NON-HEAT TREATED MAGNESIUM ALLOY SHEET WITH EXCELLENT FORMABILITY AT ROOM TEMPERATURE IN WHICH SEGREATION IS MINIMIZED
Disclosed herein is a non-heat treatable magnesium alloy sheet, including: 1˜3 wt % of aluminum (Al); 0.5˜3 wt % of tin (Sn); and a balance of magnesium, wherein the maximum deviation of average Vickers hardness (Hv) thereof, caused by center segregation and inverse segregation, is 10 Hv or less.
This application is a National Stage Application of PCT International Patent Application No. PCT/KR2012/008357 filed on Oct. 15, 2012, under 35 U.S.C. §371, which claims priority to Korean Patent Application No. 10-2011-0107405 filed on Oct. 20, 2011, which are all hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION1. Technical Field
The present invention relates to a strip-cast magnesium alloy sheet, and, more particularly, to a non-heat treatable magnesium alloy sheet, which can remarkably reduce a cast defect such as center segregation, inverse segregation or the like by alloy component control, and which can improve room-temperature formability by controlling crystal grain growth in the subsequent heat treatment procedure using a precipitate formed in a rolling procedure.
2. Description of the Related Art
A magnesium alloy, which is an alloy for structural materials having low specific gravity, high specific strength and high rigidity, has recently been increasingly used as a material for light portable electronic appliances such as mobile phones, notebooks and the like or as a material for automobiles for improving fuel efficiency. However, research into magnesium alloys has been restricted to parts for casting. Particularly, research into the improvement of high-temperature physical properties of magnesium alloys used for automobile engines or gears has attracted considerable attention, whereas research into magnesium alloys for processing, such as magnesium alloy sheets which can be used in more various fields, has not been sufficiently conducted.
Recently, for the purpose of various applications of magnesium alloys, demand for magnesium alloy products for processing has increased, and thus many research institutes have conducted research into magnesium alloys for processing. Particularly, among the magnesium alloy products, a magnesium alloy sheet, which is manufactured by twin-roll strip casting, can be applied in various fields, so research into the magnesium alloy sheet has been variously conducted, and the magnesium alloy sheet is commercially available.
However, according to the recent trend of magnesium alloy development, since various kinds of alloy elements are added or expensive rare-earth elements are used for the purpose of development of high-strength and high-formability magnesium alloys, price competitiveness has been lowered. Therefore, to date, the only magnesium alloy sheet manufactured by twin-roll strip casting is an AZ31 alloy sheet, and this AZ31 alloy sheet has mechanical properties generally used in industrial markets.
Meanwhile, considering that an aluminum alloy sheet having an yield strength (mechanical strength) of 200 MPa or less are variously used in an automobile industry and other exterior material industries, it is urgently required to develop a non-heat treatable magnesium alloy for twin-roll strip casting in order to rapidly commercialize a magnesium alloy sheet and increase the price competitiveness thereof.
SUMMARY OF THE INVENTIONAccordingly, the present invention has been devised to solve the problems of conventional magnesium alloys for twin-roll strip casting, such as cast defects, low mechanical strength and poor room-temperature formability, and an object of the present invention is to provide a non-heat treatable magnesium alloy sheet, which can reduce a cast defect such as segregation or the like by adjusting the composition of an magnesium alloy and which can obtain suitable mechanical strength and good room-temperature formability without including expensive rare-earth elements by controlling the microstructure of an magnesium alloy.
In order to accomplish the above object, an aspect of the present invention provides a non-heat treatable magnesium alloy sheet, including: 1˜3 wt % of aluminum (Al); 0.5˜3 wt % of tin (Sn); and a balance of magnesium, wherein the maximum deviation of average Vickers hardness (Hv) thereof, caused by center segregation and inverse segregation, is 10 Hv or less.
The magnesium alloy sheet may be formed by twin-roll strip casting, and may have a microstructure of an Mg2Sn secondary phase.
In the magnesium alloy sheet, the Mg2Sn secondary phase may have a volume fraction of 5% or less.
