raw material is used, a long rolled material
can be produced. Examples of the continuous casting process include
a twin-roll process, a twin-belt process, and a belt-and-wheel
process. A twin-roll process or a twin-belt process is suitable for
the production of sheet-shaped cast materials, and a twin-roll
process is particularly suitable. A continuous cast material
produced by the casting process described in Patent Literature 1 is
preferably used. The thickness, width, and length of a cast
material can be suitably selected so that a desired rolled material
(rolled sheet) is obtained. The thickness of the cast material is
preferably 10 mm or less and particularly preferably 5 mm or less
because segregation is easily caused in an excessively thick cast
material. When the obtained continuous cast material is used as a
long material, such a continuous cast material is coiled in a
cylindrical shape because the continuous cast material is easily
transferred to the next step. When the cast material is coiled
while a start-of-coiling portion of the cast material is heated to
about 100.degree. C. to 200.degree. C., even alloys such as AZ91
alloy which contain additive elements in a large amount and easily
cause cracking are easily bent. Even in the case of a small coiling
diameter, the cast material can be coiled without being cracked.
The obtained continuous cast material can be cut into sheet
materials having a desired length to obtain a raw material for
rolling. In this case, a rolled material (rolled sheet) having a
desired length is obtained.
<Solution Treatment>
[0046] By performing a solution treatment before the cast material
is rolled, the composition of the cast material can be homogenized
and elements such as Al can be sufficiently dissolved to improve
the toughness. The solution treatment is performed at a heating
temperature of 350.degree. C. or more and particularly 380.degree.
C. or more and 420.degree. C. or less for a holding time of 1 hour
or more and 40 hours or less. In the case of Mg--Al series alloys,
the holding time is preferably increased as the content of Al
increases. In a cooling step from the heating temperature after the
holding time, the precipitation of coarse precipitates can be
suppressed by increasing the cooling rate (preferably 50.degree.
C./min or more) using accelerated cooling such as water cooling or
air blast cooling.
<Rolling>
[0047] The cast material or the material subjected to a solution
treatment is rolled with multiple passes. At least one pass is
preferably performed by warm rolling which is performed while a raw
material (a cast material, a material subjected to a solution
treatment, or a worked material being subjected to rolling) is
heated to 150.degree. C. or more and 400.degree. C. or less or by
hot rolling. By heating the raw material to the above temperature,
cracking or the like is not easily caused during the rolling even
if the reduction ratio per pass is increased. As the temperature is
increased, the formation of cracks is reduced. By setting the
temperature to be 400.degree. C. or less, the degradation caused by
seizing of a surface of the raw material and the thermal
degradation of a reduction roll can be suppressed. Therefore, the
heating temperature is preferably 350.degree. C. or less, more
preferably 300.degree. C. or less, and particularly preferably
150.degree. C. or more and 280.degree. C. or less. In addition to
the raw material, the reduction roll may also be heated. The
heating temperature of the reduction roll is, for example,
100.degree. C. to 250.degree. C.
[0048] In the production method of the present invention, the
reduction ratio of each pass is set to be 25% or less. By
performing rolling with multiple passes at a relatively low
reduction ratio, plastic forming can be imparted particularly to
the surface of the raw material in a concentrated manner. The
reduction ratio of each pass can be suitably selected in the range
of 25% or less. However, if the reduction ratio is excessively low,
the number of passes to achieve a desired thickness is increased,
which decreases the productivity. Thus, the reduction ratio of each
pass is preferably 10% or more.
[0049] The conditions such as the heating temperature of a raw
material, the temperature of a reduction roll, and the reduction
ratio may be changed for each pass. Therefore, the reduction ratio
of each pass may be the same or different. An intermediate heat
treatment may be performed between the passes. By performing the
intermediate heat treatment, the strain and residual stress
introduced into the raw material before the intermediate heat
treatment can be removed or reduced to allow ease of rolling after
the intermediate heat treatment. The intermediate heat treatment
can be performed at a heating temperature of 150.degree. C. to
350.degree. C. (preferably 300.degree. C. or less, more preferably
250.degree. C. to 280.degree. C.) for a holding time of 0.5 to 3
hours. After the rolling, a final heat treatment may be performed
under the above conditions. Furthermore, the rolling is easily
performed by suitably using a lubricant because the frictional
resistance during rolling can be reduced and the seizing of the raw
material can be prevented.
[0050] In addition, the edge of the cast material before rolling
may be trimmed to prevent the extension of cracks, which may
present at the edge, during rolling. Such trimming may be performed
to appropriately adjust the width during rolling or after
rolling.
