U.S. patent application number 12/610846 was filed with the patent office on 2010-02-25 for method of producing a magnesium-alloy material.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Yoshihiro Nakai, Taichiro NISHIKAWA.
Application Number | 20100047109 12/610846 |
Document ID | / |
Family ID | 35782644 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100047109 |
Kind Code |
A1 |
NISHIKAWA; Taichiro ; et
al. |
February 25, 2010 |
METHOD OF PRODUCING A MAGNESIUM-ALLOY MATERIAL
Abstract
The invention offers (a) a method of producing a magnesium-alloy
material, the method being capable of obtaining a magnesium-alloy
material having high strength, (b) a magnesium-alloy material
having excellent strength, and (c) a magnesium-alloy wire having
high strength. A molten magnesium alloy is supplied to a continuous
casting apparatus provided with a movable casting mold to produce a
cast material. The cast material is supplied to between at least
one pair of rolls to perform an area-reducing operation (a rolling
operation). The rolling operation is performed such that pressure
is applied to the cast material using the rolls from at least three
directions in the cross section of the cast material. A
magnesium-alloy material obtained through the above-described
production method has a fine crystal structure and is excellent in
plastic processibility.
Inventors: |
NISHIKAWA; Taichiro; (Osaka,
JP) ; Nakai; Yoshihiro; (Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka
JP
|
Family ID: |
35782644 |
Appl. No.: |
12/610846 |
Filed: |
November 2, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11631361 |
Dec 29, 2006 |
|
|
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12610846 |
|
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Current U.S.
Class: |
420/409 ;
420/402; 420/407; 420/408 |
Current CPC
Class: |
B21B 1/16 20130101; B22D
11/001 20130101; B22D 11/1206 20130101; B21B 1/18 20130101; B21B
3/003 20130101; B21B 13/14 20130101; B22D 11/0602 20130101; C22C
23/06 20130101; C22C 23/02 20130101; C22C 23/04 20130101; C22F 1/06
20130101 |
Class at
Publication: |
420/409 ;
420/402; 420/407; 420/408 |
International
Class: |
C22C 23/04 20060101
C22C023/04; C22C 23/00 20060101 C22C023/00; C22C 23/02 20060101
C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2004 |
JP |
2004-194841 |
Claims
1-7. (canceled)
8. A magnesium-alloy material obtained through a production method
comprising: a casting step for obtaining a cast material by
supplying a molten magnesium alloy to a continuous casting
apparatus provided with a movable casting mold; and a rolling step
for performing an area-reducing operation by supplying the cast
material to one member selected from the group consisting of at
least one roll group and at least two pairs of rolls, in the
rolling step, pressure being applied to the cast material using the
rolls from at least three directions in the cross section of the
cast material, wherein: the rolling step is performed immediately
after the casting step as a continuous step, and the method is
performed such that structure of the magnesium-alloy material
becomes any one of a hot-rolled structure, a mixed structure
composed of a hot-rolled structure and a recrystallized structure,
and a recrystallized structure, and wherein: in the rolling step,
the rolling operation is performed by using a first pair of rolls
having first rotation axes and a second pair of rolls having second
rotation axes, and the first rotation axes are perpendicular to the
second rotation axes so that the first pair of rolls applies
pressure to the cast material from first two directions and the
second pair of rolls applies pressure to the cast material from
second two directions perpendicular to the first two
directions.
9. A magnesium-alloy wire that is obtained through performing a
drawing operation on the rolled magnesium-alloy material of claim 8
obtained through the rolling step and that has a diameter of 5 mm
or less.
10. A magnesium-alloy material that: is produced through a method
comprising: casting step for obtaining a cast material by supplying
a molten magnesium alloy to a continuous casting apparatus provided
with a movable casting mold; and a rolling step for performing an
area-reducing operation by supplying the cast material to one
member selected from the group consisting of at least one roll
group and at least two pairs of rolls, in the rolling step,
pressure being applied to the cast material using the rolls from at
least three directions in the cross section of the cast material,
wherein: the rolling step is performed immediately after the
casting step as a continuous step, and in the rolling step, the
rolling operation is performed by using a first pair of rolls
having first rotation axes and a second pair of rolls having second
rotation axes, and the first rotation axes are perpendicular to the
second rotation axes so that the first pair of rolls applies
pressure to the cast material from first two directions and the
second pair of rolls applies pressure to the cast material from
second two directions perpendicular to the first two directions;
has a crystal structure composed of any one of: a hot-rolled
structure; a hot-rolled structure and a recrystallized structure;
and a recrystallized structure; and contains 0.002 to 5.0 wt. % Ca
and the remainder being composed of any of: Mg and impurities; 0.1
to 12 wt. % Al, and Mg and impurities; 0.1 to 12 wt. % Al; at least
one constituent selected from the group consisting of 0.1 to 2.0
wt. % Mn, 0.1 to 5.0 wt. % Zn, and 0.1 to 5.0 wt. % Si; and Mg and
impurities; and 0.1 to 10 wt. % Zn, 0.1 to 2.0 wt. % Zr, and Mg and
impurities.
11. The magnesium-alloy material as defined by claim 10, the
material having a tensile strength of 200 MPa or more.
12. A magnesium-alloy material that: is produced through a method
comprising: casting step for obtaining a cast material by supplying
a molten magnesium alloy to a continuous casting apparatus provided
with a movable casting mold; and a rolling step for performing an
area-reducing operation by supplying the cast material to one
member selected from the group consisting of at least one roll
group and at least two pairs of rolls, in the rolling step,
pressure being applied to the cast material using the rolls from at
least three directions in the cross section of the cast material,
wherein: the rolling step is performed immediately after the
casting step as a continuous step, and in the rolling step, the
rolling operation is performed by using a first pair of rolls
having first rotation axes and a second pair of rolls having second
rotation axes, and the first rotation axes are perpendicular to the
second rotation axes so that the first pair of rolls applies
pressure to the cast material from first two directions and the
second pair of rolls applies pressure to the cast material from
second two directions perpendicular to the first two directions;
has a crystal structure composed of any one of: a hot-rolled
structure; a hot-rolled structure and a recrystallized structure;
and a recrystallized structure; and contains an added element other
than Mg with a content of at least 5 wt. % and at most 15 wt. % and
the remainder composed of Mg and impurities.
13. The magnesium-alloy material as defined by claim 12, wherein
the added element other than Mg is at least one element selected
from the group consisting of Al, Mn, Zn, Si, Zr, and Y.
14. The magnesium-alloy material as defined by claim 12, wherein
the content of the added element other Mg is at least 9 wt. % and
at most 15 wt. %.
15. The magnesium-alloy material as defined by claim 12, wherein
the magnesium alloy further contains 0.002 to 5.0 wt. % Ca.
