U.S. patent application number 14/189549 was filed with the patent office on 2014-09-18 for method of manufacturing cold-rolled magnesium alloy sheet for improving formability and cold-rolled magnesium alloy sheet having improved formability manufactured thereby.
This patent application is currently assigned to KOREA INSTITUTE OF MACHINERY AND MATERIALS. The applicant listed for this patent is KOREA INSTITUTE OF MACHINERY AND MATERIALS. Invention is credited to Se-Jong Kim, Young-Seon Lee.
Application Number | 20140271332 14/189549 |
Document ID | / |
Family ID | 51527791 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140271332 |
Kind Code |
A1 |
Kim; Se-Jong ; et
al. |
September 18, 2014 |
Method of Manufacturing Cold-rolled Magnesium Alloy Sheet for
Improving Formability and Cold-rolled Magnesium Alloy Sheet Having
Improved Formability Manufactured thereby
Abstract
A method of manufacturing a cold-rolled magnesium alloy sheet
for improving formability, including, (a) preparing a composite
sheet which is composed of a main sheet made of magnesium alloy and
a restrainer made of steel and functioning to suppress transverse
plastic deformation (plastic deformation in a transverse direction)
of the main sheet during a rolling process, (b) performing a
cold-rolling of the composite sheet, and (c) separating the
cold-rolled magnesium alloy sheet from the cold-rolled composite
sheet obtained from (b). The magnesium alloy sheets manufactured by
the invention has improved formability at an ambient temperature,
and thus can be usefully applied to extensive industrial fields
such as automobiles and aircrafts.
Inventors: |
Kim; Se-Jong; (Changwon-si,
KR) ; Lee; Young-Seon; (Changwon-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA INSTITUTE OF MACHINERY AND MATERIALS |
Daejeon |
|
KR |
|
|
Assignee: |
KOREA INSTITUTE OF MACHINERY AND
MATERIALS
Daejeon
KR
|
Family ID: |
51527791 |
Appl. No.: |
14/189549 |
Filed: |
February 25, 2014 |
Current U.S.
Class: |
420/409 ;
72/250 |
Current CPC
Class: |
B32B 15/01 20130101;
B21B 39/02 20130101; B21B 1/22 20130101; C22C 23/02 20130101; B21B
2001/221 20130101; B21B 2001/383 20130101; C22F 1/06 20130101 |
Class at
Publication: |
420/409 ;
72/250 |
International
Class: |
B21B 39/02 20060101
B21B039/02; B21B 1/22 20060101 B21B001/22; C22C 23/02 20060101
C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2013 |
KR |
10-2013-0026578 |
Claims
1. A method of manufacturing a cold-rolled magnesium alloy sheet
for improving formability, comprising: (a) preparing a composite
sheet which is composed of a main sheet made of magnesium alloy and
a restrainer made of steel and functioning to suppress transverse
plastic deformation (plastic deformation in a transverse direction)
of the main sheet during a rolling process, (b) performing a
cold-rolling of the composite sheet, and (c) separating the
cold-rolled magnesium alloy sheet from the cold-rolled composite
sheet obtained from (b).
2. The method according to claim 1, wherein the restrainer includes
a groove formed in a side thereof across opposite ends thereof, and
the main magnesium alloy sheet is disposed in the groove of the
restrainer.
3. The method according to claim 2, wherein both lateral sides of
the main magnesium alloy sheet are in contact with mating sides of
the groove of the restrainer without gaps therebetween.
4. The method according to claim 1, wherein the main magnesium
alloy sheet in (a) is a hot-rolled AZ31 sheet having a composition
of Al: 3 weight %, Zn: 1 weight %, Mn: 0.3 weight % and Mg:
balance.
5. The method according to claim 1, wherein (b) is performed at a
temperature of an ambient temperature -100.degree. C.
6. The method according to claim 1, wherein the main magnesium
alloy sheet of the composite sheet is coupled with the restrainer
without a gap therebetween.
7. A cold-rolled magnesium alloy sheet having an improved
formability, manufactured by a method according to any one of
claims 1 to 6.
8. The cold-rolled magnesium alloy sheet according to claim 7,
wherein the cold-rolled magnesium alloy sheet has a limit dome
height (LDH) which is increased by 20% or more, compared to a
magnesium alloy sheet which is not subjected to the cold-rolling
process.
