EP2644728A2

EP2644728A2 - Magnesium alloy sheet having superior formability at room temperature, and method for manufacturing same

Info

Publication number: EP2644728A2
Application number: EP11843068.5A
Authority: EP
Inventors: Nack Joon Kim; Jun Ho Bae; Dong Wook Kim; Do Hyang Kim
Original assignee: Academy Industry Foundation of POSTECH
Current assignee: Academy Industry Foundation of POSTECH
Priority date: 2010-11-23
Filing date: 2011-11-23
Publication date: 2013-10-02
Also published as: EP2644728A4; KR20120055304A; WO2012070870A3; KR101303585B1; WO2012070870A2

Abstract

The present invention provides a magnesium alloy plate having high formability, in which Ca is added to a Mg-Zn alloy as a precipitation enhancing alloy, and a precipitation behavior is improved by twin-roll strip casting and subsequent heat treatment to obtain high strength and low anisotropy, and thus press formability is greatly improved compared to a conventional magnesium alloy plate.

The magnesium alloy plate includes : Zn: 1 ∼ 10 wt%; Ca: 0.1 ∼ 5 wt%; and balances of magnesium (Mg) and inevitable impurities, wherein the magnesium alloy plate has a limiting dome height (LDH) of 7 mm or more.

Description

Technical Field

The present invention relates to a magnesium alloy plate having excellent formability at room temperature and a method of manufacturing the same. More particularly, the present invention relates to a method of manufacturing a magnesium alloy plate, wherein a magnesium alloy plate having excellent press formability can be realized by secondary phase control using alloy ingredients added to magnesium, strip casting and subsequent heat treatment, and wherein a magnesium alloy plate having high strength can be obtained by additional heat treatment after forming, and to a magnesium alloy plate manufactured using the method.

Background Art

A magnesium alloy is an alloy for structural materials having the lowest specific gravity, high specific strength and excellent toughness. Recently, demand for magnesium alloys has increased as cases for portable appliances and materials for automobiles, which are required to become lightweight.
Meanwhile, research into magnesium alloys has generally been conducted for the purpose of improving the high-temperature physical properties thereof in order to apply them to automobile engines, gear parts and the like, whereas research into magnesium alloys that can be applied to various fields such as plates and the like has been insufficiently conducted.
In order to use a magnesium alloy plate in various fields, it is required to develop a magnesium alloy plate having excellent formability such that it can be formed into parts having various shapes. For the purpose of reflecting this requirement, research into magnesium alloy plates having excellent formability at high temperature has recently been conducted.
Moreover, in order to use a magnesium alloy plate in a wider variety of fields, it is required to develop a magnesium alloy plate having excellent formability at room temperature.
Meanwhile, as a conventional method of manufacturing a magnesium alloy plate, there is known a method of manufacturing a magnesium alloy plate having targeted thickness by hot-extruding and hot-rolling a cast material obtained by general casting or semi-continuous casting such as die casting. This method is characterized in that a cast material having a large crystal grain size is formed into a cast material having a small crystal grain size by hot extruding. Meanwhile, since magnesium is a metal having high activity, it is easily surface-blackened or burned by the heat generated during hot extruding. Therefore, in the hot extruding process of magnesium, magnesium must be extruded such that it can be cooled to such a degree that it is not surface-blackened or burned, so there is a limitation to increase an extruding speed. That is, conventionally, a hot extruding process, which is necessarily used to manufacture a magnesium plate, has been a major cause of decreasing productivity and increasing a manufacturing cost. Moreover, since there is a limitation of miniaturizing crystal grains by only a hot extruding process, there is a problem in that it is difficult to process magnesium in complex, attractive shapes.
In order to solve such a problem, as disclosed in Korean Unexamined Patent Application Publication No. 2010-38809 , the present inventors proposed a magnesium alloy plate, the press formability of which is improved by adding yttrium (Y) to a Mg-Zn alloy in consideration of the content of zinc (Zn), microfabricating the tissue of the Mg-Zn alloy by strip casting and subsequent heat treatment and then controlling the behavior of the dispersed phase thereof.
However, this magnesium alloy plate is also problematic in that it uses expensive yttrium and has lower press formability than that of a commonly-used aluminum plate, and thus its application is limited.

Disclosure

Technical Problem

Accordingly, the present invention has been devised to solve the above-mentioned problems, and an object of the present invention is to provide a magnesium alloy plate which can be manufactured at low cost using cheap alloy elements and which can be suitably used to manufacture complicated and various parts because it has press formability equivalent to that of a commonly-used aluminum plate, and a method of manufacturing the same.

