Magnesium alloy sheet processing method and magnesium alloy sheet

Abstract

A magnesium alloy sheet processing method wherein a magnesium alloy sheet is rolled at a speed of 180 m/min or more. Particularly, a magnesium alloy sheet processing method, wherein the magnesium alloy sheet is rolled at a speed of 450 m/min or more. A magnesium alloy sheet processing method, wherein a rolling tool which is not heated is used. A magnesium alloy sheet processing method, wherein the temperature of the magnesium alloy sheet immediately before the rolling is in the range of 0° C. to 400° C. A magnesium alloy sheet, wherein the sheet has an average crystal grain of 4 μm or less, and does not internally include any unbonded interface in parallel with a direction of rolling. A magnesium alloy sheet, wherein the sheet has an average grain size of 4 μm or less, and has an internal grain boundary formed by a clean grain boundary.

Description

TECHNICAL FIELD

The present invention relates to a magnesium alloy sheet processing method and a magnesium alloy sheet, and in particular relates to a method for rolling a magnesium alloy sheet at a speed as high as 180 m/minor more, and to a magnesium alloy sheet having an excellent mechanical strength and the like.

BACKGROUND ART

Since a magnesium alloy has a high specific strength and an excellent electromagnetic wave absorption characteristic, the utilization thereof is being increased in casings or the like for portable electronic devices such as mobile phones. Actually, a magnesium alloy has a crystal lattice with a close-packed hexagonal structure, and has, at a room temperature, a deformation mechanism consisting mainly of a basal plane slip that causes deformation in a direction perpendicular to the c axis, thus making it significantly difficult to provide a plastic strain (deformation) of 10% or more at an ambient temperature. Therefore, in order to carry out such a process, e.g., rolling, not only it is necessary to heat an object to 300° C. or more, but also it becomes a multi-pass process (which means that the number of pressings performed by rolls for rolling is large. Hereinafter, “pass” will be also used as the number of pressings performed by rolls); furthermore, it is necessary to perform intermediate annealing or the like for recovering the workability during each pass (rolling). In addition, when rolling is to be carried out, a longitudinal cracking will occur in a material unless the speed at each pass is reduced. Accordingly, a magnesium alloy has been considered as an unsuitable material for rolling.

As a result, the great majority of magnesium alloy products applied to the above-described usage have been fabricated by thixo-molding, die casting or the like (see Non-Patent Documents 1 and 2).

[Non-Patent Document 1] “Magnesium Processing Technology” edited by The Japan Society for Technology of Plasticity, and published by Corona Publishing Co., Ltd. Dec. 15, 2004 (pp. 60-73)

[Non-Patent Document 2] “Handbook of Advanced Magnesium Technology” edited by Editing Committee for Handbook of Advanced Magnesium Technology, including Yo Kojima et al., and published by Kallos Publishing Co., Ltd. May 17, 2000 (pp. 241-245)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, user's desires for lighter weight and lower price of portable devices are becoming stronger in recent years.

Furthermore, the desire for good appearance is also becoming stronger.

It is becoming difficult to cope with these strong desires anymore by the above-mentioned thixo-molding and die casting. More specifically, thixo-molding is one kind of injection molding while die casting is one kind of casting; therefore, thinning of walls is limited to a certain degree, and furthermore, thixo-molding and die casting are not preferable in terms of cost reduction because a magnesium alloy is expensive. Also, there are limits to the reduction of surface irregularities of a processed surface and to the smoothing of the surface, and therefore, there are also problems in making aesthetic improvements to the appearances of products.

Thus, there has been a desire for the development of technology for fabricating a magnesium alloy sheet thinner than a conventional one.

In addition, there has been a desire for the development of technology for fabricating a magnesium alloy sheet having a finished surface smoother than that of a conventional one.

Also, there has been a desire for the development of fabricating technology or in particular a rolling method for a magnesium alloy sheet, in which process steps are simple and cost reduction can be enabled in terms of this aspect.

Moreover, there has been a desire for the development of a magnesium alloy sheet having an improved mechanical property and the like such as strength.

Solution to the Problems

As a result of conducting extensive studies for solving the above-described problems, the inventors of the present application found out that, contrary to conventional common knowledge, the execution of rolling at 180 m/minor more, i.e., at a speed (high speed) outstandingly higher than a conventional speed, achieves a high rolling reduction (degree of deformation) while suppressing the rupture and cracking of a magnesium alloy sheet, thus realizing fine crystal grains.

Furthermore, based on this, we found out more preferable processing conditions.

The claims of the present invention will be described below.

An invention according to claim 1 provides a magnesium alloy sheet processing method

wherein a magnesium alloy sheet is rolled at a speed of 180 m/min or more.

In the invention of this claim, a magnesium alloy sheet is rolled (i.e., the sheet is fed) at a speed as high as 180 m/min or more, preferably at 200 m/min or more; therefore, due to the start of a high-speed rolling and heat generation associated with the continuation of the high-speed rolling, the temperature of the magnesium alloy sheet is raised to a high level at which large deformation (plastic strain) is enabled (which, however, does not mean to exclude the possibility that an action other than heat generation is also related to this improvement of deformability), thus making it possible to carry out the rolling with a high rolling reduction, and to easily fabricate a thin magnesium alloy sheet.

Since the rolling can be carried out with a high rolling reduction in this manner, the refinement of crystal grains can be easily achieved.