16. The magnesium-alloy material as defined by claim 8, wherein
average crystal grain diameter of the magnesium-alloy material is 5
to 20 .mu.m.
17. The magnesium-alloy material as defined by claim 8, wherein a
size of precipitated-out substance is at most 10 .mu.m.
18. The magnesium-alloy material as defined by claim 10, wherein
average crystal grain diameter of the magnesium-alloy material is 5
to 20 .mu.m.
19. The magnesium-alloy material as defined by claim 10, wherein a
size of precipitated-out substance is at most 10 .mu.m.
20. The magnesium-alloy material as defined by claim 12, wherein
average crystal grain diameter of the magnesium-alloy material is 5
to 20 .mu.m.
21. The magnesium-alloy material as defined by claim 12, wherein a
size of precipitated-out substance is at most 10 .mu.m.
Description
RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 11/631,361, filed on Dec. 29, 2006, claiming priority of
Japanese Patent Application No. 2004-194841, filed on Jun. 30,
2004, the entire contents of which are hereby incorporated by
reference.
TECHNICAL FIELD
[0002] The present invention relates to (a) a magnesium-alloy
material having excellent plastic processibility and high strength,
(b) a magnesium-alloy wire having high strength and excellent
toughness, and (c) a method of producing a magnesium-alloy
material, the method being most suitable for obtaining the
foregoing magnesium-alloy material and wire.
BACKGROUND ART
[0003] Magnesium has a specific gravity (a density in g/cm.sup.3 at
20.degree. C.) of 1.74 and is the lightest metal among the metals
used as a structuring material. Consequently, in recent years,
cases have been increasing where it is used as a material for
portable apparatuses and motorcar components, both of which are
required to be light-weight. As the currently employed method of
producing a magnesium-alloy product, the injection casting process
is mainly used, such as the die casting process, the thixomolding
process, and another injection molding process.
[0004] In addition, a magnesium-alloy material having higher
strength can be obtained by performing a plastic processing on a
billet-shaped cast material obtained through the semicontinuous
casting process such as the direct-chill (DC) casting process.
However, a cast material obtained by the semicontinuous casting
process has a large crystal-grain diameter. Therefore, it is
difficult to perform the plastic processing, such as forging,
drawing, and rolling, without a pretreatment. Consequently, it is
known that it is necessary to heat the cast material again to carry
out the extrusion operation under the hot condition in order to
obtain fine crystal grains before performing the above-described
plastic processing. The performing of such a hot extrusion
increases the number of processes. In addition, the productivity
decreases greatly because a magnesium alloy is an active metal and
therefore it is necessary to determine the extrusion speed so that
sufficient cooling can be performed at the time of the extrusion.
In view of the above circumstances, Patent literature 1 has
disclosed that the employment of the continuous casting using a
movable casting mold enables the performing of the hot rolling
without carrying out the extrusion operation in advance. On the
other hand, Patent literature 2 has disclosed that a rolled wire
can be obtained by rolling an ingot of magnesium alloy using
grooved rolls under a specific rolling temperature condition.
[0005] Patent literature 1: Internationally published pamphlet
02/083341 [0006] Patent literature 2: the published Japanese patent
application Tokukai 2004-124152.
DISCLOSURE OF THE INVENTION
Problem to be Solved
[0007] As described in Patent literature 1, the performing of the
continuous casting enables the hot rolling without carrying out the
extrusion operation. However, the rolling operation disclosed in
Patent literature 1 is intended to obtain a sheet material having
excellent pressing processibility. It does not state for a
rod-shaped body. Patent literature 2 uses an ingot without studying
about the continuous casting. As described above, sufficient study
so far has not been conducted on the technique to obtain a
magnesium-alloy material, especially a long rod-shaped body, having
excellent strength and toughness.
[0008] In view of the above circumstances, a principal object of
the present invention is to offer a method of producing a
magnesium-alloy material, the method being capable of obtaining a
magnesium-alloy material having excellent mechanical properties.
Another object of the present invention is to offer a
magnesium-alloy material having excellent strength and a
magnesium-alloy wire having high strength and excellent
toughness.
Means to Solve the Problem
[0009] The present invention attains the foregoing object by
performing a rolling operation on a continuously cast material such
that pressure is applied from at least three directions in the
cross section of the material.
[0010] More specifically, according to the present invention, a
method of producing a magnesium-alloy material comprises (a) a
casting step for obtaining a cast material by supplying a molten
magnesium alloy to a continuous casting apparatus provided with a
movable casting mold and (b) a rolling step for performing an
area-reducing operation by supplying the cast material to one
member selected from the group consisting of at least one roll
group and at least one roll pair. In this case, the rolling is
performed by applying pressure using the rolls from at least three
directions in the cross section of the cast material.
[0011] The present invention is explained below in detail. The
types of the movable casting mold to be used in the production
method of the present invention include (1) a mold comprising a
pair of belts represented by the twin-belt method and (2) a mold
comprising a combination of a plurality of rolls (wheels) and a
belt represented by the wheel-and-belt method. In these movable
casting molds using a roll and/or belt, the surface making contact
with the molten metal emerges continuously. Consequently, it is
easy to obtain a smooth surface of the cast material, and the
maintenance work becomes easy. The movable casting mold described
in (2) above is composed of, for example, (a) a casting roll
provided with a groove into which the molten metal is fed, the
groove being formed at the surface portion (the surface that makes
contact with the molten metal) of the roll, (b) a plurality of
trailing rolls that follow the casting roll, and (c) a belt placed
so as to cover an opening of the groove so that the molten metal
fed into the groove can be prevented from flowing off. In addition,
a tension roller may be combined to the movable casting mold to
adjust the tension of the belt. It is desirable that the belt be
placed so as to form a closed loop through between the rolls and
over the surface of the rolls. When this method is employed, the
following advantage can be achieved. That is, when the moving speed
is adjusted in accordance with both the flow rate of the molten
metal and the cross-sectional area of the movable casting mold (the
cross-sectional area of the portion enclosed by the groove of the
casting roll and the belt), not only can the solidifying surface of
the molten metal be maintained fixed but also the cooling rate at
which the molten metal is solidified can be easily maintained
constant.
[0012] The use of the continuous casting apparatus provided with
the above-described movable casting mold enables the production of
a long cast material whose length is infinite in theory. Therefore,
the mass production of the cast material becomes possible. In
addition, as described above, the performing of the continuous
casting enables the obtaining of a cast material having not only
excellent surface property but also longitudinally uniform high
quality, in particular. In comparison with a billet-shaped cast
material obtained by the semicontinuous casting process and an
ingot obtained by the injection casting process, a cast material
obtained by the continuous casting process is advantageous in the
following points. That is, because the cooling in the cross section
becomes uniform, its crystal-grain diameter is small and therefore
it has a fine crystal structure. In addition, it decreases the
tendency to form coarse precipitated-out substances that become a
starting point of cracking. As a result, a cast material obtained
by the continuous casting process decreases the tendency to form
cracking and other defects in the following rolling step.