Technical Solution

In order to accomplish the above object, an aspect of the present invention provides a magnesium alloy plate including Zn and Ca as alloy elements. Here, the magnesium alloy has a limiting dome height (LDH) of 7 mm or more, and preferably 8 mm or more.
The limiting dome height (LDH) is referred to as an index for evaluating the formability, particularly, press formability of a plate. In the present invention, as shown in FIG. 1, the limiting dome height (LDH) means the movement distance of a punch (that is, the deformation height of a sample) taken until a disk-shaped sample is fractured when the periphery of the disk-shaped sample having a diameter of 50 mm and a thickness of 0.7 mm is fixed by force of 5 kN and is then deformed at a rate of 0.1 mm/sec by a spherical punch having a diameter of 27 mm.
The magnesium alloy plate according to the present invention may include Zn: 1 ∼ 10 wt%, and preferably 1 ∼ 7 wt%; and Ca: 0.1 ∼ 5 wt%, and preferably 0.5 ∼ 3 wt%.
Further, the magnesium alloy plate according to the present invention may have a microtexture having an average crystal grain size of 10 µm or less.
Further, the magnesium alloy plate according to the present invention may have a yield strength (YS) of 200 MPa or more, an ultimate tensile strength (UTS) of 270 MPa or more and an elongation rate (EL) of 12% or more.
Further, the magnesium alloy plate according to the present invention may have a (0002) plane having a texture intensity of 2.5 or less.
In order to accomplish the above object, another aspect of the present invention provides a method of manufacturing a magnesium alloy plate having high formability, including the steps of: (a) preparing a molten alloy including Zn: 1 ∼ 10 wt%, Ca: 0.1 ∼ 5 wt% and balances of magnesium (Mg) and inevitable impurities; (b) maintaining a temperature of the molten alloy in a range of a temperature at which a liquid fraction is 70% to a ignition temperature; (c) injecting the temperature-maintained molten alloy between two cooling rollers and strip-casting this molten alloy to form a magnesium alloy plate; (d) solution-treating the formed magnesium alloy plate at 300 ∼ 490°C for 1 ∼ 24 hours; (e) preheating the solution-treated magnesium alloy plate at 300 ∼ 400°C and then rolling this magnesium alloy plate to a thickness required at a rate of 1 ∼ 45% per pass using a heated rolling roller; and (f) solution-treating the rolled magnesium alloy plate at 300 ∼ 490°C for 0.5 ∼ 4 hours.
In the method of manufacturing a magnesium alloy plate according to the present invention, in the step (c), the interval between the two cooling rollers may be maintained at 1 ∼ 5 mm, and the rotation speed of each of the cooling rollers may be maintained at 0.2 ∼ 20 m/min, thus maintaining a cooling rate of the molten alloy at 10² ∼ 10³ K/s.
Further, in the method of manufacturing a magnesium alloy plate according to the present invention, the amount of Zn may be 1 ∼ 10 wt%, and preferably 1 ∼ 7 wt%, and the amount of Ca may be 0.1 ∼ 5 wt%, and preferably 0.5 ∼ 3 wt%.
Further, the method of manufacturing a magnesium alloy plate according to the present invention may further include the step of aging the solution-treated magnesium alloy plate at 150 ∼ 200°C for 1 ∼ 72 hours after the step of rolling the magnesium alloy plate.
Further, in the method of manufacturing a magnesium alloy plate according to the present invention, Ca may be added by the addition of a Mg-Ca matrix alloy. The reason for this is that pure Ca is not easily added in a desired amount because its melting point is high. It is preferred that the Mg-Ca matrix alloy be a Mg-Ca(2∼3.5wt%) matrix alloy.
The reasons for limitation of the alloy composition and manufacturing process in the present invention are described as follows.
The maximum solid solubility of Zn in a Mg matrix is 6.2 wt% at 340°C. When Zn is added in an amount of 1.0 wt% or more, a needle-shaped precipitate is formed by heat treatment, and thus an age-enhancing behavior occurs. Therefore, when Zn is added in an amount of less than 1.0 wt%, a precipitation enhancement phenomenon is hardly expected, and when Zn is added in an amount of more than 10 wt%, the precipitation of an equilibrium phase at a crystal grain boundary is promoted to deteriorate the mechanical properties of the aluminum alloy plate. Therefore, it is preferred that the amount of Zn be 1 ∼ 10 wt%. Meanwhile, in a Mg-Zn binary alloy, when a suitable amount of Zn is added, a non-basal plane softening phenomenon occurs to activate non-basal plane slip, but when an excess amount of Zn is added, a non-basal plane enhancing phenomenon occurs, and the mechanical properties of the magnesium alloy deteriorates. Therefore, in order to maximize the non-base plane slip and precipitation enhancing effects of Zn, it is more preferred that the upper limit of Zn be limited to 7 wt%.
Ca is an element effective at improving the high-temperature strength of a magnesium alloy. When the amount of Ca is less than 0.1 wt%, a high-temperature strength improving effect is insufficient, and when the amount of Ca is more than 5 wt%, the malleability of a magnesium alloy is deteriorated, and the flowability of a molten magnesium alloy is decreased, so the castability of the magnesium alloy is deteriorated, hot tear easily occurs, and the adhesivity between the magnesium alloy and a mold is increased during a solidification process, thereby decreasing productivity. Therefore, it is preferred that the amount of Ca be 0.1 ∼ 5 wt%. In this case, when Ca is added in an amount of 0.5 ∼ 3 wt%, the effects thereof can be maximized. Therefore, it is more preferred that the amount of Ca be 0.5 ∼ 3 wt%.
