EP0035958A2 - Mold for continuous casting - Google Patents

Abstract

1. Continuous-casting mould with a wall (21, 37, 40) which is permeable to gases and through which compressed gas issues laterally towards the melt (26), and with a hot top (16, 28, 33) which is located above the mould wall, the wall of this hot-top (18, 28, 33) being inwardly offset in relation to the wall (21, 37, 40) which is permeable to gases and the lower region of this hot-top (18, 28, 33) forming a hot-top overhang (22, 31, 34), characterized in that the hot-top (18, 28, 33) is connected to the wall (21, 37, 40) which is permeable to gases, and in that the wall (21, 37, 40) which is permeable to gases is arranged independently of a liquid-cooling system which provides the primary cooling.

Description

The invention relates to a method for the manufacture of semi-finished product from a non-ferrous metal by means of a continuous casting mold, wherein by supplying as _G between the melt on the one hand and the Heißkopf- or ' Mold wall on the other hand, a stable pressure gas cushion is built up as a separating layer.

In the context of such a process a continuous chill mold for the production of billets, _W alzbarren and ribbons _° is used any shape and alloy composition in general. The separating layer formed by the gas cushion is intended to prevent the strand surface from touching the wall of the hot head and the rest of the mold wall.

Although semi-finished products can be produced more economically with the continuous casting process than with the use of stand molds and a better quality can also be achieved, errors still occur during continuous casting that prevent immediate further processing from the as-cast state. In addition to internal defects such as cracks and possibly also porosity, this includes surface defects which can appear as waviness on the continuous casting surface, as tearing open, as cold runs, as surface segregation or as lines of friction on the strand surface. Therefore, the cast products must be post-treated before further processing, for example by milling or by turning.

In addition to the purpose of preventing contact between the wall of the hot head and the mold wall on the one hand and the strand surface on the other hand, the pressurized gas cushion is also intended for the purpose of heat dissipation or one Serve cooling, so that it comes to the so-called edge shell effect. This means the formation of a solidification zone in the edge region of the strand at the level of the mold wall. This heat dissipation or cooling in the area of the mold itself is referred to as primary cooling. Immediately after leaving the mold, the strand is further cooled by water from the outside. This is known as secondary cooling.

It is an object of the present invention, the occurrence of the above-mentioned defects, especially surface defects, to prevent the cast product and to avoid any necessary post or at least the extent of this Fehlerhafti ^g resistance and the extent of required. Reduce post-processing as much as possible.

To achieve this object, it is proposed according to the invention that the gas forming the pressure cushion is supplied at a temperature which is at least 100 ° C. above room temperature and that the mold wall remains uncooled.

The invention is based on the knowledge that the abovementioned errors can be avoided or reduced in their extent if, contrary to the knowledge previously considered valid, as little heat as possible is dissipated in the radial direction in the area of the mold and if the coarse outer shell zone is thus largely is avoided. At the same time, by preventing direct contact between the melt and the solidifying part of the strand, on the one hand, and the mold wall, on the other hand, a significantly better surface quality of the cast strand is ensured. This in turn has the advantage that the amount of post-processing required is reduced accordingly. This can save considerable costs.

Carbon dioxide, nitrogen, argon, air or mixtures of these gases will advantageously be used to form the compressed gas cushion, especially since these gases are particularly low heat conductors.

So that an initial heat dissipation in the radial direction at the beginning of the casting process can be largely or even completely avoided, it is proposed according to the invention that the mold wall is heated before the casting process begins. Accordingly, the hot head wall can also be heated before the casting process begins.

The heating can take place either electrically or by means of the compressed gas itself, a combination of these two types of heating also being conceivable.

Furthermore, it is proposed according to the invention that the gas is fed in at such a high temperature that radially outward transport of heat from the melt and thus an outer shell growth are largely prevented in the region of the compressed gas cushion. If only the gas temperature is sufficiently high, the pressure gas cushion not only serves to prevent direct contact between the melt and the solidifying part of the strand on the one hand and the mold wall on the other hand, but also at the same time as a thermal separating layer, i.e. as a shield against a mold wall, the one has a lower temperature than the melt and the solidifying part of the strand. In other words, this means that the difference between the temperature of the gas and the liquidus temperature is kept as low as possible or even made zero.

Furthermore, it is advantageous according to the invention if the gas supplied escapes downwards and if the gas is supplied at such a pressure and in such an amount that the pressure of the gas in the pressure gas cushion is equal to or greater than the sum of the atmospheric pressure and the metallostatic Pressure of the melt in the area of the compressed gas cushion.