The magnesium alloy sheet may have a yield strength of 200 MPa or more and a limit dome height (LDH) of 5 mm or more, and preferably 6 mm or more.
In the magnesium alloy sheet, the volume fraction of tension twins inclined at an angle of 85˜90° to parent grains may be 5% or more.
The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
All of the terms used in the specification are taken only to illustrate embodiments, and are not intended to limit the present invention. As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of “including, but not limited to.”
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. The above detailed descriptions of embodiments of the invention are not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
The present invention provides a non-heat treatable magnesium alloy sheet, including: 1˜3 wt % of aluminum (Al); 0.5˜3 wt % of tin (Sn); and a balance of magnesium, wherein the maximum deviation of average Vickers hardness (Hv) thereof, caused by center segregation and inverse segregation, is 10 Hv or less.
In a sheet casting process, the solidification range of a cast material influences the segregation of the cast material and degree thereof. When a molten metal is solidified while passing through the set back distance region of rollers during a plating casting process, a liquid region coexisting in a sheet is squeezed from center to surface because the sheet is pressed by two rollers, thus resulting in forming a segregated zone having high composition density on the surface thereof. This phenomenon is referred to as “inverse segregation”. Particularly, such inverse segregation severely occurs in the case of an alloy having a long solidification range. In the case of an aluminum alloy having a relatively narrow solidification range, inverse segregation does not easily occur compared to the case of a magnesium alloy having a long solidification range. Further, even in the case of a magnesium alloy, degree of segregation is changed according to the kind of alloy element. Due to such segregation, it is difficult to control the microstructure of a cast material, the cast material must be homogenized at high temperature for a long period of time, the mechanical properties of the cast material become poor, and the surface treatment of the cast material is difficult.
In order to solve the above problems of segregation, the present inventors have adjusted the solidification range of a magnesium alloy used in twin-roll strip casting to be within the range where it doesn't have mechanical properties leading to deterioration. Like the magnesium alloy according to the present invention, an alloy including 1˜3 wt % of aluminum (Al), 0.5˜3 wt % of tin (Sn) and a balance of magnesium has a solidification range of 30˜50K in an equilibrium diagram calculated by Factsage V6.2. This soldification range corresponds to half of that (80˜90K) of AZ31, which is a conventional alloy for twin-roll strip casting. Therefore, the magnesium alloy of the present invention has a solidification range of 50K or less, which is similar to that of an aluminum alloy, thus greatly reducing inverse segregation.
Degree of segregation can be evaluated by the distribution of alloy composition. In the case of an alloy having a long solidification range, center segregation and inverse segregation are highly developed, and thus the distribution of alloy composition in a cast structure appears densely at the center and edge of a sheet. The deviation in the composition of the magnesium alloy of present invention in a thickness direction does not become large. The average deviation in the composition of a commercially available strip-cast AZ31 sheet is 30˜50%, whereas the deviation in the composition of the magnesium alloy of the present invention is 10% or less.
Meanwhile, generally, when the difference in composition of a cast material is large, hardness thereof becomes partially non-uniform. Thus, when secondary phases composed of high-concentration inverse or center segregation zone and eutectic phase are clustered together, they have higher hardness. Therefore, degree of segregation can be numerically indicated, and the maximum deviation of average Vickers hardness (Hv) of the magnesium alloy sheet of the present invention in the thickness direction thereof may be 10 Hv or less.
Further, the magnesium alloy sheet of the present invention exhibits excellent mechanical properties compared to a conventional commercially available AZ31 because of the formation of Mg2Sn secondary phases. In the case of a non-heat treatable alloy, a heat treatment process for controlling the precipitates is not used, and thus a mechanism for enhancing the mechanical strength of the alloy is restricted. The volume fraction of Mg2Sn secondary phases in the magnesium alloy sheet of the present invention is 5% or less, as optical images were measured using an image pro plus 6.0 program.