In the present invention, inevitable impurities are referred to as ingredients unintentionally mixed in raw materials or unintentionally introduced in a manufacturing process. The amount of the inevitable impurities may be 0.5 wt% or less, and preferably 0.01 wt% or less, such that the inevitable impurities do not influence the physical properties of the magnesium alloy of the present invention. Particularly, among the inevitable impurities, Fe, Ni, Cr, Cu, Co and the like have a detrimental influence on the corrosion resistance of the magnesium alloy, and thus it is required to control the amount thereof to 0.005 wt% or less.
Further, when the average crystal grain size of a microtexture of the magnesium alloy plate is more than 10 µm, the strength and formability of the magnesium alloy plate are deteriorated. Therefore, it is preferred that the average crystal grain size thereof be 10 µm or less.
Further, the increase in texture intensity of a magnesium alloy deteriorates the formability of magnesium having a small amount of slip system. When the texture intensity of a (0002) plane (basal plane) of the magnesium alloy plate is more than 2.5, it is difficult to realize press formability equal to that of a magnesium alloy. Therefore, it is preferred that the texture intensity thereof be 2.5 or less, and more preferably, 2.2 or less.
According to the method of the present invention, in the step (b), when the temperature of the molten alloy is lower than the temperature at which a liquid fraction is 70%, the viscosity of the molten alloy is increased, and thus the molten alloy is solidified before it is in contact with the cooling roller in the step (c) to prevent the molten alloy from escaping from the cooling rollers. Further, when the temperature of the molten alloy is higher than the ignition temperature thereof, this process cannot be conducted. Therefore, the temperature of the molten alloy must be maintained in the above range.
Further, in the step (c), when the cooling rate of the molten alloy is less than 10² K/s, there is a problem in that, since the molten alloy is slowly cooled, this molten alloy is not greatly different from a molten alloy prepared by general mold casting in microtexture, and the flow of the molten alloy may become unstable before casting. Further, when the cooling rate thereof is more than 10³ K/s, this rate cannot be easily attained by commercial technigues, except for a rapid cooling process which is applied to formation of a thin ribbon. Therefore, it is preferred that the cooling rate thereof be maintained at 10² ∼ 10³ K/s. Further, when the interval between the two cooling rollers is maintained at 10 mm or less, it is advantageous to obtain the above cooling rate. In the present invention, when the cooling rate of the molten alloy in the step (c) is rapid, there is an advantage in that the texture of the molten alloy is microfabricated, and the segregation of the molten alloy is reduced. Further, when the cooling rate thereof is slow, there is an advantage in that intermetallic compounds having a detrimental influence on the tensile characteristics of the molten alloy are finely dispersed in a matrix. Moreover, in this case, in the step of casting the molten alloy, since a relatively thin plate can be manufactured compared to when another casting method is used, the thickness reduction ratio and roll pass in a rolling process can be reduced, so the texture generated from the rolling process can be minimized, thereby reducing the anisotropy of a plate having a bad influence on press formability.
Further, since the properties of the magnesium alloy plate formed by strip-casting the molten alloy can become non-uniform by the segregation of alloy elements at the time of post-treating this magnesium alloy, it is preferred that this magnesium alloy plate be solution-treated. In this case, the solution-treatment temperature and time of the magnesium alloy plate are set in accordance with the diffusivity and SDAS (secondary dendrite arm spacing) of Zn as a main alloy element, whether or not incipient melting exists (measured by DTA/DSC) and the oxidation degree of the magnesium alloy plate. The sufficient solution-treatment result can be obtained only when the solution treatment of the magnesium alloy plate is performed at 300 ∼ 490°C for 1 ∼ 24 hours.
Further, in the step of preheating the solution-treated magnesium alloy plate at 300 ∼ 400°C and then rolling this magnesium alloy plate to a thickness required at a rate of 1 ∼ 45% per pass using a heated rolling roller, when the above preheating temperature range (processing temperature range) is not maintained, it is difficult to obtain a strong magnesium alloy plate, so it is preferred that the above preheating temperature range be maintained. Further, as the thickness reduction ratio of the magnesium alloy plate increases, the texture of the magnesium alloy plate is enhanced, and thus the formability of the magnesium alloy plate deteriorates. Therefore, it is preferred that the thickness reduction ratio of the magnesium alloy plate per pass be maintained in a range of 1 ∼ 45%.
Further, when the magnesium alloy plate is not heat-treated at 300 ∼ 490°C for 0.5 ∼ 4 hours after rolling the magnesium alloy plate, the non-uniform characteristics of the magnesium alloy plate after post-processing cannot be sufficiently removed, so it is preferred that the above condition be maintained.
Further, in order to improve the tensile characteristics of the magnesium alloy plate, the method may further include the step of aging the solution-treated magnesium alloy plate at 150 ∼ 200°C for 1 ∼ 96 hours after rolling the magnesium alloy plate. The reason for this is that the tensile characteristics of the magnesium alloy plate can be most efficiently improved under the above heat treatment condition.