<Other Processes>
<<Polishing>>
[0051] After the rolling, polishing may be performed. The polishing
is performed in order to remove or reduce the lubricant used during
the rolling and the scratches and oxide films present on the
surface of the rolled material. The polishing is preferably
performed with a grinding belt because even a long material can be
easily polished in a continuous manner. The polishing is preferably
performed by a wet process to prevent scattering of powder.
<<Leveling>>
[0052] After the rolling or after the polishing, leveling may be
performed. The leveling is performed in order to improve the
flatness and precisely perform plastic forming such as press
forming. In the leveling, a roll leveler including a plurality of
rollers disposed in a staggered manner can be suitably used. The
leveling may be performed while the raw material is heated to, for
example, 100.degree. C. to 300.degree. C. and particularly
150.degree. C. to 280.degree. C. (warm leveling).
<<Plastic Forming>>
[0053] When the magnesium alloy material of the present invention
is processed into a formed product or a partly formed material
having a plastic formed portion, such a formed product or a partly
formed material can be produced by a production method that
includes a plastic forming step of performing plastic forming such
as press forming on at least part of the raw material (the
above-described rolled material, polished material, or leveled
material) subjected to the above rolling step. The plastic forming
is preferably performed in the temperature range of 200.degree. C.
to 300.degree. C. to improve the plastic formability of the raw
material. A heat treatment may be performed after the plastic
forming in order to remove the strain and residual stress
introduced during the plastic forming and improve the mechanical
properties. The heat treatment is performed at a heating
temperature of 100.degree. C. to 300.degree. C. for a heating time
of about 5 to 60 minutes.
<<Surface Treatment>>
[0054] When the magnesium alloy material of the present invention
includes the above-described anti-corrosion layer or coating layer,
the production can be performed by a production method that
includes a surface treating step of performing an anti-corrosion
treatment or coating on at least part of the raw material subjected
to the rolling step or at least part of the raw material subjected
to the plastic forming step. Furthermore, at least one selected
from hairline finish, diamond cutting, shot blasting, etching, and
spin cutting may be performed on at least part of the raw material.
Such a surface treatment is performed in order to improve the
corrosion resistance and a mechanical protective function and
improve the design, metal texture, and commercial value.
[0055] A specific embodiment of the present invention will now be
further described based on Test Examples.
[Test Example]
[0056] Raw materials composed of magnesium alloys having the
following compositions were subjected to rolling under various
conditions to produce magnesium alloy sheets having a thickness of
1.5 mm or more. The orientation, crystal grain size, and Vickers
hardness of the magnesium alloy sheets were measured.
[0057] In this test, a magnesium alloy sheet composed of a
magnesium alloy (Mg--9.0 mass % Al--0.6 mass % Zn) having a
composition equivalent to that of AZ91 alloy and a magnesium alloy
sheet composed of a magnesium alloy (Mg--3.1 mass % Al--0.7 mass %
Zn) having a composition equivalent to that of AZ31 alloy were
produced.
[0058] Long cast sheets (thickness 4.5 mm (4.50 to 4.51
mm).times.width 320 mm) were produced from the above magnesium
alloys having the above compositions by a twin-roll continuous
casting process. The long cast sheets were temporarily coiled to
produce cast coil stocks. Each of the cast coil stocks was
subjected to a solution treatment at 400.degree. C. for 24 hours. A
raw material obtained by uncoiling the solid solution coil stock
subjected to the solution treatment was rolled with multiple passes
under the rolling conditions shown in Table I to produce a rolled
material (magnesium alloy sheet) having a thickness of 2.0 mm (2.00
mm to 2.01 mm) or 1.5 mm. Each pass was performed by warm rolling
(the heating temperature of a raw material: 250.degree. C. to
280.degree. C., the temperature of a reduction roll: 100.degree. C.
to 250.degree. C.). The thickness of the cast material, the
thickness of a worked material being subjected to rolling, and the
thickness of the obtained magnesium alloy sheet were determined to
be the average of three thicknesses in total at the central part in
the width direction of a sheet to be measured and two points
located 50 mm from the edges in the width direction.
