EP1925681B1 - Method for electro slag remelting of metals and mould therefor - Google Patents

Description

The invention relates to a process for the electroslag remelting of metals, in particular iron and nickel-based alloys for the production of Umschmelzblöcken from one or more Abschmelzelektroden in a short, water-cooled Gleitkokille according to the preamble of patent claim 1, as these from the WO02 / 40726 A1 as well as the AT 406239 B are known. In addition, the invention comprises an improved mold compared to the prior art for carrying out this method according to claim 6.

In today's operating electric slag remelting plants according to the cited prior art, water-cooled molds are used for the formation and production of the Umschmelzblöcke whose forming the block and holding the slag bath mold wall is generally made of copper, since this material, as well as from continuous casting her known, is best suited to dissipate the liberated during the solidification of metals amounts of heat quickly and efficiently into the cooling water. While exclusively short molds are used in continuous casting, from which the solidifying strand is withdrawn more or less continuously after the formation of a first stable strand shell, in the electroslag remelting according to the state of the art both the use of so-called stand molds and the use of short sliding molds common.

For stand molds, the length of the mold corresponds to the length of the block to be produced. In the course of the remelting process, the mold is successively filled with remelted metal by melting the consumable electrode in the slag bath floating on the metal mirror, whereby there is no relative movement between the mold or the mold wall and the remelt block.

When using molds Umschmelzblöcke be produced whose length exceeds the length of the mold by a multiple. The short, water-cooled mold serves as a melting and casting mold, in which the hot slag bath is located and in which the metal melting off from the electrode is collected and subsequently solidified into the remelting block. The mold is therefore required to carry out the remelting process only in the area of the slag bath and in the area of block solidification. Once the remelt block has solidified, the mold no longer fulfills any purpose. Thus, it is possible to limit the length of the mold to this range described above and to deduct the block formed during the melting operation, for example, from the mold at an average speed corresponding to the speed of the block construction. This leads to a relative movement between the formed block and the mold wall and has the consequence that the meniscus of the metal mirror and the slag bath resting thereon-apart from during the start-up phase-remain essentially at a constant level within the mold during the entire block build-up. Instead of pulling off the forming block from a mold installed in a working platform as described above, it is also possible to use the remelting block Build up on a fixed base plate and pull the short mold upwards by means of a suitable device with a speed corresponding to the block building speed.

As is generally known, the consumption of electric melting energy in electroslag remelting is relatively high in comparison with other melting processes which also operate with electrical energy, such as scrap melting in an electric arc furnace or an induction crucible furnace, since the melting rate is controlled primarily during electroslag remelting. to ensure a perfect solidification structure of the remelting blocks. An energy saving by increasing the Abschmelzrate is therefore not possible, the direct contact of serving as a heat source, through the passage of current highly heated slag bath with the water-cooled mold wall still has a significant additional negative impact.

In order to melt iron or nickel base alloys from ambient temperature and heat them to about 1600 ° C, a theoretical energy requirement of about 400 kWh / t is required. If melting and overheating are carried out in an electric arc or induction furnace with only electrical energy, energy consumption of 500 to 700 kWh / t is to be expected due to process-related heat losses. In contrast, the energy consumption in the production of a remelt block with, for example, 1000 mm diameter at a remelt rate of 1000 kg / h, depending on the slag and height of the slag bath used between 1000 and 1800 kWh / t. This is due to the fact that the heat flow from the hot Depending on the slag composition, slag over the water-cooled mold wall into the cooling water is between 1000 and 2000 kW / m ² . The most commonly used slags with 1/3 CaO, CaF ₂ and Al ₂ O ₃ each have to be estimated at about 1100 kW / m ² . When remelting in a mold with a diameter of 1000 mm and a Schlackenbadhöhe of about 200 mm must therefore be expected with an energy loss in the slag zone of about 630 kW, which corresponds to a energy consumption of about 630 kWh / t at a remelting rate of 1000 kg / h. Based on a total energy consumption during remelting with the above-mentioned slag of about 1300 kWh / t, this corresponds to a percentage of almost 50%. In slags with higher levels of CaF ₂ , this proportion can be significantly higher.