Advantageous Effects

According to the present invention, unlike a conventional method of manufacturing a commonly-used magnesium alloy plate, there is provided a magnesium alloy plate having formability at room temperature which can be widely applied in the field of automobile and electronic industries because its strength, extensibility and formability are improved compared to those of a conventional commonly-used magnesium alloy plate by the design of alloy ingredients suitable for twin-roll strip casting, the miniaturization of crystal grains using strip casting and subsequent heat treatment, the formation of intermetallic compounds and the control of volume fraction.
Further, according to the method of manufacturing a magnesium alloy plate of the present invention, a magnesium alloy plate can be manufactured at low cost compared to a conventional commonly-used magnesium alloy plate because the number of processes in this method is decreased compared to the number of processes in a conventional method. Further, according to this method, the formation of texture can be minimized, and thus improved press formability can be obtained because the final amount of supplied magnesium alloy can be greatly reduced.

Description of Drawings

FIG. 1 is a schematic view showing a strip casting apparatus for manufacturing a magnesium alloy plate according to an embodiment of the present invention.
FIG. 2 is a schematic view showing a method of evaluating the limiting dome height of a magnesium alloy plate according to the present invention.
FIG. 3 is a photograph showing a microtexture of a magnesium alloy plate obtained by strip casting according to an embodiment of the present invention, wherein the microtexture thereof was observed by an optical microscope after the magnesium alloy plate was heat-treated at 440°CC for 1 hour.
FIG. 4 is a photograph showing a microtexture of a magnesium alloy plate obtained by strip casting according to an embodiment of the present invention, wherein the microtexture thereof was observed by an optical microscope after the magnesium alloy plate was rolled and then solution-heat-treated at 440°C for 30 minutes.
FIG. 5 is a photograph showing a microtexture of a 0.95Zn-0.9Ca alloy plate, wherein the microtexture thereof was observed by a transmission electron microscope after the 0.95Zn-0.9Ca alloy plate was rolled and then solution-heat-treated at 440°C for 30 minutes.
FIG. 6 is a photograph showing a microtexture of a 5.992n-0.98Ca alloy plate, wherein the microtexture thereof was observed by a transmission electron microscope after the 5.992n-0.98Ca alloy plate was rolled and then solution-heat-treated at 350°C for 30 minutes.
FIG. 7 is a view showing microtextures of a 0.95Zn-0.9Ca alloy plate before and after deformation, wherein the microtextures thereof were observed using EBSD (electron backscatter diffraction) after the 0.95Zn-0.9Ca alloy plate was rolled and then solution-heat-treated at 440°C for 30 minutes.
FIG. 8 is a view showing microtextures of a 5.99Zn-0.98Ca alloy plate before and after deformation, wherein the microtextures thereof were observed using EBSD (electron backscatter diffraction) after the 5.99Zn-0.98Ca alloy plate was rolled and then solution-heat-treated at 350°C for 30 minutes.
FIGS. 9a and 9b show the results of analysis of (002) basal pole figure of the magnesium alloy plate manufactured according to an embodiment of the present invention.
FIG. 10 shows the (002) texture strength and LDH of each of the magnesium alloy plates of Examples and Comparative Examples.

Best Mode

Throughout the present specification, the singular number used to explain the embodiments of the present invention includes the plural number, unless otherwise specified. Further, when it is described that any part "comprises," "includes," "contains" or "has" any constituent, it means that the part may further include other constituents, not that it excludes other constituents.
The terms and words used in the present specification and claims should not be interpreted as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present invention based on the rule according to which an inventor can appropriately define the concept of the term to describe the best method he or she knows for carrying out the invention.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. However, the scope of the present invention is not limited to the following embodiments. Therefore, it is obvious that those skilled in the art can variously modified the present invention without departing from the scope and spirit of the invention.

