U.S. patent application number 11/979048 was filed with the patent office on 2008-03-27 for magnesium alloy sheet processing method and magnesium alloy sheet.
This patent application is currently assigned to Osaka University. Invention is credited to Tetsuo Sakai, Hiroshi Utsunomiya, Norihiro Yoshida.
Application Number | 20080075624 11/979048 |
Document ID | / |
Family ID | 37481495 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080075624 |
Kind Code |
A1 |
Sakai; Tetsuo ; et
al. |
March 27, 2008 |
Magnesium alloy sheet processing method and magnesium alloy
sheet
Abstract
A magnesium alloy sheet processing method wherein a magnesium
alloy sheet is rolled at a speed of 180 m/min or more.
Particularly, a magnesium alloy sheet processing method, wherein
the magnesium alloy sheet is rolled at a speed of 450 m/min or
more. A magnesium alloy sheet processing method, wherein a rolling
tool which is not heated is used. A magnesium alloy sheet
processing method, wherein the temperature of the magnesium alloy
sheet immediately before the rolling is in the range of 0.degree.
C. to 400.degree. C. A magnesium alloy sheet, wherein the sheet has
an average crystal grain of 4 .mu.m or less, and does not
internally include any unbonded interface in parallel with a
direction of rolling. A magnesium alloy sheet, wherein the sheet
has an average grain size of 4 .mu.m or less, and has an internal
grain boundary formed by a clean grain boundary.
Inventors: |
Sakai; Tetsuo; (Osaka,
JP) ; Utsunomiya; Hiroshi; (Osaka, JP) ;
Yoshida; Norihiro; (Osaka, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Osaka University
|
Family ID: |
37481495 |
Appl. No.: |
11/979048 |
Filed: |
October 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2006/310553 |
May 26, 2006 |
|
|
|
11979048 |
Oct 30, 2007 |
|
|
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Current U.S.
Class: |
420/408 ;
148/667; 420/411 |
Current CPC
Class: |
C22C 23/04 20130101;
C22C 23/02 20130101; B21B 3/00 20130101; C22F 1/06 20130101 |
Class at
Publication: |
420/408 ;
148/667; 420/411 |
International
Class: |
C22C 23/02 20060101
C22C023/02; C22C 23/04 20060101 C22C023/04; C22F 1/06 20060101
C22F001/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2005 |
JP |
JP 2005-156813 |
Claims
1. A magnesium alloy sheet processing method wherein a magnesium
alloy sheet is rolled at a speed of 180 m/min or more.
2. A magnesium alloy sheet processing method according to claim 1,
wherein the magnesium alloy sheet is rolled at a speed of 450 m/min
or more.
3. A magnesium alloy sheet processing method according to claim 1,
wherein a rolling tool which is not heated is used.
4. A magnesium alloy sheet processing method according to claim 1,
wherein the temperature of the magnesium alloy sheet immediately
before the rolling is in the range of O.degree. C. to 400.degree.
C.
5. A magnesium alloy sheet processing method according to claim 1,
wherein the magnesium alloy sheet immediately after the rolling is
cooled at a speed of 10.degree. C./sec or more.
6. A magnesium alloy sheet processing method according to claim 1,
wherein the rolling is carried out with a rolling reduction of
25%/step or more.
7. A magnesium alloy sheet processed by the magnesium alloy sheet
processing method according to claim 1, wherein the sheet includes,
as basic blending components, 0.1 to 10.0 weight percent of Al, and
0.1 to 4 weight percent of Zn, and has a tensile strength of 250
MPa or more and an elongation of 20% or more.
8. A magnesium alloy sheet processed by the magnesium alloy sheet
processing method according to claim 1, wherein the sheet includes,
as basic blending components, 0.1 to 10.0 weight percent of Al, and
0.1 to 4 weight percent of Zn, and has an average grain size of
less than 4 .mu.m.
9. A magnesium alloy sheet, wherein the sheet has an average
crystal grain of 4 .mu.m or less, and does not internally include
any unbonded interface in parallel with a direction of rolling.
10. A magnesium alloy sheet, wherein the sheet has an average grain
size of 4 .mu.m or less, and has an internal grain boundary formed
by a clean grain boundary.
11. A magnesium alloy sheet according to claim 9, wherein the sheet
includes, as basic blending components, 0.1 to 10.0 weight percent
of Al, and 0.1 to 4 weight percent of Zn.
12. A magnesium alloy sheet according to claim 9, wherein the sheet
includes, as basic blending components, 4.0 to 8.0 weight percent
of Zn, and 0.3 to 0.8 weight percent of Zr.
13. A magnesium alloy sheet processed by the magnesium alloy sheet
processing method according to claim 1, wherein the sheet includes,
as basic blending components, 4.0 to 8.0 weight percent of Zn, and
0.3 to 0.8 weight percent of Zr, and has an average grain size of
less than 4 .mu.m.
14. A magnesium alloy sheet according to claim 10, wherein the
sheet includes, as basic blending components, 0.1 to 10.0 weight
percent of Al, and 0.1 to 4 weight percent of Zn.
15. A magnesium alloy sheet according to claim 10, wherein the
sheet includes, as basic blending components, 4.0 to 8.0 weight
percent of Zn, and 0.3 to 0.8 weight percent of Zr.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy sheet
processing method and a magnesium alloy sheet, and in particular
relates to a method for rolling a magnesium alloy sheet at a speed
as high as 180 m/minor more, and to a magnesium alloy sheet having
an excellent mechanical strength and the like.
BACKGROUND ART
[0002] Since a magnesium alloy has a high specific strength and an
excellent electromagnetic wave absorption characteristic, the
utilization thereof is being increased in casings or the like for
portable electronic devices such as mobile phones. Actually, a
magnesium alloy has a crystal lattice with a close-packed hexagonal
structure, and has, at a room temperature, a deformation mechanism
consisting mainly of a basal plane slip that causes deformation in
a direction perpendicular to the c axis, thus making it
significantly difficult to provide a plastic strain (deformation)
of 10% or more at an ambient temperature. Therefore, in order to
carry out such a process, e.g., rolling, not only it is necessary
to heat an object to 300.degree. C. or more, but also it becomes a
multi-pass process (which means that the number of pressings
performed by rolls for rolling is large. Hereinafter, "pass" will
be also used as the number of pressings performed by rolls);
furthermore, it is necessary to perform intermediate annealing or
the like for recovering the workability during each pass (rolling).
