JP2020022984A - Continuous casting method for metal - Google Patents

Abstract

To improve the cooling speed of a metal upon continuous casting.SOLUTION: A continuous casting method for a metal has a process where, while pulling up an object 6 to be cooled made of a molten metal 7 or an ingot 8 in a semi-solidified state from the inside of a mold 2, cooling water 5 added with high heat conduction particles having a heat conductivity higher than that of water is directly brought into contact with the object 6 to be cooled, and the object 6 to be cooled is cooled.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for continuously casting a metal such as aluminum.

  In the present specification and claims, the term "continuous casting" is used to mean semi-continuous casting unless otherwise specified in the text.

  With increasing demand for parts made of aluminum alloy, it is desired to increase the productivity and mechanical properties of a continuous cast rod as a material thereof. Above all, the use of Al-Si alloys having a eutectic or hypereutectic composition, mainly for casting, has been expanded, and they have come to be used as industrial materials by taking advantage of their properties such as wear resistance.

  In addition, with the tightening of fuel efficiency regulations for automobiles, aluminum alloys as materials for such parts are required to be reduced in weight and further increased in strength. Therefore, aluminum alloys having fine crystal grains are required.

  In order to obtain such an aluminum alloy, it is important to control the crystallization temperature, which is an important factor determining the mechanical properties, and to control the rapid solidification near the solidification temperature. As a casting method for obtaining a continuous casting rod having improved mechanical properties by rapid cooling, there is a direct cooling casting method (direct chill casting method) in which cooling water is directly hit against a molten metal or a semi-solid product thereof to cool the metal.

  In a continuous casting method of metal, with respect to use in a region where cooling water boils, proposals have been made to reduce a cooling rate in order to suppress cracks, warpage and constriction of an ingot (for example, Japanese Patent Application Laid-Open No. 05-057400). Japanese Patent Application Laid-Open No. H06-142847 (Patent Document 2).

  In addition, various media have been proposed with respect to a medium having improved thermal conductivity, including the following three documents, but there is no prior document for improving the cooling rate in a continuous casting method for metal.

  Japanese Patent Application Laid-Open No. 54-94481 (Patent Document 3) proposes a composition in which a fine powder of a metal such as Al is dispersed in a heat medium in a method for modifying the heat transfer properties of the heat medium. .

  JP-A-61-291388 (Patent Document 4) discloses that a medium is filled in a cavity formed by a ceramic powder having a high thermal conductivity dispersed in a medium and a three-dimensional network structure as a means for promoting heat conduction. Metal foams or ceramics have been proposed.

  JP-A-62-129378 (Patent Document 5) proposes a composition in which a metal and / or graphite is uniformly dispersed in a polyhydric alcohol in order to improve the thermal conductivity of the polyhydric alcohol-based material. Have been.

JP 05-0557400 A JP-A-06-142847 JP-A-54-94481 JP-A-61-261388 JP-A-62-129378

  In order to solve the problem of improving the mechanical properties of metal continuous cast rods, measures have been taken to increase the cooling rate, but currently only water is used as cooling water, and the improvement is limited. is there. In order to exceed this limit, further improvement in cooling rate is required.

  In the direct chill casting method, one of the important factors that determine the cooling rate is the thermal conductivity of the refrigerant, and the higher the thermal conductivity of the refrigerant, the higher the cooling rate. The cooling rate has not been improved by the improvement. In addition, most of the metals that are put into practical use are cooled from the temperature higher than the boiling point of the refrigerant, that is, water, so the Leidenfrost phenomenon occurs near the crystallization temperature and solidification temperature, which are important for determining the mechanical properties. There is a problem that a sufficient cooling rate cannot be obtained.