TABLE-US-00001 TABLE I Thickness of each pass (mm) Sample Before
Reduction ratio of each pass (%) Composition No. rolling 1 2 3 4 5
6 1 2 3 4 5 6 AZ91 A 4.50 3.25 2.50 2.00 -- -- -- 27.8 23.1 20.0 --
-- -- B 4.51 3.61 2.88 2.40 2.00 -- -- 20.0 20.2 16.7 16.7 -- -- C
4.51 3.86 3.26 2.78 2.35 2.00 -- 14.4 15.5 14.7 15.5 14.9 -- D 4.50
3.20 2.32 1.85 1.50 -- -- 28.9 27.5 20.3 18.9 -- -- E 4.51 3.45
2.69 2.12 1.70 1.50 -- 23.5 22.0 21.2 19.8 11.8 -- F 4.51 3.75 3.11
2.55 2.08 1.75 1.50 16.9 17.1 18.0 18.4 15.9 14.3 AZ31 G 4.51 3.25
2.51 2.01 -- -- -- 27.9 22.8 19.9 -- -- -- H 4.50 3.60 2.88 2.41
2.00 -- -- 20.0 20.0 16.3 17.0 -- -- I 4.51 3.85 3.26 2.77 2.35
2.01 -- 14.6 15.3 15.0 15.2 14.5 -- J 4.50 3.21 2.32 1.86 1.50 --
-- 28.7 27.7 19.8 19.4 -- -- K 4.50 3.46 2.70 2.11 1.71 1.50 --
23.1 22.0 21.9 19.0 12.3 -- L 4.51 3.76 3.10 2.55 2.09 1.75 1.50
16.6 17.6 17.7 18.0 16.3 14.3
[Orientation]
[0059] Each of the obtained magnesium alloy sheets was analyzed by
X-ray diffraction to determine the ratio O.sub.F/O.sub.C of the
basal plane peak ratio O.sub.F in the surface region to the basal
plane peak ratio O.sub.C in the internal region. Table II shows the
results. The basal plane peak ratio O.sub.F in the surface region
was measured by performing X-ray diffraction on the surface of the
magnesium alloy sheet. The basal plane peak ratio O.sub.C in the
internal region was measured by chemically removing a region
(surface region) having 1/4 the thickness of the magnesium alloy
sheet in a thickness direction from a surface of the magnesium
alloy sheet to expose the inside portion and then performing X-ray
diffraction on the exposed surface. The peak intensities of a (002)
plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane,
and a (103) plane in each of the regions were measured to determine
the ratio O.sub.F/O.sub.C.
[0060] Basal plane peak ratio O.sub.F:
I.sub.F(002)/{I.sub.F(100)+I.sub.F(002)+I.sub.F(101)+I.sub.F(102)+I.sub.F-
(110)+I.sub.F(103)}
[0061] Basal plane peak ratio O.sub.C:
I.sub.C(002)/{I.sub.C(100)+I.sub.C(002)+I.sub.C(101)+I.sub.C(102)+I.sub.C-
(110) +I.sub.C(103)}
[Average Crystal Grain Size]
[0062] The average crystal grain sizes (.mu.m) in the internal
region and surface region of each of the obtained magnesium alloy
sheets were measured in conformity with "Steels--Micrographic
determination of the apparent grain size JIS G 0551 (2005)".
Sections (cross section and longitudinal section) of the magnesium
alloy sheet in the thickness direction were taken and observed with
an optical microscope (400 times) to determine the average crystal
grain size in each of three fields of view (the total number of
fields of view in each region: 6) in the sections of the surface
region (a region having 1/4 the thickness from a surface in the
thickness direction) and internal region (a remaining region
obtained by removing the surface region). Table II shows the
average (D.sub.F) of the average crystal grain sizes in the six
fields of view in total in the surface region and the average
(D.sub.C) of the average crystal grain sizes in the six fields of
view in total in the internal region. The ratio D.sub.C/D.sub.F of
the average crystal grain size D.sub.C in the internal region to
the average crystal grain size D.sub.F in the surface region was
also determined. Table II shows the results.
[Vickers Hardness]
[0063] The Vickers hardnesses (Hv) in the internal region and
surface region of each of the obtained magnesium alloy sheets were
measured. As in the case of the measurement of the average crystal
grain size, sections (cross section and longitudinal section) of
the magnesium alloy sheet in the thickness direction were taken.
The Vickers hardness H.sub.F in the surface region was measured by
pressing an indenter against each of the sections of the surface
region. The Vickers hardness H.sub.C in the internal region was
measured by pressing an indenter against each of the sections of
the internal region. Table II shows the average (H.sub.F) of the
Vickers hardnesses in the sections of the surface region and the
average (H.sub.C) of the Vickers hardnesses in the sections of the
internal region. The ratio H.sub.C/H.sub.F of the Vickers hardness
H.sub.C in the internal region to the Vickers hardness H.sub.F in
the surface region was also determined. Table II shows the
results.
[Corrosion Test]
[0064] The corrosion resistance of each of the obtained magnesium
alloy sheets was measured. A test piece (the thickness was the same
as that of the obtained magnesium alloy sheet) was prepared in
conformity with JIS Z 2371 (2000). A salt-spray test was performed
for 96 hours to measure the corrosion weight loss (mg/cm.sup.2)
after the test. Table II shows the results.