In addition, when rolling is to be carried out, a longitudinal
cracking will occur in a material unless the speed at each pass is
reduced. Accordingly, a magnesium alloy has been considered as an
unsuitable material for rolling.
[0003] As a result, the great majority of magnesium alloy products
applied to the above-described usage have been fabricated by
thixo-molding, die casting or the like (see Non-Patent Documents 1
and 2).
[0004] [Non-Patent Document 1] "Magnesium Processing Technology"
edited by The Japan Society for Technology of Plasticity, and
published by Corona Publishing Co., Ltd. Dec. 15, 2004 (pp.
60-73)
[0005] [Non-Patent Document 2] "Handbook of Advanced Magnesium
Technology" edited by Editing Committee for Handbook of Advanced
Magnesium Technology, including Yo Kojima et al., and published by
Kallos Publishing Co., Ltd. May 17, 2000 (pp. 241-245)
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0006] However, user's desires for lighter weight and lower price
of portable devices are becoming stronger in recent years.
[0007] Furthermore, the desire for good appearance is also becoming
stronger.
[0008] It is becoming difficult to cope with these strong desires
anymore by the above-mentioned thixo-molding and die casting. More
specifically, thixo-molding is one kind of injection molding while
die casting is one kind of casting; therefore, thinning of walls is
limited to a certain degree, and furthermore, thixo-molding and die
casting are not preferable in terms of cost reduction because a
magnesium alloy is expensive. Also, there are limits to the
reduction of surface irregularities of a processed surface and to
the smoothing of the surface, and therefore, there are also
problems in making aesthetic improvements to the appearances of
products.
[0009] Thus, there has been a desire for the development of
technology for fabricating a magnesium alloy sheet thinner than a
conventional one.
[0010] In addition, there has been a desire for the development of
technology for fabricating a magnesium alloy sheet having a
finished surface smoother than that of a conventional one.
[0011] Also, there has been a desire for the development of
fabricating technology or in particular a rolling method for a
magnesium alloy sheet, in which process steps are simple and cost
reduction can be enabled in terms of this aspect.
[0012] Moreover, there has been a desire for the development of a
magnesium alloy sheet having an improved mechanical property and
the like such as strength.
Solution to the Problems
[0013] As a result of conducting extensive studies for solving the
above-described problems, the inventors of the present application
found out that, contrary to conventional common knowledge, the
execution of rolling at 180 m/minor more, i.e., at a speed (high
speed) outstandingly higher than a conventional speed, achieves a
high rolling reduction (degree of deformation) while suppressing
the rupture and cracking of a magnesium alloy sheet, thus realizing
fine crystal grains.
[0014] Furthermore, based on this, we found out more preferable
processing conditions.
[0015] The claims of the present invention will be described
below.
[0016] An invention according to claim 1 provides a magnesium alloy
sheet processing method
[0017] wherein a magnesium alloy sheet is rolled at a speed of 180
m/min or more.
[0018] In the invention of this claim, a magnesium alloy sheet is
rolled (i.e., the sheet is fed) at a speed as high as 180 m/min or
more, preferably at 200 m/min or more; therefore, due to the start
of a high-speed rolling and heat generation associated with the
continuation of the high-speed rolling, the temperature of the
magnesium alloy sheet is raised to a high level at which large
deformation (plastic strain) is enabled (which, however, does not
mean to exclude the possibility that an action other than heat
generation is also related to this improvement of deformability),
thus making it possible to carry out the rolling with a high
rolling reduction, and to easily fabricate a thin magnesium alloy
sheet.
[0019] Since the rolling can be carried out with a high rolling
reduction in this manner, the refinement of crystal grains can be
easily achieved.
[0020] More specifically, the magnesium alloy sheet is subject to
heat generation associated with the rolling that causes a large
plastic strain, and temperature reduction due to the subsequent
heat dissipation to reduction roll (since being obvious, which will
hereinafter be called "roll" in principle), the surroundings or the
like. Owing to at least one of the recrystallization and recovery
associated with these temperature changes, the grain size of the
magnesium alloy sheet becomes smaller than that of the sheet prior
to the processing, and the tensile strength, breaking elongation
and the like are improved.
[0021] Furthermore, by cooling the sheet by heat dissipation or the
like during the rolling or after the rolling, the crystal grains
are inhibited from growing, and are thus kept at a fine level.
[0022] In the invention of this claim, it is only necessary to
increase the processing speed compared with the conventional
technology, and there is no need to make changes to conventional
fabricating process steps whatsoever.
[0023] Further, there is no limit to thinning of products unlike
thixo-molding and die casting, and the process of causing large
deformation can be realized by a process with a small number of
passes, thus making it possible to contribute to lighter weight of
a material, productivity improvement, lower price, and aesthetic
improvement.
[0024] Furthermore, due to heat generation associated with the
high-speed rolling, intermediate annealing and preheating of rolls
in a multi-step rolling (multi-pass processing), which have been
conventionally required, often become unnecessary.
[0025] It should be noted that in the case of an ordinary magnesium
alloy sheet, the deformability of the material is high at a
temperature (temperature range) of about 300.degree. C. or more,
whereas the deformability of the material is low at a temperature
of about 250.degree. C. or less (including a room temperature).
[0026] In the invention of this claim, high heat generation due to
high-speed rolling is utilized as described above. The causes for
"heat generation" not only include heat generation due to
deformation resistance of a part of the sheet itself, which is just
deforming by high speed rolling, and heat generation due to
friction against the rolls, but also include contribution of heat
conduction from a high-temperature area that has already been
rolled, and is located in the top side of this part and adjacent to
this part.
[0027] In general, the rolling is carried out until the temperature
of a magnesium alloy sheet is lowered to a temperature, by heat
dissipation, at which the rolling with a high rolling reduction
becomes difficult. Such "heat dissipation" includes heat conduction
to a part to be rolled henceforward (continuously), temperature
reduction due to heat conduction to rolls, and heat conduction and
radiation to air. The temperature reduction of a rolled material
will occur when large plastic strain has been considerably
developed, new heat generation from the material is reduced, and
heat dissipation becomes greater than heat generation.
[0028] In the invention of this claim, an upper limit to the
rolling speed does not exist as long as the started high-speed
rolling allows the subsequent rolling to be carried out at a high
speed and with a high rolling reduction. It may be said that the
upper limit will not be above an actually operable speed (which is
2800 m/min in a facility used for an embodiment of the present
invention).