[Manufacture of magnesium alloy plate]

First, pure Mg (99.9%), pure Zn (99.9%) and a Mg-3wt%Ca matrix alloy were melted under a gas mixture atmosphere of CO₂ and SF₆ to prepare a molten magnesium alloy. The composition ratios of the ingredients constituting the molten magnesium alloy are given in Table 1 below. [Table 1]

Composition (wt%)

Zn Ca Mg

0.95 0.9 bal.

3.43 0.82 bal.

5.99 0.98 bal.
FIG. 1 is a schematic view showing a twin-roll strip casting apparatus for manufacturing a magnesium alloy plate according to an embodiment of the present invention. As shown in FIG. 1, the twin-roll strip casting apparatus includes a melting furnace 10, a nozzle 20 and two cooling rollers 30.
A casting method using the twin-roll strip casting apparatus is described in detail as follows. First, the temperature of the molten magnesium alloy having the above composition ratios is maintained in the range of a temperature (about 650°C) at which a liquid fraction is 70% to a temperature (about 950°C) at which the molten magnesium alloy is ignited in the melting furnace 10 under a gas mixture atmosphere of CO₂ and SF₆, and simultaneously the molten magnesium alloy is transferred to the nozzle 20. In this case, when the temperature of the molten magnesium alloy is excessively high, liquid phase matter may exist in a plate having passed through the cooling rollers 30. Therefore, in the embodiment of the present invention, considering this point, the molten magnesium alloy is transferred to the nozzle 20 while maintaining its temperature at 750°C or lower, and preferably 710°C.
The molten magnesium alloy, the temperature of which is maintained at 710°C, is injected between the two cooling rollers 30 cooled by a cooler (not shown) provided in the twin-roll strip casting apparatus through the nozzle 20. In this case, the interval between the two cooling rollers 30 is maintained at about 2 mm, and the rotation speed of each of the cooling rollers 30 is maintained at about 4 m/min at the time of injecting the molten magnesium alloy. Under such conditions, the molten magnesium alloy is cast such that the cooling rate thereof is 200 ∼ 300 K/s, thus obtaining a magnesium alloy plate having a length of about 5 m, a width of about 70 mm and a thickness of about 2 mm.
Subsequently, the obtained magnesium alloy plate is heat-treated as follows. First, the obtained magnesium alloy plate is solution-treated at 440°C for 1 hour. This solution-treatment is conducted in order to remove the cast texture and segregates formed at the time of casting before rolling and to prevent defects from being caused by nonuniform crystal grains and segregates.
Next, the solution-treated magnesium alloy plate is preheated to 300°C, and then the preheated magnesium alloy plate is hot-rolled by a rolling roller heated to 200°C.
During the hot rolling, the magnesium alloy plate is rolled at a thickness reduction ratio of 10% per pass until the final thickness reduction ratio is 50% for 5 passes, thus obtaining a magnesium alloy plate having a final thickness of 1 ∼ 0.7 mm. Then, this magnesium alloy plate is subsequently solution-treated and then aging-treated (T6 heat treatment) as given in Table 2 below.

[Microtexture of magnesium alloy plate]