  The present invention is intended to solve the problem of improving the cooling rate during continuous casting of metal, and by adding metal fine particles and the like as a heat conduction improving material to a cooling medium used during casting, a cooling medium is provided. It is an object of the present invention to provide a cooling medium in which the heat conductivity of a metal is improved and the metal to be cooled is hardly corroded by an acid or a base. In particular, by using a predetermined cooling water at the time of casting aluminum or aluminum alloy in a semi-solid state in which the surface layer is solidified and the inside is in a liquid state, as in the case of the pot top casting method, the temperature important for miniaturization of the metal structure is increased. Since the cooling rate of the region is improved and the structure is easily refined, it is expected that this will be one of the solutions to the problem of further increasing the strength of the aluminum alloy in the future.

  In addition, the present invention provides, in addition to cooling by a refrigerant, a solid having a higher thermal conductivity than the refrigerant, by adding a solid to the refrigerant. Another object is to improve the cooling rate in the vicinity.

  The present invention provides the following means.

1) A continuous casting method for cooling a cooled object by bringing cooling water into direct contact with the cooled object while pulling the cooled object formed of a molten metal or a semi-solid ingot from the inside of a mold. So,
A continuous casting method for metal using cooling water to which high thermal conductive particles having higher thermal conductivity than water are added as the cooling water.

  2) The high thermal conductive particles are at least one selected from the group consisting of metal oxide particles, metal carbide particles, metal nitride particles, metal composite oxide particles, metal composite carbide particles, metal composite nitride particles, and ceramic particles. 2. The continuous casting method for a metal according to the above item 1, wherein

  3) The continuous casting method for a metal according to the above 1 or 2, wherein the average particle diameter of the high thermal conductive particles is in a range of 10 nm to 100 µm.

  4) The continuous casting method for a metal according to any one of the above items 1 to 3, wherein the concentration of the high thermal conductive particles in the cooling water is in a range of 0.001 to 50% by mass.

  5) The continuous casting method for a metal according to any one of the above items 1 to 4, wherein the viscosity of the cooling water is 1 Pa · s or less at a temperature at the time of using the cooling water.

  6) The metal continuous casting method according to any one of the above items 1 to 5, wherein the cooling water contains a surfactant.

  7) The continuous casting method for a metal according to any one of the above items 1 to 6, wherein the metal is aluminum or an aluminum alloy.

  8) The metal continuous casting method according to the above item 7, wherein the pH of the cooling water is in the range of 4 to 9.

  9) The continuous casting method for a metal according to the above item 7, wherein the pH of the cooling water is in the range of 6 to 8.

  The present invention has the following effects.

  In the first aspect, since the thermal conductivity of the cooling water is higher than that of the water itself, the object to be cooled can be efficiently cooled. Further, even when the heat transfer between the cooling water and the object to be cooled is insufficient due to the vapor film generated by the boiling of the cooling water (for example, when the Leidenfrost phenomenon occurs), the high heat transfer in the cooling water By cooling the object to be cooled by the particles, the object to be cooled can be efficiently cooled.

  In the second aspect, the object to be cooled can be reliably and efficiently cooled.

  In the above item 3, when the average particle diameter of the high thermal conductive particles is 10 nm or more, the high thermal conductive particles are easily dispersed in the cooling water. When the average particle diameter of the high thermal conductive particles is 100 μm or less, precipitation of the high thermal conductive particles in the cooling water is easily suppressed, and the cooling effect is reduced due to a small contact area between the high thermal conductive particles and the object to be cooled. Can be suppressed.

  In the above item 4, when the concentration of the high thermal conductivity particles is 0.001% by mass or more, the cooling effect is greatly improved. When the concentration of the high thermal conductivity particles is 50% by mass or less, it is easy to handle as cooling water.

  In the preceding item 5, since the viscosity of the cooling water is 1 Pa · s or less, it is easy to handle as cooling water.

  In the above-mentioned item 6, even when the high heat conductive particles are, for example, particles that easily aggregate, the high heat conductive particles can be easily dispersed in the cooling water by including the surfactant, and the high heat conductive particles can be easily dispersed. Of the cooling capacity due to coagulation of water.

  In the above item 7, the cooling rate is improved by applying the method to a continuous casting method of aluminum or an aluminum alloy as a metal.

