JPH09122860A - Method for continuously casting aluminum - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the occurrence of defects at the bottom part of a cast block at the initial stage of continuous casting with a simple method without using a complicated equipment by film-boiling and cooling the surface of the cast block drawn out from the lower part of a mold at the initial stage of the casting and specifying the heat conduction from the surface of the cast block with a coefficient of heat conductivity. SOLUTION: In order to keep the film boiling and cooling, it is important that the heat conduction from the surface of the cast block is controlled to <=4000W/m<2> K the coefficient of heat conductivity so as not to lower the surface temp. of the cast block. Therefore, in the case of colliding the cooling water to the bottom part of the cast block at the initial stage of the continuous casting, since the film boiling and cooling are executed, the heat conduction from the surface of the cast block is suppressed, and the cooling rate is reduced, and since thermal stress is mitigated, the warping-up is not caused at the bottom part of the cast block and various kinds of the cast block defect caused by the warping-up is prevented. Particularly, in a vertical continuous casting of an aluminum ingot having rectangular cross section, the stable casting can be executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for aluminum, and more particularly, to an improvement in a continuous casting method for aluminum that reduces defects in the bottom of the ingot in the initial stage of casting in vertical continuous casting of aluminum.

[0002]

2. Description of the Related Art Vertical continuous casting of aluminum (including aluminum alloys, the same applies hereinafter) is performed by supplying molten aluminum from the upper part of a cylindrical mold opened up and down and pulling out the solidified ingot from the lower part of the mold. Done. Usually, at the start of casting, after the bottom stand is installed in the mold, the molten metal is supplied into the mold, and when the predetermined amount of molten metal is injected into the bottom stand and the mold, the bottom stand is lowered below the mold. At the same time, cooling water is supplied from the surroundings to solidify the molten metal, and the solidified ingot is continuously drawn out in the vertical direction.

In this case, particularly when casting a rolling ingot having a rectangular cross section, as shown in FIG. 1, at the initial stage of casting, the bottom of the ingot C drawn from the mold 1 is cooled by the cooling water 3. As a result of the rapid cooling, the temperature of the bottom of the ingot C sharply decreases, a rapid temperature gradient is generated in the longitudinal direction of the ingot to induce thermal stress, and the temperature between the center and the end of the ingot C is increased. Thermal stress is also generated from the steep temperature gradient, and as a result, warpage 4 occurs at the bottom of the ingot on the short side. When the bottom portion of the ingot on the short side is warped, a constriction 5 is generated at the upper portion of the ingot, which causes vertical cracking and leakage of molten metal. In FIG. 1, M is a molten aluminum and B is a solidification interface.

Further, due to the formation of the warp 4, a tensile stress is applied to the central portion of the ingot C on the long side, so that vertical cracks 6 easily occur, and the mold C is formed in the warp.
Since a void 7 is formed between the bottom of the base and the base 2, the heat removal in the direction of the base 2 is reduced, the bottom of the ingot is remelted, and a crack is generated from the remelted portion. Occurs.

Several techniques have been developed so far in order to optimize the shrinkage due to cooling of the bottom of the ingot in the initial stage of continuous casting to prevent warpage and prevent defects occurring at the bottom of the ingot. For example, in the initial stage of casting, a gas such as carbon dioxide gas is mixed in the cooling medium to reduce the rate of heat extraction from the ingot, and the amount of gas mixed is reduced as the casting progresses. Method for returning to speed (Japanese Patent Publication No. 55-42903), pulse cooling method for controlling shrinkage of the bottom of the ingot by intermittently supplying cooling water in the initial stage of casting (US Pat. No. 3,441,079) ) Have been proposed and put into practical use.

Although any of the above methods is effective for preventing defects at the bottom of the ingot at the initial stage of continuous casting of aluminum, the method of mixing carbon dioxide with the cooling medium is effective in controlling the generation of carbon dioxide and the mixing amount. Large-scale equipment for control etc. is required, the pulse cooling method requires a complicated device for controlling on / off of cooling water, and when a problem occurs in the control system, an ingot is required. As a result of the interruption of cooling, remelting occurs and the mold wall is damaged.