[0029] Moreover, "sheet" includes a coiled sheet material.
[0030] An invention according to claim 2 provides a magnesium alloy
sheet processing method based on the above-described magnesium
alloy sheet processing method,
[0031] wherein the magnesium alloy sheet is rolled at a speed of
450 m/min or more.
[0032] In the invention of this claim, the magnesium alloy sheet is
rolled at a very high speed of 450 m/min or more, preferably 500
m/min or more; therefore, the magnesium alloy sheet can be rolled
with a rolling reduction outstandingly higher than that of
conventional rolling.
[0033] Further, due to a high temperature associated with the
rolling carried out at such a very high speed, intermediate
annealing in a multi-step rolling for fabricating a thin sheet and
the like is further unnecessary.
[0034] An invention according to claim 3 provides a magnesium alloy
sheet processing method based on the above-described magnesium
alloy sheet processing method,
[0035] wherein a rolling tool which is not heated is used.
[0036] In the invention of this claim, a rolling tool which is not
heated is used, and therefore, the facility cost, electricity and
heating cost, and operation cost for preheating (i.e., increasing
the temperature of) a rolling tool prior to rolling are
unnecessary.
[0037] It should be noted that the present invention does not
prevent the preheating of a magnesium alloy sheet itself to a
certain extent.
[0038] Further, in an attempt to enable more rapid cooling right
after rolling by providing favorable heat conduction and to keep a
processed surface of the magnesium alloy sheet clean, it is
preferable to wipe off grease in advance and to use no lubricating
oil.
[0039] An invention according to claim 4 provides a magnesium alloy
sheet processing method based on the above-described magnesium
alloy sheet processing method,
[0040] wherein the temperature of the magnesium alloy sheet
immediately before the rolling is in the range of 0.degree. C. to
400.degree. C.
[0041] In the present invention, the rolling can be carried out at
a room temperature, and is thus economically advantageous. On the
other hand, it is not preferable if the temperature upper limit
prior to the start of the rolling exceeds 400.degree. C., because
crystal grains are oversized to make it impossible to obtain
favorable mechanical properties, and an oxide film is formed at a
surface to degrade the appearance.
[0042] It should be noted that depending on the other conditions
such as rolled material and rolling speed, the rolling may be
carried out after having set the temperature in the range of
180.degree. C. to 350.degree. C., or at a specific temperature
(range) such as 200.+-.20.degree. C.
[0043] In particular, in the case of rolling the magnesium alloy
sheet at 450 m/min or more, e.g., 500 m/min or about 2000 m/min,
the temperature range of the magnesium alloy sheet prior to the
start of the rolling is preferably at about 200.degree. C.
Specifically, this is because if the temperature is at such a
level, the temperature of the sheet at the start of the pressing by
rolls might be high or nearly high, which not only easily enables a
high rolling reduction, but also completely prevents the occurrence
of edge cracking. Even if edge cracking has occurred, it is small
edge cracking, and therefore, the present invention significantly
improves the yield of products, and so forth.
[0044] In particular, when a thin magnesium alloy sheet is
fabricated, the number of rolling pass is inevitably increased;
therefore, if the rolling does not allow the occurrence of edge
cracking, the yield of products is significantly improved.
[0045] An invention according to claim 5 provides a magnesium alloy
sheet processing method based on the above-described magnesium
alloy sheet processing method,
[0046] wherein the magnesium alloy sheet immediately after the
rolling is cooled at a speed of 10.degree. C./sec or more.
[0047] In the invention of this claim, the magnesium alloy sheet
heated by the high-speed rolling is rapidly cooled; thus, crystal
grains will not be grown so as to be kept at a fine level, and
eventually, the magnesium alloy sheet having excellent tensile
strength and breaking elongation is obtained.
[0048] Means for rapidly cooling the sheet include methods such as
heat conduction to a processing device, heat conduction to a
subsequent material, and spraying of water by a shower or the
like.
[0049] An invention according to claim 6 provides a magnesium alloy
sheet processing method based on the above-described magnesium
alloy sheet processing method,
[0050] wherein the rolling is carried out with a rolling reduction
of 25%/step or more.
[0051] In the invention of this claim, the rolling is carried out
with a rolling reduction of 25%/step or more, and therefore, the
grain size of sheet material is favorably refined. In particular,
the rolling reduction is preferably 40%/step or more. Such a high
rolling reduction can be realized by increasing the speed of the
rolling.
[0052] Further, since the rolling reduction is 25% or more, the
number of rolling necessary for reaching the final rolling
reduction is reduced, thus improving the productivity and promoting
the cost reduction.
[0053] Herein, "rolling reduction" refers to (sheet thickness prior
to processing--sheet thickness after processing)/(sheet thickness
prior to processing) per one pass rolling.
[0054] In the case of rolling a magnesium alloy sheet at a speed as
high as about 500 m/min, a rolling reduction of 32 to 65%/pass
(%/step) is preferable, and in the case of rolling a magnesium
alloy sheet at a speed as high as about 2000 m/min, a rolling
reduction of 35 to 55%/pass is preferable.
[0055] It should be noted that a rolled sheet is more preferably
heated in advance to about 200.degree. C., which corresponds to
about 55% of the melting point (absolute temperature) of a
magnesium alloy.
[0056] In addition, residual strain and residual stress are
preferably removed from the magnesium alloy sheet by, for example,
maintaining the sheet at 320.degree. C. to 380.degree. C. for about
12 minutes or more prior to the rolling.
[0057] It is preferable to use rolls each having a diameter of
equal to or more than 190 times as much as the thickness of a
magnesium alloy sheet to be rolled. Specifically, in the case of a
sheet having a thickness of 2.5 mm, for example, rolls each having
a diameter of 480 mm or more are preferably used, and rolls each
having a diameter of about 530 mm or more are more preferably used.
By using such rolls to carry out rolling, an angle of bite during
the rolling becomes small, and the rolling with a high rolling
reduction is easily carried out. Further, the contact area (or
length) between the material and rolls during the rolling is also
increased, thus preventing the occurrence of undue stress, such as
large compression and bending occurring locally on the magnesium
alloy sheet. It is believed that this point not only contributes to
the achievement of a high rolling reduction, but also produces an
effect on the rapid cooling after the rolling since the heat
capacity of rolls is increased.