  In the preceding items 8 and 9, corrosion of a continuous cast rod made of aluminum or an aluminum alloy can be suppressed.

FIG. 1 is a schematic cross-sectional view of a hot-top casting apparatus as a continuous casting apparatus used in a continuous casting method for metal according to an embodiment of the present invention. FIG. 2 is a schematic perspective view of the aluminum alloy sample used in the example.

  Next, an embodiment of the present invention will be described below with reference to the drawings.

  A continuous casting method for a metal according to an embodiment of the present invention is, for example, as shown in FIG. 1, a metal is continuously cast by a hot-top casting method using a hot-top casting apparatus 1 as a continuous casting apparatus. Is to manufacture.

  The hot top casting apparatus 1 includes a mold 2 and a molten metal receiving tank 3. The mold 2 is cooled by cooling water 5 as primary cooling water filled therein. Further, the mold 2 is provided with an injection port 4 for injecting the cooling water 5 in the mold 2 as secondary cooling water. The molten metal receiving tank 3 is installed above the mold 2.

  The molten metal 7 supplied into the molten metal receiving tank 3 is injected downward from the receiving tank 3 into the cooled mold 2. Then, the molten metal 7 is primarily cooled by contact with the mold 2 to form a semi-solid ingot 8. In the ingot 8 in this state, a solidified shell is generally formed on the outer periphery. Then, while continuously drawing the ingot 8 in this state downward from the inside of the mold 2, the cooling water 5 is injected from the injection port 4 of the mold 2 to the ingot 8 immediately after being drawn from the inside of the mold 2. Then, the cooling water 5 is brought into direct contact with the surface of the ingot 8. Thus, the ingot 8 is secondarily cooled at a predetermined cooling rate, and most of the ingot is solidified.

  In the metal continuous casting method described above, the cooling water 5 is obtained by adding and dispersing high heat conductive particles (not shown) having higher heat conductivity than water to water.

  As the high thermal conductive particles, at least one selected from the group consisting of metal oxide particles, metal carbide particles, metal nitride particles, metal composite oxide particles, metal composite carbide particles, metal composite nitride particles and ceramic particles is used. Can be

  Here, in the present embodiment, the common cooling water 5 is used as the primary cooling water and the secondary cooling water. However, the present invention is not limited to this. It may be used only as cooling water.

  The metal continuous casting method according to the present embodiment will be described below together with its operation and effects.

  In the continuous casting method of the metal in the present embodiment, for example, the heat conductivity of the base fluid such as water is made higher than that of the base fluid itself. ) Can be efficiently cooled. Further, even when heat transfer between the fluid and the object to be cooled is insufficient due to a vapor film generated by boiling of the fluid (that is, cooling water), particles in the fluid cool the object to be cooled. It can be expected that the object to be cooled is efficiently cooled.

  The base fluid can include any known heat transfer liquid in addition to water.

  Of course, if it contains water, it can be used as a heating medium using oil as a dispersant, but an organic solvent may be added. When an organic solvent is used, it can be used as it is unless the solvent has oxidizing power or reducing power. In the present embodiment, the purpose is to increase the cooling efficiency (cooling rate) particularly at the time of casting a metal. Therefore, it is effective to use the heat medium for heating / cooling with a large temperature difference. That is, a fluid that is stable over a wide temperature range is preferably used as the base fluid. In order to take advantage of the properties of high thermal conductivity, it is better to use a solvent with a boiling point of 100 ° C or higher and apply it over a wide temperature range, but reduce the content or make it less than the flash point or ignition point so that it is flame retardant It is preferable to use them. It is more preferable to select one having a freezing point of 0 ° C. or less.

Nanoparticles and micron particles have a relatively large specific surface area and are expected to improve the heat transfer capability of the base fluid. The metal or ceramic used may have a higher thermal conductivity than the base fluid, but is preferably Cu, Ag, SiC, Al ₂ O ₃ (alumina) having a high thermal conductivity and alloys and composites thereof, and combinations thereof. Good to choose from.