Consequently, the rolling operation can be performed sufficiently.
In addition, the obtained rolled material is suitable for plastic
processing such as drawing and forging.
[0013] It is desirable that the foregoing cast material have a
cross section whose minor axis is 60 mm or less, in particular.
When the minor axis is 60 mm or less, the cooling rate at the cross
section of the cast material is increased. Consequently, the size
of the precipitated-out substances formed at the time of the
casting can be decreased to 20 .mu.m or less. In other words, the
obtained cast material can have a finer crystal structure. As a
result, the obtained cast material can become a material more
suitable for rolling and the plastic processing performed after the
rolling.
[0014] In order to increase the cooling rate at the time of
casting, it is desirable that the continuous casting process be
performed either by the twin-belt method or the wheel-and-belt
method. In addition, it is desirable that in the movable casting
mold, at least the portion that makes contact with the molten metal
(i.e., the surface of the groove formed in the roll and the belt's
surface that makes contact with the molten metal) be formed with a
material having high thermal conductivity, such as any of iron,
iron alloy, copper, and copper alloy.
[0015] A magnesium alloy is an extremely active metal. Therefore,
it may burn by easily reacting with oxygen in the air at the time
of the melting of it. In order to effectively prevent the reaction
of a magnesium alloy with oxygen, it is desirable that the melting
be performed under the enclosed condition that is produced by
filling the melting furnace with an inert gas, such as argon gas,
or a mixed gas of air and sulfur hexafluoride (SF.sub.6) gas for
burning prevention, or the like. To achieve an effect of burning
prevention by using the foregoing mixed gas, it is recommended that
air be mixed with 0.1 to 1.0 vol. % SF.sub.6 gas.
[0016] In addition to the time of melting, a magnesium alloy may
also react with oxygen in the air at the time of casting. For
example, at the time of the pouring of the molten metal into the
movable casting mold, more specifically, in the vicinity of the
hole for pouring the molten metal, the molten metal may burn
resulting from the reaction of the magnesium alloy with oxygen in
the air. Furthermore, when the magnesium alloy is cast into the
mold, the alloy sometimes partially oxidizes simultaneously,
thereby turning black the surface of the cast material.
Consequently, it is desirable that even the vicinity of the hole
for pouring the molten metal and the movable casting mold portion
be enclosed by being filled with such a gas as an inert gas, such
as argon gas, or a mixed gas of air and a burning prevention gas,
such as SF.sub.6 gas. When a shielding gas, such as the foregoing
inert gas or air containing a burning prevention gas (a mixed gas),
is not used, it is recommended that the hole for pouring the molten
metal have an enclosed structure in which the mouth has the same
shape as the cross-sectional shape of the movable casting mold.
This structure prevents the molten metal from making contact with
the outside air in the vicinity of the hole for pouring the molten
metal. As a result, the burning and oxidation of the molten metal
can be decreased to obtain a cast material having a good surface
condition.
[0017] In addition, when a magnesium alloy added with an element
having an effect of burning protection and oxidation protection is
used, the same effect as that obtained when the shielding gas is
used can also be obtained. More specifically, the types of the
foregoing magnesium alloy include a magnesium alloy added with
0.002 to 5.0 wt. % Ca. The use of a magnesium alloy containing a
specific amount of Ca decreases the tendency to burn and oxidize at
the time of, for example, the melting and the flowing into the
movable casting mold, even when a shielding gas is not used.
Consequently, the black turning due to the partial oxidation of the
surface of the cast material can be effectively prevented. If the
Ca content is less than 0.002 wt. %, the effect of preventing the
burning and oxidation will be not sufficient. If it is more than
5.0 wt. %, this large amount will cause the generation of cracking
at the time of casting and rolling. In particular, it is desirable
that the Ca content be at least 0.01 wt. % and at most 0.1 wt. %.
Even when the hole for pouring the molten metal is designed to have
an enclosed structure in which the hole has the same shape as the
cross-sectional shape of the movable casting mold, the adding of Ca
to the magnesium alloy can effectively prevent the black turning
due to partial oxidation of the cast material. In this case, the
amount of 0.002 to 0.05 wt. % is suitable as the Ca content. In
order to prevent the black turning due to oxidation and the
cracking at the time of, for example, casting without relying on
the presence of the shielding gas and on the shape of the hole for
pouring the molten metal, it is more desirable that the Ca content
be at least 0.01 wt. % and at most 0.05 wt. %.
[0018] As described above, the use of the shielding gas and the use
of the magnesium alloy added with the oxidation-preventing element
not only suppress the burning and oxidation of the magnesium alloy
at the time of the melting and casting but also decrease the black
turning due to partial oxidation of the surface of the cast
material. The thus obtained cast material is nearly or completely
free from black-turned portions due to partial oxidation at the
surface. Consequently, the cast material has a decreased tendency
to create cracking or other defects originating from the
black-turned portions in the rolling step subsequent to the
casting.
[0019] Next, according to the production method of the present
invention, the cast material obtained by the above-described
continuous casting is processed by rolling. More specifically, the
cast material is supplied to between at least one pair of rolls
(rolling rolls) to undergo pressure application with the rolls for
the processing of area reduction. In particular, in the production
method of the present invention, a bar-shaped body is obtained by
the rolling. In this case, unlike the case where a sheet material
is obtained by rolling (rolls are applied to the cross section of
the material to be rolled from only two directions), in the
production method of the present invention, the rolling is
performed by applying rolls to the cross section of the cast
material from at least three directions. Such a rolling operation
is performed by the following methods, for example: (a) The use of
a group of rolls in which three rolls are combined in a triangular
form, and (b) A plurality of roll pairs are prepared. In each pair,
the rolls are placed in the opposite positions. The roll pairs are
placed at different places along the advancing direction of the
rolling (the direction of the length of the material to be rolled)
such that the center line of the gap between the rolls in one pair
is oriented differently from another pair.
[0020] In the case of (a) above, in which a group of rolls combined
in a triangular form are used, pressure is applied to the cast
material (the material to be rolled) from three directions at the
same place along the advancing direction of the rolling (the
direction of the length of the material to be rolled). It is
desirable to prepare a plurality of such roll groups and to place
the roll groups at different places along the advancing direction
of the rolling such that the orientations of the triangles differ
from one another, because the pressure is applied uniformly onto
the circumferential surface of the cast material (the material to
be rolled). In addition, when a plurality of roll groups are placed
at different places along the advancing direction of the rolling, a
rolled material having an intended size (cress-sectional area) can
be obtained.