Description
CROSS REFERENCE RELATED APPLICATION
[0001] This application claims foreign priority of Korean Patent
Application No. 10-2013-0026578, filed on Mar. 13, 2013, which is
incorporated by reference in its entirety into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a method of
manufacturing a cold-rolled magnesium alloy sheet for improving
formability and a cold-rolled magnesium alloy sheet having improved
formability manufactured thereby, and more particularly, to a
method of manufacturing a cold-rolled magnesium alloy sheet using a
composite sheet including a restrainer functioning to suppress
transverse plastic deformation of a magnesium alloy sheet so as to
improve formability and a cold-rolled magnesium alloy sheet having
improved formability manufactured thereby.
[0004] 2. Description of the Related Art
[0005] A variety of sheet-working processes such as a press working
are being used for the production of sheets in industrial fields
such as automobiles and aircrafts. During a sheet-working process,
a sheet workpiece partially or completely undergoes a complicated
deformation such as stretching, bending, flanging, deep drawing or
combinations thereof.
[0006] Therefore, in a process of working sheets, formability,
which indicates a degree to which a sheet can be plastically
deformed without fracture, is critical above all physical
properties.
[0007] Although a magnesium alloy is getting a lot of attention as
a structural alloy material which exhibits a low specific gravity
and superior specific strength and stiffness, the magnesium alloy
does not have a sufficient number of slip systems but has a limited
number of active slip systems because it has a hexagonal close
packing crystal structure. In particular, since the slip system in
the basal texture, which is formed in the magnesium alloy by
hot-rolling or extrusion, has a Schmid factor close to zero for a
deformation along a c-axis, formability at an ambient temperature
is deteriorated which inhibits its extensive applications.
[0008] Accordingly, in order to employ the magnesium alloy sheet in
more various industrial fields than current application fields,
there is a compelling need for development of a magnesium alloy
sheet having superior formability at an ambient temperature.
SUMMARY OF THE INVENTION
[0009] Accordingly, the present invention has been made keeping in
mind the above problems occurring in the prior art, and an object
of the present invention is to provide a method of manufacturing a
cold-rolled magnesium alloy sheet for improving formability and a
cold-rolled magnesium alloy sheet having improved formability
manufactured thereby.
[0010] In order to accomplish the above object, the present
invention provides a method of manufacturing a cold-rolled
magnesium alloy sheet for improving formability, comprising: (a)
preparing a composite sheet which is composed of a main sheet made
of magnesium alloy and a restrainer made of steel and functioning
to suppress transverse plastic deformation (plastic deformation in
a transverse direction) of the main sheet during a rolling process,
(b) performing a cold-rolling of the composite sheet, and (c)
separating the cold-rolled magnesium alloy sheet from the
cold-rolled composite sheet obtained from (b).
[0011] Furthermore, the present invention provides a cold-rolled
magnesium alloy sheet having improved formability manufactured by
the method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0013] FIG. 1 is a schematic view illustrating an embodiment of a
composite sheet used in a method of manufacturing a cold-rolled
magnesium alloy sheet for improving formability, according to the
present invention;
[0014] FIGS. 2A to 2C are optical photomicrographs which show
microscopic textures at a center point, a 1/4 point and an edge
point positioned in a transverse direction (TD) across an end face
of the specimen prepared in an example, in which the end face is
perpendicular to the rolling direction (RD);
[0015] FIGS. 3A and 3B are an inverse pole figure (IPF) map in a
normal direction and a (0001) pole figure which are taken at an
edge area of an end face of the specimen prepared in the example
using an electron back scatter diffraction (EBSD), in which the end
face is perpendicular to the rolling direction (RD);
[0016] FIG. 4 is a graph showing tension test results of magnesium
alloy specimens prepared in an example according to the present
invention and in a comparative example; and
[0017] FIG. 5 is a graph showing a limit dome height (LDH) results
of the magnesium alloy specimens prepared in the example according
to the present invention and in the comparative example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The invention will be more particularly described with
reference to the accompanying drawings.
[0019] A method of manufacturing a cold rolled magnesium alloy
sheet for improving formability comprises (a) preparing a composite
sheet which is composed of a main sheet made of magnesium alloy and
a restrainer made of steel and functioning to suppress transverse
plastic deformation (plastic deformation in a transverse direction)
of the main sheet during a rolling process, (b) performing a
cold-rolling of the composite sheet, and (c) separating the
cold-rolled magnesium alloy sheet from the cold-rolled composite
sheet obtained from the operation (b).
[0020] The operations of the method according to the present
invention will now be more specifically described.
[0021] Operation (a) of a method according to the present invention
is performed in order to produce a composite sheet which is
composed of a main sheet made of magnesium alloy and a restrainer
made of steel and functioning to suppress transverse plastic
deformation (deformation in a transverse direction) of the main
sheet during a rolling process.