[0058] Furthermore, most suitable magnesium alloy sheets include a
sheet equivalent to ASTM AZ31B or AZ91D, and a magnesium alloy
having a material composition similar to these materials (i.e., a
composition whose basic blending components are more or less
different from the above-mentioned ASTM specification, or a
composition in which the other kind of metal is slightly
blended).
[0059] An invention according to claim 7 provides a magnesium alloy
sheet processed by the magnesium alloy sheet processing method
according to claim 1,
[0060] wherein the sheet includes, as basic blending components,
0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of
Zn, and has a tensile strength of 250 MPa or more and an elongation
of 20% or more.
[0061] The invention of this claim is centered on a magnesium alloy
AZ31B sheet processed by the magnesium alloy sheet processing
method according to claim 1, and including a magnesium alloy sheet
whose deformability in rolling is determined to be not
significantly different therefrom, in the aspects of material and
mechanical properties. Therefore, the sheet of this invention
contains magnesium as a principal material, and basic blending
components (which are essential blending components for allowing
the intrinsic properties of the alloy to be achieved) such as 0.1
to 10.0 weight percent, preferably 2.5 to 3.5 weight percent of Al,
0.1 to 4 weight percent, preferably 2.5 to 3.5 weight percent of
Zn, 0 to 0.5 weight percent of Mn, and the other components, e.g.,
impurity such as 0.04 to 0.01 weight percent or less of Fe, Si, Cu,
Ni, and Ca (which are inevitably mingled more or less in the
current technology).
[0062] Further, the sheet has a tensile strength of 250 MPa or more
and an elongation of 20% or more.
[0063] An invention according to claim 8 provides a magnesium alloy
sheet processed by the magnesium alloy sheet processing method
according to claim 1,
[0064] wherein the sheet includes, as basic blending components,
0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of
Zn, and has an average grain size of less than 4 .mu.m.
[0065] The invention of this claim is centered on a magnesium alloy
AZ31B sheet processed by the magnesium alloy sheet processing
method according to claim 1, and including a magnesium alloy sheet
whose deformability in rolling is determined to be not
significantly different therefrom, in the aspects of material and
metallographic structure. Therefore, the material aspect is same as
that of the invention of claim 7.
[0066] Further, the average grain size is less than 4 .mu.m,
preferably less than 3.2 .mu.m, and more preferably less than 2.4
.mu.m. Therefore, the sheet of this invention is improved not only
in mechanical strength and the like, but also in deformability when
fabricating a casing by a press forming.
[0067] It should be noted that the average grain size in the
present invention refers to an average grain size measured by a
method called an intercept method described below. Specifically, on
an optical microscope microstructure photograph or an electron
microscope microstructure photograph, a line segment having a total
length L (for which a value converted to an actual length by the
magnification of the microstructure photograph is used) is drawn,
and the total number of crystal grains cut by the line segment is
defined as N. When an end of the line segment is located within a
crystal grain, this will be counted as 0.5. And in this case, L/N
is defined as the average grain size. It should be noted that N
represents the number of crystal grains concerning the measurement,
and is desirably 100 or more.
[0068] An invention according to claim 9 provides a magnesium alloy
sheet
[0069] wherein the sheet has an average crystal grain size of 4
.mu.m or less, and does not internally include any unbonded
interface in parallel with a direction of rolling.
[0070] In the magnesium alloy sheet according to the invention of
this claim, the average grain size of a metal is 4 .mu.m or less,
and one piece of sheet is rolled and fabricated; thus, no unbonded
interface of crystal grains can exist within the sheet (however, in
the case of carrying out rolling at a room temperature, a fine
internal crack might occur at 45 degrees with respect to the
rolling direction). Therefore, the magnesium alloy sheet of this
invention has excellent mechanical properties. Such a magnesium
alloy sheet can be fabricated by the method according to any one of
claims 1 to claims 6, for example. In particular, the sheet is
preferably rolled at a relatively low temperature.
[0071] An upper limit to the average grain size in this claim is
defined as 4 .mu.m or less in view of dramatic improvements of
mechanical properties.
[0072] Specifically, in the case of using the processing method of
claim 1, although the average grain size might be, for example, 4.8
.mu.m, which is above 4 .mu.m, depending on the rolling conditions,
the average grain size is preferably 4 .mu.m or less in view of
dramatic improvements of mechanical properties. It should be noted
that even if the average grain size exceeds 4 .mu.m, a magnesium
alloy sheet fabricated by the invention of claim 1, for example,
has excellent mechanical properties compared with a conventional
magnesium alloy sheet, and is included in the invention of the
other claim such as claim 1 as long as the processing method
thereof satisfies the requirements of the invention of claim 1, for
example.
[0073] A lower limit to the average grain size is 2.2 .mu.m in the
embodiment. However, by optimizing the combination of rolling
conditions such as temperature, speed, rolling reduction, and
cooling speed, finer crystal grain structure may be obtained.
Specifically, there is a study example in which a massive sample
having an average grain size of 0.36 .mu.m is obtained by hot or
warm multi-axial forging process on a laboratory scale. Therefore,
also in this invention, by conducting an in-depth experiment in
which more detailed various rolling conditions are set while
centering on a rolling condition that reduces the average grain
size, a rolling condition that further reduces the average grain
size may be found out. Hence, a lower limit to the average grain
size is not defined in the invention of this claim. In other words,
if a magnesium alloy sheet having an average grain size of less
than 2.2 .mu.m, e.g., a magnesium alloy sheet having an average
grain size of 0.36 .mu.m, is proved to be obtainable by further
limiting the rolling conditions such as speed, temperature and
rolling reduction defined in claim 1, for example, so as to create
a new invention, the invention is created by utilizing the
invention of this claim or the inventions of the other claims due
to numerical limitation, and is therefore included in the scope of
the inventions of these claims.
[0074] It should be noted that there is a report about a magnesium
alloy sheet having an average grain size of less than 4 .mu.m which
can be fabricated by an accumulative roll-bonding process (J. A.
del Valle, M. T. Perez-Prado and O. A. Ruano, "Accumulative roll
bonding of a Mg-based AZ61 alloy", Materials Science and
Engineering: A, Volumes 410-411, 25 Nov. 2005, Pages 353-357).