[0021] In the case of (b) above, in which a plurality of roll pairs
are used and the roll pairs are placed such that when viewed from a
front position in the advancing direction of the rolling, the
center line of the gap between the rolls of one pair crosses that
of another pair. When the roll pairs are placed as described above,
pressure is applied by the rolls to the cast material (the material
to be rolled) from at least four directions (two directions at two
or more places) at different places along the advancing direction
of the rolling (the direction of the length of the material to be
rolled). For example, two roll pairs are prepared. In one roll
pair, the rolls are placed such that the center line of the gap
between the rolls is oriented horizontally, and in the other roll
pair, the rolls are placed such that the center line of the gap
between the rolls is oriented vertically. In this case, one roll
pair applies the pressure to the cast material (the material to be
rolled) from two directions (i.e., from left and right), and the
other roll pair applies the pressure to the cast material from
different two directions (i.e., from above and down). When a
plurality of such roll pairs are prepared and placed at different
places along the advancing direction of the rolling (the direction
of the length of the material to be rolled), a rolled material
having an intended size (cross-sectional area) can be obtained.
[0022] It is desirable that the above-described rolling be a hot
rolling. A magnesium alloy has a hexagonal close-packed (hcp)
structure, which has poor processibility at room temperature or so.
Therefore, to improve the plastic processibility, it is desirable
to heat the cast material for the rolling operation. More
specifically, it is desirable that the temperature of the cast
material be at least 100.degree. C. and at most 500.degree. C. If
the processing temperature is less than 100.degree. C., cracking
may be created on the surface of the magnesium-alloy material
(which is under the rolling operation) during the rolling,
rendering the rolling impossible. On the other hand, if the
processing temperature is more than 500.degree. C., not only may
the surface of the material be oxidized during the rolling to turn
black but also heat generation and another undesirable phenomenon
accompanying the processing may burn the material in the course of
the processing. In particular, it is desirable that the processing
temperature be at least 150.degree. C. and at most 400.degree. C.
The heating of the cast material may be performed by either of the
following two methods: [0023] (a) a method of heating the cast
material directly by using a heating means such as a heater or a
high-frequency induction heater, and [0024] (b) a method of heating
the cast material indirectly by using a heated rolling roll that is
provided with a heating means such as a heater. In addition, even
when the cast material is heated directly, the rolling roll may be
provided with a heating means so as to be operated under heated
condition. When this system is employed, the magnesium-alloy
material in contact with the rolling roll decreases the tendency to
cool itself, further facilitating the rolling operation.
[0025] The rolling step may be performed immediately after the
casting step as a continuous step. The continuous operation of the
casting step and rolling step enables the utilization of the
remaining heat in the casting step. Consequently, the consumption
of the heat energy can be decreased at the time of the heating of
the cast material in the rolling step. As a result, the continuous
operation can not only decrease the load of the heating means that
directly heats the cast material and the heating means that is
provided in the rolling roll but also reduce the cost. In addition,
the utilization of the remaining heat in the casting step can not
only bring the cast material to a sufficiently heated state but
also decrease the variations in the temperature of the cast
material. Therefore, because the rolling condition, such as the
pressure, is stabilized, the cracking and other defects in the
material at the time of the rolling can also be decreased.
Furthermore, when the continuous casting apparatus and the rolling
apparatus are linearly arranged so that the cast material can be
linearly supplied to the rolling apparatus, the application of
bending and other undesirable effects onto the cast material is
decreased at the time of the supply. As a result, the surface
cracking of the material due to bending can be prevented. When the
rolling is performed immediately after the casting, a heating
means, such as a heater or a high-frequency induction heater, may
be placed between the continuous casting apparatus and the rolling
apparatus provided with the foregoing rolling roll so that the cast
material can be heated.
[0026] The rolling step may conduct a plurality of passes by
providing multiple stages of the above-described roll group or roll
pair or the like. In this case, it is desirable that the total
reduction of area be at least 20%. In particular, it is desirable
that the total reduction of area be at least 50%. When the
processing is performed at a total reduction of area of at least
20%, the cast structure of the magnesium alloy disappears nearly
completely and the structure becomes any one of (a) a hot-rolled
structure, (b) a mixed structure composed of a hot-rolled structure
and a recrystallized structure, and (c) a recrystallized structure.
All of these structures are a fine crystal structure (the average
crystal grain diameter is at most 50 .mu.m). Consequently, the
obtained rolled material has excellent plastic processibility for a
drawing operation and a forging operation, for example. Therefore,
when such a rolled material is further processed by drawing or
forging or the like, a magnesium-alloy material can be easily
obtained, such as a wire and a forged material. In the case of the
recrystallized structure, when the average crystal grain diameter
is 30 .mu.m or less, in particular, the drawing processibility and
the forging processibility are further improved. To improve the
plastic processibility of the rolled material, it is recommendable
to obtain a finer crystal structure. To further decrease the
average crystal grain diameter, it is recommendable to increase the
total reduction of area. On the other hand, if the total reduction
of area is less than 20%, the crystal structure of the rolled
material remains to be the cast structure, which has a large
crystal grain diameter. As a result, such a rolled material tends
to have a poor plastic processibility in the processing to be
performed after the rolling, such as drawing and forging.
[0027] It is desirable that the rolled material produced by the
above-described continuous casting and rolling have a tensile
strength of 200 MPa or more. In particular, it is desirable that
the tensile strength be 250 MPa or more. The rolled material having
such a high strength can improve the processibility in a plastic
processing such as drawing and forging. If the tensile strength is
less than 200 MPa, the foregoing plastic processibility tends to
decrease. Consequently, in comparison with the magnesium-alloy
material obtained by the injection casting process, such as the die
casting and the thixomolding, and the semicontinuous casting
process, the rolled material loses the advantage in strength. The
tensile strength can be varied by controlling the rolling
conditions. For example, the tensile strength can be controlled by
properly selecting not only the rolling temperature and the
reduction of area in one pass but also the total reduction of
area.
[0028] A magnesium-alloy material of the present invention obtained
by the foregoing continuous casting and rolling can be long bodies
(bar-shaped bodies) having various cross-sectional shapes by
variously changing the shape of the rolling roll. For example, it
can have a multiangular bar shape or a circular bar shape.
