JPH0242575B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous casting method for aluminum or an aluminum alloy.

Generally, slabs or billets of aluminum or aluminum alloys as columnar ingots for plastic working are cast by a vertical semi-continuous casting method.

As shown in FIG. 1, the semi-continuous casting apparatus is comprised of a forced cooling annular mold 1 with a water-cooled jacket, a lower mold 2 that fits into the inner wall of the mold, and is movable up and down.
In casting, molten aluminum is continuously injected into the mold 1 from above, the outer shell of the molten metal is solidified by removing heat from the molten metal within the mold, and water is continuously injected directly below the mold using a spray device 4. The lower die 2 is lowered as the ingot is formed.

In FIG. 1, numeral 3 is a portion of molten aluminum called a sump, and numeral 5 is a portion of a solidified aluminum ingot.

In semi-continuous aluminum casting, center cracks called cold cracks often occur at the start of casting. This is the 2000 series, 7000
This phenomenon is particularly likely to occur in alloys of various grades, and the larger the cross-section of the ingot, the more likely it is to occur.

Cold cracks occur when there is a large temperature difference between the surface and the inside of the ingot during the thermally unstable state at the initial stage of casting, resulting in compressive stress on the surface and tensile stress in the center. It is said that then.

Cold cracks are easier to prevent when the casting speed is slow, so a method is often used to prevent the occurrence of cold cracks by slowing down the casting speed at the start and then returning it to the normal casting speed. If it is slow, a cold welding phenomenon (cold shut) will occur. Since the casting surface portion of the cold shaft becomes a defective product, the yield of the product decreases in this method. In addition, changing the casting speed makes the work complicated.

When casting aluminum alloys that are prone to cold cracks, a method has been proposed in which aluminum with low stress concentration is first cast, and then aluminum alloys with high stress concentration are cast (Japanese Patent Application Laid-open No.
54-128936), however, this method also has the disadvantage of complicating the work and reducing the yield of the product, since the composition of the aluminum alloy is changed during the casting process.

An object of the present invention is to provide a method that prevents the occurrence of cold cracks at the start of casting of aluminum or aluminum alloy, and easily performs stable casting.

As mentioned above, cold cracks are more likely to occur as the temperature difference between the surface and inside of the ingot increases, and therefore, it is said that cold cracks do not occur in the case of unidirectional solidification.
In this state, the shape of the solidification interface of the ingot is
becomes flat.

From this point of view, the inventor focused on the shape of the solidification interface of the ingot at the initial stage of casting, and found that the closer the shape is to the flatness, the less likely cold cracks will occur.

The shape of the bottom generally used is either flat or concave as shown in FIG. 1, and the solidification interface is concave due to the cooling method.

FIG. 2 shows an example of the change over time in the shape of the solidification interface of an ingot (billet) cast using a lower die with a raised curved surface that expands downward.

As shown in Figure 2, the shape of the solidification interface of an ingot cast using a lower die with a raised curved surface that expands downward is that immediately after casting, a It showed an approximate raised shape, passed through a W-shape, then became a concave shape, and finally became a concave shape, d. In the middle, the solidification interface had a shape that was close to flat, and it was confirmed that cold cracks were unlikely to occur. . The position of this flat solidification interface is desirably 1/2R or more (R is the diameter of the billet) for billets, and 1/4H or more for slabs (H is the length of the short side of the slab).

We considered the shape of the lower die in more detail,
It has been found that a shape in which the solidification interface of the ingot is substantially inverted is most preferable.

Here, the details of the present invention will be explained with reference to billets and slabs, which are typical ingots, based on the drawings.

FIG. 3 shows a sectional view of the lower die for billet casting.
The downwardly expanding raised curved surface drawn by dotted lines and solid lines is an inverted version of the shape of the solidification interface of the ingot, and the solid line portion is the lower mold. Judging from the properties of the solidification interface of the ingot, this downwardly expanding protruding curved surface has its apex at the axis of the annular mold. However, the downwardly expanding raised curved surface may be a truncated raised surface as described later.

R is the diameter of the lower mold, γ ₁ is the diameter of the lower end of the raised portion, and γ ₂ is the diameter of the upper end. h is the height of the raised portion.