More specifically, the magnesium alloy sheet is subject to heat generation associated with the rolling that causes a large plastic strain, and temperature reduction due to the subsequent heat dissipation to reduction roll (since being obvious, which will hereinafter be called “roll” in principle), the surroundings or the like. Owing to at least one of the recrystallization and recovery associated with these temperature changes, the grain size of the magnesium alloy sheet becomes smaller than that of the sheet prior to the processing, and the tensile strength, breaking elongation and the like are improved.

Furthermore, by cooling the sheet by heat dissipation or the like during the rolling or after the rolling, the crystal grains are inhibited from growing, and are thus kept at a fine level.

In the invention of this claim, it is only necessary to increase the processing speed compared with the conventional technology, and there is no need to make changes to conventional fabricating process steps whatsoever.

Further, there is no limit to thinning of products unlike thixo-molding and die casting, and the process of causing large deformation can be realized by a process with a small number of passes, thus making it possible to contribute to lighter weight of a material, productivity improvement, lower price, and aesthetic improvement.

Furthermore, due to heat generation associated with the high-speed rolling, intermediate annealing and preheating of rolls in a multi-step rolling (multi-pass processing), which have been conventionally required, often become unnecessary.

It should be noted that in the case of an ordinary magnesium alloy sheet, the deformability of the material is high at a temperature (temperature range) of about 300° C. or more, whereas the deformability of the material is low at a temperature of about 250° C. or less (including a room temperature).

In the invention of this claim, high heat generation due to high-speed rolling is utilized as described above. The causes for “heat generation” not only include heat generation due to deformation resistance of a part of the sheet itself, which is just deforming by high speed rolling, and heat generation due to friction against the rolls, but also include contribution of heat conduction from a high-temperature area that has already been rolled, and is located in the top side of this part and adjacent to this part.

In general, the rolling is carried out until the temperature of a magnesium alloy sheet is lowered to a temperature, by heat dissipation, at which the rolling with a high rolling reduction becomes difficult. Such “heat dissipation” includes heat conduction to a part to be rolled henceforward (continuously), temperature reduction due to heat conduction to rolls, and heat conduction and radiation to air. The temperature reduction of a rolled material will occur when large plastic strain has been considerably developed, new heat generation from the material is reduced, and heat dissipation becomes greater than heat generation.

In the invention of this claim, an upper limit to the rolling speed does not exist as long as the started high-speed rolling allows the subsequent rolling to be carried out at a high speed and with a high rolling reduction. It may be said that the upper limit will not be above an actually operable speed (which is 2800 m/min in a facility used for an embodiment of the present invention).

Moreover, “sheet” includes a coiled sheet material.

An invention according to claim 2 provides a magnesium alloy sheet processing method based on the above-described magnesium alloy sheet processing method,

wherein the magnesium alloy sheet is rolled at a speed of 450 m/min or more.

In the invention of this claim, the magnesium alloy sheet is rolled at a very high speed of 450 m/min or more, preferably 500 m/min or more; therefore, the magnesium alloy sheet can be rolled with a rolling reduction outstandingly higher than that of conventional rolling.

Further, due to a high temperature associated with the rolling carried out at such a very high speed, intermediate annealing in a multi-step rolling for fabricating a thin sheet and the like is further unnecessary.

An invention according to claim 3 provides a magnesium alloy sheet processing method based on the above-described magnesium alloy sheet processing method,

wherein a rolling tool which is not heated is used.

In the invention of this claim, a rolling tool which is not heated is used, and therefore, the facility cost, electricity and heating cost, and operation cost for preheating (i.e., increasing the temperature of) a rolling tool prior to rolling are unnecessary.

It should be noted that the present invention does not prevent the preheating of a magnesium alloy sheet itself to a certain extent.

Further, in an attempt to enable more rapid cooling right after rolling by providing favorable heat conduction and to keep a processed surface of the magnesium alloy sheet clean, it is preferable to wipe off grease in advance and to use no lubricating oil.

An invention according to claim 4 provides a magnesium alloy sheet processing method based on the above-described magnesium alloy sheet processing method,

wherein the temperature of the magnesium alloy sheet immediately before the rolling is in the range of 0° C. to 400° C.

In the present invention, the rolling can be carried out at a room temperature, and is thus economically advantageous. On the other hand, it is not preferable if the temperature upper limit prior to the start of the rolling exceeds 400° C., because crystal grains are oversized to make it impossible to obtain favorable mechanical properties, and an oxide film is formed at a surface to degrade the appearance.

It should be noted that depending on the other conditions such as rolled material and rolling speed, the rolling may be carried out after having set the temperature in the range of 180° C. to 350° C., or at a specific temperature (range) such as 200±20° C.

In particular, in the case of rolling the magnesium alloy sheet at 450 m/min or more, e.g., 500 m/min or about 2000 m/min, the temperature range of the magnesium alloy sheet prior to the start of the rolling is preferably at about 200° C. Specifically, this is because if the temperature is at such a level, the temperature of the sheet at the start of the pressing by rolls might be high or nearly high, which not only easily enables a high rolling reduction, but also completely prevents the occurrence of edge cracking. Even if edge cracking has occurred, it is small edge cracking, and therefore, the present invention significantly improves the yield of products, and so forth.

In particular, when a thin magnesium alloy sheet is fabricated, the number of rolling pass is inevitably increased; therefore, if the rolling does not allow the occurrence of edge cracking, the yield of products is significantly improved.

An invention according to claim 5 provides a magnesium alloy sheet processing method based on the above-described magnesium alloy sheet processing method,

wherein the magnesium alloy sheet immediately after the rolling is cooled at a speed of 10° C./sec or more.

In the invention of this claim, the magnesium alloy sheet heated by the high-speed rolling is rapidly cooled; thus, crystal grains will not be grown so as to be kept at a fine level, and eventually, the magnesium alloy sheet having excellent tensile strength and breaking elongation is obtained.