[0029] When the above-described continuously cast and rolled
material is further processed by plastic processing such as drawing
and forging, a magnesium-alloy material having higher strength can
be obtained. The magnesium-alloy material obtained by further
performing plastic processing on a continuously cast and rolled
material, as described above, has a higher strength than (a) a cast
material produced by the casting other than the continuous casting
and (b) a rolled material produced by rolling the cast material
just described in (a) above. Consequently, when the alloy material
of the present invention is used to produce component parts or the
like, it can produce a small, thin component, thereby enabling not
only a decrease in the number of alloy materials but also a further
decrease in the weight of the component. In other words, the
present invention can offer at low cost a magnesium-alloy material
for flattened or expanded materials. In addition, as described
above, a magnesium-alloy material of the present invention obtained
through the continuous casting and rolling has a plastic
processibility superior to that of an extruded material and
consequently has a large degree of freedom in shape. Therefore,
various shapes can be drawn. For example, when a drawing operation
is performed on an alloy material of the present invention, by
using a specially formed die or roller, a specially formed wire (a
linearly shaped body) can be obtained whose cross section is not
only circular but also noncircular such as elliptical, rectangular,
polygonal, and so on. Furthermore, when a drawing operation is
performed on an alloy material of the present invention with dies
placed in multiple stages, a wire having a diameter as small as 5
mm or less can be obtained.
[0030] A wire obtained by performing a drawing operation on an
alloy material of the present invention obtained through the
continuous casting and rolling can have a strength higher than that
of a wire obtained by performing a drawing operation on an extruded
material produced by extruding an injection cast material or a
semicontinuously cast material. This is attributable to the fact
that because the cooling rate at the time of the continuous casting
is sufficiently higher than that of the injection casting and
semicontinuous casting, the concentration of the solid solution of
the below-described added elements becomes relatively high. In
addition, because a wire obtained by drawing also has excellent
plastic processibility, another plastic processing such as forging
can be further performed. In other words, the wire can be used as a
material for forging operation.
[0031] In the present invention, a magnesium alloy is defined as an
alloy that contains an added element other than Mg and the
remainder composed of Mg and impurities. The use of a magnesium
alloy containing an added element other than Mg can improve the
strength, elongation, high-temperature strength, resistance to
corrosion, and so on of (a) a rolled material produced by the
continuous casting and rolling and (b) a processed material
produced by a plastic processing after the continuous casting and
rolling. The types of such an element to be added include Al, Zn,
Mn, Si, Cu, Ag, Y, and Zr. It is desirable that the total content
of the added elements be 20 wt. % or less. If the total content of
the added elements is more than 20 wt. %, cracking and other
defects in the material may be caused at the time of casting. More
specific compositions are shown below, for example: [0032] (a) a 5
to 15 wt. % element other than Mg and the remainder of Mg and
impurities, [0033] (b) 0.1 to 12 wt. % Al and the remainder of Mg
and impurities, [0034] (c) 0.1 to 12 wt. % Al; at least one
constituent selected from the group consisting of 0.1 to 2.0 wt. %
Mn, 0.1 to 5.0 wt. % Zn, and 0.1 to 5.0 wt. % Si; and the remainder
of Mg and impurities, and [0035] (d) 0.1 to 10 wt. % Zn, 0.1 to 2.0
wt. % Zr, and the remainder of Mg and impurities. The impurities
may either be only the elements contained unintentionally or
contain intentionally added elements (the added elements).
[0036] As the foregoing alloy composition, the family expressed in
the representative symbol in the American Society for Testing and
Materials (ASTM) Specification such as the AZ, AS, AM, and ZK
families may be used. More specifically, the types of the AZ family
include AZ10, AZ21, AZ31, AZ61, AZ80, and AZ91, for example. The
types of the AS family include AS21 and AS41, for example. The
types of the AM family include AM60 and AM100, for example. The
types of the ZK family include ZK40 and ZK60, for example. The Al
content may either be as low concentration as 0.1 wt. % to less
than 2.0 wt. % or as medium or high concentration as 2.0 to 12.0
wt. %.
[0037] A magnesium alloy having an added element other than Mg with
a content of 5 wt. % or more has a tendency to improve the strength
in comparison with the case where the content of the added element
is less than 5 wt. %. Consequently, when such an alloy is used as
the material, the effect of decreasing the weight is great. For
example, AZ61, AZ80, and AZ91 alloys have a strength superior to
that of an AZ31 alloy. The types of such an added element include
at least one element selected from the group consisting of Al, Zn,
Mn, Si, Zr, and Y. It is desirable to contain these elements with a
total content of 5 wt. % or more, particularly desirably 9 wt. % or
more. In addition, when the content of the added elements other
than Mg is increased, it can be expected to further improve the
high-temperature strength and resistance to corrosion. As for the
resistance to corrosion, when the Al content is 8 wt. % or more,
the effect is particularly great. Such a magnesium alloy can have a
resistance to corrosion comparable to that of an Al alloy.
Furthermore, when an alloy contains yttrium with the
above-described content range, the alloy can have excellent tensile
strength and high-temperature strength.
[0038] On the other hand, in the case of the magnesium alloy
containing added elements with high concentration as described
above, when the semicontinuous casting process, such as DC casting,
is performed, precipitated-out substances as large as several tens
of micrometers or so tend to be included. Such coarse inclusions
will cause the creation of cracking at the time of (a) the rolling
operation after the casting and (b) the plastic processing after
the rolling operation, thereby decreasing the productivity
considerably. On the other hand, in the present invention, because
the continuous casting is performed using a movable casting mold,
it is easy to increase the rate of cooling at the time of casting.
More specifically, a rate of 1.degree. C./sec or more, in
particular, 10.degree. C./sec or more, can be easily achieved. As a
result, the size of the precipitated-out substances can be
decreased to 20 .mu.m or less, in particular, 10 .mu.m or less.
Therefore, by performing the continuous casting as in the present
invention, even a magnesium-alloy material containing a high
concentration of added elements can produce a cast material that
has nearly no possibility of creating cracking that originates from
the above-described precipitated-out substance during (a) the
rolling operation after the casting and (b) the plastic processing
after the rolling operation. In addition, in the case of the
continuous casting, as described above, the amount of the solid
solution of the added element will increase after the casting.
Consequently, even when the processing temperature for the rolling
after the casting is increased to as high as 350.degree. C. or
more, the tendency to coarsen the crystal grain will be decreased.
As a resuit, the obtained rolled material has excellent plastic
processibility, facilitating the plastic processing after the
rolling. More over, this obtained rolled material has, as described
above, a fine and uniform crystal structure (not the cast
structure). This fact also gives superior plastic processibility to
this material. The added element has such various effects.
Nevertheless, as described above, when it is added excessively, the
material will increase the tendency to generate cracking and other
defects. Therefore, it is desirable that the content of the added
element be 20 wt. % or less, particularly desirably 15 wt. % or
less.
[0039] In addition, it is desirable that 0.002 to 5.0 wt. % Ca be
added to the above-described composition, because the material can
be prevented from burning and oxidizing at the time of, for
example, the melting and the casting, as described above.
EFFECT OF THE INVENTION
[0040] As explained above, the production method of the present
invention carries out a rolling operation on a cast material
produced by the continuous casting such that pressure is applied
from at least three directions in the cross section of the
material. This method can offer a specific effect that a
magnesium-alloy material can be obtained that has excellent
mechanical properties such as strength. In particular, a long
magnesium-alloy material can be obtained that has a decreased
tendency to produce cracking and other defects during the casting
and rolling and that has an excellent surface property over its
length.