However, in this method, in addition to the necessity of
complicated and numerous steps, one piece of sheet is fabricated by
stacking sheets to carry out rolling and by utilizing solid phase
bonding due to rolling deformation; therefore, this method has a
disadvantage that an unbonded interface, which is in parallel with
a direction of rolling, remains within the sheet. A magnesium alloy
is susceptible to formation of an oxide film at its surface, and
thus has difficulty in avoiding a residual unbonded interface in
particular.
[0075] In the invention of this claim, "does not internally include
any unbonded interface in parallel with a direction of rolling"
purports to remove an unbonded interface formed in parallel with
the rolling direction within the sheet by an accumulative
roll-bonding process or the like as described above. Herein,
"parallel" naturally not only includes the case where something is
geometrically in parallel with something, but also includes the
case where something is metallographically perceived as being in
parallel with the rolling direction.
[0076] Further, "does not include any unbonded interface" does not
purport to remove, from the invention of this claim, the case where
a trace of an unbonded interface partially slightly exists, but
means that an unbonded interface can be perceived as not being
included substantially in terms of metallography.
[0077] An invention according to claim 10 provides a magnesium
alloy sheet
[0078] wherein the sheet has an average grain size of 4 .mu.m or
less, and has an internal grain boundary formed by a clean grain
boundary.
[0079] In the magnesium alloy sheet according to the invention of
this claim, the average grain size of a metal is 4 .mu.m or less,
and an internal grain boundary is formed by a clean grain boundary,
thus dramatically improving mechanical properties of the magnesium
alloy sheet.
[0080] As described above, an accumulative roll-bonding process is
a method for fabricating one piece of sheet, which is a completed
article, by stacking sheets, serving as raw materials, to carry out
rolling and by utilizing solid phase bonding due to rolling
deformation. In the case of performing such a rolling method, a
surface of a sheet serving as a raw material becomes a bonded
interface, and therefore, an oxide formed at the surface of the
sheet serving as a raw material is taken inside one piece of sheet
that is a completed article. Further, such an oxide exists at a
grain boundary without penetration into the inside of crystal
grains.
[0081] A clean grain boundary in the invention of this claim refers
to a grain boundary which is not formed by rolling one piece of
sheet to fabricate a magnesium alloy sheet, but is formed generally
by normal raw material fabricating steps such as melting, casting,
plastic processing, and heat treatment, and which does not include
an oxide or the like formed, for example, in the case of stacking
and rolling a plurality of sheets as described above.
[0082] On the other hand, a grain boundary, at which a small amount
of oxides exist due to slight oxygen inevitably taken in during the
fabrication of a raw material prior to rolling, is included in a
clean grain boundary according to this claim.
[0083] In the invention of this claim, the grain boundary is a
clean grain boundary as described above, and therefore, the sheet
of this invention has mechanical properties superior to those of a
magnesium alloy sheet whose grain boundary is not clean.
[0084] Moreover, in the invention of this claim, the average grain
size is 4 .mu.m or less, and therefore, the magnesium alloy sheet
has extremely excellent mechanical properties.
[0085] The sheet according to the invention of this claim as
described above can be fabricated by using the processing method of
claim 1, for example. It is to be noted that if the processing
method of claim 1 is used as mentioned above, the average grain
size might be, for example, 4.8 .mu.m, which is above 4 .mu.m,
depending on the rolling conditions; however, the invention of this
claim is limited to a magnesium alloy sheet having an average grain
size of 4 .mu.m or less in view of dramatic improvements of
mechanical properties.
[0086] It should be noted that a lower limit to the average grain
size is not defined for the same reason as described in claim
9.
[0087] An invention according to claim 11 provides a magnesium
alloy sheet based on the above-described magnesium alloy sheet,
[0088] wherein the sheet includes, as basic blending components,
0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of
Zn.
[0089] The magnesium alloy sheet according to the invention of this
claim is a magnesium alloy sheet such as AZ31B or AZ91D containing
Al and Zn, which is selected from the magnesium alloy sheets
according to claims 9 and 10, and which has excellent mechanical
properties such as a tensile strength of 250 MPa or more and an
elongation of 20% or more.
[0090] An invention according to claim 12 provides a magnesium
alloy sheet based on the above-described magnesium alloy sheet,
[0091] wherein the sheet includes, as basic blending components,
4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of
Zr.
[0092] The magnesium alloy sheet according to the invention of this
claim is a magnesium alloy sheet such as ZK60A containing Zn and
Zr, which is selected from the magnesium alloy sheets according to
claims 9 and 10, and which has excellent mechanical properties.
[0093] An invention according to claim 13 provides a magnesium
alloy sheet processed by the magnesium alloy sheet processing
method according to claim 1,
[0094] wherein the sheet includes, as basic blending components,
4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of
Zr, and has an average grain size of less than 4 .mu.m.
[0095] The invention of this claim is centered on a magnesium alloy
ZK60A sheet processed by the magnesium alloy sheet processing
method according to claim 1, and including a magnesium alloy sheet
whose deformability in rolling is determined to be not
significantly different therefrom, in the aspects of material and
metallographic structure.
[0096] Further, the average grain size is less than 4 .mu.m,
preferably less than 3.2 .mu.m, and more preferably less than 2.4
.mu.m. Therefore, the sheet of this invention is improved not only
in mechanical strength or the like, but also in deformability when
forming a casing by a press.
[0097] Documents concerning rolling of a magnesium alloy sheet, in
particular a rolling speed thereof, include the following
documents, for example: TABLE-US-00001 (A) USP 5087304 (B) USP
2314010 (C) GB 601388 (D) JP 2004-60048A
[0098] However, in Document (A), the rolling speed is described as
the number of revolutions, and is thus unclear since no absolute
rolling speed is defined.
[0099] Further, the rolling speed is described as being 180 m/min
in Document (B), while the rolling speed is described as being 180
m/min or more in Document (C).
[0100] However, the present invention is based on the following
concept. The magnesium alloy sheet is rolled at a high speed as
described above; therefore, due to the start of the high-speed
rolling and heat generation associated with the continuation of the
high-speed rolling, the temperature of the magnesium alloy sheet is
raised to a high level at which large deformation (plastic strain)
is enabled, thereby making it possible to easily carry out the
rolling with a high rolling reduction, and to easily fabricate a
thin magnesium alloy sheet. Thus, crystal grains can be easily
refined by enabling the rolling with a high rolling reduction in
this manner.
[0101] However, such a concept is not disclosed in Documents (B)
and (C).