The microtexture of the magnesium alloy plate manufactured as above was analyzed. FIG. 3 is a photograph showing a microtexture of the manufactured magnesium alloy plate, wherein the microtexture thereof was observed by an optical microscope after the magnesium alloy plate was heat-treated at 440°C for 1 hour.
FIG. 4 is a photograph showing a microtexture of the manufactured magnesium alloy plate, wherein the microtexture thereof was observed by an optical microscope after the magnesium alloy plate was rolled and then solution-heat-treated at 440°C for 30 minutes. As shown in FIG. 4, it can be seen that the average crystal grain size of the microtexture of the magnesium alloy plate is about 11 µm, and microprecipitates are uniformly distributed in the microtexture thereof.
FIGS. 5 and 6 are photographs showing the microtextures of the magnesium alloy plates manufactured according to the present invention, wherein each of the microtextures thereof were observed by a transmission electron microscope after each of the magnesium alloy plates was rolled and then solution-heat-treated.
In an embodiment of the present invention, a precipitated phase is differently formed according to the amount of Zn. When the magnesium alloy plate is manufactured while setting the amount of Ca at 1 wt% and changing the amount of Zn to 1 wt%, 4 wt% or 6 wt%, as shown in FIG. 5, it can be ascertained that a Mg₂Ca phase is formed when the amount of Zn is 1 wt%, and that a Mg₆Zn₃Ca₂ phase is formed when the amount of Zn is 6 wt% (4 wt% or more). Considering that each of the magnesium alloy plates has similar basal pole intensity, as given in Table 3 below, even though the precipitated phases are different from each other, it is determined that the difference in formability of the magnesium alloy plates is not influenced by the difference in the precipitated phase of the magnesium alloy plates.
FIGS. 7 and 8 are views showing the deformation behaviors of the microtextures of the 0.95Zn-0.9Ca alloy plate and 5.99Zn-0.98Ca alloy plate of the magnesium alloy plates manufactured according to the present invention, wherein the deformation behaviors thereof were analyzed using EBSD (electron backscatter diffraction) after the 0.95Zn-0.9Ca alloy plate and 5.99Zn-0.98Ca alloy plate were rolled and then solution-heat-treated. As shown in FIGS. 7 and 8, it is inferred that crystal orientation is changed before and after deformation, and, owing to the crystal orientation difference, the formability of the 0.95Zn-0.9Ca alloy plate is high compared to other alloy plates.
As described above, the method of manufacturing a magnesium alloy plate according to an embodiment of the present invention is characterized in that precipitated phases uniformly dispersed in the microtexture of the magnesium alloy plate can be obtained using a hot extruding process that is simple compared to a conventional hot extruding process.
Further, as shown in Table 2 below, it is determined that heat treatment temperature is lowered with the increase of the amount of Zn. In the magnesium alloy according to an embodiment of the present invention, optimum heat treatment temperature at which precipitated phases are uniformly distributed in each crystal grain is set. Therefore, when heat treatment is conduced for a long period of time at a temperature higher than the optimum temperature, a crystal grain boundary is partially melted, and a large amount of precipitated phases is distributed in the crystal grain boundary, thus deteriorating the tensile property and formability of the magnesium alloy plate at room temperature.

[Evaluation of physical properties of magnesium alloy plate]

In order to evaluate the tensile characteristics of the magnesium alloy plate manufactured as above, a sample having a gauge length of 12.6 mm, a gauge width of 5 mm and a thickness of 1 mm was fabricated, and the tensile characteristics of the sample was tested at a deformation rate of 6.4x10^-4s^-1.
Further, in order to evaluate the press formability of the manufactured magnesium alloy plate, a limiting dome height (LDH) test was carried out.
FIG. 2 is a schematic view showing a method of obtaining the limiting dome height (LDH) selected as an index for evaluating the formability (particularly, press formability) of the magnesium alloy plate according to an embodiment of the present invention.
The limiting dome height (LDH) test was carried out as follows. First, a disk-shaped sample having a diameter of 50 mm and a thickness of 0.7 mm was fabricated, interposed between an upper die and a lower die, and then fixed by a force of 5 kN. Press oil was used as a lubricant. Subsequently, the disk-shaped sample was deformed at rate of 0.1 mm/sec by a spherical punch having a diameter of 27 mm until the disk-shaped sample is fractured. At this time, the deformation height of the disk-shaped sample was measured. For comparison, LDH tests for commercially available magnesium alloy plates (AZ31 H24, ZW41) and an aluminum plate (Al5052), as well as LDH test of the magnesium alloy plate, were carried out.

The tensile characteristics and formation characteristics measured by the above method are given in Table 2 below.

[Table 2]

Composition (wt%)			Heat treatment	Crystal grain size (um)	UTS (MPa)	YS (MPa)	EL (mm)	LDH (mm)	Remark
Zn	Ca	Mg	Heat treatment	Crystal grain size (um)	UTS (MPa)	YS (MPa)	EL (mm)	LDH (mm)	Remark
0.95	0.9	bal.	440°C/1h+5pass+440/30m	11.6	229.5	151.7	11.4	8.8	Ex.
			470°C/2h+5pass+470/30m	20	222.7	126.9	13.1	8	Ex.
			470°C/2h+5pass+380/30m	7.8	236	168.4	13.8	6.6	Ex.
3.43	0.82	bal.	400°C/1h+5pass+400/30m	11.2	258.2	151.9	14.5	7.1	Ex.
3.43	0.82	bal.	380°C/4h+5pass+380/30m	13.2	254.4	158.8	15.5	7.4	Ex.
5.99	0.98	bal.	350°C/1h+5pass+350/30m	10.9	258.9	163.6	17.2	7.5	Ex.
			380°C/4h+5pass+380/30m	12.7	258.4	152.4	14.3	8	Ex.
			380°C/4h+5pass+300/1h	-	247.7	154	14.8	8.6	Ex.
Commercially available AZ31B H24				-	290	220	15	2.7	Comp. Ex.
ZW41				4	223	89	21	6.6	Comp. Ex.
Al5052				29.2	189	82	16.9	7.7	Comp. Ex.