[0041] In addition, the containment of a specified amount of
element for preventing burning can effectively prevent the burning
and oxidation of the material at the time of the melting, the
pouring of the molten metal, and the casting.
[0042] A magnesium-alloy material of the present invention obtained
through the above-described continuous casting and rolling has a
fine structure. Consequently, it is excellent in plastic
processibility and therefore can undergo plastic processing such as
drawing and forging. A magnesium-alloy material of the present
invention having undergone the plastic processing has high strength
and high toughness and is light-weight. Because it has these
features, it can be used in various fields. In addition, a
magnesium-alloy material of the present invention having undergone
a plastic processing can be further processed by forging and the
like. In other words, a magnesium-alloy material of the present
invention can be used as a material for forging, for example.
BEST MODE FOR CARRYING OUT THE INVENTION
[0043] Embodiments of the present invention are explained
below.
Test Example 1
[0044] A cast material was produced by performing a continuous
casting on a molten magnesium alloy using a wheel-and-belt-type
continuous casting apparatus. The obtained cast material was
examined to clarify the surface property and the structure.
[0045] The magnesium alloy used in this test was an AZ31 alloy
equivalent material. Its composition was analyzed by chemical
analysis. The result was shown in wt. % as follows: Al: 3.0%, Zn:
1.0%, Mn: 0.15%, and the remainder: Mg and impurities including
0.0013% Ca, which was not added intentionally.
[0046] FIG. 1 shows a continuous casting apparatus used in this
test. FIG. 1 emphasizes a cast material 1 in showing it. This is
also applicable to FIG. 2 described below. A continuous casting
apparatus 10 comprises (a) a casting roll 11 provided with a groove
11a into which a molten metal is poured, the groove 11a being
formed at the surface portion that makes contact with the molten
metal, (b) two trailing rolls 12a and 12b that move following the
casting roll 11, (c) a belt 13 provided so as to cover an opening
of the groove 11a so that the molten metal poured into the groove
11a can be prevented from flowing out, and (d) a tension roll 12c
for adjusting the tension of the belt 13. In this example, as shown
in FIG. 1(A), the trailing rolls 12a and 12b are placed at the
opposite positions in terms of the casting roll 11. The tension
roll 12c is placed behind the three rolls 11, 12a, and 12b (the
right-hand side in FIG. 1(A)). The belt 13 is placed so as to form
a closed loop by circulating it between the rolls 11 and 12a,
between the rolls 11 and 12b, and over the circumference of the
roll 12c. In this structure, when the casting roll 11 rotates in a
direction shown by an arrow, the rolls 12a to 12c rotate in turn
through the belt 13. A supplying section (nozzle) 14 is placed
between the casting roll 11 and the trailing rolls 12a. The
supplying section 14 is provided with a hole for pouring the molten
metal (a spout) to which the molten metal is fed from a melting
furnace (see FIG. 2 described below). The molten metal fed from the
melting furnace to the supplying section 14 flows into the groove
11a of the casting roll 11 through the hole for pouring the molten
metal. The opening is covered with the belt 13. Thus, the cast
material 1 having a rectangular cross section as shown in FIG. 1(B)
is obtained.
[0047] In this example, the surface portion of the groove 11a with
which the molten metal makes contact was formed with SUS430, which
has excellent resistance to heat. The groove 11a had a
cross-sectional area of about 300 mm.sup.2 (width: 18 mm, height:
17 mm). The belt 13 was formed of pure copper (C1020) and had a
thickness of 2 mm. Furthermore, in this example, cooling water was
fed to the inside of the casting roll 11 so that the roll 11 could
be cooled. In this example, the flow rate or the cooling water was
set to be 30 liter/min. In addition, in this example, the hole for
pouring the molten metal, which was provided at the supplying
section 14, was designed to have the same cross-sectional shape as
that of the groove 11a of the casting roll 11. What is more, the
section between the hole for pouring the molten metal and the
casting roll 11 was made to be an enclosed structure, so that the
molten metal in this section could not make contact with the
outside air.
[0048] In this example, the melting furnace had a mixed-gas
atmosphere in which air is mixed with 0.2 vol. % SF.sub.6 gas. The
magnesium alloy having the above-described alloy composition was
melted at 700 to 800.degree. C. A molten metal composed of the
magnesium alloy was poured into a tundish through a launder heated
at about 500.degree. C. Then, the molten metal was fed from the
tundish and was poured into the movable casting mold through the
supplying section and the hole for pouring the molten metal to
perform the continuous casting at a speed of 3 m/min. In this
example, because the melting of the magnesium alloy was conducted
in an atmosphere having mixed SF.sub.6 gas, problems such as
combustion of the alloy during the melting were not created.
Although a mixed gas of SF.sub.6 gas and air was used in this
example, an inert gas such as argon gas may be employed to fill the
melting furnace with an inert atmosphere.
[0049] The cross section of the obtained cast material was examined
under an optical microscope. Although precipitated-out substances
were observed, their size was 10 .mu.m at the most. It had a fine
crystal structure. However, it was found that in the obtained cast
material, only a small part of the surface was turned black due to
oxidation. This is attributable to the fact that although Ca was
unavoidably contained in the magnesium alloy, because only the
section between the hole for pouring the molten metal and the
casting roll was made to be an enclosed structure, the molten metal
was brought into contact with outside air at a place such as the
launder portion, so that the molten metal was oxidized. In view of
the above result, another cast material containing Ca was produced
by adding 0.01 wt. % Ca to the foregoing alloy structure and by
carrying out the continuous casting under the same condition as
above. When the surface of the Ca-containing cast material was
examined, no black turning due to oxidation was observed. In
addition, by varying the Ca content, cast materials were produced
by carrying out the continuous casting under the same condition.
The examination of the surface property revealed that as the Ca
content increases, the cast material decreases the tendency to be
oxidized. Nevertheless, when the Ca content exceeds 5 wt. %, it was
observed that some cast materials created surface cracking. The
result shows that when a magnesium alloy is used that contains a
specific amount of Ca, the oxidation can be prevented effectively
without producing surface cracking.
Test Example 2
[0050] The continuous casting apparatus (see FIG. 1(A)) used in
Test example 1 above was provided, in the vicinity of it, with a
rolling apparatus comprising pairs of rolls. A cast material
obtained by the continuous casting was subjected to a rolling
operation directly after the casting operation to produce a rolled
material. The magnesium alloy used in this test was produced by
adding 0.01 wt. % Ca to the AZ31 alloy equivalent material used in
Test example 1 above.
[0051] FIG. 2 shows a production line used in this test. The line
comprises a continuous casting apparatus and a rolling apparatus.
