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.

Description

CROSS REFERENCE RELATED APPLICATION

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

1. Field of the Invention

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.

2. Description of the Related Art

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.

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.

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.

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

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.

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).

Furthermore, the present invention provides a cold-rolled magnesium alloy sheet having improved formability manufactured by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

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;

FIGS. 2A to 2C are optical photomicrographs which show microscopic textures at a center point, a ¼ 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);

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);

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

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

The invention will be more particularly described with reference to the accompanying drawings.

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).

The operations of the method according to the present invention will now be more specifically described.

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.

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.

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.

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.

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.

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.

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.

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° 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° C., but higher than ambient temperature, can be produced.

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.

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.

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.

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.

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.

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.

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

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×12 mm×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.

Thereafter, the composite sheet was homogenized at a low temperature of 100° 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° 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.

FIGS. 2A to 2C are optical photomicrographs which show microscopic textures at three points (a center point, a ¼ 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.

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

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×12 mm×1.2 mm.

EXPERIMENTAL EXAMPLE

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.

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⁻¹ and a temperature of 100° C., and then the measurement results were represented in FIG. 4.

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.

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.

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.

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° 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.