  In the metal continuous casting method in the present embodiment, it is desirable that the average particle size of the high thermal conductive particles be in the range of 10 nm to 100 μm. The reason is as follows.

  When the average particle diameter (diameter) of the particles is large, for example, when it is larger than 100 μm, the particles tend to precipitate in the base fluid, and the load on a pump or screw used during mechanical transportation described later increases, or when the particles are not used. This is not preferable because the particles cannot easily be conveyed or the machine is likely to be damaged due to the precipitation or aggregation of the particles. Further, for example, when a solid having an average particle size of 10 mm is used, use of the solid is not preferable because the pipe may be blocked. On the other hand, if the average particle size of the particles is too large, the contact area between the high thermal conductive particles and the object to be cooled is small, and the cooling effect is reduced. On the other hand, if the average particle size of the particles is small, for example, less than 10 nm, aggregation at the gas-liquid interface or solid-liquid interface is likely to occur, and it may be difficult to disperse the particles in the medium.

  In the metal continuous casting method according to the present embodiment, the concentration of the high thermal conductive particles in the cooling water is desirably in the range of 0.001 to 50% by mass. The reason is as follows.

  The concentration of the high thermal conductive particles with respect to the heat medium (that is, cooling water) is preferably low from the viewpoint of requiring fluidity, but is preferably high from the viewpoint of requiring cooling efficiency. Is determined. The concentration of the high thermal conductive particles is preferably 0.001% by mass or more and 50% by mass or less based on the mass of the entire heat medium. When the content is less than 0.001% by mass, the ratio of the high heat conductive particles present in the heat medium is low, and the effect of adding the high heat conductive particles as the heat medium is poor. If it exceeds 50% by mass, the filling rate is too large, and the fluidity as a liquid may decrease.

  In the metal continuous casting method according to the present embodiment, it is desirable that the viscosity of the cooling water be 1 Pa · s or less at the temperature at the time of using the cooling water. The reason is as follows.

  The viscosity of the heating medium (that is, cooling water) becomes important when mechanically moving the heating medium. Of course, it can be used in a situation where natural convection is used, but it is not efficient. In particular, in the widely used casting process of metals including aluminum and its alloys, for which hot-top casting and float casting have been proposed, the pressure and flow rate of the heat medium are increased by pumps and screws to reduce the heat medium. When colliding with the metal to be cast, the cooling speed is improved by increasing the flow velocity. Here, if the viscosity of the heat medium is large, the heat medium does not have a sufficient flow velocity at the time of collision with the metal, which causes resistance to heat transfer. In addition, since the state may be a laminar flow state or a state close to a laminar flow state, if the viscosity is low, which is a resistance to heat transfer, a sufficient flow velocity can be maintained, and the resistance to heat transfer is greatly reduced. Therefore, the lower the viscosity of the heat medium during use, the better. In the present embodiment, it is recommended that the use condition be 1 Pa · s or less at the use temperature. Preferably, the viscosity of the cooling water in the hot-top casting method is such that the flow rate is at least 1 L / min per inch of the diameter of the casting rod. The reason why the viscosity of the cooling water is defined by the temperature at the time of use is that the viscosity of the cooling water may greatly change depending on the temperature.

  In the metal continuous casting method according to the present embodiment, the cooling water desirably contains a surfactant. The reason is as follows.

  When using high thermal conductive particles that are likely to cause aggregation, the use of a surfactant reduces the interfacial tension and improves wettability, thereby promoting dispersion and reducing the possibility of reduced cooling capacity and pipe blockage due to aggregation. You can do it.

  In the continuous casting method of the metal in the present embodiment, the metal is preferably aluminum or an aluminum alloy. The reason is as follows.

  In particular, when casting the aluminum or aluminum alloy by the direct chill casting method, if the cooling water is replaced with the above-described cooling water 5 of the present embodiment, an improvement in the cooling rate can be expected. Further, since existing equipment can be used as it is, the investment amount for introducing new equipment is small.