From the test results given in Table 2 above, it can be ascertained that the LDH of AZ31 H24, which is a commercially available magnesium alloy, is only 2.7 mm, whereas the LDH of ZW41, which is known as a magnesium alloy having excellent formability, is 6.6 mm, so ZW41 exhibits excellent formability compared to AZ31 H24, and that the LDH of A15052, which is an aluminum alloy having excellent formability compared to an magnesium alloy, is 7.7 mm, so the formability of A15052 is excellent compared to those of the two kinds of magnesium alloys.
In contrast, it can be ascertained that the LDH of each of the magnesium alloy plates manufactured according to Examples of the present invention is 6.6 ∼ 8.8 mm. Considering that formability increases as LDH increases, it can be ascertained that the magnesium alloy plates manufactured by Examples of the present invention exhibit excellent formability by three or times compared to that of a commercially available AZ31 H24 alloy plate, and that the LDH of some of the magnesium alloy plates manufactured by Examples of the present invention is greatly improved compared to that of a ZW41 alloy plate generally known to have excellent formability. Further, it can be ascertained that all of the magnesium alloy plates manufactured by Examples of the present invention exhibit formability equal to that of an aluminum-based A15052 plate or some of the magnesium alloy plates exhibit excellent formability compared to that of the an aluminum-based A15052 plate.
FIGS. 9a and 9b show the results of analysis of (002) basal pole figure of the magnesium alloy plate manufactured according to an example of the present invention. Generally, while a magnesium alloy plate is rolled, the pole intensity of a basal plate thereof becomes high, and such increase in texture intensity deteriorates the formability of magnesium having a small amount of a slip system.

Thus, conventionally, research has been conducted into process and heat treatment for lowering the maximum intensity of a basal pole and providing a random texture.

[Table 3]

Composition (wt%)			Heat treatment	Eu (mm)	LDH (mm)	(002) texture intensity	Remark
Zn	Ca	Mg	Heat treatment	Eu (mm)	LDH (mm)	(002) texture intensity	Remark
0.95	0.9	bal.	440°C/1h+5pass+440/30m	11.4	8.8	2.0	Ex.
3.43	0.82	bal.	400°C/1h+5pass+400/30m	14.5	7.1	2.0	Ex.
5.99	0.98	bal.	350°C/1h+5pass+350/30m	17.2	7.5	2.1	Ex.
Commercially available AZ31B H24				15	2.7	-	Comp. Ex.
AZ31				15.9	4.1	9.3	Comp. Ex.
ZW41				21	6.6	3.0	Comp. Ex.
Al5052				16.9	7.7	-	Comp. Ex.

Referring to FIG. 9a, it is shown that the texture of a basal plane of the magnesium alloy plate according to an example of the present invention exhibits low intensity of 3.8 even when it is rolled. Further, referring to FIG. 9b, it is shown that the heat-treated alloy sample having a maximum of LDH exhibits low intensity of 2.0. As shown in Table 3 above, the magnesium alloy plate of the present invention exhibits low intensity compared to that of a conventional magnesium plate.
FIG. 10 shows the ratio of a basal plane (002) texture and a pyramid plane (10-11) texture of each of the magnesium alloy plates of Examples and Comparative Examples. The pyramid plane texture of the magnesium alloy plate according to an example of the present invention is relatively strong compared to that of an AZ31 alloy plate. This result means that a random texture is formed in the magnesium alloy plate according to an example of the present invention.

When the magnesium alloy plate having the composition given in Table 2 above is additionally heat-treated (age-hardened), a magnesium alloy plate having higher strength than yield strength of the solution-treated magnesium alloy plate can be manufactured. The results of comparing the tensile characteristics of the additionally age-hardened magnesium alloy plate with those of a commonly known magnesium alloy plate treated in the same manner as in an example of the present invention and those of a commercially available AZ31 H24.

[Table 4]

Composition (wt%)			Heat treatment	UTS (MPa)	YS (MPa)	EL (mm)	Remark
Zn	Ca	Mg	Heat treatment	UTS (MPa)	YS (MPa)	EL (mm)	Remark
0.95	0.9	bal.	440°C/1h+5pass+440/30m+150/16h	252.4	194.2	8	Ex.
			470°C/2h+5pass+470/2h+150/48h	256.4	186.9	9.2	Ex.
			470°C/2h+5pass+470/2h+200/1h	256.3	201.3	7.2	Ex.
3.43	0.82	bal.	400°C/1h+5pass+440/30m+150/8h	262.7	180.5	16.2	Ex.
			380°C/4h+5pass+380/4h+150/16h	245.1	172.3	11.6	Ex.
			380°C/4h+5pass+380/4h+200/1h	253.7	174.7	15.9	Ex.
5.99	0.98	bal.	350°C/1h+5pass+350/30m+150/24h	263.7	175.8	13.9	Ex.
			380°C/4h+5pass+380/4h+150/48h	278.6	208.8	12	Ex.
			380°C/4h+5pass+380/4h+200/8h	262.5	205	8.4	Ex.
Commercially available AZ31B H24				290	220	15	Comp. Ex.
AZ31				235	131	15.9	Comp. Ex.
ZW41				223	89	25.5	Comp. Ex.

