EP0275228A1 - Process and device for melting and continuously casting metals - Google Patents

Abstract

Apparatus for melting and continuously casting metals, comprising a vertical, conductive cold crucible (1) with at least part of the height of its wall in the form of longitudinal sectors (2) which are electrically insulated from one another and have a cooling fluid running through them; an inductor (6) with coils helically surrounding the crucible (1) over part of its height and supplied with alternating current both for heating and confining the metal; a system for drawing down the ingot; and possibly a chamber with a controlled external atmosphere, separating at least the contents of the crucible (1) from the external atmosphere, characterized in that the crucible (1) has an upper zone (4) divided into sectors, with parallel vertical generating lines, and a lower zone (5) divided into sectors and joined to the upper zone (4), the generating lines of the lower zone spreading apart in a downward direction from the join (45) between the two zones (4 and (5), and that the lowest coil (60) of the inductor (6) is at the level of that join (45). The apparatus of the invention is advantageously used for melting and casting refractory metals and other materials.

Description

The invention relates to a vertical device for melting and continuous casting of metals, of the type called a cold crucible with induction heating.

The cold crucible has a conductive wall, often of copper, consisting of several longitudinal sectors, in number ranging from 4 to more than 20, juxtaposed, electrically isolated from each other, and traversed by an internal circulation of coolant.

This wall is thus maintained at a temperature much lower than that of the melt.

The crucible is surrounded, over part of its height, by a cooled coaxial helical inductor, traversed by an alternating current at medium or high frequency. The division into sectors of the wall of the crucible allows the alternating magnetic field of the inductor to induce in the metallic mass to be treated currents which heat it and stir it when it is melted.

In a first category of cold-crucible continuous casting devices, the molten metal is gradually evacuated through an orifice, generally located at the bottom of the crucible. The device is then used exclusively for heating, solidification taking place in a separate ingot mold. The contamination of the metal by the wall can be avoided by the formation of a film of solidified slag in contact with the wall, which constitutes a sheath around the liquid metal.
Another solution is electromagnetic confinement using an alternating magnetic field which develops forces causing the lateral surface of the melt to peel off from the wall. These two methods are mentioned in patent FR 2497050 (= US 4432093).

In a second category of cold crucible continuous casting devices, the metal is evacuated progressively in the solid state, by pulling down.

The device then ensures the heating (melting) of the load, its cooling (solidification) and its extraction. This is the case of US Pat. No. 3,775,091.
The metal is never in contact with the vertical cylindrical crucible, because on the one hand it is subjected to electromagnetic confinement forces, on the other hand a layer of solidified slag is interposed between the metal (liquid or solid) and the wall over the entire height of the appliance. There are several disadvantages here = on the one hand, the long contact length between the solid mass and the wall requires a great tensile force, and precautions to avoid the tearing of material on the wall of the crucible; on the other hand, it is necessary to peel the layer of slag which adheres to the ingot before transforming it. Finally, the slag is difficult to handle, constitutes a risk of metal pollution and corrosion of the crucible, requires additional oven cleaning operations, because it vaporizes when working under vacuum, and prevents the obtaining of an ingot shape. other than cylindrical.

The invention which relates to a device of the 2nd category above, overcomes these drawbacks.
The device according to the invention, for continuous metal casting, comprising a vertical conductive cold crucible, the wall of which is formed at least over part of its height by longitudinal sectors electrically isolated from each other and traversed by a cooling, a helical coil inductor surrounding the crucible over part of its height and supplied with alternating current, serving both for heating and confining the metal, an ingot drawing system from below and possibly an enclosure with controlled insulating atmosphere at least the contents of the crucible of the external atmosphere, is characterized in that the crucible comprises a sectored upper zone with parallel vertical generators and a sectorized lower zone which is connected to the upper zone and whose generatrices deviate downwards from the connection of these two zones and in that the lowest turn of the inductor is at this connection.

This structure allows, by means of the electric adjustment of the inductor, to obtain the electromagnetic confinement of the liquid mass away of the wall, except in a portion of very low height, preferably not exceeding 1 cm, at the connection of the 2 zones of the crucible, where the side wall or skin of the metal is made to solidify in contact with the cold wall from the crucible.
Below this level, the thickness of the solidified metal increases until it concerns the entire section of the ingot.
Due to the change in cross section of the crucible passing from the upper zone to the lower zone, the solid metal only touches the wall at the low height indicated.
In this way the drawing of the ingot is facilitated, the pollution of the solidified metal by the metal of the crucible is nonexistent, the risk of tearing of metal from said wall almost zero and the ingot therefore has a good surface condition.
This device makes it possible to operate without slag, hence the simplification of the feeding system, possibility of working easily under vacuum or under inert atmosphere and elimination of the peeling operation before ingot working.
It is necessary for correct operation of the device, that there is a metal-wall contact zone to form the skin of the ingot.