Means for rapidly cooling the sheet include methods such as heat conduction to a processing device, heat conduction to a subsequent material, and spraying of water by a shower or the like.

An invention according to claim 6 provides a magnesium alloy sheet processing method based on the above-described magnesium alloy sheet processing method,

wherein the rolling is carried out with a rolling reduction of 25%/step or more.

In the invention of this claim, the rolling is carried out with a rolling reduction of 25%/step or more, and therefore, the grain size of sheet material is favorably refined. In particular, the rolling reduction is preferably 40%/step or more. Such a high rolling reduction can be realized by increasing the speed of the rolling.

Further, since the rolling reduction is 25% or more, the number of rolling necessary for reaching the final rolling reduction is reduced, thus improving the productivity and promoting the cost reduction.

Herein, “rolling reduction” refers to (sheet thickness prior to processing—sheet thickness after processing)/(sheet thickness prior to processing) per one pass rolling.

In the case of rolling a magnesium alloy sheet at a speed as high as about 500 m/min, a rolling reduction of 32 to 65%/pass (%/step) is preferable, and in the case of rolling a magnesium alloy sheet at a speed as high as about 2000 m/min, a rolling reduction of 35 to 55%/pass is preferable.

It should be noted that a rolled sheet is more preferably heated in advance to about 200° C., which corresponds to about 55% of the melting point (absolute temperature) of a magnesium alloy.

In addition, residual strain and residual stress are preferably removed from the magnesium alloy sheet by, for example, maintaining the sheet at 320° C. to 380° C. for about 12 minutes or more prior to the rolling.

It is preferable to use rolls each having a diameter of equal to or more than 190 times as much as the thickness of a magnesium alloy sheet to be rolled. Specifically, in the case of a sheet having a thickness of 2.5 mm, for example, rolls each having a diameter of 480 mm or more are preferably used, and rolls each having a diameter of about 530 mm or more are more preferably used. By using such rolls to carry out rolling, an angle of bite during the rolling becomes small, and the rolling with a high rolling reduction is easily carried out. Further, the contact area (or length) between the material and rolls during the rolling is also increased, thus preventing the occurrence of undue stress, such as large compression and bending occurring locally on the magnesium alloy sheet. It is believed that this point not only contributes to the achievement of a high rolling reduction, but also produces an effect on the rapid cooling after the rolling since the heat capacity of rolls is increased.

Furthermore, most suitable magnesium alloy sheets include a sheet equivalent to ASTM AZ31B or AZ91D, and a magnesium alloy having a material composition similar to these materials (i.e., a composition whose basic blending components are more or less different from the above-mentioned ASTM specification, or a composition in which the other kind of metal is slightly blended).

An invention according to claim 7 provides a magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1,

wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn, and has a tensile strength of 250 MPa or more and an elongation of 20% or more.

The invention of this claim is centered on a magnesium alloy AZ31B sheet processed by the magnesium alloy sheet processing method according to claim 1, and including a magnesium alloy sheet whose deformability in rolling is determined to be not significantly different therefrom, in the aspects of material and mechanical properties. Therefore, the sheet of this invention contains magnesium as a principal material, and basic blending components (which are essential blending components for allowing the intrinsic properties of the alloy to be achieved) such as 0.1 to 10.0 weight percent, preferably 2.5 to 3.5 weight percent of Al, 0.1 to 4 weight percent, preferably 2.5 to 3.5 weight percent of Zn, 0 to 0.5 weight percent of Mn, and the other components, e.g., impurity such as 0.04 to 0.01 weight percent or less of Fe, Si, Cu, Ni, and Ca (which are inevitably mingled more or less in the current technology).

Further, the sheet has a tensile strength of 250 MPa or more and an elongation of 20% or more.

An invention according to claim 8 provides a magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1,

wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn, and has an average grain size of less than 4 μm.

The invention of this claim is centered on a magnesium alloy AZ31B sheet processed by the magnesium alloy sheet processing method according to claim 1, and including a magnesium alloy sheet whose deformability in rolling is determined to be not significantly different therefrom, in the aspects of material and metallographic structure. Therefore, the material aspect is same as that of the invention of claim 7.

Further, the average grain size is less than 4 μm, preferably less than 3.2 μm, and more preferably less than 2.4 μm. Therefore, the sheet of this invention is improved not only in mechanical strength and the like, but also in deformability when fabricating a casing by a press forming.

It should be noted that the average grain size in the present invention refers to an average grain size measured by a method called an intercept method described below. Specifically, on an optical microscope microstructure photograph or an electron microscope microstructure photograph, a line segment having a total length L (for which a value converted to an actual length by the magnification of the microstructure photograph is used) is drawn, and the total number of crystal grains cut by the line segment is defined as N. When an end of the line segment is located within a crystal grain, this will be counted as 0.5. And in this case, L/N is defined as the average grain size. It should be noted that N represents the number of crystal grains concerning the measurement, and is desirably 100 or more.

An invention according to claim 9 provides a magnesium alloy sheet

wherein the sheet has an average crystal grain size of 4 μm or less, and does not internally include any unbonded interface in parallel with a direction of rolling.

In the magnesium alloy sheet according to the invention of this claim, the average grain size of a metal is 4 μm or less, and one piece of sheet is rolled and fabricated; thus, no unbonded interface of crystal grains can exist within the sheet (however, in the case of carrying out rolling at a room temperature, a fine internal crack might occur at 45 degrees with respect to the rolling direction). Therefore, the magnesium alloy sheet of this invention has excellent mechanical properties. Such a magnesium alloy sheet can be fabricated by the method according to any one of claims 1 to claims 6, for example. In particular, the sheet is preferably rolled at a relatively low temperature.