In FIG. 2, the same sign as used in FIG. 1 shows the same item.
This production line is provided with the following units in this
order for the production: a melting furnace 15, a continuous
casting apparatus 10, (guide rolls 40), a heating means 30, a
rolling apparatus 20, and a take-up device 50. The continuous
casting apparatus 10 and the rolling apparatus 20 were placed such
that the cast material 1 having left the continuous casting
apparatus 10 is linearly introduced into the rolling apparatus 20.
The rolling apparatus 20 comprises linearly arranged four two-stage
rolling machines 20A to 20D, each of which is provided with two
rolling-roll pairs 21a and 21b. In each of the two-stage rolling
machines 20A to 20D, the two rolling-roll pairs are placed such
that the center line of the gap between the rolls 21 of one pair is
oriented to a direction different from that of the other pair (the
two center lines cross each other). More specifically, of the two
rolling-roll pairs, in the rolling-roll pair 21a, the rolls 21 are
placed such that the center line of the gap between the rolls 21 is
oriented horizontally, and in the other rolling-roll pair 21b, the
rolls 21 are placed such that the center line of the gap between
the rolls 21 is oriented vertically. In other words, the
rolling-roll pair 21a was placed in the vertical position (the
up-and-down position in FIG. 2) to the cast material 1. On the
other hand, the rolling-roll pair 21b was placed in the horizontal
position (the position perpendicular to the sheet of paper in FIG.
2) to the cast material 1. Each of the rolling-rolls 21 was
provided with a heater (not shown) at the inside of it to enable
the heating of the rolling-roll 21. In addition, because the
temperature of the cast material 1 in the vicinity of the exit of
the continuous casting apparatus 10 became about 150.degree. C.,
the heating means 30 was placed in front of the rolling apparatus
20. As a result, it was possible to directly heat the cast material
1 using the heating means 30 before the rolling operation. In this
example, as the heating means 30, a high-frequency induction heater
was used.
[0052] As with Test example 1, the melting furnace 15 had a
mixed-gas atmosphere in which air is mixed with 0.2 vol. % SF.sub.6
gas. A magnesium alloy containing Ca was melted at 700 to
800.degree. C. in the furnace 15. The obtained molten metal was
poured into a tundish 17 through a launder 16 heated at about
500.degree. C. The molten metal was fed from the tundish 17 to the
supplying section 14, to the hole for pouring the molten metal, and
to the continuous casting apparatus 10 to obtain a cast material 1
(cross-sectional area: about 300 mm.sup.2). The casting speed was
set to be 3 m/min. Subsequently, the obtained cast material 1 was
sent to the heating means 30 through the guide rolls 40 to heat the
cast material 1 up to about 400.degree. C. The heated cast material
1 was then sent to the rolling apparatus 20 to be processed by
rolling. In this example, the rolling operation was performed while
the individual rolling rolls 21 were being heated at 150.degree. C.
with the heater. In each of the rolling machines 20A to 20D, the
reduction of area was set to be 15% to 20%. The total reduction of
area was about 56%. The obtained rolled material 2 was a long body
(a rod-shaped body) having a circular cross section with a diameter
of 13 mm. The long body was wound up with the take-up device
50.
[0053] The thus obtained continuously cast and rolled material was
subjected to the observation under an optical microscope. When its
structure was examined at the cross section, the cast structure
disappeared completely and the structure was composed of a
hot-rolled structure and a recrystallized structure. The average
crystal grain diameter of the rolled material was measured to be 20
.mu.m. Although precipitated-out substances were observed in the
rolled material, their size was 10 .mu.m at the most. The tensile
strength of the rolled material was measured to be 250 MPa. In
other words, it was confirmed that the material had a strength that
satisfied the desirable value of 200 MPa or more.
[0054] A specimen having a diameter of 8 mm and a length of 12 mm
was taken from the above-described continuously cast and rolled
material. The specimen was subjected to a hot upsetting at a
temperature of 300.degree. C. (upsetting speed: 12 mm/sec,
upsetting rate: 70% (height: 3.6 mm)). The result showed that the
upsetting was successfully performed without creating cracking and
another defect on the surface of the specimen. On the other hand,
for comparison, a commercially available extruded material
(diameter: 8 mm, length: 12 mm) made of an AZ31 alloy was also
subjected to the hot upsetting under the same condition. The result
showed that the processing at an upsetting rate of 70% created
surface cracking. When the crystal structure at a cross section of
the extruded material was examined under an optical microscope,
precipitated-out substances having a size of about 30 .mu.m were
observed. Therefore, the precipitated-out substances are considered
to be the cause of the cracking.
Test Example 3
[0055] The continuously cast and rolled material obtained in Test
example 2 (the long body having a diameter of 13 mm) was processed
by drawing using drawing dies to obtain a wire. The strength and
toughness of the wire were examined. In this test, the processing
temperature was set to be 200.degree. C., and the reduction of area
for one pass was 10% to 15%. In every two to three passes, a heat
treatment was conducted at 300.degree. C. for 30 min. Thus, a wire
was obtained that had a circular cross section with a diameter of
2.8 mm (total reduction of area: about 95%) The tensile strength
and elongation of the obtained wire were examined. The wire had a
tensile strength of 310 MPa and an elongation of 15%. In other
words, the wire was excellent in both strength and toughness. The
number of breakings of the wire during the drawing operation was
0.5 times per kg.
[0056] For comparison, a commercially available extruded material
(diameter: 13 mm) made of an AZ31 alloy was also processed by
drawing under the same condition as above to obtain a wire having a
diameter of 2.8 mm. The tensile strength and elongation of the
obtained wire were examined. The wire had a tensile strength of 290
MPa and an elongation of 15%. As described above, the result showed
that the wire produced by using the continuously cast and rolled
material had a property superior to that of the extruded wire. In
addition, when the extruded wire was used, the number of breakings
of the wire during the drawing operation was 2.0 times per kg. This
result showed that the use of the continuously cast and rolled
material is superior in drawing processibility. In other words, the
above test confirmed that the use of the continuously cast and
rolled material can improve the tensile strength without reducing
the elongation.
Test Example 4
[0057] Magnesium alloys were prepared that had a composition
different from that of the magnesium alloy used in the
above-described Test examples. Using the prepared magnesium alloys,
continuously cast and rolled materials were produced through the
same method as above. The compositions of the alloys used are shown
below.
(Alloy Composition)
[0058] An AM60 alloy (a magnesium alloy): Al: 6.1 wt. %, Mn: 0.44
wt. %, and the remainder: Mg and impurities. An AZ61 alloy (a
magnesium alloy): Al: 6.4 wt. %, Zn: 1.0 wt. %, Mn: 0.28 wt. %, and
the remainder: Mg and impurities. An AZ91 alloy (a magnesium
alloy): Al: 9.0 wt. %, Zn: 1.0 wt. %, and the remainder: Mg and
impurities. A ZK60 alloy (a magnesium alloy): Zn: 5.5 wt. %, Zr:
0.45 wt. %, and the remainder: Mg and impurities. A Y-containing
alloy (a magnesium alloy): Zn: 2.5 wt. %, Y: 6.8 wt. %, and the
remainder: Mg and impurities.