It has been surprisingly found that, as long as the height of this contact zone remains low, the electrical contact that the metal produced between the sectors of the crucible does not disturb the electrical operation of the system. The height of this zone is therefore limited to less than 1 cm, preferably 2 to 5 mm.
The level of the lowest coil of the inductor is very important. If it is located above the connection between the two zones of the crucible, it is not possible to limit the height of contact of the metal with the wall sufficiently, hence difficulties in the electrical field, and for drawing the ingot. On the other hand, if it is located below the connection, the risk of liquid metal running down the wall increases appreciably.

When a fairly short curved zone of connection between the cylindrical part and the conical part of the crucible is provided, the reference level for the lowest turn of the inductor is that of the intersection of the extensions of these 2 zones.

The angle of inclination of the oblique generatrices of the lower zone of the crucible relative to the vertical generatrices of the upper sectorized wall of this crucible depends on the coefficient of contraction of the material on solidification. It must be chosen so that the ingot remains as close as possible to the wall so that it can continue to cool while avoiding touching it.
An angle between 1 ° and 5 ° and preferably of the order of 2 ° is generally chosen.

In established operating conditions, the device contains as constant a quantity of metal as possible, the supply and extraction being precisely regulated. The top of the dome of liquid metal (form due to electromagnetic confinement) is maintained at a constant level which depends on the electrical and magnetic characteristics of the system and the nature of the metal.
The height of the inductor is preferably chosen such that its upper turn is at the level of the top of the liquid dome. If the height of the inductor is smaller, there is an instability of the dome, with the risk of contact of the metal with the wall in unwanted areas. It is advantageous for the sectored upper zone of the crucible to exceed the top of the liquid dome by an amount of the order of 1/6 of the internal transverse dimension of the crucible.

The internal transverse dimension is half of the smallest dimension of the crucible. In the case of a circular section, it is the radius. In the case of an ellipse, it is the half minor axis. In the case of a square, it is the half side. In the case of a rectangle it is half width. Finally, in the case of a complex section, it is half the distance between the closest parallel segments or half the distance between the closest parallel tangent points.

Alternatively, the crucible can be extended upward by a non-sectored area. Then, the total height of the crucible above the highest turn of the inductor is at least equal to half of the interior transverse dimension of the crucible.
The internal transverse dimension of the crucible is measured in the upper zone with vertical generators surrounded by the inductor.

The sectorized lower zone of the crucible has a total height at least equal to half the internal transverse dimension of the crucible to avoid a screen effect causing a drop in energy efficiency.
Its wall is either entirely oblique or initially oblique and then extended downwards by a vertical portion.

In the latter case, the height of the oblique part is at least equal to a quarter of the internal transverse dimension of the crucible. The crucible can also be extended downwards by a non-sectored zone with a vertical or oblique cooled wall connecting to the sectorized zone which is above it.
Its height will preferably be between half the internal transverse dimension of the crucible and this dimension. Its role is above all to continue cooling the ingot.

The wall of the crucible is made of a material which is a good thermal and electrical conductor (for example, copper, aluminum) so as to have good energy efficiency.

The continuous slag-free casting according to the invention, which requires a low metal-wall direct contact zone, requires a connection angle between the liquid and the wall corresponding to poor wetting. It is then necessary in certain cases to provide the internal surface of the crucible with a surface coating, for example metallic, or to subject it to a surface treatment, so as to obtain an excellent surface condition for the ingot.

The device of the invention is suitable for the production of cylindrical ingots. It is also suitable for the dairy-free preparation of ingots of non-circular cross-section, for example polygonal, the inner wall of the upper zone of the crucible then being of polygonal cylindrical shape, ingots which cannot be obtained in the presence of slag, because the solidification of this in the angles harms the good filling of the section with metal.

In order to obtain an effective value of the uniform magnetic field along the internal wall of the crucible, the inductors must be modified. In a first embodiment (fig. 3), the distance between inductor and wall is varied in the vicinity of the angles to reduce the intensity of the field.
In a second embodiment (fig. 4), the magnetic circuit is arranged in the rectilinear parts of the crucible section, for example by partially surrounding the inductor with magnetic sheets or ferrites, possibly cooled, to increase the field in these areas.