[0022] In this description, the composite sheet is defined as
sheet-shaped product composed of different kinds of materials, that
is, magnesium and steel, in which the different materials are
separately prepared and then coupled to each other thus providing
the sheet-shaped product.
[0023] In the composite sheet, the magnesium sheet, which is the
target to be formed through a cold-rolling process, is coupled to a
restrainer that functions to suppress plastic deformation in a
transverse direction (MD) perpendicular to a rolling direction (RD)
during cold-rolling process, with the result that the magnesium
sheet is contained in the composite sheet according to the present
invention.
[0024] In this context, the restrainer may have any appropriate
shape without any limitation as long as it can apply a compressive
stress which is capable of suppressing plastic deformation in a
transverse direction of the magnesium sheet during a rolling
process.
[0025] In a case where a composite sheet 1 which is configured to
have a shape and a structure, for example, as illustrated in FIG.
1, operation (a) may be fulfilled in such a way that a restrainer 3
having a groove is prepared by processing a hot-rolled sheet, a
main sheet 2 is prepared by processing a hot-rolled magnesium alloy
sheet to have dimensions corresponding to the shape of the groove,
and the magnesium sheet 2 is disposed in the groove of the
restrainer 3. In case of necessity, the main sheet 2 may be
temporarily bonded to the restrainer 3 such that the final
composite sheet can be effectively produced in a cold-rolling
operation to be described later.
[0026] The classes of magnesium alloy constituting the main sheet
and steel constituting the restrainer are not limited to any
specific class as long as they can be rolled at a desired rolling
reduction and stiffness of the steel is higher that of the
magnesium alloy. When the stiffness of the steel constituting the
restrainer is higher than that of the magnesium alloy constituting
the main sheet, the restrainer will apply a compressive stress
against expansion of the magnesium sheet in a transverse direction
during a subsequent rolling operation of the composite sheet, and
thus rolling properties of the composite sheet can be effectively
improved when the cold-rolling operation is performed in operation
(b) to be fully described later.
[0027] Operation (b) of the method according to the present
invention, which is intended to cold-roll the composite sheet which
is prepared in operation (a), may be fulfilled using a well-known
rolling machine and a well-known rolling technology.
[0028] In this operation, the composite sheet, which has been
prepared in a manner described in detail above, is cold-rolled.
Thanks to this operation (b), edge cracks do not occur even in the
magnesium alloy sheet which must be conventionally hot-rolled at a
sufficiently high temperature of higher than 230.degree. C. in
order to avoid occurrence of cracks, and a sheet, which has an
intact and fine texture even when being cold-rolled at a low
temperature of lower than 100.degree. C., but higher than ambient
temperature, can be produced.
[0029] More specifically, the restrainer, which is composed of
steel, applies a compressive stress to the magnesium alloy sheet in
a transverse direction (TD) during a cold-rolling process to thus
inhibit occurrence of edge cracks and cause tensile twinning, with
the result that a ductility of the sheet can be significantly
improved and thus a high-quality sheet can be produced even through
a cold-rolling process.
[0030] According to the present invention, magnesium alloy, which
is inevitably subjected to a hot-rolling process because it has a
low ductility and thus a low workability at an ambient temperature,
can also be manufactured into a high-quality sheet. Therefore, it
is possible to prevent the shortening of service life of a roll
caused by the heating of working material and rolling rolls, and it
is possible to reduce production costs owing to energy saving and
thus to manufacture an economical and high-quality sheet.
[0031] Operation (c) of the method according to the present
invention, which is intended to separate the cold-rolled magnesium
alloy sheet from the composite sheet obtained in operation (b), may
employ any solution as long as it can separate the cold-rolled
magnesium alloy sheet from the restrainer.
[0032] Since the cold-rolled magnesium alloy sheet manufactured by
the method according to the present invention has an ambient
temperature formability which is remarkably improved compared to
the magnesium alloy sheet which is not subjected to the
cold-rolling process, it is expected that it gives advantages in
economic efficiency and simplifies processing in various industrial
fields such as automobiles and aircrafts and thus the application
field of the magnesium alloy sheet is enormously broadened.
[0033] Specifically, the cold-rolled magnesium alloy sheet having
an improved formability, according to the present invention, is
characterized in that it has a limit dome height (LDH) at an
ambient temperature which is increased by 20% or more, compared to
the magnesium alloy sheet which is not subjected to the
cold-rolling process.