[0102] Further, although the applications of the inventions of
Documents (B) and (C) are filed in 1941 and 1945, respectively,
these old inventions are not yet put to practical use. To the
contrary, the effects of the present invention are clearly
confirmed as described in the following embodiments.
[0103] Furthermore, in the invention of Document (D), the rolling
speed is described as being 1.0 m/min or more in claim 4, and is
described as being 3.0 to 21.0 m/min at the best in an embodiment
thereof, thus providing no suggestion of a high-speed rolling
method as described in the invention of this claim, which is
basically different in inventive ideas.
EFFECT OF THE INVENTION
[0104] In the present invention, rolling is carried out at a speed
as high as 180 m/min or more; therefore, even a magnesium alloy
sheet, which has been considered as being unable to be rolled to a
high rolling reduction in one pass due to its low intrinsic
deformability at a low temperature, can be rolled to a high rolling
reduction in one pass at a low temperature.
[0105] Further, in the magnesium alloy sheet, crystal grains are
refined during the rolling that applies a high rolling reduction,
and the sheet is rapidly cooled so that the crystal grains are kept
refined, thus improving mechanical properties such as strength and
the like.
[0106] Furthermore, reheating (intermediate annealing) during each
pass is unnecessary.
[0107] Thus, the mass production of thin magnesium alloy sheets and
outstanding cost reduction are enabled.
[0108] Moreover, the present invention can provide thin sheets and
casings for portable devices made of magnesium alloy, which are
lightweight, have smooth surfaces and good appearances.
BRIEF DESCRIPTION OF THE DRAWINGS
[0109] [FIG. 1] A graph showing the temperature balance of the
sheet and temperature change as and results thereof, when the
rolling speed of an AZ31 sheet is changed with a rolling reduction
of 60% (r=60%) at 20.degree. C.
[0110] [FIG. 2] A graph showing the temperature balance of the
sheet and temperature change as and results thereof, when the
rolling speed of an AZ31 sheet is changed with a rolling reduction
of 60% (r=60%) at 100.degree. C.
[0111] [FIG. 3] A graph showing the temperature balance of the
sheet and temperature change as and results thereof, when the
rolling speed of an AZ31 sheet is changed with a rolling reduction
of 60% (r=60%) at 200.degree. C.
[0112] [FIG. 4] Diagrams showing the appearances of samples rolled
at respective rolling reductions according to embodiments of the
present invention and comparative examples.
[0113] [FIG. 5] A representation (microscope photograph) showing
the crystal grain size of a sheet according to an embodiment of the
present invention.
[0114] [FIG. 6] A representation (microscope photograph) showing
the crystal grain size of the other sheet according to another
embodiment of the present invention.
[0115] [FIG. 7] A representation (microscope photograph) showing
the crystal grain size of a sheet rolled by a conventional
technology.
[0116] [FIG. 8] A representation (microscope photograph) showing
the crystal grain size of a sheet fabricated by an accumulative
roll-bonding process, and the state of an unbonded interface.
DESCRIPTION OF THE REFERENCE CHARACTERS
[0117] 1 cracking [0118] 2 discolored portion [0119] 3 fine flaw or
concave at edge
BEST MODE FOR CARRYING OUT THE INVENTION
[0120] Hereinafter, the present invention will be described based
on the best mode for carrying out the invention. It should be noted
that the present invention would not be limited to the following
embodiments. Various modifications can be made to the following
embodiments within the scope identical to the present invention and
the scope of its equivalence.
[0121] (Rolled Material)
[0122] As a rolled material, a magnesium alloy sheet equivalent to
ASTM AZ31B was prepared. The composition of the sheet includes the
following basic blending materials: 3.0 weight percent of Al; 1.0
weight percent of Zn; and 0.4 weight percent of Mn, and the
remainder of which is magnesium. It is to be noted that the
composition includes 0.3 weight percent of unavoidable impurity at
the maximum, and 0.01 weight percent or less of Si.
[0123] Furthermore, the sample sheet has the following dimensions:
a thickness of 2.5 mm; a width of 30 mm; and a length of 300
mm.
[0124] Moreover, in order to remove residual strain, the sample was
held at 350.degree. C. for 15 minutes prior to rolling, and was
then water-cooled.
[0125] (Rolling Mill)
[0126] A rolling mill used is an experimental high-speed rolling
mill fabricated by Tokushu Kinzoku Excel Co., Ltd., in which the
rolling speed can be adjusted in the range of 100 to 2800 m/min.
Further, the diameter of each of upper and lower reduction rolls is
530 mm, which is 212 times as much as the thickness of the
magnesium alloy sheet to be rolled.
[0127] Furthermore, a low-speed rolling mill fabricated by Tokyo
Roll was also used for a comparative experiment. It should be noted
that the diameter of each roll of this rolling mill is 310 mm.
[0128] (Rolling Method)
[0129] The above-described magnesium alloy sheet was rolled using
the above-mentioned two types of rolling mills while changing the
rolling speed, the rolling temperature of each sample and the
rolling reduction for each rolling. The results are shown in Table
1.
[0130] It should be noted that although the rolling temperature of
the sample was changed from a room temperature to 350.degree. C. at
the maximum, no roll was heated prior to rolling. Therefore, the
temperature of roll surface was at a room temperature (8 to
25.degree. C.) when rolling was started, and was kept at a room
temperature during rolling because the heat capacity of rolls is
large.
[0131] In addition, no lubricating oil was used, and furthermore,
the roll surface was degreased by ethanol prior to rolling.
Moreover, all rolling was carried out for 1 pass (only once). All
the samples after rolling were each rapidly cooled by a
water-cooling shower provided at an exit of the rolling mill.