• Les figures 1a et 1b représentent les coupes schématiques transversale et axiale d'un creuset froid selon l'invention.
• La figure 2 représente une installation de fusion et coulée semi-continue, selon l'invention en atmosphère contrôlée.
• Les figures 3 et 4 représentent des schémas en coupe transversale de creusets polygonaux avec les inducteurs adaptés.

The device of the invention, particularly advantageous for the remelting and casting of refractory metals of groups IV, V and VI or their alloys, is also usable for the remelting and casting of other metals or alloys, in particular earth rare, aluminum, copper, silicon and nickel or cobalt base alloys. It is also suitable for the production of metal by chemical reaction, in particular when the other product formed by this reaction is gaseous or volatile.

• Figures 1a and 1b show the schematic transverse and axial sections of a cold crucible according to the invention.
• FIG. 2 represents a semi-continuous melting and casting installation according to the invention in a controlled atmosphere.
• Figures 3 and 4 show cross-sectional diagrams of polygonal crucibles with suitable inductors.

EXAMPLE 1

In Figure 1b, the electrical and fluid connections have not been shown.
1 is a copper crucible with a circular section 180 mm high.
The upper 125 mm (a + b + c) consist of 16 hollow sectors 2 which are each of substantially trapezoidal cross section (Figure 1a), cooled by internal circulation of water, the lower 55 mm (d) being constituted by a skirt 3 also cooled by internal circulation of water (Figure 1b).
The upper zone 4 of the crucible 1 is in the form of a cylinder 80 mm high and 60 mm in internal diameter.
The lower zone 5 is in the form of a truncated cone 100 mm high and at an angle at the top 4 °, sectorized over a height c = 45 mm, the rest of the height d = 55 mm not being sectorized.
The inductor 6 is a copper tube 1 mm thick and 6 mm in internal diameter, wound helically over a height of 7 substantially contiguous and isolated turns, with a diameter of 85 mm.
7 is the false bottom of the cylindrical part of the crucible, on which the solidified metal 8 of the ingot rests and which is drawn downwards in steady state.

The whole is in an enclosure isolated under argon at atmospheric pressure. Titanium shavings are purified by reflow. At start-up, the titanium false back is in position such that its upper face is located at mid-height of the inductor.
The electric power is gradually increased until the upper part of the false bottom melts. The false bottom is pulled slightly, titanium shavings are fed and the power is further increased to its nominal value.

When the top 9 of the liquid dome 10 arrives at the top 11 of the inductor 6, it is supplied at the normal operating rate, ie 200 g / min of titanium shavings, and the false bottom 7 is drawn at the rate of 1 , 6 cm / min. During the entire operation, the metal-wall contact height remained between 2 and 5 mm. In 32 min, a 6.5 kg ingot was obtained having the following composition:
WHERE 2000 ppm
C 230 ppm
N₂ 105 ppm
Cu <20 ppm
Ti stay
and an excellent surface finish.

Figure 2 shows the semi-continuous casting installation used. The crucible 20 is placed inside the sealed enclosure 21 under argon at atmospheric pressure. The means for introducing inert gas or placing under vacuum are not shown. The hopper 22 contains the material which is fed into the crucible via the distributor 23. The false bottom 7 which supports the ingot 25 is linked to the rod 26 which is driven by the device 27 and which passes tightly through the wall of the enclosure 21. The operation of the supply and extraction devices are synchronized by means of a regulator not shown, controlled by laser measurement of the level of the dome of liquid metal in the crucible.

EXAMPLE 2

To treat zirconium waste, a crucible was produced having substantially the same dimensions as that of Example 1, with only 2 differences: the angle of the cone was 2.5 ° and the height of the lower conical skirt not sectored was 70 mm.

The operating power is 35 kW at the terminals of the inductor, with a frequency current of 9 kHz. We work under argon at atmospheric pressure.

The procedure is the same as in Example 1. The metal-wall contact height is between 2 and 8 mm during the whole operation. Under steady conditions, the supply of zirconium chips is 175. g / min, the drawing speed of 1 cm / min. In 54 min, a 9.4 kg ingot is obtained, having a good surface condition and the following impurity contents:
WHERE 700 ppm
C 30 ppm
N₂ 80 ppm
Cu <10 ppm

EXAMPLE 3

To purify TA6V titanium alloy chips, a copper crucible with 16 sectors was produced, with an internal diameter of 100 mm, with a total height of 280 mm.
The sectors extend over a height of 230 mm from the top.
The upper part is cylindrical and 130 mm high, the lower part is tapered with an angle of 2 ° and with a sectored height of 100 mm.