If γ ₁ = R and γ ₂ = 0, the shape of the lower mold will be the same as that of the solidified ingot with the solidified interface inverted, but if the condition of Rγ ₁ 1/5R 4/5Rγ ₂ 0 h1/4R is If it satisfies the above requirements, it may be a raised curved surface with the upper part cut off, that is, a truncated raised surface, as shown in FIG.

4 and 5 are respectively a perspective view of the lower die for slab casting and a sectional view taken along the line a-a along a plane parallel to the short side of the slab. The downwardly expanding raised curved surface drawn by dotted lines and solid lines has a shape obtained by inverting the solidification interface of the ingot, and the solid line portion is the actual lower die.

H, t ₁ and t ₂ are the width of the slab as seen from the short side direction of the slab and the width of the lower end of the raised portion of the lower die, respectively.

In the case of slab casting, similarly to the situation in billet casting, the upper part of a downwardly expanding raised curved surface may be cut off as long as the following conditions are met.

1/2Ht ₁ 1/10H 2/5Ht ₂ 0 h1/8H Furthermore, in order to make these effects even greater, it is also possible to form a cavity inside the lower mold and perform forced cooling. As a cooling method, there are, for example, a method in which the cavity is filled with cooling water, a method in which the cooling water is injected, etc. (FIG. 6).

Next, the present invention will be further described with reference to Examples, but it goes without saying that the present invention is not limited to the following Examples.

EXAMPLE A 12 inch diameter billet of 2024 alloy (AA standard) was cast by a vertical semi-continuous casting method using a gas pressurized hot top method. Under the casting conditions of a casting speed of 80 mm/min and a cooling water flow rate of 120/min, the lower mold has the following properties:
Using the invention shown in FIG. 6, 90/
Min of cooling water was supplied through the supply pipe 6 to water-cool the lower mold. During casting, no cold cracks occurred and a sound ingot was obtained.

On the other hand, the same billet as in the example was cast using the known lower mold shown in FIG. 1 under the same casting equipment and under the same casting conditions (however, the lower mold was not water-cooled). A cold crack occurred in the early stages of casting.

As explained above, according to the present invention, by using a lower mold having a raised curved surface that expands downward with the axis of the annular mold as its apex, cold molding at the start of casting of aluminum or aluminum alloy can be achieved. This can prevent cracks from occurring.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of a vertical semi-continuous casting device;
Figure 2 is a cross-sectional view showing the change over time of the solidification interface of an ingot cast using a lower mold with a raised curved surface that expands downward with the axis of the annular mold at its apex;
The figure is a sectional view of a lower die for billet casting to explain the present invention, FIG. 4 is a perspective view of a lower die for slab casting to explain the present invention, and FIG.
The a-a sectional view of the lower mold in the figure, FIG. 6, has a protruding curved surface expanding downward, and a cavity for forced cooling is bored inside the lower mold according to the method of the present invention. FIG. 3 is a cross-sectional view of the lower mold for billet casting. 1... Forced cooling annular mold with water cooling jacket, 2
... lower mold, 3 ... molten aluminum, 5 ... solidified aluminum ingot, 6 ... lower mold cooling water supply pipe.

Claims (1)

[Scope of Claims] 1. Aluminum or aluminum alloy molten metal is continuously injected into the forcibly cooled annular mold from above, and solidified columnar ingots are formed such that the upper surface thereof is aligned with the apex of the axis of the annular mold. A semi-continuous casting method for aluminum or aluminum alloy in which the aluminum or aluminum alloy is supported on a lower die which can be lowered and lowered and which is formed with a raised curved surface that expands downward. 2. The aluminum according to claim 1, wherein the raised curved surface expanding downward on the upper surface of the lower mold has a shape substantially inverted from the solidification interface at the top of the solidified ingot during continuous casting. or semi-continuous casting of aluminum alloys. 3. The semi-continuous casting method for aluminum or aluminum alloy according to claim 1, wherein the lower die has a cavity therein, and the inner wall of the cavity is forcibly cooled by a cooling medium. 4. The semi-continuous casting method for aluminum or aluminum alloy according to claim 3, characterized in that forced cooling is performed by directly injecting a liquid cooling medium onto the inner wall surface of the cavity.