There are several options for supplying the gas used to build up the compressed gas cushion. The. Gas can also be supplied at several points in the hot head cross section. It is advantageous according to the invention that the heated compressed gas is supplied both on the underside of the hot head and on the mold wall itself. Thus, the gas pressure can be built up partly in an approximately vertical direction from above and partly in an approximately horizontal direction from the mold wall underneath.

An upper limit for the gas temperature to be selected will generally be given by the liquidus temperature or, at most, only a few degrees above this temperature. Since the melt has a temperature during casting that is higher than the liquidus temperature, such a quantity of heat can be released from the melt to the gas without disadvantages that the. corresponds to this temperature difference.

Some advantageous embodiments of the invention are described in more detail below with reference to a drawing.

• Fig. 1 einen Teilschnitt durch eine Kokille;
• Fig. 2 in einer der Darstellung nach Figur 1 entsprechende Darstellung einen Teilschnitt durch eine andere Ausführungsform der Kokille.
• _Fig. 3 einen Teilbereich aus einem solchen Teilschnitt, der eine andere Form des Heißkopfüberhanges darstellt.

In detail show:

• 1 shows a partial section through a mold.
• 2 in one of the representations according to FIG. 1 corresponding representation of a partial section through another embodiment of the mold.
• _F ig. 3 shows a partial area from such a partial section, which represents a different shape of the hot head overhang.

For all embodiments it is common that the part of a mold located to the left of a central axis 10 is shown, whereby this has a circular cross section. Of course, the invention can also be used for molds which have a different cross-sectional shape.

The continuous casting mold shown in FIG. 1 has an outer jacket part 11 and an insert part 12. Together with a base part 13 and a wall part 14, these form an annular space 15 which is closed in the circumferential direction and in which the water required for direct cooling is contained. Between the insert part 12 on the one hand and the bottom part 13 and the wall part 14 on the other hand, an annular gap-shaped channel 16 is formed, through which water is distributed over the circumference of a casting strand 17 to the outer circumferential surface thereof. The wall part 14 ensures a uniform supply of the water fed into the annular space 15 at any point over the circumference.

The mold has a hot head 18 made of heat insulation the material, which also serves to prevent the liquid melt introduced into the mold from cooling. The hot head 18 has an inwardly projecting wall 19 in contact with the melt, which at its deepest point merges into an obliquely upward and inclined underside 20. A porous wall 21 adjoins the wall 19 offset to the outside. The part of the hot head 18 protruding inwards from this represents the hot head overhang 22. On the rear side of the wall 21, an annular space 23 is formed by the insert part 12, the hot head 18 and by the wall 21 itself, to which gas flows via at least one channel 24 is fed under pressure. The supply can alternatively also take place wholly or partly via cavities which are formed here in the form of a network 25 in the hot head 18 and in which heat can be exchanged between the gas and the hot head 18. If necessary, the mold and in particular its area formed by the hot head 18 can be additionally heated, for example by means of an electrical heater (not shown here).

A solidification front, designated by reference number 27, runs between the liquid melt, which is designated here by reference number 26, and the casting strand 17, which is made of solidified material.

During casting, the casting strand 17 is lowered in accordance with the progress of solidification, with new melt 26 being added at the same time. The gas supplied to the annular space 23 has a temperature which corresponds approximately to the liquidus temperature and is therefore only slightly lower than the temperature of the supplied melt 26. The pressure and the quantity of gas supplied in the annular space 23 are dimensioned such that the gas pressure in the compressed gas cushion
corresponds approximately to the sum of the atmospheric pressure and the metallostatic pressure of the melt 26 in the region of the wall 21. Thus, the pressure gas emerging from this can make contact between the wall 21 and the part of the underside 20 of the hot head 18 adjoining it, on the one hand, and the oxide skin on the outside of the casting strand 17 and in the region of the not yet solidified melt 26, on the other hand, by forming a corresponding pressure gas cushion prevent. Heat dissipation in the radial direction to the outside is prevented by the temperature of the gas. The solidification front 27 therefore also has a relatively flat course in the outer region, which shows that the edge shell effect which is otherwise caused by primary cooling is not or at least barely present. As can also be seen in FIG. 1, the gas can only escape downward, since the lowest point of the wall 19 is lower than the highest point of the cross-sectional part of the melt located below the hot head overhang 22.

Only below the porous wall 21 and thus below the solidification front 27 is there cooling by the water emerging from the annular gap-shaped channel 16. Immediately in the exit region of the channel 16, the cooling effect on the outer surface of the casting strand 17 is initially somewhat reduced by the fact that the compressed gas exits there at a relatively high temperature. For example, for aluminum continuous casting, a temperature in the order of 700 ° C applies.

