JP2008105038A - Apparatus for continuously casting magnesium alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for continuously casting a magnesium alloy with which crystal grain over the whole zone in a billet can uniformly be made to be fine. <P>SOLUTION: This apparatus is provided with a tundish 2 for holding the molten magnesium alloy 1, a mold for setting at the bottom part of the tundish and an impressing horn 5 for impressing ultrasonic vibration 4 from the upper part of the molten magnesium alloy flowing into the mold, and the impressing horn is formed as a curving surface shape bulged toward the lower part of the tip end part 6 and the tip end part of the impressing horn is positioned in the mold. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnesium alloy continuous casting apparatus.

Magnesium alloys are the lightest among practical metals and are being used for automobile parts and electrical equipment parts. When applying a magnesium alloy as a forging material, it is essential to refine the crystal grains in order to improve the workability of the material. Until now, it has been extremely difficult to apply a coarse cast material (billet) having a crystal grain size of 300 to 600 μm as a material for forging, and forging can be performed unless secondary processing such as extrusion is performed. It was impossible to obtain a desired crystal grain size (100 μm or less). For this reason, extruded rods have been mostly adopted as forging materials for magnesium alloys.
On the other hand, attempts have been made to refine billet crystal grains by, for example, heat treatment of molten metal or addition of a micronizing agent. Although there is a possibility that a crystal grain size applicable to forging can be obtained by these methods, the former increases safety and equipment load during casting operations, and the latter decreases the internal quality of the billet and reduces the environmental load. There was a problem in terms of increase.

Patent Document 1 describes that crystal grains can be refined by applying ultrasonic vibration to molten metal during the casting process. Conventionally, an application horn for applying ultrasonic vibration to molten metal has a flat tip, and when ultrasonic vibration is applied using such an application horn, the crystal grains become finer at the center of the billet. Thus, the billet could not be uniformly refined over the entire area of the billet.
JP-A-2-247314

  An object of the present invention is to provide a continuous casting apparatus of a magnesium alloy capable of uniformly refining crystal grains over the entire area of the billet in view of the above situation.

  In order to achieve the above object, a magnesium alloy continuous casting apparatus according to the first aspect of the present invention includes a tundish containing molten magnesium alloy, a mold installed at the bottom of the tundish, and magnesium flowing into the mold. An application horn that applies ultrasonic vibration to the molten alloy from above, and the application horn has a curved shape with the tip bulging downward, and the tip of the application horn is positioned in the mold. Features. Here, “the tip of the application horn is positioned in the mold” includes the case where the tip of the application horn is at the same height as the upper end or the lower end of the mold.

  In the magnesium alloy continuous casting apparatus according to the first aspect of the present invention, since the tip of the application horn has a curved shape that bulges downward, ultrasonic vibration is irradiated from the entire tip of the application horn to the surroundings. In addition, since the region where the ultrasonic vibration is irradiated is expanded and the tip of the application horn is positioned in the mold, the ultrasonic vibration can be more uniformly irradiated to the solidification interface of the magnesium alloy melt in the mold. As a result, the crystal grains can be uniformly refined over the entire area of the billet.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view showing an outline of a magnesium alloy continuous casting apparatus of the present invention, and FIG. 2 is an enlarged longitudinal sectional view showing the vicinity of a mold of the continuous casting apparatus. As shown in FIG. 1, the continuous casting apparatus includes a tundish 2 that houses a molten magnesium alloy 1, a cylindrical mold 3 installed at the bottom of the tundish 2, a cooling water jacket 7, and a cradle that moves up and down. 8, the magnesium alloy melt 1 is continuously drawn downward from the mold 3 by lowering the cradle 8, and solidified by the cooling water 9 sprayed from the cooling water jacket 7, and the billet 10 having a circular cross section is continuously formed. It is for casting. The periphery of the mold 3 is held by a refractory material 11. As shown in FIG. 2, the mold 3 has a tapered surface in which the inner peripheral surface 15 gradually increases in diameter from the inlet side toward the outlet side, and a relief portion 16 is provided below the inner peripheral surface 15. It is. Reference numeral 12 in the figure indicates a solidification interface of the molten magnesium alloy 1.