[0007]

DISCLOSURE OF THE INVENTION The present invention aims to develop a method for overcoming the drawbacks of the above-mentioned conventional methods for preventing the defects of the ingot in the initial stage of casting in the vertical continuous casting of aluminum. This was done as a result of a basic experimental study on the cooling stage of a high-temperature aluminum ingot, the purpose of which is to use a simple method and a simple method without using complicated equipment. An object of the present invention is to provide a continuous casting method for aluminum that can suppress the occurrence of bottom defects.

[0008]

A method for continuous casting of aluminum according to the present invention for achieving the above object is a vertical continuous casting of aluminum in which molten metal is supplied from an upper part of a mold and a solidified ingot is drawn out from a lower part of the mold. In the early stage of casting, the surface of the ingot pulled out from the lower part of the mold is cooled by film boiling, and the heat extraction from the surface of the ingot is 400 with a thermal conductivity coefficient.
It is characterized in that it is set to 0 W / m ² K or less.

INDUSTRIAL APPLICABILITY The present invention is effective particularly in the case of casting a rectangular ingot for rolling among the vertical continuous casting of aluminum in which molten aluminum is supplied from the upper part of the mold and the solidified ingot is drawn out to the lower part of the mold. As shown in FIG. 1, a bottom stand 2 is installed in a mold 1, a molten metal M is injected onto the bottom stand 2 in the mold, and cooling water 3 is supplied to form a bottom part of an ingot C at an initial stage of casting. In, the surface of the ingot is cooled by film boiling.

According to the film boiling cooling, the cooling water boils and vaporizes on the surface of the hot ingot to form a vapor film, and the ingot is directly cooled through the vapor film without being exposed to water. Therefore, heat extraction from the surface of the ingot is significantly suppressed. After that, when the temperature of the ingot surface decreases, the vapor film partly shakes, the transition boiling stage in which the part comes into contact with water to generate bubbles, the vapor film entirely disappears and the ingot surface becomes cooling water. The nucleate boiling stage where they come into contact with each other and generate bubbles occurs, heat extraction from the surface of the ingot is increased to a predetermined cooling rate, and steady continuous casting is performed.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the continuous casting of aluminum of the present invention, as a method for maintaining the film boiling cooling state at the initial stage of casting, a method of increasing the casting speed and a method of reducing the supply amount of cooling water in the initial stage Alternatively, there is a method of combining these, but in order to achieve the effect of the present invention, heat extraction at the film boiling limit is used as a thermal conductivity coefficient of 4,000.
It is necessary to set it to W / m ² K or less.

The above thermal conductivity coefficient is 5 m from the surface of the ingot.
Thermocouples are embedded at positions inside m and 15 mm, temperature changes at these positions are measured at predetermined intervals to obtain temperature changes, and the water temperature is calculated as 20 ° C. (Sumitomo Light Metal Technical Report,
Volume 35, No. 1, No. 2, June 1994, No. 99
Page, "Cooling characteristics of various cooling methods for continuous casting of aluminum", 2.2 2.2 Evaluation method of cooling performance)

In order to maintain the film boiling cooling, heat extraction from the surface of the ingot is performed as a heat transfer coefficient of 4000 W / m ² K.
Controlling the following, it is important not to lower the surface temperature of the ingot, when the heat extraction amount is large, the cooling form transitions from film boiling to transition boiling, nucleate boiling, and the effect of the present invention is obtained. Disappear. However, for example, if the amount of cooling water is too small, the temperature of the ingot surface rises, and there is a risk of molten metal leakage, so the amount of cooling water for obtaining film boiling cooling, the casting speed, the material of the aluminum to be cast, the casting It is determined experimentally taking into account the size of the mass.

According to the present invention, in the initial stage of continuous casting, when cooling water collides with the bottom of the ingot, film boiling cooling is performed, so that heat extraction from the ingot surface is suppressed,
Since the cooling rate is reduced and the thermal stress is relieved, the bottom of the ingot does not warp, and various ingot defects caused by the warp are prevented.