An upper limit to the average grain size in this claim is defined as 4 μm or less in view of dramatic improvements of mechanical properties.

Specifically, in the case of using the processing method of claim 1, although the average grain size might be, for example, 4.8 μm, which is above 4 μm, depending on the rolling conditions, the average grain size is preferably 4 μm or less in view of dramatic improvements of mechanical properties. It should be noted that even if the average grain size exceeds 4 μm, a magnesium alloy sheet fabricated by the invention of claim 1, for example, has excellent mechanical properties compared with a conventional magnesium alloy sheet, and is included in the invention of the other claim such as claim 1 as long as the processing method thereof satisfies the requirements of the invention of claim 1, for example.

A lower limit to the average grain size is 2.2 μm in the embodiment. However, by optimizing the combination of rolling conditions such as temperature, speed, rolling reduction, and cooling speed, finer crystal grain structure may be obtained. Specifically, there is a study example in which a massive sample having an average grain size of 0.36 μm is obtained by hot or warm multi-axial forging process on a laboratory scale. Therefore, also in this invention, by conducting an in-depth experiment in which more detailed various rolling conditions are set while centering on a rolling condition that reduces the average grain size, a rolling condition that further reduces the average grain size may be found out. Hence, a lower limit to the average grain size is not defined in the invention of this claim. In other words, if a magnesium alloy sheet having an average grain size of less than 2.2 μm, e.g., a magnesium alloy sheet having an average grain size of 0.36 μm, is proved to be obtainable by further limiting the rolling conditions such as speed, temperature and rolling reduction defined in claim 1, for example, so as to create a new invention, the invention is created by utilizing the invention of this claim or the inventions of the other claims due to numerical limitation, and is therefore included in the scope of the inventions of these claims.

It should be noted that there is a report about a magnesium alloy sheet having an average grain size of less than 4 μm which can be fabricated by an accumulative roll-bonding process (J. A. del Valle, M. T. Perez-Prado and O. A. Ruano, “Accumulative roll bonding of a Mg-based AZ61 alloy”, Materials Science and Engineering: A, Volumes 410-411, 25 Nov. 2005, Pages 353-357). However, in this method, in addition to the necessity of complicated and numerous steps, one piece of sheet is fabricated by stacking sheets to carry out rolling and by utilizing solid phase bonding due to rolling deformation; therefore, this method has a disadvantage that an unbonded interface, which is in parallel with a direction of rolling, remains within the sheet. A magnesium alloy is susceptible to formation of an oxide film at its surface, and thus has difficulty in avoiding a residual unbonded interface in particular.

In the invention of this claim, “does not internally include any unbonded interface in parallel with a direction of rolling” purports to remove an unbonded interface formed in parallel with the rolling direction within the sheet by an accumulative roll-bonding process or the like as described above. Herein, “parallel” naturally not only includes the case where something is geometrically in parallel with something, but also includes the case where something is metallographically perceived as being in parallel with the rolling direction.

Further, “does not include any unbonded interface” does not purport to remove, from the invention of this claim, the case where a trace of an unbonded interface partially slightly exists, but means that an unbonded interface can be perceived as not being included substantially in terms of metallography.

An invention according to claim 10 provides a magnesium alloy sheet

wherein the sheet has an average grain size of 4 μm or less, and has an internal grain boundary formed by a clean grain boundary.

In the magnesium alloy sheet according to the invention of this claim, the average grain size of a metal is 4 μm or less, and an internal grain boundary is formed by a clean grain boundary, thus dramatically improving mechanical properties of the magnesium alloy sheet.

As described above, an accumulative roll-bonding process is a method for fabricating one piece of sheet, which is a completed article, by stacking sheets, serving as raw materials, to carry out rolling and by utilizing solid phase bonding due to rolling deformation. In the case of performing such a rolling method, a surface of a sheet serving as a raw material becomes a bonded interface, and therefore, an oxide formed at the surface of the sheet serving as a raw material is taken inside one piece of sheet that is a completed article. Further, such an oxide exists at a grain boundary without penetration into the inside of crystal grains.

A clean grain boundary in the invention of this claim refers to a grain boundary which is not formed by rolling one piece of sheet to fabricate a magnesium alloy sheet, but is formed generally by normal raw material fabricating steps such as melting, casting, plastic processing, and heat treatment, and which does not include an oxide or the like formed, for example, in the case of stacking and rolling a plurality of sheets as described above.

On the other hand, a grain boundary, at which a small amount of oxides exist due to slight oxygen inevitably taken in during the fabrication of a raw material prior to rolling, is included in a clean grain boundary according to this claim.

In the invention of this claim, the grain boundary is a clean grain boundary as described above, and therefore, the sheet of this invention has mechanical properties superior to those of a magnesium alloy sheet whose grain boundary is not clean.

Moreover, in the invention of this claim, the average grain size is 4 μm or less, and therefore, the magnesium alloy sheet has extremely excellent mechanical properties.

The sheet according to the invention of this claim as described above can be fabricated by using the processing method of claim 1, for example. It is to be noted that if the processing method of claim 1 is used as mentioned above, the average grain size might be, for example, 4.8 μm, which is above 4 μm, depending on the rolling conditions; however, the invention of this claim is limited to a magnesium alloy sheet having an average grain size of 4 μm or less in view of dramatic improvements of mechanical properties.

It should be noted that a lower limit to the average grain size is not defined for the same reason as described in claim 9.

An invention according to claim 11 provides a magnesium alloy sheet based on the above-described magnesium alloy sheet,

wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn.

The magnesium alloy sheet according to the invention of this claim is a magnesium alloy sheet such as AZ31B or AZ91D containing Al and Zn, which is selected from the magnesium alloy sheets according to claims 9 and 10, and which has excellent mechanical properties such as a tensile strength of 250 MPa or more and an elongation of 20% or more.

An invention according to claim 12 provides a magnesium alloy sheet based on the above-described magnesium alloy sheet,

wherein the sheet includes, as basic blending components, 4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of Zr.

The magnesium alloy sheet according to the invention of this claim is a magnesium alloy sheet such as ZK60A containing Zn and Zr, which is selected from the magnesium alloy sheets according to claims 9 and 10, and which has excellent mechanical properties.

An invention according to claim 13 provides a magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1,

wherein the sheet includes, as basic blending components, 4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of Zr, and has an average grain size of less than 4 μm.

The invention of this claim is centered on a magnesium alloy ZK60A sheet processed by the magnesium alloy sheet processing method according to claim 1, and including a magnesium alloy sheet whose deformability in rolling is determined to be not significantly different therefrom, in the aspects of material and metallographic structure.

Further, the average grain size is less than 4 μm, preferably less than 3.2 μm, and more preferably less than 2.4 μm. Therefore, the sheet of this invention is improved not only in mechanical strength or the like, but also in deformability when forming a casing by a press.

Documents concerning rolling of a magnesium alloy sheet, in particular a rolling speed thereof, include the following documents, for example:

(A) USP 5087304 (B) USP 2314010
(C) GB 601388   (D) JP 2004-60048A

However, in Document (A), the rolling speed is described as the number of revolutions, and is thus unclear since no absolute rolling speed is defined.

Further, the rolling speed is described as being 180 m/min in Document (B), while the rolling speed is described as being 180 m/min or more in Document (C).

However, the present invention is based on the following concept. The magnesium alloy sheet is rolled at a high speed as described above; therefore, due to the start of the high-speed rolling and heat generation associated with the continuation of the high-speed rolling, the temperature of the magnesium alloy sheet is raised to a high level at which large deformation (plastic strain) is enabled, thereby making it possible to easily carry out the rolling with a high rolling reduction, and to easily fabricate a thin magnesium alloy sheet. Thus, crystal grains can be easily refined by enabling the rolling with a high rolling reduction in this manner.

However, such a concept is not disclosed in Documents (B) and (C).

Further, although the applications of the inventions of Documents (B) and (C) are filed in 1941 and 1945, respectively, these old inventions are not yet put to practical use. To the contrary, the effects of the present invention are clearly confirmed as described in the following embodiments.

Furthermore, in the invention of Document (D), the rolling speed is described as being 1.0 m/min or more in claim 4, and is described as being 3.0 to 21.0 m/min at the best in an embodiment thereof, thus providing no suggestion of a high-speed rolling method as described in the invention of this claim, which is basically different in inventive ideas.

EFFECT OF THE INVENTION

In the present invention, rolling is carried out at a speed as high as 180 m/min or more; therefore, even a magnesium alloy sheet, which has been considered as being unable to be rolled to a high rolling reduction in one pass due to its low intrinsic deformability at a low temperature, can be rolled to a high rolling reduction in one pass at a low temperature.

Further, in the magnesium alloy sheet, crystal grains are refined during the rolling that applies a high rolling reduction, and the sheet is rapidly cooled so that the crystal grains are kept refined, thus improving mechanical properties such as strength and the like.

Furthermore, reheating (intermediate annealing) during each pass is unnecessary.

Thus, the mass production of thin magnesium alloy sheets and outstanding cost reduction are enabled.

Moreover, the present invention can provide thin sheets and casings for portable devices made of magnesium alloy, which are lightweight, have smooth surfaces and good appearances.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A graph showing the temperature balance of the sheet and temperature change as and results thereof, when the rolling speed of an AZ31 sheet is changed with a rolling reduction of 60% (r=60%) at 20° C.

[FIG. 2] A graph showing the temperature balance of the sheet and temperature change as and results thereof, when the rolling speed of an AZ31 sheet is changed with a rolling reduction of 60% (r=60%) at 100° C.

[FIG. 3] A graph showing the temperature balance of the sheet and temperature change as and results thereof, when the rolling speed of an AZ31 sheet is changed with a rolling reduction of 60% (r=60%) at 200° C.

[FIG. 4] Diagrams showing the appearances of samples rolled at respective rolling reductions according to embodiments of the present invention and comparative examples.

[FIG. 5] A representation (microscope photograph) showing the crystal grain size of a sheet according to an embodiment of the present invention.

[FIG. 6] A representation (microscope photograph) showing the crystal grain size of the other sheet according to another embodiment of the present invention.

[FIG. 7] A representation (microscope photograph) showing the crystal grain size of a sheet rolled by a conventional technology.

[FIG. 8] A representation (microscope photograph) showing the crystal grain size of a sheet fabricated by an accumulative roll-bonding process, and the state of an unbonded interface.

DESCRIPTION OF THE REFERENCE CHARACTERS

• • 1 cracking
  • 2 discolored portion
  • 3 fine flaw or concave at edge

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described based on the best mode for carrying out the invention. It should be noted that the present invention would not be limited to the following embodiments. Various modifications can be made to the following embodiments within the scope identical to the present invention and the scope of its equivalence.

(Rolled Material)

As a rolled material, a magnesium alloy sheet equivalent to ASTM AZ31B was prepared. The composition of the sheet includes the following basic blending materials: 3.0 weight percent of Al; 1.0 weight percent of Zn; and 0.4 weight percent of Mn, and the remainder of which is magnesium. It is to be noted that the composition includes 0.3 weight percent of unavoidable impurity at the maximum, and 0.01 weight percent or less of Si.

Furthermore, the sample sheet has the following dimensions: a thickness of 2.5 mm; a width of 30 mm; and a length of 300 mm.

Moreover, in order to remove residual strain, the sample was held at 350° C. for 15 minutes prior to rolling, and was then water-cooled.

(Rolling Mill)

A rolling mill used is an experimental high-speed rolling mill fabricated by Tokushu Kinzoku Excel Co., Ltd., in which the rolling speed can be adjusted in the range of 100 to 2800 m/min. Further, the diameter of each of upper and lower reduction rolls is 530 mm, which is 212 times as much as the thickness of the magnesium alloy sheet to be rolled.

Furthermore, a low-speed rolling mill fabricated by Tokyo Roll was also used for a comparative experiment. It should be noted that the diameter of each roll of this rolling mill is 310 mm.

(Rolling Method)

The above-described magnesium alloy sheet was rolled using the above-mentioned two types of rolling mills while changing the rolling speed, the rolling temperature of each sample and the rolling reduction for each rolling. The results are shown in Table 1.

It should be noted that although the rolling temperature of the sample was changed from a room temperature to 350° C. at the maximum, no roll was heated prior to rolling. Therefore, the temperature of roll surface was at a room temperature (8 to 25° C.) when rolling was started, and was kept at a room temperature during rolling because the heat capacity of rolls is large.

In addition, no lubricating oil was used, and furthermore, the roll surface was degreased by ethanol prior to rolling. Moreover, all rolling was carried out for 1 pass (only once). All the samples after rolling were each rapidly cooled by a water-cooling shower provided at an exit of the rolling mill.

TABLE 1
ROLLING	ROLLING			GRAIN
SPEED	TEMPERATURE	ROLLING REDUCTION		SIZE
(m/min)	° C.	PER PASS %	SAMPLE APPEARANCE	μm
2660	8˜15	62.4	EDGE CRACKING
2000	350	61.2	SOUND	4.8
2000	350	54.4	FINE EDGE CRACKING	4.7
2000	350	45.8	FINE EDGE CRACKING	4.3
2000	350	38.0	FINE EDGE CRACKING	4.6
2000	200	62.4	SOUND	3.1
2000	200	51.1	SOUND	2.2
2000	200	43.7	SOUND	2.2
2000	200	36.3	SOUND
2000	100	61.1	EDGE CRACKING	2.9
2000	100	54.7	EDGE CRACKING
2000	100	46.7	EDGE CRACKING
2000	100	42.2	LARGE EDGE CRACKING
2000	100	33.7	LARGE EDGE CRACKING
2000	8˜15	61.6	EDGE CRACKING
2000	8˜15	60.5	EDGE CRACKING
2000	8˜15	53.1	EDGE CRACKING
2000	8˜15	42.4	EDGE CRACKING
2000	8˜15	32.3	EDGE CRACKING
2000	8˜15	30.0	EDGE CRACKING
500	200	58.2	FINE EDGE CRACKING	3.2
500	200	44.1	FINE EDGE CRACKING
500	100	54.7	EDGE CRACKING	2.4
500	100	41.8	LARGE EDGE CRACKING
500	8˜15	53.3	LARGE EDGE CRACKING
500	8˜15	44.1	LARGE EDGE CRACKING
186	100	50.0	EDGE CRACKING
103	100	50.0	RUPTURE
17.5	350	49	RUPTURE
17.5	350	30.7	SOUND
17.5	350	13.2	SOUND
17.5	200	40.7	RUPTURE
17.5	200	32.1	RUPTURE
17.5	200	26.1	FINE EDGE CRACKING

(Rolling Test Results)

First, “edge cracking” will be described.

In Table 1, “edge cracking” refers to a situation in which a crack having a size of about 3 to 5 mm has occurred at a widthwise end of a rolled sheet. “Fine edge cracking” refers to a situation in which a crack having a size of less than 3 mm has similarly occurred. Further, “large edge cracking” refers to a situation in which a crack having a size of greater than 5 mm has similarly occurred. It is to be noted that the widthwise center of the sheet was sound in each case. It should be noted that the length of edge cracking has nothing to do with the sheet width. In other words, even if the sheet width is increased, edge cracking will not be enlarged. Therefore, the greater the sheet width, the smaller the adverse effect of edge cracking on the yield. In actual rolling, a sheet having a width greater than that of a test piece is rolled in most cases, and as a result, the occurrence of small edge cracking will not be a significant problem from a practical standpoint.

From Table 1, the following facts are determined. Even at a temperature of 200° C. or less, which has been considered to be unable to achieve a rolling reduction of 20% or more per pass, the rolling is carried out only to the extent that edge cracking occurs at a rolling speed of 186 m/min and with a rolling reduction of 50%. Thus, the rolling is enabled even at a rolling speed of 180 m/min or more and with a rolling reduction of 60%, and the rolling is enabled without any problems if it is carried out at a higher rolling speed and a higher rolling temperature. Further, it can be seen from Table 1 that the rolling is sufficiently enabled from a practical standpoint even with an extremely high rolling reduction of 40% or more if the rolling speed is 500 m/min or more in particular, i.e., the rolling is enabled while maintaining a sufficient yield. Furthermore, it can be seen that, if the rolling speed is 2000 m/min, the rolling is sufficiently enabled even with an extremely high rolling reduction of 62.4%.

The reasons for the above facts will be described with reference to FIGS. 1, 2 and 3. FIGS. 1, 2 and 3 are graphs showing the temperature balance of the sheet and temperature change as and results thereof, when the rolling speed of an AZ31 sheet is changed with a rolling reduction of 60% (r=60%) at 20° C., 100° C. and 200° C., respectively.

From FIG. 1, it can be seen that, even though the rolling speed is relatively as low as 180 m/min (which is, however, higher than a conventional rolling speed, and is indicated by the vertical line), the temperature of the rolled material, which has been at 20° C., is raised to 120° C. during the rolling due to the heat generation caused by processing heat and frictional heat. From FIG. 2, it can be seen that when the rolling speed is 180 m/min (indicated by the vertical line), the temperature of the rolled material, which has been at 100° C., is raised to 150° C. during the rolling as a result of deducting the heat dissipation to rolls from the heat generation caused by processing heat and frictional heat. From FIG. 3, it can be seen that when the rolling speed is 180 m/min (indicated by the vertical line), the temperature of the rolled material, which has been at 200° C., remains at 200° C. as a result of the heat generation caused by processing heat and frictional heat, and the heat dissipation caused by conduction to rolls.

If the rolling reduction and rolling speed are increased, the amount of heat generation caused by the processing is increased because the deformation resistance is increased in accordance with an increase in deformation speed associated with the rolling. Further, the frictional heat generation is also increased similarly since friction is increased due to an increase in rolling pressure. On the other hand, the heat dissipation due to rolls is decreased because the contact time is reduced. Therefore, it can be seen that if the rolling is enabled at 180 m/min at a room temperature, the rolling is enabled at higher temperature and speed without any problems also in terms of temperature rise of the rolled material.

In particular, when the temperature of the sample is at 200° C., a very excellent deformability in rolling is achieved even if the rolling speed is as extremely high as 500 m/min or 2000 m/min, and even if the rolling reduction is as extremely high as 40 to 60% as shown in Table 1.

Moreover, as shown in Table 1, even at a room temperature, no rupture occurs although edge cracking or large edge cracking occurs in the material, as long as the rolling reduction is about 30%. Therefore, it can be assumed that if the rolling speed is 180 m/min or more, the rolling reduction is not large, e.g., about 40 to 60% as shown in Table 1, and if the rolling reduction is about 25%, the rolling can be sufficiently carried out even at a room temperature. Thus, it can be seen that, even in the range of 0 to 200° C., a rolling reduction of 25% or more per pass is enabled by setting the rolling speed at 180 m/min or more.

Next, the appearances of samples, rolled at 200° C. and at rolling speeds of 17.5 m/min and 2000 m/min, are conceptually shown in FIG. 4. The left side of FIG. 4 shows the appearances of the samples each rolled at 17.5 m/min, and the rolling reduction is 26.1% in (a), 32.1% in (b) and 40.7% in (c). The right side of FIG. 4 shows the appearances of the samples each rolled at 2000 m/min, and the rolling reduction is 36.3% in (a), 51.1% in (b) and 61.1% in (c). The length of each arrow extending vertically at the center is 1 cm.

Also from FIG. 4, it can be seen that, when the rolling speed is as low as 17.5 m/min and the rolling reduction is 25% or more, cracking extending to the entire sheet width or the entire sheet depth occurs at intervals of 3 to 5 mm. Therefore, the sheet is cut at intervals of 3 to 5 mm, or even if the sheet is not cut, the sheet is barely connected. In the diagrams (a), (b) and (c) at the left side of FIG. 4, each solid line in the sheets, which is indicated by the reference numeral 1, represents cracking.

However, it can be seen that when the rolling is carried out at a speed as very high as 2000 m/min, substantially no flaw is found even if the rolling reduction is as high as 36.3%, and that the surface is smooth and the rolling is soundly carried out even if the rolling reduction is as considerably high as 51.1%, although slight concaves each having a length of about 1 mm are observed at intervals of 2 to 3 mm at edges of the sheet. Furthermore, even when the rolling reduction is as very high as 61.6%, only slight concaves each having a length of about 1 mm are observed at intervals of 2 to 3 mm. In the diagrams (a), (b) and (c) at the right side of FIG. 4, the black portions, indicated by the reference numeral 2, represent fine discolored portions, while the small lines indicated by the reference numeral 3 represent fine flaws or concaves at edges.

(Structure of Rolled Material)

Next, the structure of a rolled material will be described.

The rolled material is completely recrystallized to almost uniform grain size since it is rapidly cooled by a shower after rolling. Further, as can be seen from Table 1, the grain size is 5 μm or less at a rolling temperature of 350° C., and is extremely fine, e.g., 3 μm or less, at 200° C. in most cases. Therefore, the mechanical properties and corrosion resistance of the rolled material are excellent. Although it was difficult for a conventional fabricating method to achieve such a fine structure, it was realized by increasing the rolling speed.

FIGS. 5 and 6 show microscope photographs of the structures of sheets each rolled by the method of the present invention.

FIG. 5 shows the case where the sheet was rolled at a rolling speed of 2000 m/min, a rolling temperature of 200° C. and a rolling reduction of 61%, and the average grain size in this case is 3.1 μm. FIG. 6 shows the case where the sheet was rolled at a rolling speed of 2000 m/min, a rolling temperature of 200° C. and a rolling reduction of 51%, and the average grain size in this case is 2.2 μm. In addition, no unbonded interface exists.

For reference purposes, FIG. 7 shows an example in which a conventional rolling method, i.e., hot rolling, was carried out. The example shown in FIG. 7 is described in Materials Science and Engineering A, Vol. 410-411 (2005) pp. 308-311, in which rolling conditions include a temperature of 375° C. and a rolling reduction of 70%. As can be seen from the photograph, in addition to that the average grain size is 8 μm, i.e., the grain size is far larger than that of the sheet rolled by the method of the present invention, there is formed a so-called “duplex grain structure” in which crystal grains far larger than the average size mixedly exist.

Similarly, for reference purposes, FIG. 8 shows the photograph of structure of a magnesium alloy AZ61 to which an accumulative roll-bonding process proposed as a process for refining crystal grains of a sheet (Patent No. 2961263) was applied. The example shown in FIG. 8 is described in Materials Science and Engineering A, Vol. 410-411 (2005) pp. 353-357. The black line extending laterally at the center of the photograph indicates an unbonded interface at which stacked sheets remain unbonded. Accordingly, the strength is not sufficient as compared with the sheet of the present invention.

(Mechanical Properties of Rolled Material)

Four types of samples were rolled under the following four conditions: a rolling speed of 2000 m/min; a rolling speed of 500 m/min; a rolling temperature of 200° C.; and a rolling temperature of 100° C., with a rolling reduction of 60%, and were then rapidly cooled and recrystallized. Then, samples each having a size of one fifth (a parallel part length of 10 mm and a parallel part width of 5 mm) of a JIS Z2201-5 test piece were cut out from the four types of samples, and were subjected to tensile test at 0.5 mm/min.

Each of the test pieces had a tensile strength of 250 MPa or more and a breaking elongation of 20% or more, while some of them had a breaking elongation of 25% or more. Therefore, it was found out that they are superior to MP1 (tensile strength: 220 MPa or more, breaking elongation: 11% or more) equivalent to AZ31B specified by JIS H4201 in tensile strength and breaking elongation.

(Other Material: No. 1)

AZ91D whose aluminum content is higher than that of AZ31B was used as a sample, and was subjected to high-speed rolling to test whether or not it is possible to perform high-speed rolling on a magnesium alloy other than AZ31B.

AZ91D contains: 9.0 weight percent of Al; 0.7 weight percent of Zn; 0.24 weight percent of Mn; and 0.025 weight percent of Si at the maximum, and the remainder of which is constituted by Mg and unavoidable impurity.

From the test results, it was confirmed that a sound rolled sheet can be obtained at a rolling temperature of 200° C., a rolling speed of 2000 m/min, and a rolling reduction of 50%/pass.

(Other Material: No. 2)

ZK60A was subjected to high-speed rolling at a rolling temperature of 100° C. to 350° C., a rolling reduction of 30 to 60%/step, and a rolling speed as high as 1000 m/min, and the results similar to those of AZ31B or the like were obtained. Specifically, at 100° C., the rolling was enabled with a rolling reduction of 60% although edge cracking had occurred. At 150° C. and 200° C., the rolling was enabled with a rolling reduction of 60% although fine edge cracking had occurred.

Further, the average grain size was as fine as 2.5 μm at a rolling temperature of 350° C. and a rolling reduction of 50%/step.

ZK60A contains 6.0 weight percent of Zn and 0.5 weight percent of Zr, and the remainder of which is constituted by Mg and unavoidable impurity.

Thus, it was confirmed that the high-speed rolling according to the present invention is applicable to a wide variety of magnesium alloys.

Claims

1. A magnesium alloy sheet processing method wherein a magnesium alloy sheet is rolled at a speed of 180 m/min or more.

2. A magnesium alloy sheet processing method according to claim 1, wherein the magnesium alloy sheet is rolled at a speed of 450 m/min or more.

3. A magnesium alloy sheet processing method according to claim 1, wherein a rolling tool which is not heated is used.

4. A magnesium alloy sheet processing method according to claim 1, wherein the temperature of the magnesium alloy sheet immediately before the rolling is in the range of O° C. to 400° C.

5. A magnesium alloy sheet processing method according to claim 1, wherein the magnesium alloy sheet immediately after the rolling is cooled at a speed of 10° C./sec or more.

6. A magnesium alloy sheet processing method according to claim 1, wherein the rolling is carried out with a rolling reduction of 25%/step or more.

7. A magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1, wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn, and has a tensile strength of 250 MPa or more and an elongation of 20% or more.

8. A magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1, wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn, and has an average grain size of less than 4 μm.

9. A magnesium alloy sheet, wherein the sheet has an average crystal grain of 4 μm or less, and does not internally include any unbonded interface in parallel with a direction of rolling.

10. A magnesium alloy sheet, wherein the sheet has an average grain size of 4 μm or less, and has an internal grain boundary formed by a clean grain boundary.

11. A magnesium alloy sheet according to claim 9, wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn.

12. A magnesium alloy sheet according to claim 9, wherein the sheet includes, as basic blending components, 4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of Zr.

13. A magnesium alloy sheet processed by the magnesium alloy sheet processing method according to claim 1, wherein the sheet includes, as basic blending components, 4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of Zr, and has an average grain size of less than 4 μm.

14. A magnesium alloy sheet according to claim 10, wherein the sheet includes, as basic blending components, 0.1 to 10.0 weight percent of Al, and 0.1 to 4 weight percent of Zn.

15. A magnesium alloy sheet according to claim 10, wherein the sheet includes, as basic blending components, 4.0 to 8.0 weight percent of Zn, and 0.3 to 0.8 weight percent of Zr.