The inductor with 10 turns in tube of outside diameter 8 mm, and thickness 1 mm, has a height of 85 mm and an inside diameter of 150 mm. We work under argon at atmospheric pressure, with a power of 50 kW and a frequency of 3 kHz, a supply of 466 g / min and an extraction speed of 1.3 cm / min. The metal / wall contact height remains between 5 and 10 mm.
In 75 min, a 35 kg ingot is obtained.

EXAMPLE 4 (fig. 3)

Bars of rectangular section 75 x 18 mm were obtained from TA6V alloy chips.
The crucible 100 is rectangular, with sides 75 and 1 = 18 mm. The corresponding internal transverse dimension 1/2 is 9 mm. Its total height is 110 mm. It comprises, from top to bottom, a sectorized cylindrical part of 65 mm in height, a sectorized conical part of 15 mm and a non-sectorized conical part of 30 mm. The angle of the cone is 2 °. The number of sectors is 18. The inductor 106 with 6 turns has a height of 50 mm. It is made of the same copper tube as in the previous examples. The space between crucible and inductor is 10 mm, except in the vicinity of the angles where it is increased.
We work under argon at atmospheric pressure, with a power of 35 kW at the terminals of the inductor, a frequency of 100 kHz, a feed rate of 175 g / min and a drawing speed of 2.9 cm / min . The metal-wall contact height is between 5 and 10 mm. In 11 min, a 1.8 kg ingot is obtained.

EXAMPLE 5 (Figure 4)

This figure represents a variant of Example 4, where the inductor 206 of substantially constant spacing with the sectors of the crucible 200, is surrounded by magnetic sheets 2060 on its rectilinear parts, so as to increase the field in the corresponding zones.

Claims (10)

1. Device for melting and continuous casting of metals, comprising a cold crucible (1) vertical conductor whose wall is formed over at least part of its height by longitudinal sectors (2) electrically isolated from each other and traversed by a cooling fluid, an inductor (6) with helical coils surrounding the crucible (1) over part of its height and supplied with alternating current at medium or high frequency serving both for heating and for confining the metal, a draft system (7 and 26 and 27) of the ingot downwards and possibly an enclosure (21) with a controlled atmosphere isolating from the atmosphere at least the contents of the crucible (1), characterized in that the crucible (1) has an upper zone sectorized (4) with parallel vertical generators and a sectorized lower zone (5) which connects to the upper zone (4) and whose generators move away from the base, from the connection (45) of these two zones (4 and 5), and in that the lowest turn (60) of the inductor (6) is at the level of said connection (45). 2. Device according to claim 1 wherein the lower sectorized area (5) with oblique generators of the crucible (1) is extended downwards by a sectorized area with vertical generators, the height of the 1st area being at least equal to 1/4 of the internal transverse dimension of the crucible. 3. Device according to one of claims 1 or 2, wherein the sectorized lower zone (5) of the crucible is extended downwards by a non-sectorized zone (3) cooled. 4. Device according to one of claims 1 to 3, wherein the internal surface of the upper zone (4) of the crucible (1) is provided with a surface coating or is treated so as to obtain poor wetting of this surface by liquid metal. 5. Device according to one of claims 1 to 4, where the angle of inclination of the oblique generatrices of the sectorized lower zone (5) of the crucible (1) is between 1 ° and 5 °. 6. Device according to one of claims 1 to 5, in which the interior wall of the upper zone (4) of the crucible (1) is of circular cylindrical shape, and in which the interior wall of the sectorized lower zone (5) is tapered. 7. Device according to one of claims 1 to 5, wherein the inner wall of the upper region (4) of the crucible (1) is of cylindrical polygonal shape. 8. A continuous process for implementing the device of one of claims 1 to 7, in which the solid metal is continuously supplied in divided form, where it is heated and it is melted by induction, where the electromagnetically is confined. liquid metal, where it is cooled by the circulation of coolant in the wall, where it is extracted in the solidified state by pulling down at a speed corresponding to the feed, characterized in that, in the absence of slag, the metal is in contact with the wall over a limited height not exceeding 1 cm, above the connection (45) between the upper (4) and lower (5) sectors of the crucible (1). 9. The method of claim 8 wherein the supply and extraction are kept constant, so that the top (9) of the dome of liquid metal (10) is substantially at the highest turn (11) of l 'inductor (6). 10. Use of the device of any one of claims 1 to 7 for the remelting and casting of one of the metals or alloys of the group formed by: the refractory metals of groups IV, V and VI and their alloys, earths -rare, aluminum, copper, silicon, nickel-based alloys, cobalt-based alloys.

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