Particularly, in the case of AZ31 having low alloy element content, only thermo-mechanical treatment such as hot rolling may be used as a mechanism for enhancing mechanical strength. In this case, after rolling, due to crystal grain growth together with inner strain, the mechanical strength of AZ31 is rapidly lowered with the passage of annealing time, and this phenomenon becomes remarkable with the increase of annealing temperature.
The volume fraction of secondary phases in the magnesium alloy of the present invention is greatly decreased in the homogenization process after casting. However, secondary phases are distributed in the microstructure thereof again with dynamic precipitation during rolling, and the secondary phase distribution controls crystal grain growth during annealing, thus preventing the mechanical strength of the magnesium alloy from being rapidly lowered.
The reasons for limiting the composition ratio of the magnesium alloy to the above range are as follows. When the amount of Al is less than 1 wt %, the effect of improving fluidity and the effect of enhancing strength are insufficient, and, when the amount thereof is more than 3 wt %, the solidification range of the molten magnesium alloy is enlarged, and thus the effect of controlling segregation is not sufficient.
Further, when the amount of Sn is less than 0.5 wt %, the volume fraction of Mg2Sn secondary phases in the magnesium alloy is low, the contribution to the improvement of mechanical properties of the magnesium alloy is insufficient, and, when the amount thereof is more than 3 wt %, the homogenization treatment temperature and time increase, and secondary phases formed during a rolling process are locally distributed in the magnesium alloy in large amounts, thus exerting a negative influence on the improvement of formability and elongation.
Generally, the formability of magnesium at room temperature is poor due to the absence of a slip system, and thus it is important that twins be used as a deformation factor replacing the slip system.
The magnesium alloy sheet according to the present invention exhibits excellent room-temperature formability compared to that of a conventional AZ31 alloy because of tension twins inclined at an angle of 85˜90° to parent grains.
Hereinafter, the present invention will be described in more detail with reference to the following Examples.
Manufacture of Magnesium Alloy Sheet
First, pure Mg (99.9%), pure Al (99.9%) and pure Sn (99.9%) were melted by a furnace 10 of a twin-roll strip casting apparatus shown in
Then, in order to evaluate the degree of segregation of the obtained magnesium alloy sheet in the solidification range thereof, EPMA (electron probe X-ray microanalysis) and hardness measurement were carried out.
As shown in
Further, from the result of EPMA analysis shown in
Thermo-Mechanical Treatment
The sheet manufactured as above was heat-treated as follows. First, the sheet was solution-treated at 400° C. for 3 hours. Subsequently, the solution-treated sheet was preheated to 200° C., and was then hot-rolled by rollers heated to 200° C.
During the hot rolling, the preheated sheet was hot-rolled five times at a reduction ratio of 10% per pass to a final reduction ratio of 50%, thereby finally obtaining a sheet having a thickness of 1˜0.7 mm.
Evaluation of Mechanical Properties
The above strip-cast and heat-treated magnesium alloy sheet was annealed as shown in Table 1 below, and then the mechanical properties and formability thereof were evaluated.
In order to evaluate the tensile characteristics of the magnesium alloy sheet, a sample having a length of 12.6 mm, a width of 5 mm and a thickness of 1 mm was fabricated, and then the tensile characteristics of the sample were tested at a deformation ratio of 6.4×10−4 s−1.
Further, in order to evaluate the formability of the magnesium alloy sheet, a limit dome height (LDH) test was carried out.
The magnesium alloy sheet, which is a non-heat treatable magnesium alloy sheet, similarly to AZ31 (typical non-heat treated alloy), is characterized in that its mechanical strength is decreased with the increase of annealing time and annealing temperature, and its elongation and formability (that is, LDH) is increased with the increase of annealing time and annealing temperature.
Generally, tensile elongation is used as an alternative item to formability, but, as given in Table 1 above, elongation is not absolutely proportional to LDH representing formability. Therefore, it is preferred that a test accompanying an actual forming procedure is conducted compared to when a uniaxial tensile elongation is used as an index representing formability.