  In the metal continuous casting method in the present embodiment, when the metal is aluminum or an aluminum alloy, the pH of the cooling water is preferably in the range of 4 to 9. The reason is as follows.

  This is because, as a property of aluminum or an aluminum alloy, when the pH is outside the range of 4 to 9, corrosion tends to occur due to reactions such as oxidation and reduction. Even when the pH is in the range of 4 to 9, corrosion may progress slowly, but the time that the cooling medium touches is only the time during casting, and corrosion hardly progresses due to the oxide film on the aluminum surface. So there is no problem.

  Further, when casting aluminum or aluminum alloy, particularly when casting 2000 series or 7000 series alloy, which is particularly easily corroded, or when using a metal which is easily corroded in the piping, the pH is 6 to 8. It is desirable to use a cooling medium that is within the range.

  Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and can be variously modified without departing from the gist of the present invention.

  In the present invention, the cooling water may be used for the production of ingots of metals (including alloys) without using any special equipment. However, existing cooling methods such as hot top casting and float casting may be used. The apparatus may be used to produce a metal ingot, and there is no limitation in the method and equipment.

  Further, the continuous casting method for metal according to the present invention may be a horizontal continuous casting method for metal.

  Specific examples and comparative examples of the present invention are shown below. However, the present invention is not limited to the following examples.

  As shown in FIG. 2, after heating a thermocouple 11, a truncated cone-shaped sample 10 of an aluminum alloy containing 14% by mass of Si, 4% by mass of Cu, and 0.5% by mass of Mg. After cooling, the cooling rate at that time was measured. The conditions are as follows.

  In addition, SiC particles (270 W / (m · K)) having higher thermal conductivity than water were selected as the high thermal conductive particles to be added to water, and the effect was verified.

1) Sample size: upper surface φ105mm × lower surface φ62mm × height 125mm
2) Temperature measurement part: inside the ingot (35 mm from the top surface, 10 mm from the center)
3) Temperature measurement range: 450 to 300 ° C
4) Temperature raising condition: The sample 10 is kept in an electric furnace at 500 ° C. for 2 hours, and the sample temperature is raised to 480 ° C.

5) Cooling water volume: 10L
6) the coolant vessel dimensions: top Fai269.66Mm × lower surface Fai245mm × height 205.5mm- sample volume mm ³
7) Cooling water temperature: start 10 ° C
8) Cooling conditions: After the sample temperature has risen to 450 ° C., the sample is taken out with a fire scoop and immediately immersed in cooling water immediately. At this time, the sample 10 and the cooling water container are not in contact with each other, and the sample 10 is held in a state of being suspended in the cooling water with a fire scissor until the sample 10 is cooled to a temperature lower than 300 ° C. At this time, the time from 450 ° C. to 300 ° C. was defined as the cooling time.

  9) High thermal conductive particles: SiC particles, average particle size 4 μm (# 3000), 1% by mass, medium containing only SiC particles, 10 L.

10) Temperature measurement conditions-Thermocouple 11: K thermocouple class 2
-Measurement frequency: 100 ms.

  Further, simulation conditions are shown below.

1) Calculation software: ANSYS fluent 18.0
2) Calculation method: finite element method, double precision, pressure-based solver, multiphase flow model: VOF method, turbulence model: k-ωSST, unsteady calculation 3) Physical property values: shown in Table 1.

  In Table 1, the viscosity and thermal conductivity of water were calculated with reference to the Thermophysical Properties Handbook, The Society of Thermophysical Properties of Japan, Yokendo, 2008 Table.