As shown in Table 4 above, it can be ascertained that the magnesium alloy plates according to examples of the present invention have very high tensile strength compared to that of the magnesium alloy plate manufactured by striping casting, and that some of these magnesium alloy plates have somewhat low tensile strength compared to that of a commercially available AZ31 H24.
As described above, according to the magnesium alloy plate of the present invention, the mechanical properties of high formability and high strength of the magnesium alloy plate can be controlled such that this magnesium alloy plate has mechanical properties equal to those of aluminum (lightweight metal) by heat treatment after rolling.
First, since casting and hot rolling are simultaneously conducted in one process by a twin-roll strip casting method as the method of manufacturing a magnesium alloy plate, a magnesium alloy is very rapidly cooled compared to a conventional method, and thus particles can be microfabricated, thereby improving the strength of the magnesium alloy plate.
Meanwhile, the strength of a conventional magnesium alloy plate is low compared to that of a heat-treated aluminum plate, whereas the strength of the magnesium alloy plate of the present invention is high compared to that of the heat-treated aluminum plate. Therefore, the magnesium alloy plate of the present invention can be applied to the field of automobile and structural materials, and can be used in the various fields requiring magnesium alloy plates having a complicated shape, to which a conventional alloy plate cannot be applied, because its formability is very excellent compared to that of a conventional magnesium plate.

Claims

A magnesium alloy plate having high formability, comprising: Zn: 1 ∼ 10 wt%; Ca: 0.1 ∼ 5 wt%; and balances of magnesium (Mg) and inevitable impurities, wherein the magnesium alloy plate has a limiting dome height (LDH) of 7 mm or more.
The magnesium alloy plate of claim 1, wherein an amount of Zn is 1 ∼ 7 wt%, and an amount of Ca is 0.5 ∼ 3 wt%.
The magnesium alloy plate of claim 1 or 2, wherein a microtexture of the magnesium alloy plate has an average crystal grain size of 10 µm or less.
The magnesium alloy plate of claim 1 or 2, wherein the magnesium alloy plate has a limiting dome height (LDH) of 8 mm or more.
The magnesium alloy plate of claim 1 or 2, wherein the magnesium alloy plate has a yield strength (YS) of 200 MPa or more, an ultimate tensile strength (UTS) of 270 MPa or more and an elongation rate (EL) of 12% or more.
The magnesium alloy plate of claim 1 or 2, wherein a (0002) plane of the magnesium alloy plate has a texture intensity of 2.5 or less
A method of manufacturing a magnesium alloy plate having high formability, comprising the steps of:
(a) preparing a molten alloy including Zn: 1 ∼ 10 wt%, Ca: 0.1 ∼ 5 wt% and balances of magnesium (Mg) and inevitable impurities;

(b) maintaining a temperature of the molten alloy in a range of a temperature at which a liquid fraction is 70% to a ignition temperature;

(c) injecting the temperature-maintained molten alloy between two cooling rollers and strip-casting this molten alloy to form a magnesium alloy plate;

(d) solution-treating the formed magnesium alloy plate at 300 ∼ 490°C for 1 ∼ 24 hours;

(e) preheating the solution-treated magnesium alloy plate at 300 ∼ 400°C and then rolling this magnesium alloy plate to a thickness required at a rate of 1 ∼ 45% per pass using a heated rolling roller; and

(f) solution-treating the rolled magnesium alloy plate at 300 ∼ 490°C for 0.5 ∼ 4 hours.
The method of claim 7, wherein, in the step (c), an interval between the two cooling rollers is maintained at 1 ∼ 5 mm, and a rotation speed of each of the cooling rollers is maintained at 0.2 ∼ 20 m/min, thus maintaining a cooling rate of the molten alloy at 10² ∼ 10³ K/s.
The method of claim 7 or 8, wherein an amount of Zn is 1 ∼ 7 wt%, and an amount of Ca is 0.5 ∼ 3 wt%.
The method of claim 7 or 8, further comprising the step of aging the solution-treated magnesium alloy plate at 150 ∼ 200°C for 1 ∼ 72 hours after the step of rolling the magnesium alloy plate.
The method of claim 7 or 8, wherein Ca is added by the addition of a Mg-Ca matrix alloy.