  The continuous casting apparatus further includes an ultrasonic vibration generator 13 and an application horn 5 attached to the ultrasonic vibration generator 13, and the magnesium alloy melt 1 flowing into the mold 3 from the tip 6 of the application horn 5. The ultrasonic vibration 4 in the vertical direction can be directly applied. The application horn 5 has a cylindrical shape in which the tip 6 is hemispherical and made of steel. The application horn 5 is vertically provided with its center coincident with the center of the mold 3 and can be moved in the vertical direction by the lifting device 14. In this way, by directly applying the ultrasonic vibration 4 to the molten magnesium alloy 1 in the mold 3 during continuous casting, solidification nucleation of the molten magnesium alloy 1 in the mold 3 is promoted. Since the strong intermetallic compound that crystallizes is crushed, the crystal grains of the billet 10 can be refined.

Below, the result of the experiment conducted in order to verify the crystal grain refinement | miniaturization effect by application of an ultrasonic vibration is shown. As the magnesium alloy, AZ91-1% Ca magnesium alloy was used. Table 1 shows the chemical composition of the AZ91-1% Ca magnesium alloy.
The experimental procedure is as follows. A magnesium alloy is melted in an electric furnace, and molten metal 1 is poured into a preheated tundish. When the molten metal temperature in the tundish 2 reached 700 ° C., downward drawing was started, and continuous casting was performed at a constant speed. The drawing speed was 80 mm / min, and the amount of cooling water was 14.0 liters / min. The billet 10 to be cast had a diameter D of 55 mm and a length of 300 mm. The resonance frequency of the irradiated ultrasonic vibration was 19.5 kHz, and the amplitude was 6.5 μm. The observation of the structure of the billet 10 and the measurement of the average particle diameter were performed by polishing and etching a cylindrical sample cut out from the vicinity of the center of the billet. The crystal grain size was measured at the midpoint of the line segment connecting the center and end.

In order to verify the influence due to the difference in the diameter d of the applied horn 5 and the shape of the tip 6 under the above casting conditions, the applied horn 5 having a diameter of 40 mm and a tip shape of a hemisphere, the tip shape of the tip 30 having a diameter of 30 mm and a hemisphere Continuous casting was performed while applying ultrasonic vibration with an application horn 5 of a mold, an application horn 5 with a diameter of 40 mm and a flat tip shape, and an application horn 5 with a diameter of 30 mm and a flat tip shape. Observation of the structure of the billet 10 and measurement of the average crystal grain size were performed. The tip of the application horn 5 was flush with the upper end of the mold 3. The experimental results are shown in Table 2. 3A shows a macro structure of a billet continuously cast while applying ultrasonic vibration with an application horn having a diameter of 40 mm and a hemispherical tip, and FIG. 3B applies ultrasonic vibration. The macrostructure of a billet continuously cast without a metal is shown.

  As a result of this experiment, the crystal grains are made finer when the diameter of the applied horn 5 is larger, and the crystal grains are finer when the tip 6 of the applied horn 5 is hemispherical than when the applied horn 5 is flat. In addition, it can be seen that the material is refined almost uniformly over the entire area of the billet. When ultrasonic vibration is applied with an application horn 5 having a diameter of 40 mm and a hemispherical tip, the average crystal grain size is refined to 75 μm, and a uniform grain size distribution is obtained over the entire billet. A billet applicable as a material was obtained. When the tip shape of the applied horn is flat, the particle size distribution becomes finer toward the center of the billet, whereas when the tip shape is a hemispherical shape, the ultrasonic vibration 4 is applied evenly throughout the entire area. Since irradiation is performed vertically from the bottom surface of the horn 5, when the application horn tip 6 is a flat type, the ultrasonic vibration 4 is applied only to a region directly below the application horn 5, whereas the application horn tip 6 is In the case of the hemispherical type, as shown in FIG. 2, the ultrasonic vibration 4 is irradiated vertically from the entire hemispherical surface, the region irradiated with the ultrasonic vibration 4 is expanded, and the magnesium alloy melt 1 in the mold 3 is expanded. This is considered to be because the ultrasonic vibration 4 can be irradiated more uniformly on the solidification interface 12 of the solid phase.

