GB2177633A - A block for starting the process of continuous casting of molten metal - Google Patents

Abstract

The block (10) has a centrally located cavity (12) so that a substantially reduced amount of heat is withdrawn from the bottom of an ingot being cast from the mold, compared with the case of the solid block of prior practice. The cavity (12) is separated by a thin wall (30) from the upper surface of the block which is of steel. The upper surface is concavely curved in both directions cross-sectionally of the block. <IMAGE>

Description

SPECIFICATION Insulating starting block The present invention relates generally to a metal block for starting the process of casting an ingot and particularly to an insulating block that reduces substantially the amount of heat withdrawn from the molten metal in the process of starting an ingot.

Generally, in starting the process of making a rectangular-shaped ingot for rolling in a rolling mill, a solid aluminium or steel block has been used to close the end of the casting mold from which molten metal is withdrawn in making the ingot. Such solid blocks chill the molten metal in the mold, such that when casting ingots of high strength alloys, for example, 7075 and 2024, the ingots frequently crack. As a rectangular ingot leaves a mold, the edges of the ingot cooi faster than the wide surfaces of the ingot, i.e., the surfaces that engage the rolls of a rolling mill, for example, in "breaking down" the ingot. This means that the edges solidify first such that when the molten center of the bottom of the ingot solidifies, the edges have been solid for some time. It is well known that when metal solidifies it shrinks.The center of the ingot thus shrinks against the outside solid edges such that a high stress condition develops in the solid bottom of the ingot. An ingot can literally destroy itself when this stress exceeds the strength of the material of the ingot.

To cure the problem of cracking an insulating material, fiberglass or Refrasil (a trademark of Hitco Company, Irvine, California) may be placed on the center of the block so that the molten metal is insulated from the central mass of the block. In this manner, the chill on the molten metal caused by the mass of the block is reduced.

However, such insulating pads create conditions that cause alligator fractures and separations at the bottom of the ingot in the process of reducing the thickness of the ingot in a hot rolling mill. The edges of such pads often do not remain flat when the molten metal is poured on them. Rather, the pad edges tend to curl and fold into the end or butt of the ingot. When the ingot is rolled in a reversing mill to reduce its thickness, the folded edges of the pad in the ingot end act as stress risers which, in turn, provide paths for crack (alligator) propagation. Such alligator separations of the ingot damage the mill and produce a substantial amount of scrap material, as the alligator ends must be removed before further rolling of the ingot can be accomplished.

Damage to the mill is caused when chunks and pieces of the alligatored end break off and wedge in portions of the mill. A critical problem and area involves "the stripper", which is a device that pushes the front end of an ingot or slab being rolled away from the work rolls so that the slab does not wrap around the rolls. Strippers are costly pieces of equipment, and an alligator end can tear out a stripper in short order.

A method for reducing edge cracks in a cast ingot is disclosed in U.S. Patent 3,948,310 to Deschapelles. Deschapelles provides a starting block with corner projections that limit the solidification rate at the corners of the ingot by preventing excessive amounts of coolant from reaching the corners during the initial solidification process. The corner projections also stabilize the position of the ingot as it leaves the mold. Such corner projections are, however detrimental to casting hard alloy ingots.

The corner projections tend to confine the material of the ingot during the cooling process such that the stresses in the ingot are enhanced rather than relieved.

The present invention is directed to an insulating block for starting the process of casting molten metal contained in a casting mold, said block having: a centrally located cavity therein containing air, peripheral wall portions located about said cavity, and a thin wall portion integral with and joining said peripheral wall portions; said peripheral and thin wall portions having a concave surface located to contact the molten metal, and extending lengthwise of the block. The insulating nature of such a block functions to limit the amount of heat withdrawn from the butt of the ingot as it leaves the casting mold. In this manner, insulating pads are eliminated, with their attendant problems of alligator fractures during hot rolling.

The center mass of the ingot is, in other words, insulated from the block so that the center of the ingot will remain hot until cooled by water running down the sides of the ingot in the casting process.

The objectives and advantages of the invention will best be understood from consideration of the following detailed description and the accompanying drawing in which: Figure 1 is a longitudinal section and partial perspective view of the starting block of the invention: Figure 2 is a cross-sectional and partial perspective view of the ingot of Figure 1: Figure 3 is a somewhat diagrammatic depiction of a mold in the process of casting a rectangular ingot on a starting block of the prior art, the figure viewing the ingot from its broad or breadth dimension: and Figure 4 is the apparatus and ingot of Figure 3 viewed from the edge of the ingot.