Comparing AT alloys with a conventional AZ31 alloy, it can be ascertained that the formability of an AT alloy is more excellent than that of the conventional AZ31 alloy under the condition that they have similar yield strengths to each other, and that the yield strength of an AT33 alloy further including Sn is higher than those of other AT alloys under the condition that they have similar LDH values to each other. Entirely, yield strength is inversely proportional to LDH, but an AT alloy has more excellent mechanical properties than those of the conventional AZ31 alloy and have higher LDH values than that of the conventional AZ31 alloy.
From
As shown in
This difference is caused by the amount of tension twins formed in the deformation process thereof. These tension twins contribute greatly to the improvement of formability of the magnesium alloy sheet lacking a slip system.
The present invention provides a magnesium alloy sheet, which can make a uniform cast structure by controlling alloy elements having a narrow solidification range, which can make up for defects caused by segregation, and which has excellent mechanical strength and formability without using expensive rare-earth elements. Further, the magnesium alloy sheet according to the present invention can be used in various application fields because it is a non-heat treatable magnesium alloy sheet and its yield strength and LDH value are linearly changed depending on annealing time.
As described above, the magnesium alloy sheet according to the present invention is advantageous in that it can remarkably improve defects, such as inverse segregation, center segregation and the like, occurring in a conventional magnesium alloy sheet prepared by a twin-roll strip casting process due to the change of a solidification range by the addition of alloy elements, and in that its mechanical strength is excellent compared to that of a conventional non-heat treatable strip-cast magnesium alloy sheet even after it is heat-treated to have high formability.
Further, the magnesium alloy sheet according to the present invention can exhibit excellent formability without using expensive rare-earth elements added to impart high formability and high strength, thus increasing competitiveness thereof in the light structural materials market.
Although the preferred embodiments of the present invention have 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 non-heat treatable magnesium alloy sheet, comprising:
- 1˜3 wt % of aluminum (Al); 0.5˜3 wt % of tin (Sn); and a balance of magnesium, wherein a maximum deviation of average Vickers hardness (Hv) thereof, caused by center segregation and inverse segregation, is 10 Hv or less.
2. The non-heat treatable magnesium alloy sheet of claim 1, wherein the magnesium alloy sheet is formed by twin-roll strip casting, and has a microstructure of a Mg2Sn secondary phase.
3. The non-heat treatable magnesium alloy sheet of claim 2, wherein the Mg2Sn secondary phase has a volume fraction of 5% or less.
4. The non-heat treatable magnesium alloy sheet of claim 1, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 5 mm or more.
5. The non-heat treatable magnesium alloy sheet of claim 1, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 6 mm or more.
6. The non-heat treatable magnesium alloy sheet of claim 1, wherein a volume fraction of tension twins inclined at an angle of 85˜90° to parent grains is 5% or more.
7. The non-heat treatable magnesium alloy sheet of claim 2, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 5 mm or more.
8. The non-heat treatable magnesium alloy sheet of claim 3, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 5 mm or more.
9. The non-heat treatable magnesium alloy sheet of claim 2, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 6 mm or more.
10. The non-heat treatable magnesium alloy sheet of claim 3, wherein the magnesium alloy sheet has a yield strength of 200 MPa or more and a limit dome height (LDH) of 6 mm or more.
11. The non-heat treatable magnesium alloy sheet of claim 2, wherein a volume fraction of tension twins inclined at an angle of 85˜90° to parent grains is 5% or more.
12. The non-heat treatable magnesium alloy sheet of claim 3, wherein a volume fraction of tension twins inclined at an angle of 85˜90° to parent grains is 5% or more.
Type: Application
Filed: Oct 15, 2012
Publication Date: Jul 24, 2014
Inventors: Nack-Joon Kim (Pohang-si), Myoeng-Shik Shim (Jeollanam-do), Byeong-Chan Suh (Seoul), Jun-Ho Bae (Pohang-si), Dong-Wook Kim (Pohang-si)
Application Number: 14/237,892
International Classification: C22C 23/02 (20060101); C22C 23/00 (20060101);