Next, in order to verify the influence on the refinement of billet crystal grains due to the difference in the immersion depth of the applied horn 5, the applied horn 5 having a diameter of 30 mm and a hemispherical tip is used. The position of was continuously cast with X = −5, 0, 10, 40 mm, and the average particle size of each cast billet was measured and the structure was observed. The applied horn tip position X corresponds to the distance from the lower end of the inner peripheral surface 15 of the mold 3 to the tip of the applied horn 5 as shown in FIG. 2, and when X = −5 mm, the tip of the applied horn 5 Is 5 mm lower than the lower end of the inner peripheral surface 15 of the mold 3, and when X = 0 mm, the tip of the applied horn 5 is at the same height as the lower end of the inner peripheral surface 15 of the mold 3, and X = 40 mm Sometimes the tip of the application horn 5 is at the same height as the upper end of the mold 3. Table 3 shows the measurement results of the average particle diameter of each billet, and FIG. 4 shows the microstructure of each billet.

  From this experimental result, it can be seen that when X = -5 mm where the applied horn 5 is immersed most deeply, the average crystal grain size is as fine as 24 μm, and the effect of miniaturization becomes smaller as the applied horn is raised. That is, the closer the tip of the application horn 5 is to the solidification interface 12 of the molten magnesium alloy 1, the finer the crystal grains. Thus, since the effect of crystal grain refinement is greatly affected by the difference in immersion depth of the applied horn, the applied horn can be moved up and down by enabling the applied horn to move up and down as in this continuous casting apparatus. The ultrasonic vibration can be applied in an optimum state by moving.

  The present invention is not limited to the embodiments described above. The application horn may be any material as long as it can transmit ultrasonic vibrations to the molten magnesium alloy in the mold, and the material and the like can be changed as appropriate. The shape of the tip portion 6 of the application horn 5 is not limited to a hemispherical shape, and as shown in FIG. 5A, the shape of the tip portion 6 may be a spherical shape curved with a constant curvature R larger than the radius of the application horn 5. As shown in b), it may be a curved surface shaped like an ellipse cut in half when viewed from the front-rear direction or the left-right direction. The application horn can also be fixedly provided at a certain height. The mold may have a straight inner peripheral surface.

It is a longitudinal cross-sectional view which shows the outline | summary of the continuous casting apparatus of the magnesium alloy of this invention. It is a longitudinal cross-sectional view which expands and shows the mold vicinity of the continuous casting apparatus. (A) shows a macro structure of a billet cast while applying ultrasonic vibration with an application horn having a diameter of 40 mm and a hemispherical tip. (B) shows a macro structure of a billet cast without applying ultrasonic vibration. Is shown. It is a photograph of the microstructure of a billet cast by changing the tip position X of the applied horn, where (a) is for X = -5 mm, (b) is for X = 0 mm, and (c) is for X = 10 mm. (D) is for X = 40 mm. It is a front view of the application horn front-end | tip part which shows the other example of the shape of an application horn front-end | tip part.

Explanation of symbols

1 Magnesium alloy molten metal 2 Tundish 3 Mold 4 Ultrasonic vibration 5 Applied horn 6 Tip of applied horn

Claims (1)

  A tundish that contains molten magnesium alloy, a mold installed at the bottom of the tundish, and an application horn that applies ultrasonic vibration from above to the molten magnesium alloy that has flowed into the mold. A magnesium alloy continuous casting apparatus, characterized in that it has a curved shape that bulges downward, and the tip of an application horn is positioned in a mold.