TABLE-US-00002 TABLE II Surface roughness Corrosion Crystal Vickers
Ra in bent weight Basal plane grain size hardness portion loss in
peak ratio Surface/ (.mu.m) Internal/ (Hv) Internal/ after press
salt-spray Sample Surface Internal Internal Surface Internal
Surface Surface Internal Surface forming test Composition No.
O.sub.F O.sub.C O.sub.F/O.sub.C D.sub.F D.sub.C D.sub.C/D.sub.F
H.sub.F H.sub.C H.sub.C/H.sub.F (.mu.m) (mg/cm.sup.2) AZ91 A 0.861
0.841 1.02 5.2 5.1 1.02 88 89 1.01 0.65 0.31 B 0.870 0.814 1.07 4.9
7.5 1.53 90 75 0.83 0.49 0.13 C 0.883 0.805 1.10 4.8 8.8 1.83 89 73
0.82 0.49 0.12 D 0.878 0.860 1.02 5.2 5.1 1.04 90 89 0.99 0.56 0.33
E 0.880 0.836 1.052 5.0 7.6 1.52 89 75 0.84 0.52 0.12 F 0.882 0.820
1.08 4.8 8.5 1.77 91 75 0.82 0.47 0.11 AZ31 G 0.711 0.691 1.03 6.3
6.4 1.07 68 66 0.97 1.09 0.62 H 0.720 0.682 1.06 5.3 8.1 1.53 70 59
0.84 0.47 0.40 I 0.731 0.668 1.09 4.9 8.9 1.82 69 57 0.83 0.44 0.41
J 0.721 0.718 1.00 6.4 6.5 1.05 67 66 0.99 1.02 0.65 K 0.729 0.685
1.06 5.2 7.9 1.52 70 58 0.83 0.46 0.42 L 0.734 0.671 1.09 4.8 9.0
1.88 71 57 0.80 0.44 0.40
[0065] As is clear from Tables I and II, by rolling a continuous
cast material with multiple passes at a reduction ratio of 25% or
less per pass, there is provided a thick magnesium alloy sheet
(magnesium alloy material) which has a thickness of 1.5 mm or more
and in which the structure (basal plane peak ratio) in the internal
region in the thickness direction and the structure (basal plane
peak ratio) in the surface region are different from each other. In
this magnesium alloy sheet, the mechanical properties in the
internal region are different from the mechanical properties in the
surface region.
[0066] When each of the obtained magnesium alloy sheets was
subjected to press forming (the heating temperature of the
magnesium alloy sheet: 250.degree. C. to 270.degree. C.), all the
samples were successfully press formed. In the measurement of the
structure of a flat portion in each of the press formed materials,
the structure of a flat portion was substantially the same as that
of the magnesium alloy sheet before press forming, and the basal
plane peak ratio and average crystal grain size were substantially
equivalent to those of the magnesium alloy sheet before press
forming. Furthermore, the surface roughness Ra in a bent portion of
the press formed material was measured. Table II shows the results,
which make it clear that samples having different structures
between the surface region and the internal region, specifically,
sample Nos. B, C, E, F, H, I, K, and L constituted by a structure
in which the grain size in the surface region is small have a small
surface roughness Ra of about 0.5 .mu.m or less and thus have a
smooth surface.
[0067] It was confirmed from the above test results that a
magnesium alloy material which has a thick sheet-shaped portion
with a thickness of 1.5 mm or more and in which the structure of
the sheet-shaped portion varies in the thickness direction and has
a particular orientation had high surface roughening resistance. It
was also confirmed that, when the magnesium alloy material had a
surface region constituted by a relatively fine structure, the
magnesium alloy material had high corrosion resistance.
[0068] The above-described embodiments can be suitably modified
without departing from the scope of the present invention and are
not limited to the above configurations. For example, the
composition of the magnesium alloy, the thickness and shape of the
magnesium alloy material, and the reduction ratio of each pass and
the number of passes in the rolling step can be suitably
changed.
INDUSTRIAL APPLICABILITY
[0069] The magnesium alloy material of the present invention can be
suitably used for members in various fields that require corrosion
resistance and wear resistance, such as parts of automobiles, parts
of railway vehicles, parts of airplanes, parts of bicycles, and
parts of electric and electronic devices; constituent materials for
the members; and bags and raw materials thereof The method for
producing a magnesium alloy material according to the present
invention can be suitably used for the production of the above
magnesium alloy material of the present invention.
* * * * *