[0015]

【Example】

Example 1 An aluminum alloy having a composition equivalent to JIS 3004 alloy was produced by a normal vertical continuous casting method and had a cross-sectional dimension of 55 mm in length.
The ingot was formed into a rectangular ingot having a size of 0 mm and a width of 170 mm.
The casting conditions are shown in Table 1. In addition, the casting temperature was 700 ° C., an effective mold length of 65 mm was used as a mold, and 50 ingots were cast under each condition.

In the above continuous casting, the ingot surface is cooled at the initial stage by film boiling cooling, and as shown in Table 2, the average amount of warpage is as small as 4.6 mm or less, which is caused by the warpage. There were no cracks at the center of the long side of the ingot, cracks at the short side of the ingot, leak of molten metal, or cracks resulting from remelting of the bottom. In Table 2, the value of the heat transfer coefficient is the casting length 1 from the start of casting corresponding to the initial stage of casting.
Up to 50 mm.

[0017]

[Table 1]

[0018]

[Table 2]

Comparative Example 1 An aluminum alloy having the same composition as in Example 1 was cast into an ingot having the same cross-sectional dimensions as in Example 1 according to the normal vertical continuous casting method as in Example 1. Table 3 shows the casting conditions. The casting temperature and the effective length of the mold used were 700 ° C. and 65 m, respectively, as in Example 1.
m, and 50 ingots were cast under each condition. In casting under each condition, the heat transfer coefficient up to a casting length of 150 mm, which corresponds to the initial stage of casting, was obtained, and the average amount of warpage and the crack occurrence state were examined. Table 4 shows the results.

[0020]

[Table 3]

[0021]

[Table 4]

As shown in Table 4, the test No. In Nos. 6 to 9, film boiling cooling was not performed, the amount of warpage was large, and cracks and molten metal leak occurred. For example,
Test No. In 6, the warp amount is 15 to 19 m
m, the average amount of warpage was as large as 17.1 mm, and due to the warpage, two ingots were cracked at the center of the ingot in the long side direction, and one ingot was cracked at the short side of the ingot. As a result, defective molten metal leakage occurred due to the remelting of the necked portion in the two ingots.

Further, the test No. 1 and test No. FIG. 2 shows the result of calculating the change in the thermal conductivity coefficient with respect to the casting length for No. 6. Test No. in which film boiling cooling is performed In Test No. 1, which has a thermal conductivity coefficient of 4000 W / m ² K or less up to a casting length of 150 mm corresponding to the initial stage of casting, film boiling cooling is not performed. 6 up to 16000
Quenching to reach W / m ² K is taking place.

[0024]

As described above, according to the present invention, in the continuous casting of aluminum, particularly the vertical continuous casting of an aluminum ingot having a rectangular cross section, the cooling rate in the initial stage of casting is moderated and the ingot bottom warps. Since it is suppressed, defects that occur in the casting bottom due to warpage can be prevented, and stable casting can be performed.

[Brief description of the drawings]

FIG. 1 is a vertical cross-sectional view showing the outline of vertical continuous casting of aluminum.

FIG. 2 is a graph showing changes in a heat transfer coefficient with respect to a casting length, when film boiling cooling is performed and when film boiling cooling is not performed.

[Explanation of symbols]

 1 Mold 2 Bottom 3 Cooling Water 4 Warpage 5 Constriction 6 Vertical Crack 7 Gap C Ingot M Aluminum Molten Metal B Solidification Interface

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Watanabe 5-11-3 Shimbashi, Minato-ku, Tokyo Sumitomo Light Metal Industry Co., Ltd.

Claims (1)

[Claims] 1. In vertical continuous casting of aluminum, in which molten aluminum is supplied from the upper part of the mold and solidified ingot is drawn from the lower part of the mold, film boiling is performed on the surface of the ingot drawn from the lower part of the mold in the initial stage of casting. A continuous casting method for aluminum, which comprises cooling and extracting heat from the surface of the ingot to a heat transfer coefficient of 4000 W / m ² K or less.