[0059] Alloys produced by further adding 0.01 wt. % Ca individually
to the foregoing AM60 alloy, AZ61 alloy, AZ91 alloy, ZK60 alloy,
and Y-containing alloy.
[0060] The thus obtained individual continuously cast and rolled
materials were subjected to the examination under an optical
microscope. When their structure was examined at the cross section,
in all of the rolled materials, the cast structure disappeared
completely and the structure was composed of any one of (a) a
hot-rolled structure, (b) a mixed structure having a hot-rolled
structure and a recrystallized structure, and (c) a recrystallized
structure. The average crystal grain diameter of these rolled
materials was measured to be 5 to 20 .mu.m. The maximum grain
diameter of the precipitated-out substances was 3 to 10 .mu.m. In
other words, they had a fine structure. In addition, all of the
continuously cast and rolled materials had a tensile strength of
200 MPa or more. In other words, they had an excellent strength.
These continuously cast and rolled materials were processed by
drawing as with Test example 3. The obtained wires had high
strength and excellent toughness as with Test example 3. Some of
the alloys having no added Ca showed partial black turning due to
oxidation on the surface of the cast material. On the other hand,
the alloys having added Ca showed no oxidation on the surface of
the cast material.
[0061] It is commonly known that an AZ91 alloy material is usually
difficult to process by extrusion. Nevertheless, in the present
invention, by performing a rolling operation immediately after the
continuous casting, it was possible to obtain a rod-shaped material
and a multiangular material by using even an AZ91 alloy equivalent
material. This is attributable to the fact that because the cooling
rate at the time of the continuous casting is sufficiently higher
than that of a semicontinuous casting, the increase in the amount
of the solid solution of the added element, such as Al or Zn,
decreases the tendency to grow the crystal grains even at the
temperature range for the hot rolling operation, which is
350.degree. C. or more.
Test Example 5
[0062] The continuous casting apparatus and rolling apparatus shown
in FIG. 2 were used to produce a continuously cast material and a
continuously cast and rolled material. The obtained continuously
cast material was subjected to an examination of the structure. The
obtained continuously cast and rolled material was subjected to an
examination of the structure, strength, and plastic
processibility.
[0063] The magnesium alloy used in this test was an AZ91 alloy
equivalent mateal. Its composition was analyzed by chemical
analysis. The result was shown in wt. % as follows: Al: 9.0%, Zn:
1.0%, Mn: 0.2%, and the remainder: Mg and impurities including
0.0013% Ca, which was not added intentionally.
[0064] The specification of the continuous casting apparatus was
the same as that in Test example 1. The specification of the
melting furnace and the like was the same as that in Test example
2. A continuous casting was performed under the following
conditions: melting temperature: 700.degree. C., casting speed: 3
m/min, and cooling rate: 50 to 100.degree. C./sec. Thus, a cast
material having a cross-sectional area of about 300 mm.sup.2
(width: 18 mm, height: 17 mm). The cross section of the obtained
cast material was examined under an optical microscope. Although
precipitated-out substances were observed, their size was 10 .mu.m
or less. It had a fine crystal structure.
[0065] The specification of the rolling apparatus was the same as
that in Test example 2. The obtained cast material was heated at
about 400.degree. C. using a heating means and was sent to the
rolling apparatus. The rolling operation was performed under the
same condition as that in Test example 2. Thus, a long rolled
material having a circular cross section with a diameter of 13 mm
was obtained. The obtained continuously cast and rolled material
was subjected to an examination under an optical microscope. When
its structure was examined at the cross section, the cast structure
disappeared completely and the structure was composed of a
hot-rolled structure and a recrystallized structure. The average
crystal grain diameter of the rolled material was measured to be 9
.mu.m. In addition, although precipitated-out substances were
observed in the rolled material, their size was 10 .mu.m at the
most. The tensile strength of the rolled material was measured to
be 300 MPa.
[0066] The obtained continuously cast and rolled material was
subjected to a processing of hot upsetting. More specifically, a
specimen having a diameter of 8 mm and a length of 12 mm was taken
from the above-described continuously cast and rolled material. The
specimen was subjected to a hot upsetting at a temperature of
300.degree. C. (upsetting speed: 12 mm/sec, upsetting rate: 80%
(height: 2.4 mm)). The result showed that the upsetting was
successfully performed without creating cracking and another defect
on the surface of the specimen. On the other hand, for comparison,
a commercially available extruded material (diameter: 8 mm, length:
12 mm) made of an AZ91 alloy was also subjected to the hot
upsetting under the same condition. The result showed that the
processing at an upsetting rate of 50% created surface
cracking.
Industrial Applicability
[0067] The present invention can offer a method of producing a
magnesium-alloy material. The method can be utilized suitably for
the production of a magnesium-alloy material having high strength
and excellent plastic processibility. The method can offer the
alloy material with high productivity. In addition, a continuously
cast and rolled material obtained through the production method of
the present invention has excellent strength and toughness and
therefore can be used suitably as a material for plastic
processing. Furthermore, a magnesium-alloy material of the present
invention obtained by performing a plastic processing on the
continuously cast and rolled material not only has high strength
and high toughness but also is light-weight. Consequently, it is
suitable as a material for components of a portable apparatus, a
motorcar, and the like. In particular, a magnesium-alloy wire of
the present invention obtained by performing a drawing operation is
suitable as a welding wire, a material for a screw, and a material
for forging operation.
BRIEF DESCRIPTION OF THE DRAWING
[0068] FIG. 1(A) is a schematic diagram showing the constitution of
a continuous casting apparatus used in Test examples 1 to 5, and
FIG. 1 (B) is a partial cross section explaining a state in which a
belt is placed on a casting roll.
[0069] FIG. 2 is a schematic diagram showing the constitution of a
production line system used in Test examples 2 to 5, the production
line system being provided with a continuous casting apparatus and
a rolling apparatus in tandem.
EXPLANATION OF THE SIGN
[0070] 1: cast material; 2: rolled material; 10: continuous casting
apparatus; 11: casting roll; 11a: groove; 12a, 12b: trailing roll;
12c: tension roll; 13: belt; 14: supplying section; 15: melting
furnace; 16: launder; 17: tundish; 20: rolling apparatus; 20A, 20B,
20C, 20D: two-stage rolling machine; 21: rolling roll; 21a, 21b:
rolling roll pair; 30: heating means; 40: guide roll; and 50:
take-up device.
* * * * *