Compared to the embodiment according to FIG. 1, the embodiment shown in FIG. 2 contains only one differently designed hot head 28 with a side wall 29 protruding into the melt 26 downwardly open annular recess 30 of approximately U-shaped cross-section forms the underside of the hot head overhang 31. The supply of the heated and pressurized gas takes place through one or more channels 32. As can be seen in FIG. 2, the lowest point of the wall also lies here 29 below the highest point of the cross-sectional area of the melt 26 located in the area of the recess 30. The lower area of the hot head 28 is designed as a porous wall 37, behind which an annular space 38 is located. Channels 39 open into these, through which heated compressed gas can be supplied, as can channels 32. As can be seen in FIG. 2, the gas supplied can only escape downwards along the outside of the casting strand 17 and cannot escape upwards along the wall 29.

Finally, FIG. 3 shows a further embodiment only for a hot head 33, the hot head overhang 34 of which now has an obliquely downward nose from which the wall 35 projecting into the melt 26 extends upwards. A portion of the heated and pressurized gas is supplied via channels 36 on the underside of the hot head overhang 34. For the rest, the lower region of the hot head 33 is also designed as a porous wall 40, behind which an annular space 41 is located. The latter is also supplied with heated compressed gas via channels 42.

Otherwise, unless expressly described here with reference to FIGS. 2 and / or 3, what has been said in connection with the embodiment according to FIG. 1 applies.

The method proposed by the invention can be used advantageously not only for the continuous casting of aluminum and aluminum alloys, but generally for non-ferrous metals, for example for copper and magnesium, and their alloys.

By accommodating cooling water in the annular space 15 and by draining it via the annular-shaped channel 16, indirect cooling of the melt on the way over the mold wall is never brought about. A heat flow of a certain size through the hot head, in particular in the direction of the cooling water, is unavoidable, but is otherwise of no importance for the process. However, it is advantageous to design the cooling system and to bring the cooling into effect in a precisely defined and sharply delimited zone of the cast strand with the aid of the annular gap-shaped channel 16 in order to achieve the most uniform possible shrinkage of the cast strand over the circumference.

Reference number list

• 10 central axis
• 11 jacket part
• 12 insert
• 13 bottom part
• 14 wall part
• 15 annulus
• 16 channel
• 17 casting strand
• 18 hot head
• 19 wall
• 20 bottom.
• 21 wall
• 22 Hot head overhang
• 23 annulus
• 24 channel
• 25 network
• 26 melt
• 27 Solidification front
• 28 hot head
• 29 wall
• 30 recess
• 31 Hot head overhang
• 32 channel
• 33 hot head
• 34 Hot head overhang
• 35 wall
• 36 channel
• 37 wall
• 38 annulus
• 39 channel
• 40 wall
• 41 annulus
• 42 channel

Claims (10)

1. A process for the production of semi-finished products from a non-ferrous metal by continuous casting using a mold, a stable pressure gas cushion being built up as a separating layer by supplying gas between the melt on the one hand and the hot-head or mold wall on the other hand, characterized in that the gas with is supplied to a temperature which is at least 100 ° C above room temperature, and that the mold wall remains uncooled. 2. The method according to claim 1, characterized in that the mold wall is heated before the start of the casting process. 3. The method according to claim 1 or 2, characterized in that the hot head wall is heated before the start of the casting process. 4. The method according to claim 2 or 3, characterized in that the heating takes place electrically. 5. The method according to claim 2 or 3, characterized in that the heating is carried out by the compressed gas itself. 6. The method according to any one of the preceding claims, characterized in that the gas is supplied at such a high temperature that a radially outward transport of heat from the melt and thus an edge shell growth are largely prevented in the region of the pressure gas cushion. 7. The method according to any one of the preceding claims, characterized in that the supplied gas escapes downwards and that the gas is supplied under such a pressure and in such an amount that the pressure of the gas in the pressure gas cushion is equal to or greater than the sum of the atmospheric pressure and the metallostatic pressure of the melt in the area of the compressed gas cushion. 8. The method according to any one of the preceding claims, characterized in that carbon dioxide, nitrogen, argon, air or mixtures of these gases are used to form the compressed gas cushion. 9. The method according to any one of the preceding claims, characterized in that the heated compressed gas is supplied both on the underside of the hot head and on the mold wall itself. 1 ₀ . Method according to one of the preceding claims, characterized in that the heated compressed gas is supplied to the compressed gas cushion through porous wall parts.

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