TABLE-US-00002 TABLE 1 ROLLING ROLLING GRAIN SPEED TEMPERATURE
ROLLING REDUCTION SIZE (m/min) .degree. C. PER PASS % SAMPLE
APPEARANCE .mu.m 2660 8.about.15 62.4 EDGE CRACKING 2000 350 61.2
SOUND 4.8 2000 350 54.4 FINE EDGE CRACKING 4.7 2000 350 45.8 FINE
EDGE CRACKING 4.3 2000 350 38.0 FINE EDGE CRACKING 4.6 2000 200
62.4 SOUND 3.1 2000 200 51.1 SOUND 2.2 2000 200 43.7 SOUND 2.2 2000
200 36.3 SOUND 2000 100 61.1 EDGE CRACKING 2.9 2000 100 54.7 EDGE
CRACKING 2000 100 46.7 EDGE CRACKING 2000 100 42.2 LARGE EDGE
CRACKING 2000 100 33.7 LARGE EDGE CRACKING 2000 8.about.15 61.6
EDGE CRACKING 2000 8.about.15 60.5 EDGE CRACKING 2000 8.about.15
53.1 EDGE CRACKING 2000 8.about.15 42.4 EDGE CRACKING 2000
8.about.15 32.3 EDGE CRACKING 2000 8.about.15 30.0 EDGE CRACKING
500 200 58.2 FINE EDGE CRACKING 3.2 500 200 44.1 FINE EDGE CRACKING
500 100 54.7 EDGE CRACKING 2.4 500 100 41.8 LARGE EDGE CRACKING 500
8.about.15 53.3 LARGE EDGE CRACKING 500 8.about.15 44.1 LARGE EDGE
CRACKING 186 100 50.0 EDGE CRACKING 103 100 50.0 RUPTURE 17.5 350
49 RUPTURE 17.5 350 30.7 SOUND 17.5 350 13.2 SOUND 17.5 200 40.7
RUPTURE 17.5 200 32.1 RUPTURE 17.5 200 26.1 FINE EDGE CRACKING
[0132] (Rolling Test Results)
[0133] First, "edge cracking" will be described.
[0134] In Table 1, "edge cracking" refers to a situation in which a
crack having a size of about 3 to 5 mm has occurred at a widthwise
end of a rolled sheet. "Fine edge cracking" refers to a situation
in which a crack having a size of less than 3 mm has similarly
occurred. Further, "large edge cracking" refers to a situation in
which a crack having a size of greater than 5 mm has similarly
occurred. It is to be noted that the widthwise center of the sheet
was sound in each case. It should be noted that the length of edge
cracking has nothing to do with the sheet width. In other words,
even if the sheet width is increased, edge cracking will not be
enlarged. Therefore, the greater the sheet width, the smaller the
adverse effect of edge cracking on the yield. In actual rolling, a
sheet having a width greater than that of a test piece is rolled in
most cases, and as a result, the occurrence of small edge cracking
will not be a significant problem from a practical standpoint.
[0135] From Table 1, the following facts are determined. Even at a
temperature of 200.degree. C. or less, which has been considered to
be unable to achieve a rolling reduction of 20% or more per pass,
the rolling is carried out only to the extent that edge cracking
occurs at a rolling speed of 186 m/min and with a rolling reduction
of 50%. Thus, the rolling is enabled even at a rolling speed of 180
m/min or more and with a rolling reduction of 60%, and the rolling
is enabled without any problems if it is carried out at a higher
rolling speed and a higher rolling temperature. Further, it can be
seen from Table 1 that the rolling is sufficiently enabled from a
practical standpoint even with an extremely high rolling reduction
of 40% or more if the rolling speed is 500 m/min or more in
particular, i.e., the rolling is enabled while maintaining a
sufficient yield. Furthermore, it can be seen that, if the rolling
speed is 2000 m/min, the rolling is sufficiently enabled even with
an extremely high rolling reduction of 62.4%.
[0136] The reasons for the above facts will be described with
reference to FIGS. 1, 2 and 3. FIGS. 1, 2 and 3 are graphs showing
the temperature balance of the sheet and temperature change as and
results thereof, when the rolling speed of an AZ31 sheet is changed
with a rolling reduction of 60% (r=60%) at 20.degree. C.,
100.degree. C. and 200.degree. C., respectively.
[0137] From FIG. 1, it can be seen that, even though the rolling
speed is relatively as low as 180 m/min (which is, however, higher
than a conventional rolling speed, and is indicated by the vertical
line), the temperature of the rolled material, which has been at
20.degree. C., is raised to 120.degree. C. during the rolling due
to the heat generation caused by processing heat and frictional
heat. From FIG. 2, it can be seen that when the rolling speed is
180 m/min (indicated by the vertical line), the temperature of the
rolled material, which has been at 100.degree. C., is raised to
150.degree. C. during the rolling as a result of deducting the heat
dissipation to rolls from the heat generation caused by processing
heat and frictional heat. From FIG. 3, it can be seen that when the
rolling speed is 180 m/min (indicated by the vertical line), the
temperature of the rolled material, which has been at 200.degree.
C., remains at 200.degree. C. as a result of the heat generation
caused by processing heat and frictional heat, and the heat
dissipation caused by conduction to rolls.
[0138] If the rolling reduction and rolling speed are increased,
the amount of heat generation caused by the processing is increased
because the deformation resistance is increased in accordance with
an increase in deformation speed associated with the rolling.
Further, the frictional heat generation is also increased similarly
since friction is increased due to an increase in rolling pressure.
On the other hand, the heat dissipation due to rolls is decreased
because the contact time is reduced. Therefore, it can be seen that
if the rolling is enabled at 180 m/min at a room temperature, the
rolling is enabled at higher temperature and speed without any
problems also in terms of temperature rise of the rolled
material.
[0139] In particular, when the temperature of the sample is at
200.degree. C., a very excellent deformability in rolling is
achieved even if the rolling speed is as extremely high as 500
m/min or 2000 m/min, and even if the rolling reduction is as
extremely high as 40 to 60% as shown in Table 1.
[0140] Moreover, as shown in Table 1, even at a room temperature,
no rupture occurs although edge cracking or large edge cracking
occurs in the material, as long as the rolling reduction is about
30%. Therefore, it can be assumed that if the rolling speed is 180
m/min or more, the rolling reduction is not large, e.g., about 40
to 60% as shown in Table 1, and if the rolling reduction is about
25%, the rolling can be sufficiently carried out even at a room
temperature. Thus, it can be seen that, even in the range of 0 to
200.degree. C., a rolling reduction of 25% or more per pass is
enabled by setting the rolling speed at 180 m/min or more.
[0141] Next, the appearances of samples, rolled at 200.degree. C.
and at rolling speeds of 17.5 m/min and 2000 m/min, are
conceptually shown in FIG. 4. The left side of FIG. 4 shows the
appearances of the samples each rolled at 17.5 m/min, and the
rolling reduction is 26.1% in (a), 32.1% in (b) and 40.7% in (c).
The right side of FIG. 4 shows the appearances of the samples each
rolled at 2000 m/min, and the rolling reduction is 36.3% in (a),
51.1% in (b) and 61.1% in (c). The length of each arrow extending
vertically at the center is 1 cm.