[0034] In this regard, LDH, which refers to an index which is
extensively used to evaluate a press-formability among formability
by various processes, is defined in such a way that a disc-shaped
specimen having a predetermined dimension is held at a
circumference thereof by a certain force, the specimen is deformed
by a spherical punch which has a predetermined diameter and moves a
predetermined speed, a deformed height of the specimen is taken as
the LDH when the specimen is broken.
[0035] The present invention will now be described in detail with
reference to an example. The example is set forth to illustrate,
but is not to be construed as the limit of the present
invention.
EXAMPLE
[0036] A commercial hot-rolled AZ31 (Al: 3 weight %, Zn: 1 weight
%, Mn: 0.3 weight % and Mg: balance) sheet having a thickness of
1.2 mm was processed to prepare a specimen having a dimension of 90
mm.times.12 mm.times.1.2 mm, and a groove was formed between the
opposite ends on a side of a hot-rolled steel sheet (C: 0.12 weight
%, Si: 0.20 weight %, Mn: 0.87 weight % and Fe: balance). The
specimen was fitted in the groove of the steel sheet to prepare a
composite sheet having a shape and a structure as shown in FIG.
1.
[0037] Thereafter, the composite sheet was homogenized at a low
temperature of 100.degree. C. for 20 minutes, and was then
cold-rolled at a rolling reduction of 3% using a rolling machine
equipped with rolls having a diameter of 250 mm under the
conditions of a rolling temperature of 100.degree. C. equal to the
homogenization temperature and a rolling speed of 2 rpm.
Subsequently, the magnesium alloy specimen was separated from the
cold-rolled composite sheet.
[0038] FIGS. 2A to 2C are optical photomicrographs which show
microscopic textures at three points (a center point, a 1/4 point
and an edge point) positioned in a transverse direction (TD) across
an end face of the specimen prepared in the example, in which the
end face is perpendicular to the rolling direction (RD). From the
optical photomicrographs, it is ascertained that densities of
twinning are similar to each other regardless of measurement
location.
[0039] FIGS. 3A and 3B are an inverse pole figure (IPF) map in a
normal direction and a (0001) pole figure which are taken at an
edge area of an end face of the specimen prepared in the example
using an electron back scatter diffraction (EBSD), in which the end
face is perpendicular to the rolling direction (RD). From the
figures, it is found that most of twinnings are of {10-12} tensile
twinning. Furthermore, it is also ascertained that a compressive
stress was applied to the magnesium alloy sheet in a transverse
direction (TD) during the rolling process due to the steel sheet
which has a strength higher than the magnesium alloy sheet in a
transverse direction (TD), from the fact that face {0001} was
turned in a transverse direction (TD).
COMPARATIVE EXAMPLE
[0040] A commercial hot-rolled AZ31 (Al: 3 weight %, Zn: 1 weight
%, Mn: 0.3 weight % and Mg: balance) sheet having a thickness of
1.2 mm was processed to prepare a specimen having a dimension of 90
mm.times.12 mm.times.1.2 mm.
EXPERIMENTAL EXAMPLE
[0041] Measurement of tensile strength and formability are measured
for the magnesium alloy sheet specimen prepared in the example and
the comparative example is executed as follows.
[0042] For the tension test, a tension test specimen which has a
gauge length of 25 mm, a gauge width of 6 mm and a thickness of
1.16 mm was prepared to be configured to be extended in a rolling
direction. The tension test was executed at a strain rate of 0.001
s.sup.-1 and a temperature of 100.degree. C., and then the
measurement results were represented in FIG. 4.
[0043] For an additional LDH test, a disc-shaped specimen which has
a diameter of 50 mm and a thickness 1.16 mm was prepared. The
specimen was interposed between upper and lower dies of an LDH test
equipment, and was then held by a force of 15 kN. Subsequently, the
specimen was deformed by a spherical punch having a diameter of 25
mm which moves toward the specimen at a speed of 0.02 mm/sec. When
the specimen was broken, a deformed height of the specimen was
measured, and the measurement results were represented in FIG.
5.
[0044] From FIGS. 4 and 5, it is ascertained that the magnesium
alloy specimen prepared in the example and the magnesium alloy
specimen prepared in the comparative example have tension strengths
of 275 MPa and 272 MPa, respectively, which are almost same as each
other, whereas yield strengths of both the specimens were 187 MPa
and 144 MPa in which the yield strength of the specimen prepared in
the example was increased compared to that of the specimen prepared
in the comparative example. As for formability, it is ascertained
that the cold-rolled magnesium alloy specimen prepared in the
example exhibits a remarkable formability which is improved by 23%
compared to the magnesium alloy specimen prepared in the
comparative example.
[0045] Although the preferred example of the present invention has
been disclosed for illustrative purposes, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *