Producing method for magnesium alloy material

Abstract

A magnesium alloy material such as a magnesium alloy cast material or a magnesium alloy rolled material, excellent in mechanical characteristics and surface precision, a producing method capable of stably producing such material, a magnesium alloy formed article utilizing the rolled material, and a producing method therefor. The magnesium material includes a melting step of melting a magnesium alloy in a melting furnace to obtain a molten metal, a transfer step of transferring the molten metal from the melting furnace to a molten metal reservoir, and a casting step of supplying a movable mold with the molten metal from the molten metal reservoir, through a pouring gate, and solidifying the molten metal to continuously produce a cast material. Parts are formed by a low-oxygen material having an oxygen content of 20 mass % or less. The cast material is given a thickness of from 0.1 to 10 mm.

Claims

1. A magnesium alloy cast material, comprising: a surface part; a central part; and a ripple mark present on the surface part, wherein the ripple mark present on the surface part of the cast material satisfies a relation rwrd<1.0 for a maximum width rw and a maximum depth rd, rw and rd being in units of mm, wherein during casting, a molten metal is supplied from a pouring gate with a supply pressure of equal to or larger than 101.8 kPa and less than 118.3 kPa, and supplied such that a distance from a distal end of the pouring gate to a position where the molten metal first contacts a movable mold substantially becomes less than 10% of a distance between a plane containing the rotary axes of rolls and the distal end of the pouring gate, and wherein the magnesium alloy comprises magnesium and at least a first additional element and at least a second additional element, wherein the at least first additional element is selected from the group of Al, and Zn, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least first additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material, and the at least second additional element includes Sr, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least second additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material.

2. The magnesium alloy cast material of claim 1, wherein an intermetallic compound has an average size of 20 m or less.

3. The magnesium alloy cast material of claim 1, wherein a dendrite arm spacing (DAS) is from 0.5 m to 5.0 m.

4. The magnesium alloy cast material of claim 1, wherein a depth of a surface defect is less than 10% of a thickness of the cast material.

5. The magnesium alloy cast material of claim 1, wherein a plate thickness of the cast material is from 0.1 to 10.0 mm.

6. The magnesium alloy cast material of claim 1, wherein the cast material, upon being subjected to rolling, has an average size of a crystal grain of from 0.5 m to 30 m.

7. The magnesium alloy cast material of claim 6, wherein a difference between the average size of a crystal grain in a surface part of the cast material subjected to rolling and the average size of a crystal grain in a central part thereof is 20% or less.

8. The magnesium alloy cast material of claim 6, wherein an average size of an intermetallic compound is from 20 m or less.

9. The magnesium alloy cast material of claim 6, wherein a depth of a surface defect is less than 10% of a thickness of the cast material subjected to rolling.

10. A magnesium alloy cast material, comprising: a surface part; a central part; and a ripple mark present on the surface part, wherein the ripple mark present on the surface part of the cast material satisfies a relation rwrd<1.0 for a maximum width rw and a maximum depth rd, rw and rd being in units of mm, wherein during casting, a molten metal is supplied from a pouring gate with a supply pressure of equal to or larger than 101.8 kPa and less than 118.3 kPa, and supplied such that a distance from a distal end of the pouring gate to a position where the molten metal first contacts a movable mold substantially becomes less than 10% of a distance between a plane containing the rotary axes of rolls and the distal end of the pouring gate, and wherein the magnesium alloy comprises magnesium and at least a first additional element and at least a second additional element, wherein the at least first additional element is selected from the group of Al, Zn, Mn, Y, Zr, Cu, Ag and Si, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least first additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material, and the at least second additional element is selected from the group of Ni, Au, Pt, Sr, Ti, B, Bi, Ge, In, Nd, La and Re, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least second additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material.

11. A magnesium alloy cast material, comprising: a surface part; a central part; and a ripple mark present on the surface part, wherein the ripple mark present on the surface part of the cast material satisfies a relation rwrd<1.0 for a maximum width rw and a maximum depth rd, rw and rd being in units of mm, wherein during casting, a molten metal is supplied from a pouring gate with a supply pressure of equal to or larger than 101.8 kPa and less than 118.3 kPa, and supplied such that a distance from a distal end of the pouring gate to a position where the molten metal first contacts a movable mold substantially becomes less than 10% of a distance between a plane containing the rotary axes of rolls and the distal end of the pouring gate, and wherein the magnesium alloy comprises magnesium and at least a first additional element and at least a second additional element, wherein the at least first additional element is selected from the group of Al, Zn, Y and Si, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least first additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material, and the at least second additional element is selected from the group of Ni, Au, Pt, Sr, Ti, B, Bi, Ge, In, Nd, La and Re, in an amount of 0.5 mass % or more per element wherein the difference in mass of the at least second additional element is less than 10% between the surface part and the central part of the magnesium alloy cast material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a continuous casting apparatus for a magnesium alloy.

(2) FIG. 2(A) is a partial magnified view showing a structure in the vicinity of a pouring gate, indicating a state where a solidification completion point exists within an offset section.

(3) FIG. 2(B) is a partial magnified view showing a structure in the vicinity of a pouring gate, indicating a state where a solidification completion point does not exist within an offset section.

(4) FIG. 3(A) is a cross-sectional view along a line X-X in FIG. 2(A), showing an example in which a pouring gate has a rectangular cross section.

(5) FIG. 3(B) is a cross-sectional view along a line X-X in FIG. 2(A), showing an example in which a pouring gate has a trapezoidal cross section.

(6) FIG. 4(A) is a partial schematic view of a movable mold, showing an example having a cover layer on a surface of the movable mold, in which the cover layer is contacted with and fixed to the surface of the movable mold.

(7) FIG. 4(B) is a partial schematic view of a movable mold, showing an example having a cover layer on a surface of the movable mold, in which the cover layer is movably provided on the surface of the movable mold.

(8) FIG. 5 is a schematic view of a continuous casting apparatus for a magnesium alloy, in which a molten metal is supplied by a weight thereof to a movable mold.

BEST MODE FOR CARRYING OUT THE INVENTION

(9) In the following, embodiments of the present invention will be explained with reference to the accompanying drawings. In the drawings, same components are represented by same symbols and will not be explained in duplication. Also dimensional ratios in the drawings doe not necessarily match those in the description.

(10) FIG. 1 is a schematic view of a continuous casting apparatus for a magnesium alloy. The continuous casting apparatus includes a pair of rolls 14 as a movable mold, and produces a cast material by supplying the movable mold with a molten metal 1 of a magnesium alloy, utilizing a pump 11b and a pump 12e. The apparatus is equipped with a melting furnace 10 for melting a magnesium alloy to form a molten metal 1, a molten metal reservoir 12 for temporarily storing the molten metal 1 from the melting furnace 10, a transfer gutter 11 provided between the melting furnace 10 and the molten metal reservoir 12 for transporting the molten metal 1 from the melting furnace 10 to the molten metal reservoir 12, a supply part 12d including a pouring gate 13 for supplying the molten metal 1 from the molten metal reservoir 12 to a gap between a pair of rolls 14, and a pair of rolls 14 for casting the supplied molten metal 1 thereby forming a cast material 2.

(11) In the example shown in FIG. 1, the melting furnace 10 includes a crucible 10a for melting the magnesium alloy and storing the molten metal 1, a heater 10b provided on the external periphery of the crucible 10a for maintaining the molten metal 1 at a constant temperature, and a casing 10c storing the crucible 10a and the heater 10b. Also a temperature measuring device (not shown) and a temperature controller (not shown) are provided for regulating the temperature of the molten metal 1. Also the crucible 10a is provided, for controlling an atmosphere in the interior thereof by a gas to be explained later, with a gas introducing pipe 10d, an exhaust pipe 10e and a gas controller (not shown). Also the crucible 10a is equipped with a fin (not shown) for agitating the molten metal 1 thereby rendered capable of agitation.

(12) In the example shown in FIG. 1, the transfer gutter 11 is inserted at an end thereof into the molten metal 1 in the crucible 10a and connected at the other end to the molten metal reservoir 12, and is provided on an external periphery with a heater 11a in order that the temperature of the molten metal 1 is not lowered in transporting the molten metal 1. Also a pump 11b is provided for supplying the molten metal 1 to the molten metal reservoir 12. On an external periphery of the transfer gutter 11, an ultrasonic agitating apparatus (not shown) is provided, thereby enabling to agitate the molten metal 1 during the transport.

(13) In the example shown in FIG. 1, the molten metal reservoir 12 is equipped, on an external periphery thereof, with a heater 12a, a temperature measuring instrument (not shown) and a temperature controller (not shown). The heater 12a is principally used at the start of operation, for heating the molten metal reservoir 12 in order that the molten metal 1 transported from the melting furnace 10 is maintained at least at a non-solidifying temperature. During a stable operation, the heater 12a may be suitably used in consideration of a heat input from the molten metal 1 transferred from the melting furnace 10 and a heat output dissipated from the molten metal reservoir 12. Also as in the crucible 10a, the molten metal reservoir 12 is provided, for the purpose of atmosphere control by a gas, with a gas introducing pipe 12b, an exhaust pipe 12c and a gas controller (not shown). Also, as in the crucible 10a, the molten metal reservoir 12 is equipped with a fin (not shown) for agitating the molten metal 1 thereby rendered capable of agitation.

(14) In the example shown in FIG. 1, the supply part 12d is inserted, at an end thereof, into the molten metal 1 of the molten metal reservoir 12, and is provided, at the other end (at a side of the rolls 14 constituting the movable mold), with a pouring gate 13. In the vicinity of the pouring gate 13, a temperature measuring device (not shown) is provided for a temperature management of the molten metal 1 supplied to the pouring gate 13. The temperature measuring device is so positioned as not to hinder the flow of the molten metal 1. The pouring gate 13 is provided separately with heating means such as a heater and is preferably heated, before the operation is started, to a temperature range in which the molten metal 1 does not solidify. Also in order to reduce a temperature fluctuation of the molten metal 1 in a transversal cross-sectional direction of the pouring gate 13, it is possible to confirm the temperature suitably with the temperature measuring device and to heat the pouring gate 13 by the heating means. The temperature fluctuation may also be reduced by forming the pouring gate 13 with a material having an excellent thermal conductivity. For the purpose of supplying the molten metal 1 from the pouring gate 13 to the movable mold (gap between the rolls 14), the supply part 12d includes a pump 12e between the molten metal reservoir 12 and the pouring gate 13. A pressure of the molten metal 1 supplied from the pouring gate 13 to the gap between the rolls 14 can be regulated, by regulating an output of the pump 12e.

(15) In the example shown in FIG. 1, the movable mold is constituted of a pair of rolls 14. The rolls 14 are provided in an opposed relationship with a gap therebetween, and are rendered rotatable by an unillustrated drive mechanism in mutually different directions (clockwise in a roll and counterclockwise in the other). The molten metal 1 is supplied into the gap between the rolls 14, and, under rotation of the rolls 14, the molten metal 1 supplied from the pouring gate 13 solidifies while in contact with the rolls 14, and discharged as a cast material 2. In the present example, as the casting direction is vertically upwards, a molten metal dam 17 (cf. FIGS. 3(A) and 3(B)) is provided in order that the molten metal does not leak downwards from a gap between the movable mold and the pouring gate 13. Each roll 14 incorporates a heating-cooling mechanism (not shown) for arbitrarily regulating the surface temperature, and is equipped with a temperature measuring instrument (not shown) and a temperature controller (not shown).

(16) Then, the present invention is characterized in employing, as a material for forming parts contacted by the molten metal 1 in the process from the melting step to the continuous casting, a low-oxygen material having an oxygen content in a volumic ratio of 20 mass % or less. As such material, the present example employed a cast iron (oxygen concentration: 100 ppm or less in weight proportion) for the crucible 10a, a stainless steel (SUS 430, oxygen concentration: 100 ppm or less in weight proportion) for the transfer gutter 11, the molten metal reservoir 12, the supply part 12d, the pouring gate 13 and the molten metal dam 17 (cf. FIGS. 3(A) and 3(B), and a copper alloy (composition (mass %): copper 99%, chromium 0.8% and impurities as remainder, oxygen concentration: 100 ppm or less in weight proportion) for the rolls 14.

(17) As the manufacture of the cast material with such continuous casting apparatus allows to reduce a bonding of the molten metal with oxygen, it is possible to reduce a formation of magnesium oxide or a chipping of the oxygen-deprived material, which lead to a deterioration in the surface properties of the cast material. Also as the molten metal is less contaminated by magnesium oxide or an oxygen-deprived material, a deterioration in the secondary working property caused by the presence of these foreign substances can also be reduced.

(18) Particularly in the continuous casting apparatus shown in FIG. 1, the interior of the crucible 10a and the interior of the molten metal reservoir 12 may be maintained in a low-oxygen atmosphere by sealing a gas of a low oxygen concentration therein. In such state, the bonding of the molten metal with oxygen can be reduced more effectively. Examples of the gas for constituting the low-oxygen atmosphere include an argon gas with an oxygen content less than 5 vol %, and a mixed gas of carbon dioxide and argon. Also a flame-resisting gas such as SF.sub.6 may be mixed.

(19) Also in the continuous casting apparatus shown in FIG. 1, a solidification completion point may be positioned within a region to a discharge from the movable mold, by executing such a control as to sufficiently lower the mold temperature and to regulate a driving speed of the movable mold, in consideration of a desired alloy composition and a desired plate thickness and of a material constituting the mold. FIGS. 2(A) and 2(B) are partial magnified views showing a structure in the vicinity of the pouring gate, and FIG. 2(A) indicates a state where the solidification completion point exists within an offset section, while FIG. 2(B) indicates a state where the solidification completion point does not exist within an offset section. A section between a plane including the center axes of the rolls 14 (the plane being hereinafter called a mold center 15) and a distal end of the pouring gate 13 is called an offset 16. As shown in FIG. 2(A), the molten metal 1, supplied from the supply part 12d, through the pouring gate 13, to the gap between the rolls 14, is released in a closed space surrounded by the pouring gate 13, the rolls 14 and the unillustrated molten metal dam, and is cooled by contacting the rolls 14 under formation of a meniscus 20 whereby a solidification is initiated. Along the casting direction (upwards in FIGS. 2(A) and 2(B)), the rolls 14 are positioned closer, so that the gap between the rolls 14 becomes smaller. More specifically, when the molten metal 1 supplied from the pouring gate 13 comes into an initial contact with the rolls 14 in an initial stage of the casting, the gap is largest at an initial gap m1 between portions initially contacted by the molten metal 1, and, as the solidified material passes through the mold center 15, the gap becomes a minimum gap m2 where the rolls 14 are positioned closest. Therefore, without generating a gap between a solidified shell formed by a solidification and the rolls 14 by a solidification shrinkage, the solidified shell remains in close contact with the rolls 14 and a cooling effect thereof until the solidification is completed at a solidification completion point 21. Also in a section from the solidification completion point 21 to the mold center 15, the gap between the rolls 14 becomes even smaller. Therefore, the solidified magnesium alloy is subjected to a compressive deformation by a reducing force from the rolls 14, and is discharged from the gap between the rolls 14, thereby providing a cast material 2 with smooth surfaces as in a rolled material. The solidification state is preferably controlled in such a manner that the solidification completion point 21 exists within the section of offset 16. Also a high cooling effect is obtained by selecting the distance of the initial gap m1 as from 1 to 1.55 times of the minimum gap m2.

(20) On the other hand, in a case of not executing a solidification control as described above, the molten metal 1, supplied from the supply part 12d, through the pouring gate 13, to the gap between the rolls 14 as shown in FIG. 2(B), is released in a closed space surrounded by the pouring gate 13, the rolls 14 and the unillustrated molten metal dam, and is cooled by contacting the rolls 14 under formation of a meniscus 20 whereby a solidification is initiated. However, it passes through the mold center 15, with a large amount of an unsolidified part in the central part. Thus, a solidification completion point 23 is present in a position after the section of offset 16. Since the magnesium alloy after passing the mold center 15 is separated from the rolls 14, the solidification proceeds not by the cooling by the rolls 14 but by a cooling by heat radiation from the surfaces of the cast material 2. Therefore the solidification rate becomes slower at the central part of the cast material 2, thus causing a center-line segregation.

(21) FIGS. 3(A) and 3(B) are cross-sectional views along a line X-X in FIG. 2(A), wherein FIG. 3(A) shows an example in which a pouring gate has a rectangular cross section, and FIG. 3(B) shows an example in which a pouring gate has a trapezoidal cross section. Also in the continuous casting apparatus shown in FIG. 1, a region where a meniscus 20 is formed (cf. FIGS. 2(A) and 2(B)) may be made sufficiently small by regulating the pressure of the molten metal 1, supplied from the pouring gate 13 to the gap between the rolls 14, by the pump 12e. Also by a control so as to minimize the temperature fluctuation in the molten metal 1 in the transversal cross-sectional direction of the pouring gate 13, the molten metal 1 is immediately filled in the meniscus-forming region thereby providing a satisfactory cast material 2. For example, the temperature measuring device 13a as shown in FIG. 3(A) is used to regulate a temperature of separate heating means, such as a heater, in such a manner that a temperature fluctuation in the molten metal 1 in the transversal cross-sectional direction of the pouring gate 13 becomes 10 C. or less, and the pump 12e (cf. FIG. 1) is regulated in such a manner that the pressure of the molten metal 1 supplied to the gap between the rolls 14 becomes equal to or larger than 101.8 kPa and less than 118.3 kPa (equal to or larger than 1.005 atm and less than 1.168 atm). In this manner, the molten metal 1 can be sufficiently filled as shown in FIG. 3(A). An example shown in FIG. 3(B) is merely different in the shape of the pouring gate 13, and, as in the example shown in FIG. 3(A), the molten metal 1 can be filled sufficiently by regulating the pressure of the molten metal 1, supplied from the pouring gate 13 to the bag between the rolls 14, by the pump 12e (cf. FIG. 1), and by controlling the temperature fluctuation of the molten metal 1 in the transversal cross-sectional direction of the pouring gate 13.

(22) In the continuous casting apparatus shown in FIG. 1, a cover layer may be provided on the movable mold, in order to further increase the cooling rate. FIGS. 4(A) and 4(B) are partial schematic views of a movable mold, showing examples having a cover layer on a surface of the movable mold, wherein FIG. 4(A) shows an example in which the cover layer is contacted with and fixed to the surface of the movable mold, and FIG. 4(B) shows an example, in which the cover layer is movably provided on the surface of the movable mold. A movable mold 30 shown in FIG. 4(A) is provided, on an external periphery of rolls 14a, with a cover layer 14b of material having a low oxygen content and excellent in thermal conductivity. The cover layer 14b is provided in such a manner that the molten metal 1 supplied from the pouring gate 13 and the cast material 2 obtained by solidification do not come into contact with the roll 14a. Examples of a material for forming such cover layer 14b include copper and a copper alloy. The material for forming the cover layer 14b is a material only required to have a low oxygen content and an excellent thermal conductivity as described above, a material that is not strong enough as the material for the rolls 14a may also be used. The cover layer 14b, having an excellent thermal conductivity, efficiently dissipate the heat of the molten metal 1 when contacted by the molten metal 1, thereby contributing to increase the cooling rate of the molten metal 1. Also because of the excellent thermal conductivity, it also provides an effect of preventing a dimensional change in the roll 14a due to a deformation by the heat from the molten metal 1. Also in case the cover layer 14b is formed by a material similar to that of the roll 14a, the cover layer 14b alone may be replaced economically when it is damaged in the operation.

(23) Although the cover layer 14b may be contacted with and fixed to the roll 14a as described above, as shown in FIG. 4(B), a cover layer 19 may be provided so as to be movable on the external periphery of the roll 14a. The cover layer 19 is formed as a belt-shaped member with a material having a low oxygen content and excellent in thermal conductivity as in the cover layer 14b, and is constructed in a closed loop structure as shown in FIG. 4(B). Such closed-loop cover layer 19 is supported by a roll 14a and a tensioner 18, in such a manner that the cover layer 19 is movable on the external periphery of the roll 14a. The cover layer 19, having an excellent thermal conductivity as in the cover layer 14, sufficiently increases the cooling rate of the molten metal 1 and suppresses a dimensional change of the roll 14a by a thermal deformation. Also in case the cover layer 19 is formed by a material similar to that of the roll 14a, the cover layer 19 alone may be replaced when it is damaged in the operation. Also the cover layer 19, so constructed as to displace between the roll 14a and the tensioner 18, it may be subjected to a surface cleaning or a correction of a deformation by a thermal strain, after contacting the molten metal 1 and before a next contact. Also heating means for heating the cover layer 19 may be provided between the roll 14a and the tensioner 18.

(24) FIG. 5 is a schematic view of a continuous casting apparatus for a magnesium alloy, in which a molten metal is supplied to a movable mold, utilizing the weight of the molten metal. The continuous casting apparatus is similar in a basic structure to the apparatus shown in FIG. 1. More specifically, it is equipped with a melting furnace 40 for melting a magnesium alloy to form a molten metal 1, a molten metal reservoir 42 for temporarily storing the molten metal 1 from the melting furnace 40, a transfer gutter 41 provided between the melting furnace 40 and the molten metal reservoir 42 for transporting the molten metal 1 from the melting furnace 40 to the molten metal reservoir 42, a supply part 42d including a pouring gate 43 for supplying the molten metal 1 from the molten metal reservoir 42 to a gap between a pair of rolls 44, and a pair of rolls 44 for casting the supplied molten metal 1 thereby forming a cast material 2. A difference lies in a fact that the molten metal 1 is supplied by the weight thereof to the gap between the rolls 44.

(25) In the apparatus shown in FIG. 5, the melting furnace 40, as in the melting furnace 10 shown in FIG. 1, includes a crucible 40a, a heater 40b, and a casing 40c, a temperature measuring device (not shown) and a temperature controller (not shown). Also the crucible 40a is provided with a gas introducing pipe 40d, an exhaust pipe 40e and a gas controller (not shown). Also the crucible 40a is equipped with a fin (not shown) for agitating the molten metal 1 thereby rendered capable of agitation. The transfer gutter 41 is connected, at an end thereof, with the crucible 40a, and, at the other end with the molten metal reservoir 42, and is provided in an intermediate part with a heater 41a and a valve 41b for supplying the molten metal 1 to the molten metal reservoir 42. On an external periphery of the transfer gutter 41, an ultrasonic agitating apparatus (not shown) is provided.

(26) In the example shown in FIG. 5, the molten metal reservoir 42 is equipped, on an external periphery thereof, with a heater 42a, a temperature measuring instrument (not shown) and a temperature controller (not shown). Also the molten metal reservoir 42 is provided with a gas introducing pipe 42b, an exhaust pipe 42c and a gas controller (not shown). Also the molten metal reservoir 42 is equipped with a fin (not shown) for agitating the molten metal 1 thereby rendered capable of agitation. The supply part 42d is connected, at an end thereof, with the molten metal reservoir 42, and is provided, at the other end (at a side of the rolls 44 constituting the movable mold), with a pouring gate 43. In the vicinity of the pouring gate 43, a temperature measuring device (not shown) is provided for a temperature management of the molten metal 1 supplied to the pouring gate 43. The temperature measuring device is so positioned as not to hinder the flow of the molten metal 1. In order that the molten metal 1 is supplied from the pouring gate 43 to the gap between the rolls 44 by the weight of the molten metal 1, a center line 50 to be explained later of the gap between the rolls 44 is positioned horizontally, and the molten metal reservoir 42, the pouring gate 43 and rolls 44 are positioned in such a manner that the molten metal is supplied from the molten metal reservoir 42, through the pouring gate 43, in a horizontal direction to the gap between the rolls 44 and that the cast material 2 is formed in a horizontal direction. Also the supply part 42d is positioned lower than a liquid level of the molten metal 1 in the molten metal reservoir 42. A sensor 47 for detecting the liquid level is provided, for executing a regulation that the liquid level of the molten metal 1 in the molten metal reservoir 42 comes to a predetermined height h from the center line 50 of the gap between the rolls 44. The sensor 47 is connected to an unillustrated controller, which regulates the valve 41b in response to a detection result of the sensor 47 to control the flow rate of the molten metal 1, thereby regulating the pressure of the molten metal 1 in the supply from the pouring gate 43 to the gap between the rolls 44. More specifically, a height of a point distant by 30 mm from the center line 50 is selected as a set value for the liquid level of the molten metal 1, and the liquid level is preferably so controlled to be positioned at such set value10%. Also the pressure of the molten metal 1 is desirably made equal to or larger than 101.8 kPa and less than 118.3 kPa (equal to or larger than 1.005 atm and less than 1.168 atm).

(27) In the example shown in FIG. 5, the movable mold is constituted of a pair of rolls 44. The rolls 44 are provided in an opposed relationship with a gap therebetween, and are rendered rotatable by an unillustrated drive mechanism in mutually different directions (clockwise in a roll and counterclockwise in the other). Particularly, the rolls 44 are disposed such that the center line 50 of the gap between the rolls is positioned horizontally. The molten metal 1 is supplied into the gap between the rolls 44, and, under rotation of the rolls 44, the molten metal 1 supplied from the pouring gate 43 solidifies while in contact with the rolls 44, and discharged as a cast material 2. In the present example, the casting direction is horizontal. Each roll 44 incorporates a heating-cooling mechanism (not shown) for arbitrarily regulating the surface temperature, and is equipped with a temperature measuring instrument (not shown) and a temperature controller (not shown).

(28) In the present example, graphite (oxygen concentration: 50 ppm or less in weight proportion (excluding oxygen in pores) is employed as a low-oxygen material having an oxygen content of 20% by mass for forming the crucible 40a, the transfer gutter 41, the molten metal reservoir 42, the supply part 42d and the pouring gate 43. Also as a material for forming the rolls 44, a copper alloy (composition (mass %): copper 99%, chromium 0.8% and impurities as remainder, oxygen concentration: 100 ppm or less in weight proportion) is employed.

(29) The manufacture of the cast material with such continuous casting apparatus allows, as in the apparatus shown in FIG. 1, to reduce drawbacks resulting from a bonding of the molten metal with oxygen, namely a deterioration of the surface properties of the cast material and a loss in the secondary working property. Also in the apparatus shown in FIG. 5, a low-oxygen atmosphere is maintained in the interior of the crucible 40a and the interior of the molten metal reservoir 42 to effectively reduce the bonding of the molten metal with oxygen.

Test Example 1

(30) Continuous casting is conducted with the continuous casting apparatus shown in FIG. 5 to produce cast materials (plate materials). Characteristics of the obtained cast materials are investigated. Composition, cast conditions and characteristics of the investigated magnesium alloys are shown in Tables 1 to 5. Tables 1-5 show the material of the mold only, and materials for constituents other than the mold are same as those (carbon) shown in FIG. 5. In Table 1 to 5, a maximum temperature, a minimum temperature and a fluctuation of molten metal mean the temperatures at the pouring gate and the fluctuation in the transversal cross-sectional directional direction of the pouring gate. An offset mean a distance (offset 46) between the plane including the central axes of the rolls 44 (hereinafter mold center 45) and the distal end of the pouring gate 43 in FIG. 5. An atmosphere is constituted of oxygen in a content shown in Tables 1 to 5 and a mixed gas of argon and nitrogen in the remainder. A gap at pouring gate means a gap between parts of rolls initially contacted by the molten metal supplied from the pouring gate. A roll gap at the mold center means a minimum gap where the rolls are positioned closest. A reduction rate is defined by (gap at pouring gate/minimum gap)100. A supply pressure means a compression load applied from the molten metal (including solidified portion) to the rolls. A temperature of cast material means a surface temperature of the magnesium alloy material immediately after discharge from the rolls. A fluctuation in components is determined based on set contents corresponding to the composition of each sample shown in Tables 1 to 5.

(31) TABLE-US-00001 TABLE 1 sample No., composition (mass %) No. 1 No. 3 No. 4 Mg No. 2 Mg Mg 3 mass % Al Mg 3 mass % Al 6 mass % Al 1 mass % Zn 3 mass % Al 1 mass % Zn 1 mass % Zn item unit 0.03 mass % Ca 1 mass % Zn 0.05 mass % Ca 0.03 mass % Ca Casting conditions melting point ( C.) 630 630 630 610 conductivity x (% IACS) 18 18 18 12 oxygen content in atmosphere (vol %) 4 4 4 4 molten metal liquid level from roll gap center line (mm) 50 50 50 50 converted supply pressure (molten metal pressure) (kPa) 102.1 102.1 102.1 102.1 molten metal max temperature ( C.) 705 700 700 695 molten metal min temperature ( C.) 700 695 695 690 molten metal temperature fluctuation ( C.) 5 5 5 5 movable mold (roll) diameter (mm) 400 400 400 400 offset (mm) 15 15 15 15 ratio of offset/roll circumferential length (%) 1.2 1.2 1.2 1.2 gap at pouring gate (mm) 4.6 5.1 5.1 4.6 roll gap at mold center (mm) 3.5 4 4 3.5 reduction rate (times) 1.31 1.28 1.28 1.31 solidification completion point/offset (%) 40 38 38 40 cooling rate (K/sec) 636 783 523 2129 roll load (N) 670000 630000 630000 650000 plate width (mm) 200 200 200 200 load per plate width (N/mm) 3350 3150 3150 3250 cast plate temperature ( C.) 270 270 300 250 mold material copper alloy copper alloy copper copper electroconductivity y of mold material (% IACS) 80 80 10 100 melting point of mold material (K) 1256 1256 1766 1356 relation 100 y > x 10 (/X) cover layer none none none none electroconductivity y of cover layer (% IACS) thickness of cover layer (m) melting point of cover layer (K) relation 100 y > x 10 (/X) melting point of surface material of movable mold (K) 1256 1256 1766 1356 surface temperature of movable mold (K) 423 423 423 423 relation (movable mold surface temp./surface mat. m.p.) (/X) 34%: 34%: 24%: 31%: Cast material characteristics thickness (mm) 4.3 4.8 4.8 4.3 DAS (m) 4.8 4.5 5.1 3.3 max size of intermetallic compounds (m) <1 <1 <1 4.0 component element contained at least by 0.5% Al, Zn Al, Zn Al, Zn Al, Zn fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/2.70-2.78 Al/2.70-2.78 Al/5.95-6.07 element/compositional average (%) Al/2.7% Al/2.7% Al/2.7% Al/2.0% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 element/compositional average (%) Zn/8.0% Zn/8.0% Zn/8.0% Zn/8.0% relation: fluctuation 10% (/X) surface defect depth (mm) 0.06 0.05 0.06 0.06 surface defect depth/plate thickness (%) 1.3% 1.1% 1.2% 1.5% ripple mark max width rw (mm) 0.5 mm 0.5 mm 0.5 mm 0.6 mm ripple mark max depth rd (mm) 0.01 mm 0.01 mm 0.01 mm 0.01 mm relation: rw rd (/X) 0.005: 0.005: 0.005: 0.006: tensile strength (MPa) 213 215 208 215 breaking elongation (%) 3.5 3.2 3.6 2.5

(32) TABLE-US-00002 TABLE 2 sample No., composition (mass %) No. 5 No. 6 Mg Mg No. 7 No. 8 8 mass % Al 9 mass % Al Mg Mg 0.6 mass % Zn 1 mass % Zn 4 mass % Al 2.5 mass % Zn item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 7 mass % Y Casting conditions melting point ( C.) 610 595 617 600 conductivity x (% IACS) 11 10 12 10 oxygen content in atmosphere (%) 4 4 4 4 molten metal liquid level from roll gap center line (mm) 75 75 75 75 converted supply pressure (molten metal pressure) (kPa) 102.6 102.6 102.6 102.6 molten metal max temperature ( C.) 670 680 700 685 molten metal min temperature ( C.) 662 671 695 680 molten metal temperature fluctuation ( C.) 8 9 5 5 movable mold (roll) diameter (mm) 400 400 400 400 offset (mm) 15 15 20 17 ratio of offset/roll circumferential length (%) 1.2 1.2 1.6 1.4 gap at pouring gate (mm) 4.1 5.1 6.0 5.5 roll gap at mold center (mm) 3 4 4 4 reduction rate (times) 1.37 1.28 1.50 1.38 solidification completion point/offset (%) 40 25 40 30 cooling rate (K/sec) 523 557 1933 2895 roll load (N) 700000 630000 430000 350000 plate width (mm) 200 200 130 130 load per plate width (N/mm) 3500 3150 3310 2690 cast plate temperature ( C.) 270 270 250 250 mold material copper copper copper Copper electroconductivity y of mold material (% IACS) 10 10 100 100 melting point of mold material (K) 1766 1766 1356 1356 relation 100 y > x 10 (/X) cover layer copper alloy copper alloy Mg none electroconductivity y of cover layer (% IACS) 20 25 38 thickness of cover layer (m) 20 50 50 melting point of cover layer (K) 1173 1173 923 relation 100 y > x 10 (/X) melting point of surface material of movable mold (K) 1173 1173 923 1356 surface temperature of movable mold (K) 423 423 423 353 relation (movable mold surface temp./surface mat. m.p.) (/X) 36%: 36%: 46%: 26%: Cast material characteristics thickness (mm) 3.9 4.8 4.5 4.4 DAS (m) 5.1 5 3.4 3 max size of intermetallic compounds (m) 5.0 5.0 15.0 6.7 component element contained at least by 0.5% Al, Zn Al, Zn Al, Si Zn, Y fluctuation element/min.-max. (mass %) Al/8.00-8.15 Al/8.82-9.08 Al/4.10-4.21 Zn/2.35-2.51 element/compositional average (%) Al/1.9% Al/2.9% Al/2.8% Zn/6.4% element/min.-max. (mass %) Zn/0.62-0.65 Zn/0.81-0.89 Si/1.05-1.08 Y/6.51-6.73 element/compositional average (%) Zn/5.0% Zn/8.0% Si/3.0% Y/3.1% relation: fluctuation 10% (/X) surface defect depth (mm) 0.06 0.08 0.16 0.19 surface defect depth/plate thickness (%) 1.6% 1.6% 3.5% 4.3% ripple mark max width rw (mm) 0.3 mm 0.5 mm 1.0 mm 0.2 mm ripple mark max depth rd (mm) 0.01 mm 0.01 mm 0.01 mm 0.01 mm relation: rw rd (/X) 0.003: 0.005: 0.010: 0.002: tensile strength (MPa) 230 241 205 260 breaking elongation (%) 1.2 1.1 1.1 1.1

(33) TABLE-US-00003 TABLE 3 sample No., composition (mass %) No. 9 No. 11 No. 12 Mg Mg Mg 3 mass % Al No. 10 3 mass % Al 3 mass % Al 1 mass % Zn Mg 1 mass % Zn 1 mass % Zn item unit 0.03 mass % Ca 0.03 mass % Ca 0.03 mass % Ca 0.03 mass % Ca Casting conditions melting point ( C.) 630 650 630 630 conductivity x (% IACS) 18 38 18 18 oxygen content in atmosphere (%) 4 4 15 4 molten metal liquid level from roll gap center line (mm) 155 155 155 155 converted supply pressure (molten metal pressure) (kPa) 104.0 104.0 104.0 104.0 molten metal max temperature ( C.) 705 700 705 697 molten metal min temperature ( C.) 700 695 700 697 molten metal temperature fluctuation ( C.) 5 5 5 3 movable mold (roll) diameter (mm) 400 400 400 400 offset (mm) 15 10 18 15 ratio of offset/roll circumferential length (%) 1.2 0.8 1.4 1.2 gap at pouring gate (mm) 4.1 1.6 4.6 4.6 roll gap at mold center (mm) 3 1 3 3.5 reduction rate (times) 1.37 1.55 1.53 1.31 solidification completion point/offset (%) 30 35 30 30 cooling rate (K/sec) 595 3617 1472 2604 roll load (N) 360000 300000 1600000 250000 plate width (mm) 130 80 500 80 load per plate width (N/mm) 2770 3750 3200 3130 cast plate temperature ( C.) 300 250 250 250 mold material copper copper copper copper electroconductivity y of mold material (% IACS) 10 100 100 100 melting point of mold material (K) 1766 1356 1356 1356 relation 100 y > x 10 (/X) cover layer copper alloy none none none electroconductivity y of cover layer (% IACS) 25 thickness of cover layer (m) 50 melting point of cover layer (K) 1173 relation 100 y > x 10 (/X) melting point of surface material of movable mold (K) 1173 1356 1356 1356 surface temperature of movable mold (K) 353 423 423 423 relation (movable mold surface temp./surface mat. m.p.) (/X) 30%: 31%: 31%: 31%: Cast material characteristics thickness (mm) 3.5 1.4 5.0 3.8 DAS (m) 4.9 2.8 3.7 3.1 max size of intermetallic compounds (m) 20.0 <1 <1 <1 component element contained at least by 0.5% Al, Zn Al, Zn Al, Zn fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/2.70-2.78 Al/2.70-2.78 element/compositional average (%) Al/2.7% Al/2.7% Al/2.7% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 element/compositional average (%) Zn/8.0% Zn/8.0% Zn/8.0% relation: fluctuation 10% (/X) surface defect depth (mm) 0.04 0.00 0.06 0.05 surface defect depth/plate thickness (%) 1.2% 0.1% 1.2% 1.4% ripple mark max width rw (mm) 0.5 mm 0.2 mm 0.5 mm 0.5 mm ripple mark max depth rd (mm) 0.01 mm 0.01 mm 0.01 mm 0.01 mm relation: rw rd (/X) 0.005: 0.002: 0.005: 0.005: tensile strength (MPa) 220 195 215 213 breaking elongation (%) 3.6 2.8 3.4 3.6

(34) TABLE-US-00004 TABLE 4 sample No., composition (mass %) No. 13 No. 14 No. 15 No. 16 Mg Mg Mg Mg 4 mass % Al 4 mass % Al 9 mass % Al 6 mass % Zn item unit 2 mass % Si 5 mass % Si 2 mass % Si 0.4 mass % Zr Casting conditions melting point ( C.) 630 680 595 635 conductivity x (% IACS) 11 10 10 10 oxygen content in atmosphere (%) 4 4 4 15 molten metal liquid level from roll gap center line (mm) 155 155 75 75 converted supply pressure (molten metal pressure) (kPa) 104.0 104.0 102.6 102.6 molten metal max temperature ( C.) 710 730 680 690 molten metal min temperature ( C.) 680 700 671 665 molten metal temperature fluctuation ( C.) 5 5 9 5 movable mold (roll) diameter (mm) 400 400 400 400 offset (mm) 15 15 15 15 ratio of offset/roll circumferential length (%) 1.2 1.2 1.2 1.2 gap at pouring gate (mm) 4.1 4.1 5.1 4.1 roll gap at mold center (mm) 3 3 4 3 reduction rate (times) 1.37 1.37 1.28 1.37 solidification completion point/offset (%) 30 30 25 30 cooling rate (K/sec) 636 636 783 636 roll load (N) 460000 460000 730000 560000 plate width (mm) 130 130 200 150 load per plate width (N/mm) 3540 3540 3650 3730 cast plate temperature ( C.) 300 300 300 300 mold material copper copper copper copper electroconductivity y of mold material (% IACS) 100 100 100 100 melting point of mold material (K) 1356 1356 1356 1356 relation 100 y > x 10 (/X) cover layer none none none none electroconductivity y of cover layer (% IACS) thickness of cover layer (m) melting point of cover layer (K) relation 100 y > x 10 (/X) melting point of surface material of movable mold (K) 1356 1356 1356 1356 surface temperature of movable mold (K) 423 423 423 423 relation (movable mold surface temp./surface mat. m.p.) (/X) 31%: 31%: 31%: 31%: Cast material characteristics thickness (mm) 3.5 3.5 4.8 3.5 DAS (m) 4.8 4.8 4.5 4.8 max size of intermetallic compounds (m) 0.9 0.9 3 1.2 component element contained at least by 0.5% Al, Si Al, Si Al, Si Zn fluctuation element/min.-max. (mass %) Al/3.99-4.11 Al/3.99-4.11 Al/8.79-9.06 Zn/5.70-5.78 element/compositional average (%) Al/2.8% Al/2.8% Al/3.0% Zn/1.3% element/min.-max. (mass %) Si/1.83-1.95 Si/4.83-4.95 Si/1.83-1.95 element/compositional average (%) Si/6.0% Si/2.4% Si/6.0% relation: fluctuation 10% (/X) surface defect depth (mm) 0.02 0.02 0.07 0.12 surface defect depth/plate thickness (%) 0.6% 0.6% 1.5% 3.4% ripple mark max width rw (mm) 0.5 mm 0.5 mm 0.5 mm 0.5 mm ripple mark max depth rd (mm) 0.01 mm 0.01 mm 0.01 mm 0.01 mm relation: rw rd (/X) 0.005: 0.005: 0.005: 0.005: tensile strength (MPa) 260 290 287 269 breaking elongation (%) 3.6 1.6 2.4 2.1

(35) TABLE-US-00005 TABLE 5 sample No., composition (mass %) No. 20 No. 17 No. 18 No. 19 Mg Mg Mg Mg 4 mass % Al 9 mass % Al 5 mass % Al 5 mass % Al 2 mass % Si item unit 1.5 mass % Ca 3 mass % Ca 10 mass % Ca 0.8 mass % Ca Casting conditions melting point ( C.) 590 600 610 610 conductivity x (% IACS) 11 10 10 11 oxygen content in atmosphere (%) 4 4 15 4 molten metal liquid level from roll gap center line (mm) 75 75 75 155 converted supply pressure (molten metal pressure) (kPa) 102.6 102.6 102.6 104.0 molten metal max temperature ( C.) 690 680 700 710 molten metal min temperature ( C.) 670 677 680 680 molten metal temperature fluctuation ( C.) 5 5 5 5 movable mold (roll) diameter (mm) 400 400 400 400 offset (mm) 15 15 15 15 ratio of offset/roll circumferential length (%) 1.2 1.2 1.2 1.2 gap at pouring gate (mm) 4.1 4.1 4.1 4.1 roll gap at mold center (mm) 3 3 3 3 reduction rate (times) 1.37 1.37 1.37 1.37 solidification completion point/offset (%) 30 30 30 30 cooling rate (K/sec) 783 783 636 636 roll load (N) 560000 780000 780000 460000 plate width (mm) 150 250 250 130 load per plate width (N/mm) 3730 3120 3120 3540 cast plate temperature ( C.) 300 300 300 300 mold material copper copper copper copper electroconductivity y of mold material (% IACS) 100 100 100 100 melting point of mold material (K) 1356 1356 1356 1356 relation 100 y > x 10 (/X) cover layer none none none none electroconductivity y of cover layer (% IACS) thickness of cover layer (m) melting point of cover layer (K) relation 100 y > x 10 (/X) melting point of surface material of movable mold (K) 1356 1356 1356 1356 surface temperature of movable mold (K) 423 423 423 423 relation (movable mold surface temp./surface mat. m.p.) (/X) 31%: 31%: 31%: 31%: Cast material characteristics thickness (mm) 3.5 3.5 3.5 3.5 DAS (m) 4.5 4.5 4.8 4.8 max size of intermetallic compounds (m) 0.9 1.2 2.1 0.9 component element contained at least by 0.5% Al, Ca Al, Ca Al, Ca Al, Si fluctuation element/min.-max. (mass %) Al/8.70-8.78 Al/4.70-4.78 Al/4.70-4.78 Al/3.99-4.11 element/compositional average (%) Al/0.9% Al/1.6% Al/1.6% Al/2.8% element/min.-max. (mass %) Ca/1.43-1.51 Ca/2.99-3.05 Ca/9.81-9.89 Si/1.83-1.95 element/compositional average (%) Ca/5.3% Ca/2.0% Ca/0.8% Si/6.0% relation: fluctuation 10% (/X) surface defect depth (mm) 0.01 0.02 0.07 0.02 surface defect depth/plate thickness (%) 0.3% 0.6% 1.5% 0.6% ripple mark max width rw (mm) 0.5 mm 0.5 mm 0.5 mm 0.5 mm ripple mark max depth rd (mm) 0.01 mm 0.01 mm 0.01 mm 0.01 mm relation: rw rd (/X) 0.005: 0.005: 0.005: 0.005: tensile strength (MPa) 265 275 265 245 breaking elongation (%) 1.7 1.1 0.5 3.6

(36) As a result, the casting could be executed without causing a cracking or the like, and the obtained cast materials are found, as shown in Tables 1 to 5, to have a uniform composition, an excellent surface quality, fine intermetallic compounds and excellent mechanical characteristics.

Test Example 2

(37) Thus obtained cast materials are subjected to a rolling work to prepare rolled materials. Each rolled material is subjected, after the rolling work, to a heat treatment (for about 1 hour, at a temperature suitably selected according to the composition, within a temperature range of from 100 to 350 C.). The rolled materials obtained after the heat treatment are investigated for characteristics. Rolling conditions and characteristics are shown in Tables 6 to 10. The rolling work is conducted by plural passes, with a one-pass reduction rate within a range of from 1 to 50% and at a temperature of from 150 to 350 C., and a rolling is conducted in a final pass under conditions shown in Tables 6 to 10. A commercial rolling oil is employed as a lubricating agent.

(38) TABLE-US-00006 TABLE 6 sample No., composition (mass %) No. 1 No. 3 No. 4 Mg No. 2 Mg Mg 3 mass % Al Mg 3 mass % Al 6 mass % Al 1 mass % Zn 3 mass % Al 1 mass % Zn 1 mass % Zn item unit 0.03 mass % Ca 1 mass % Zn 0.05 mass % Ca 0.03 mass % ca Rolling conditions plate thickness before rolling (mm) 4.3 4.8 4.8 4.3 total reduction rate (%) 88% 92% 92% 88% max value of 1-pass reduction rate c (%) 25 25 25 15 min value of 1-pass reduction rate c (%) 9 9 9 6 step meeting relation 50 c 1 present? (/X) surface temp of rolling rolls in last pass ( C.) 175 175 175 175 material temp. t1 before rolling in last pass ( C.) 20 20 20 20 material temp. t2 after rolling in last pass ( C.) 165 165 165 165 T ( C.) 165 165 165 165 reduction rate c in last pass (%) 9 9 9 6 relation T/c (/X) 18.3 18.3 18.3 27.5 Rolled material characteristics thickness (mm) 0.5 0.4 0.4 0.5 average crystal grain size (m) 3.3 3.3325 3.57 3.36 average crystal grain size in surface part (m) 3 3.1 3.4 3.2 average crystal grain size in central part (m) 3.6 3.565 3.74 3.52 difference in average crystal grain size between surface (m) 0.6 0.465 0.34 0.32 and central parts relation (difference in average crystal grain size between (%) 18.2%: 14.0%: 9.5%: 9.5%: surface and central parts 20%) max size of intermetallic compounds (m) none none none 4 component element contained at least by 0.5% Al, Zn Al, Zn Al, Zn Al, Zn fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/2.70-2.78 Al/2.70-2.78 Al/5.95-6.07 element/compositional average (%) Al/2.7% Al/2.7% Al/2.7% Al/2.0% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 element/compositional average (%) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 relation: fluctuation 10% (/X) surface defect depth/plate thickness (%) 0.80% 0.90% 1.05% 1.20% tensile strength (MPa) 296 288 301 331 breaking elongation (%) 10.4 9.6 8.5 7.8

(39) TABLE-US-00007 TABLE 7 sample No., composition (mass %) No. 5 No. 6 Mg Mg No. 7 No. 8 8 mass % Al 9 mass % Al Mg Mg 0.6 mass % Zn 1 mass % Zn 4 mass % Al 2.5 mass % Zn item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 7 mass % Y Rolling conditions plate thickness before rolling (mm) 3.9 4.8 4.5 4.4 total reduction rate (%) 87% 90% 89% 89% max value of 1-pass reduction rate c (%) 15 15 15 15 min value of 1-pass reduction rate c (%) 6 6 6 6 step meeting relation 50 c 1 present? (/X) surface temp of rolling rolls in last pass ( C.) 175 175 175 175 material temp. t1 before rolling in last pass ( C.) 20 20 20 20 material temp. t2 after rolling in last pass ( C.) 165 165 165 165 T ( C.) 165 165 165 165 reduction rate c in last pass (%) 6 6 6 6 relation T/c (/X) 27.5 27.5 27.5 27.5 Rolled material characteristics thickness (mm) 0.5 0.5 0.5 0.5 average crystal grain size (m) 3.52 3.504 3.74 3.3 average crystal grain size in surface part (m) 3.2 3.2 3.4 3 average crystal grain size in central part (m) 3.84 3.808 4.08 3.6 difference in average crystal grain size between surface (m) 0.64 0.608 0.68 0.6 and central parts relation (difference in average crystal grain size between (%) 18.2%: 17.4%: 18.2%: 18.2%: surface and central parts 20%) max size of intermetallic compounds (m) 5 5 15 6.7 component element contained at least by 0.5% Al, Zn Al, Zn Al, Si Zn, Y fluctuation element/min.-max. (mass %) Al/8.00-8.15 Al/8.82-9.08 Al/4.10-4.21 Zn/2.35-2.51 element/compositional average (%) Al/1.9% Al/2.9% Al/2.8% Zn/6.4% element/min.-max. (mass %) Zn/0.62-0.65 Zn/0.81-0.89 Si/1.05-1.08 Y/6.51-6.73 element/compositional average (%) Zn/0.62-0.65 Zn/0.81-0.89 Si/1.05-1.08 Y/6.51-6.73 relation: fluctuation 10% (/X) surface defect depth/plate thickness (%) 1.10% 0.60% 1.20% 3.20% tensile strength (MPa) 360 395 350 345 breaking elongation (%) 8.2 8.6 5.1 5.3

(40) TABLE-US-00008 TABLE 8 sample No., composition (mass %) No. 9 No. 11 No. 12 Mg Mg Mg 3 mass % Al No. 10 3 mass % Al 3 mass % Al 1 mass % Zn Mg 1 mass % Zn 1 mass % Zn item unit 0.03 mass % Ca 0.03 mass % Ca 0.03 mass % Ca 0.03 mass % Ca Rolling conditions plate thickness before rolling (mm) 3.5 1.4 5 3.8 total reduction rate (%) 97% 86% 98% 47% max value of 1-pass reduction rate c (%) 25 25 25 25 min value of 1-pass reduction rate c (%) 9 9 9 9 step meeting relation 50 c 1 present? (/X) surface temp of rolling rolls in last pass ( C.) 175 175 175 175 material temp. t1 before rolling in last pass ( C.) 20 20 20 20 material temp. t2 after rolling in last pass ( C.) 165 165 165 165 T ( C.) 165 165 165 165 reduction rate c in last pass (%) 9 9 9 9 relation T/c (/X) 18.3 18.3 18.3 18.3 Rolled material characteristics thickness (mm) 0.1 0.2 0.1 2 average crystal grain size (m) 3.255 3.36 3.255 3.255 average crystal grain size in surface part (m) 3.1 3.2 3.1 3.1 average crystal grain size in central part (m) 3.41 3.52 3.41 3.41 difference in average crystal grain size between surface (m) 0.31 0.32 0.31 0.31 and central parts relation (difference in average crystal grain size between (%) 9.5%: 9.5%: 9.5%: 9.5%: surface and central parts 20%) max size of intermetallic compounds (m) 20 none none none component element contained at least by 0.5% Al, Zn Al, Zn Al, Zn fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/2.70-2.78 Al/2.70-2.78 element/compositional average (%) Al/2.7% Al/2.7% Al/2.7% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 element/compositional average (%) Zn/0.81-0.89 Zn/0.81-0.89 Zn/0.81-0.89 relation: fluctuation 10% (/X) surface defect depth/plate thickness (%) 0.09% 0.10% 0.90% 1.15% tensile strength (MPa) 286 275 296 265 breaking elongation (%) 10.4 11.2 10.2 8.7

(41) TABLE-US-00009 TABLE 9 sample No., composition (mass %) No. 13 No. 14 No. 15 No. 16 Mg Mg Mg Mg 4 mass % Al 4 mass % Al 9 mass % Al 6 mass % Zn item unit 2 mass % Si 5 mass % Si 2 mass % Si 0.4 mass % Zr Rolling conditions plate thickness before rolling (mm) 3.5 3.5 3.5 3.5 total reduction rate (%) 86% 86% 90% 86% max value of 1-pass reduction rate c (%) 25 25 25 25 min value of 1-pass reduction rate c (%) 9 9 8 9 step meeting relation 50 c 1 present? (/X) surface temp of rolling rolls in last pass ( C.) 175 175 175 175 material temp. t1 before rolling in last pass ( C.) 20 20 20 20 material temp. t2 after rolling in last pass ( C.) 165 165 165 165 T ( C.) 165 165 165 165 reduction rate c in last pass (%) 9 9 8 9 relation T/c (/X) 18.3 18.3 18.3 18.3 Rolled material characteristics thickness (mm) 0.5 0.5 0.5 3.5 average crystal grain size (m) 4.255 4.255 4.36 4.255 average crystal grain size in surface part (m) 4.10 4.10 4.20 4.10 average crystal grain size in central part (m) 4.41 4.41 4.52 4.41 difference in average crystal grain size between surface (m) 0.31 0.31 0.32 0.31 and central parts relation (difference in average crystal grain size between (%) 7.5%: 7.5%: 7.0%: 7.5%: surface and central parts 20%) max size of intermetallic compounds (m) 0.9 0.9 3 1.2 component element contained at least by 0.5% Al, Si Al, Si Al, Si Zn fluctuation element/min.-max. (mass %) Al/3.99-4.11 Al/3.99-4.11 Al/8.79-9.06 Zn/5.70-5.78 element/compositional average (%) Al/2.8% Al/2.8% Al/3.0% Zn/1.3% element/min.-max. (mass %) Si/1.83-1.95 Si/4.83-4.95 Si/1.83-1.95 element/compositional average (%) Si/6.0% Si/2.4% Si/6.0% relation: fluctuation 10% (/X) surface defect depth/plate thickness (%) 0.02 0.02 0.07 0.12 tensile strength (MPa) 314 364 410 322 breaking elongation (%) 13.4 8.4 7.2 12.2

(42) TABLE-US-00010 TABLE 10 sample No., composition (mass %) No. 20 No. 17 No. 18 No. 19 Mg Mg Mg Mg 4 mass % Al 9 mass % Al 5 mass % Al 5 mass % Al 2 mass % Si item unit 1.5 mass % Ca 3 mass % Ca 10 mass % Ca 0.8 mass % Ca Rolling conditions plate thickness before rolling (mm) 3.5 3.5 3.5 3.5 total reduction rate (%) 86% 90% 87% 86% max value of 1-pass reduction rate c (%) 25 25 15 25 min value of 1-pass reduction rate c (%) 9 8 8 9 step meeting relation 50 c 1 present? (/X) surface temp of rolling rolls in last pass ( C.) 175 175 175 175 material temp. t1 before rolling in last pass ( C.) 20 20 20 20 material temp. t2 after rolling in last pass ( C.) 165 165 165 165 T ( C.) 165 165 165 165 reduction rate c in last pass (%) 9 8 8 9 relation T/c (/X) 18.3 18.3 18.3 18.3 Rolled material characteristics thickness (mm) 0.5 0.5 0.5 0.5 average crystal grain size (m) 4.255 4.36 4.010 4.255 average crystal grain size in surface part (m) 4.10 4.20 3.90 4.10 average crystal grain size in central part (m) 4.41 4.52 4.21 4.41 difference in average crystal grain size between surface (m) 0.31 0.32 0.71 0.31 and central parts relation (difference in average crystal grain size between (%) 7.5%: 7.0%: 7.3%: 7.5%: surface and central parts 20%) max size of intermetallic compounds (m) 1.5 1.2 2.1 0.9 component element contained at least by 0.5% Al, Ca Al, Ca Al, Ca Al, Si fluctuation element/min.-max. (mass %) Al/8.70-8.78 Al/4.70-4.78 Al/4.70-4.78 Al/3.99-4.11 element/compositional average (%) Al/0.9% Al/1.6% Al/1.6% Al/2.8% element/min.-max. (mass %) Ca/1.43-1.51 Ca/2.99-3.05 Ca/9.81-9.89 Si/1.83-1.95 element/compositional average (%) Ca/5.3% Ca/2.0% Ca/0.8% Si/6.0% relation: fluctuation 10% (/X) surface defect depth/plate thickness (%) 0.01 0.02 0.07 0.02 tensile strength (MPa) 405 321 341 325 breaking elongation (%) 12.2 9.3 8.7 13.5

(43) As shown in Tables 6 to 10, the obtained rolled materials are excellent in the surface quality and also in the strength and tenacity. Also the materials had a fine crystal structure and showed fine intermetallic compounds. Also when the cast materials of Nos. 1 to 20 are subjected to a solution treatment at a temperature suitable for each composition within a temperature range of from 300 to 600 C. for 1 hour or longer, and are further subjected to a rolling and a heat treatment under similar conditions as above, and the characteristics are investigated in a similar manner. As a result, unexpected cracking, strain or deformation did not occur at all during the rolling, and the rolling work could be executed in more stable manner.

Test Example 3

(44) The obtained rolled materials are subjected to a pressing work (into an ordinary case shape) at 250 C. to prepare magnesium alloy formed articles. As a result, the formed articles utilizing the aforementioned rolled materials had an excellent dimensional precision, without cracking. Also among the rolled materials, certain samples are selected (Nos. 1-4, 9-13, 15, 16, 18 and 20 being selected) and subjected to a pressing work of various shapes at 250 C. These rolled materials are capable of pressing in any shape, and are excellent in external appearance and dimensional precision. As a comparison, a commercially available AZ31 alloy material is similarly subjected to pressing works in various shapes. As a result, the AZ31 alloy material is incapable of pressing due to cracking, or provided a product of an inferior appearance even when the pressing work is possible.

Test Example 4

(45) Also among the rolled materials, certain samples are selected (Nos. 5 and 6 being selected) and investigated for corrosion resistance. These samples are confirmed to have a corrosion resistance, comparable to that of an AZ91 alloy material, prepared by an ordinary thixomold method.

Test Example 5

(46) Also among the rolled materials, certain samples are selected (Nos. 1, 6, 7, 13 and 18 being selected) and evaluated for a bending amount. On two parallel projections, which are positioned at a distance of 150 mm, has a height of 20 mm and a sharp upper end, a sample of a width of 30 mm, a length of 200 mm and a thickness of 0.5 mmt is placed perpendicularly to the projections, and a decrease in the height at a center, when a predetermined load is applied at the center of the projections, is divided by a decrease in the height, measured in a same method on a commercial AZ31 alloy plate of 0.5 mmt, and is represented by a percentage. As a result, as shown in Table 12, the samples prepared by a twin-roll casting are confirmed to have a bending resistance, equal to or higher than that of the commercial AZ31 alloy.

Test Example 6

(47) Furthermore, among the rolled materials, certain samples are selected (Nos. 1, 6, 7, 13 and 18 being selected), and same compositions are molten with a carbon crucible in an argon atmosphere, then cast in a SUS316 mold, coated with a graphite releasing agent, with a cooling rate of from 1 to 10 K/sec so as to obtain a shape of 100 mm200 mm20 mmt, then subjected to a homogenization process at 400 C. for 24 hours in the air, and subjected to a cutting work to obtain test pieces of a thickness of 4 mmt, without defects on the surface and in the interior (in Table 11, represented as Nos. 1_M1, 6_M1, 7_M1, 13_M1 and 18_M1). The prepared test piece is subjected to a rolling work to 0.5 mmt so as to satisfy a relation 100>(T/c)>5 wherein c (%) is a one-pass reduction rate, and T ( C.) is a higher one of a temperature t1 ( C.) of the material before the rolling and a temperature t2 ( C.) of the material at the rolling operation. As a result, as shown in Table 11, the magnesium alloys cast with a cooling rate of from 1 to 10 K/sec showed cracking in the rolling process and could not be rolled, except for the alloy of the composition No. 1.

Test Example 7

(48) Furthermore, among the rolled materials, certain samples are selected (Nos. 1, 6, 7, 13 and 18 being selected), and same compositions are molten with a carbon crucible in an argon atmosphere, then cast in a SUS316 mold, coated with a graphite releasing agent, with a cooling rate of from 1 to 10 K/sec so as to obtain a shape of 100 mm200 mm20 mmt, then subjected to a homogenization process at 400 C. for 24 hours in the air, and subjected to a cutting work to obtain test pieces of a thickness of 0.5 mmt, without defects on the surface and in the interior (in Table 11, represented as Nos. 1_M2, 6_M2, 7_M2, 13_M2 and 18_M2). Among thus prepared samples and the aforementioned rolled materials, certain samples (Nos. 1, 6, 7, 13, 18 and 1_M1 being selected) are investigated for mechanical characteristics at the room temperature, 200 C. and 250 C., and for a creep property at 150 C. The creep property is evaluated after holding the test piece in an environment of 1502 C. for 20 hours, and is represented by a percentage to a creep stress (a stress (MPa) generating a creep rate of 0.1%/1000 h at a constant temperature) of a commercial AZ31 alloy plate. As a result, as shown in Table 12, the samples prepared by the twin-roll casting are confirmed to show an excellent heat resistance.

(49) TABLE-US-00011 TABLE 11 sample No., composition (mass %) No. 1 No. 6 Mg Mg No. 7 No. 13 No. 18 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca Twin-roll cast-rolled material plate thickness before rolling (mm) 4.3 4.8 4.5 3.5 3.5 total reduction rate (%) 88% 90% 89% 86% 90% thickness (mm) 0.5 0.5 0.5 0.5 0.5 average crystal grain size (m) 3.3 3.504 3.74 4.255 4.36 max size of intermetallic compounds (m) none 5 15 0.9 1.2 component element contained at least by 0.5% Al, Zn Al, Zn Al, Si Al, Si Al, Ca fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/8.82-9.08 Al/4.10-4.21 Al/3.99-4.11 Al/4.70-4.78 element/compositional average (%) Al/2.7% Al/2.9% Al/2.8% Al/2.8% Al/1.6% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/1.83-1.95 Ca/2.99-3.05 element/compositional average (%) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/6.0% Ca/2.0% relation: fluctuation 10% (/X) sample No., composition (mass %) No. 1_M1 No. 6_M1 Mg Mg No. 7_M1 No. 13_M1 No. 18_M1 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca SUS mold cast-rolled material plate thickness before rolling (mm) 4.0 4.0 4.0 4.0 4.0 total reduction rate (%) 87% cracked in rolling work to 0.5 mmt thickness (mm) 0.5 average crystal grain size (m) 3.52 max size of intermetallic compounds (m) 20 component element contained at least by 0.5% Al, Zn Al, Zn Al, Si Al, Si Al, Ca fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/8.82-9.08 Al/4.10-4.21 Al/3.99-4.11 Al/4.70-4.78 element/compositional average (%) Al/2.7% Al/2.9% Al/2.8% Al/2.8% Al/1.6% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/1.83-1.95 Ca/2.99-3.05 element/compositional average (%) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/6.0% Ca/2.0% relation: fluctuation 10% (/X) sample No., composition (mass %) No. 1_M2 No. 6_M2 Mg Mg No. 7_M2 No. 13_M2 No. 18_M2 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca SUS mold cast-cut material thickness (mm) 0.5 0.5 0.5 0.5 0.5 average crystal grain size (m) 25 28 25 25 25 max size of intermetallic compounds (m) 20 35 15 15 30 component element contained at least by 0.5% Al, Zn Al, Zn Al, Si Al, Si Al, Ca fluctuation element/min.-max. (mass %) Al/2.70-2.78 Al/8.82-9.08 Al/4.10-4.21 Al/3.99-4.11 Al/4.70-4.78 element/compositional average (%) Al/2.7% Al/2.9% Al/2.8% Al/2.8% Al/1.6% element/min.-max. (mass %) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/1.83-1.95 Ca/2.99-3.05 element/compositional average (%) Zn/0.81-0.89 Zn/0.81-0.89 Si/1.05-1.08 Si/6.0% Ca/2.0% relation: fluctuation 10% (/X)

(50) TABLE-US-00012 TABLE 12 sample No., composition (mass %) Twin-roll cast-rolled material No. 1 No. 6 Mg Mg No. 7 No. 13 No. 18 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca tensile strength (room temp.) (MPa) 296.2 395.1 350.0 314.3 321.0 breaking elongation (room temp.) (%) 10.4 8.6 5.1 13.4 9.3 mechanical tensile strength (200 C.) (MPa) 108.4 131.2 120.2 129.7 128.5 characteristics breaking elongation (200 C.) (%) 98.1 90.1 89.3 73.6 85.2 tensile strength (250 C.) (MPa) 69.1 75.5 86.7 92.9 81.2 breaking elongation (250 C.) (%) 144.5 214.3 119.4 95.1 128.7 creep property (%) 110 150 780 1020 1130 bend resistance bending amount 95 90 85 80 80 sample No., composition (mass %) SUS mold cast-rolled material No. 1_M1 No. 6_M1 Mg Mg No. 7_M1 No. 13_M1 No. 18_M1 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca mechanical tensile strength (room temp.) (MPa) 268.2 cracked in rolling work to 0.5 mmt characteristics breaking elongation (room temp.) (%) 9.6 tensile strength (200 C.) (MPa) 98.4 breaking elongation (200 C.) (%) 65.9 tensile strength (250 C.) (MPa) 60.1 breaking elongation (250 C.) (%) 78.3 creep property (%) 101 sample No., composition (mass %) SUS mold cast-cut material No. 1_M2 No. 6_M2 Mg Mg No. 7_M2 No. 13_M2 No. 18_M2 3 mass % Al 9 mass % Al Mg Mg Mg 1 mass % Zn 1 mass % Zn 4 mass % Al 4 mass % Al 5 mass % Al item unit 0.03 mass % Ca 0.03 mass % Ca 1 mass % Si 2 mass % Si 3 mass % Ca mechanical tensile strength (room temp.) (MPa) 132.3 258.8 134.6 138.3 125.6 characteristics breaking elongation (room temp.) (%) 5.6 8.1 3.2 2.8 3.4 tensile strength (200 C.) (MPa) 85.1 107.5 102.2 110.9 122.2 breaking elongation (200 C.) (%) 28.4 28.0 25.1 16.1 16.8 tensile strength (250 C.) (MPa) 57.3 64.1 78.7 70.5 73.2 breaking elongation (250 C.) (%) 38.1 72.1 35.9 19.6 23.2 creep property (%) 80 85 300 500 600

INDUSTRIAL APPLICABILITY

(51) The producing method of the present invention for magnesium alloy material is capable of stably producing magnesium alloy materials such as a magnesium alloy cast material and a magnesium alloy rolled material, excellent in mechanical characteristics, a surface quality, a bending resistance, a corrosion resistance, and a creep property. The obtained rolled material has an excellent plastic working property as in a pressing or a forging, and is optimum as a material for such molding process. Also the obtained magnesium alloy molded article can be utilized in structural members and decorative articles in the fields relating to household electric appliances, transportation, aviation-space, sports-leisure, medical-welfare, foods, and construction.