[0142] Also from FIG. 4, it can be seen that, when the rolling
speed is as low as 17.5 m/min and the rolling reduction is 25% or
more, cracking extending to the entire sheet width or the entire
sheet depth occurs at intervals of 3 to 5 mm. Therefore, the sheet
is cut at intervals of 3 to 5 mm, or even if the sheet is not cut,
the sheet is barely connected. In the diagrams (a), (b) and (c) at
the left side of FIG. 4, each solid line in the sheets, which is
indicated by the reference numeral 1, represents cracking.
[0143] However, it can be seen that when the rolling is carried out
at a speed as very high as 2000 m/min, substantially no flaw is
found even if the rolling reduction is as high as 36.3%, and that
the surface is smooth and the rolling is soundly carried out even
if the rolling reduction is as considerably high as 51.1%, although
slight concaves each having a length of about 1 mm are observed at
intervals of 2 to 3 mm at edges of the sheet. Furthermore, even
when the rolling reduction is as very high as 61.6%, only slight
concaves each having a length of about 1 mm are observed at
intervals of 2 to 3 mm. In the diagrams (a), (b) and (c) at the
right side of FIG. 4, the black portions, indicated by the
reference numeral 2, represent fine discolored portions, while the
small lines indicated by the reference numeral 3 represent fine
flaws or concaves at edges.
[0144] (Structure of Rolled Material)
[0145] Next, the structure of a rolled material will be
described.
[0146] The rolled material is completely recrystallized to almost
uniform grain size since it is rapidly cooled by a shower after
rolling. Further, as can be seen from Table 1, the grain size is 5
.mu.m or less at a rolling temperature of 350.degree. C., and is
extremely fine, e.g., 3 .mu.m or less, at 200.degree. C. in most
cases. Therefore, the mechanical properties and corrosion
resistance of the rolled material are excellent. Although it was
difficult for a conventional fabricating method to achieve such a
fine structure, it was realized by increasing the rolling
speed.
[0147] FIGS. 5 and 6 show microscope photographs of the structures
of sheets each rolled by the method of the present invention.
[0148] FIG. 5 shows the case where the sheet was rolled at a
rolling speed of 2000 m/min, a rolling temperature of 200.degree.
C. and a rolling reduction of 61%, and the average grain size in
this case is 3.1 .mu.m. FIG. 6 shows the case where the sheet was
rolled at a rolling speed of 2000 m/min, a rolling temperature of
200.degree. C. and a rolling reduction of 51%, and the average
grain size in this case is 2.2 .mu.m. In addition, no unbonded
interface exists.
[0149] For reference purposes, FIG. 7 shows an example in which a
conventional rolling method, i.e., hot rolling, was carried out.
The example shown in FIG. 7 is described in Materials Science and
Engineering A, Vol. 410-411 (2005) pp. 308-311, in which rolling
conditions include a temperature of 375.degree. C. and a rolling
reduction of 70%. As can be seen from the photograph, in addition
to that the average grain size is 8 .mu.m, i.e., the grain size is
far larger than that of the sheet rolled by the method of the
present invention, there is formed a so-called "duplex grain
structure" in which crystal grains far larger than the average size
mixedly exist.
[0150] Similarly, for reference purposes, FIG. 8 shows the
photograph of structure of a magnesium alloy AZ61 to which an
accumulative roll-bonding process proposed as a process for
refining crystal grains of a sheet (Patent No. 2961263) was
applied. The example shown in FIG. 8 is described in Materials
Science and Engineering A, Vol. 410-411 (2005) pp. 353-357. The
black line extending laterally at the center of the photograph
indicates an unbonded interface at which stacked sheets remain
unbonded. Accordingly, the strength is not sufficient as compared
with the sheet of the present invention.
[0151] (Mechanical Properties of Rolled Material)
[0152] Four types of samples were rolled under the following four
conditions: a rolling speed of 2000 m/min; a rolling speed of 500
m/min; a rolling temperature of 200.degree. C.; and a rolling
temperature of 100.degree. C., with a rolling reduction of 60%, and
were then rapidly cooled and recrystallized. Then, samples each
having a size of one fifth (a parallel part length of 10 mm and a
parallel part width of 5 mm) of a JIS Z2201-5 test piece were cut
out from the four types of samples, and were subjected to tensile
test at 0.5 mm/min.
[0153] Each of the test pieces had a tensile strength of 250 MPa or
more and a breaking elongation of 20% or more, while some of them
had a breaking elongation of 25% or more. Therefore, it was found
out that they are superior to MP1 (tensile strength: 220 MPa or
more, breaking elongation: 11% or more) equivalent to AZ31B
specified by JIS H4201 in tensile strength and breaking
elongation.
[0154] (Other Material: No. 1)
[0155] AZ91D whose aluminum content is higher than that of AZ31B
was used as a sample, and was subjected to high-speed rolling to
test whether or not it is possible to perform high-speed rolling on
a magnesium alloy other than AZ31B.
[0156] AZ91D contains: 9.0 weight percent of Al; 0.7 weight percent
of Zn; 0.24 weight percent of Mn; and 0.025 weight percent of Si at
the maximum, and the remainder of which is constituted by Mg and
unavoidable impurity.
[0157] From the test results, it was confirmed that a sound rolled
sheet can be obtained at a rolling temperature of 200.degree. C., a
rolling speed of 2000 m/min, and a rolling reduction of
50%/pass.
[0158] (Other Material: No. 2)
[0159] ZK60A was subjected to high-speed rolling at a rolling
temperature of 100.degree. C. to 350.degree. C., a rolling
reduction of 30 to 60%/step, and a rolling speed as high as 1000
m/min, and the results similar to those of AZ31B or the like were
obtained. Specifically, at 100.degree. C., the rolling was enabled
with a rolling reduction of 60% although edge cracking had
occurred. At 150.degree. C. and 200.degree. C., the rolling was
enabled with a rolling reduction of 60% although fine edge cracking
had occurred.
[0160] Further, the average grain size was as fine as 2.5 .mu.m at
a rolling temperature of 350.degree. C. and a rolling reduction of
50%/step.
[0161] ZK60A contains 6.0 weight percent of Zn and 0.5 weight
percent of Zr, and the remainder of which is constituted by Mg and
unavoidable impurity.
[0162] Thus, it was confirmed that the high-speed rolling according
to the present invention is applicable to a wide variety of
magnesium alloys.
* * * * *