Preferred embodiment of the invention Referring now to the figures of the drawing, a double curvature starting block 10, which may be of steel, is shown in section and partial perspective. The block is provided with a cavity 12 located centrally in the structure of the block. As shown, cavity 12 opens downwardly and opposite of upper face 14 of the block. The cavity can be covered by a metal plate (not shown), which can be attached to the wall portions of the block that surround the cavity by suitable fastening means.

As shown in Figure 1,the upper face 14 of the block is mildly concave in a longitudinal direction, while the face in a cross-wire section of the block, as depicted in Figure 2, is concaved in a more sub stantial manner. Such double curvature is known and is particularly suitable for vertical casting in gots of hard aluminum alloys. This is appreciated in Figures 3 and 4 of the drawings showing an in got 16 being cast. A coolant 15 directed at high ve locity against the sides of the ingot in the process of being cast extracts rapidly large amounts of heat from the ingot.

More particularly, Figures 3 and 4 depict an ingot 16 being formed from molten metal 18 in the lower portion of a casting mold 20. A solid metal starting block of the prior art is shown receiving the bot tom end of the ingot.

The large amount of heat extracted by the cool ant causes rapid solidification of the outer portion of the molten metal leaving mold 20 to form a sub stantially thick outer shell or wall of metal 24.

When the molten metal reaches starting block 22, the block withdraws considerable heat from the molten metal to form a wall 26 of solid metal at the bottom of the ingot.

As the outer shell 24 is formed, it shrinks, as disr cussed above, and curls away from block 22. This is shown in Figures 3 and 4 of the drawings. The curling is substantially greater across the rolling breadth of ingot 16, as shown in Figure 3, in com parison with the narrow edges of the ingot, as shown in Figure 4. The shrinkage and curling of the thick side wall 24 relative to lower wall 26, across the breadth of the ingot, develops a high stress condition in the lower wall of the ingot, par ticularly at locations of transition between the side wall of 24 and the lower wall 26. In Figure 3, the approximate locations of transition are indicated by reference numeral 28. As discussed earlier, such a stress can literally destroy an ingot.

The present invention involves the removal of the center mass of a starting block, to provide the insulating block 10 of Figures 1 and 2, such that the rate of heat withdrawal from the bottom of the ingot is substantially reduced over that of the solid starting block. This results in little or no solid wall being formed at the ingot bottom such that the solid outer portion 24 of the ingot can now shrink against a relatively soft and essentially liquid metal center at the bottom of the ingot. In this manner, stresses are substantially reduced if not eliminated altogether in the process of casting an ingot.

As shown further in Figure 1, the length of the cavity of the block of the invention is somewhat greater than one-half the total length of the block.

The width of the cavity, however, as shown in Fig ure 2, is substantially greater than one-half the width of the block.

For example, a block that was successfully tested had an overall length of about 50 inches, while the length of the cavity was 28 inches. The thickness of the block at the ends thereof was 8 inches; at the center the block was 5.219 inches. The depth of the cavity was 3.72 inches, thereby leaving an upper wall portion 20 of approximately 1-1/2 inches in thickness. This wall and surface 14 form the bot tom of the mold in casting apparatus for continu ous casting of ingot by the direct chill method. The entire surface 14 of the block is covered with molten metal during the start of an ingot drop.

The width of the block that was successfully tested was about 16-1/2 inches wide, with cavity 12 being on the order of 11 inches wide.

The starting block structure of the invention is such that ample peripheral wall portions 22 are provided that can extract heat from the molten metal as it is withdrawn from the mold in the process of casting an ingot. However, cavity 12, with ambient air located therein acts as an insulator that reduces the rate of heat extraction from the butt of the ingot leaving the mold to thereby prevent cracking of the butt. Since no separate insulating materials, such as fiberglass, need now be placed on the surface of the block contacting the molten metal and ingot butt, the source of stress risers is eliminated and alligatoring due to this cause is eliminated.

Claims (5)

1. An insulating block for starting the process of casting molten metal contained in a casting mold, said block having: a centrally located cavity therein containing air, peripheral wall portions located about said cavity, and a thin wall portion integral with and joining said peripheral wall portions, said peripheral and thin wall portions having a concave surface located to contact the molten metal, and extending lengthwise of the block.

2. A block according to claim 1, in which the concave surface is mildly concave in a direction crosswise of the block.

3. A block according to claim 1 or 2, in which the material of the peripheral and thin wall portions is steel.

4. An insulating block substantially as hereinbefore described and as illustrated in Figures 1 and 2 of the accompanying drawings.

5. A process for casting molten metal contained in a casting mold in which in starting the process there is used an insulating block according to any one of the preceding claims.