4) Method for calculating physical properties of a mixture (Tc: thermal conductivity of a mixture, _c: mixture, _s: solute, _l: solvent, a: volume fraction, k: thermal conductivity, ρ: density)
Ρ_c = ρ_s × a + ρ_l (1-a)
Cp_c = (ρ_s × a × Cp_s + ρ_l (1-a) Cp_l) / ρ_c
· Tc_c = k_l {k_l (1 -a (2/3)) + k_s × a (2/3)} / {k_l (1-a (2/3) + a) + k_s (a (2/3) -a) ｝
・ Calculate with one-dimensional heat conduction by substituting the volume fraction with a cube. ・ Μ_c: For slurry, it is unknown, so calculate with water. 5) Sample size: upper surface φ105 mm × lower surface φ62 × height 125 mm
6) the coolant volume: top Fai269.66Mm × lower surface Fai245mm × height 205.5mm- sample volume mm ³
7) Sample position: refrigerant center 8) Initial temperature: refrigerant 18 ° C, sample 500 ° C
9) Temperature measurement position: 35 mm from top surface, 10, 20, 30 mm from center
10) Cooling time: Time from when the 10 mm point reaches 450 ° C. to when it reaches 300 ° C.

  Table 2 shows the above results.

  As is clear from Table 2, in Experimental Example 2, the cooling rate was improved by 7% and 2.3 ° C./s as compared with Experimental Example 1. In addition, when the experimental example 2 and the simulation example 2 are compared, both have different cooling rates, but have the same relative cooling rate. For this reason, it was confirmed that the addition of particles having a higher thermal conductivity than water to the cooling water improves the cooling rate.

  Further, from simulation examples 2 and 3, it was found that the cooling rate was further improved by increasing the amount of SiC particles added. Also, from the simulation examples 4 and 5, the cooling capacity is improved by adding not only the SiC particles but also the alumina particles. Therefore, it was found that the cooling ability of the cooling water was improved by adding particles having a higher thermal conductivity than the water to the cooling water.

  From the results of the prior patent (Japanese Patent Application Publication No. 2014-534273), not only SiC particles and alumina particles, but also metal oxide particles having higher thermal conductivity than water, not reacting with water, and having the same particle size It is considered that cooling water with improved cooling capacity can be obtained by adding metal nitride particles and ceramic particles to water.

  Therefore, in the continuous casting method of metal, as cooling water (eg, at least secondary cooling water of primary cooling water and secondary cooling water), cooling water to which high heat conductive particles having higher thermal conductivity than water is added. By using, the cooling rate of the object to be cooled in the vicinity of the crystallization temperature and the solidification temperature of the metal can be improved.

  INDUSTRIAL APPLICATION This invention can be utilized for the continuous casting method of metals, such as aluminum.

1: Hot-top casting apparatus 2: Mold 5: Cooling water 6: Object to be cooled 7: Molten metal 8: Ingot

Claims (9)

A metal continuous casting method for cooling a cooled object by directly contacting cooling water with the cooled object while pulling the cooled object formed of a molten metal or a semi-solid ingot from the inside of a mold, ,
A continuous casting method for metal using cooling water to which high thermal conductive particles having higher thermal conductivity than water are added as the cooling water.   As the high thermal conductive particles, at least one selected from the group consisting of metal oxide particles, metal carbide particles, metal nitride particles, metal composite oxide particles, metal composite carbide particles, metal composite nitride particles and ceramic particles is used. The method for continuous casting of a metal according to claim 1, which is performed.   The method for continuously casting metal according to claim 1, wherein the average particle diameter of the high thermal conductive particles is in a range of 10 nm to 100 μm.   The continuous casting method for a metal according to any one of claims 1 to 3, wherein a concentration of the high thermal conductive particles with respect to the cooling water is in a range of 0.001 to 50% by mass.   5. The continuous casting method for a metal according to claim 1, wherein the viscosity of the cooling water is 1 Pa · s or less at a temperature when the cooling water is used.   The method according to any one of claims 1 to 5, wherein the cooling water contains a surfactant.   The continuous casting method for a metal according to any one of claims 1 to 6, wherein the metal is aluminum or an aluminum alloy.   The method for continuously casting metal according to claim 7, wherein the pH of the cooling water is in the range of 4 to 9.   The continuous casting method for a metal according to claim 7, wherein the pH of the cooling water is in a range of 6 to 8.

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