For the reasons described above, it would therefore be obvious to isolate the mold in the region of the slag bath against heat loss, so as to reduce the melting energy consumption. After the Austrian patent 287215 there has already been a corresponding proposal in 1968, according to which in electroslag remelting by controlling the position of the molten metal level in the mold, all the slag floating on the molten metal is collected as a liquid slag layer in a thermally insulated zone of the mold; the temperature of the liquid slag layer is held by the thermal insulation above or at least at the melting temperature of the metal. Accordingly, the liquid metal accumulated in the mold reaches into the region of the heat insulation, and the dividing line between the isolated and the water-cooled mold part is located below the metal mirror.

This arrangement, which corresponds to the current state of the art, has the disadvantage that the onset of solidification is not adequately defined and thus considerable difficulties in operational use can occur. Thus, it is possible that with appropriate overheating of the metal penetrates this into the gap between the insulated and water-cooled part of the mold and solidifies there in contact with the water-cooled lower part and forms a metal lug that gets stuck in the gap. Depending on the strength of the flag, the block can now ever get stuck in the mold and a block deduction become impossible, whereby the remelting would be finished. If the flag is of low thickness and not formed over the entire block circumference, so cracks will form in the solidifying shell, which complicate at least one further processing of the block. If deeper cracks form, liquid metal and the slag may leak, thus ending the remelting process. These problems of transition from an insulated container to a water-cooled solidification mold in contact with liquid or eventually solidifying metal are known from horizontal continuous casting. There, the problem is solved in that at the transition point insulation - water cooling a so-called breaking ring of boron nitride is incorporated, which due to its specific properties in terms of thermal conductivity, wettability by the liquid metal, etc., a progression of solidification over the boundary line insulation - Water cooling prevented and allows easy detachment of the solidified strand shell. However, boron nitride is an expensive, complex to produce and only in relatively small dimensions up to usual Stanggussdimensionen in the range up to about 200 mm diameter available material and thus comes for the For the electroslag remelting interesting dimensions of 500 mm diameter and considerably more out of consideration. For all these reasons, the above-described prior art method, despite the obvious economic advantages, has not found practical application to date.

From the DE 18 17 124 A1 Furthermore molds are known according to the preamble of claim 1, in which the upper, in direct contact with the slag bath standing part of the mold is formed partly of heat-insulating material in the form of an annular insulating layer. Due to the direct contact of the heat-insulating material with the slag bath, it must be adapted to the temperature of the slag bath in terms of its temperature resistance and formation, so that the insulating layer is not destroyed or removed by the slag bath.

The object of the present invention is to use, on the one hand, the economic advantage of thermal insulation in the area of the slag bath in the case of electroslag remelting, while avoiding the problems described above, so that a technical application is possible in a meaningful manner.

To achieve this object, the teaching of the independent claim; the dependent claims indicate favorable developments. In addition, all combinations of at least two of the features disclosed in the description, the drawings and / or the claims fall within the scope of the invention. For given designation ranges, values lying within the specified limits should also be disclosed as limit values and used as desired.

According to the invention, the solution of this problem using a known short, two-part Gleitkokille whose lower, the casting cross-section forming part in the usual manner water cooled and the upper part against heat dissipation is completely or partially isolated, achieved in that except in the start-up phase during the regular block structure of the metal mirror by a corresponding control of the relative movement between the mold and remelt always in the lower, water-cooled part of the mold - that is below the dividing line between water-cooled and insulated part of the same - held that the distance between the surface of the metal mirror on the one hand and the through the dividing line 5 mm, but not more than 100 mm, and that the slag bath floating on the metal mirror is at least 75% of its height in the region of the insulated part.

The relative movement can be either stepwise or continuously in a conventional manner; in the case of a step-by-step movement, the movement step followed by a break should correspond at least to twice the block-building speed. In principle, a step in the opposite direction, whose step length amounts to a maximum of 30% of the length of the original movement step, can be connected to each such movement step. A possible continuous withdrawal movement can also be superimposed on an oscillating movement.

For the production of a heat dissipation obstructing layer has been mentioned in the AT 287215 a lining with a heat-insulating, preferably ceramic layer - for example, porcelain - proposed. The problem with this material, but also with other ceramic materials, is that they are soluble in the superheated reactive slag and thus would be consumed by this within a short time. Further advantages, features and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing; this shows in its single figure a schematic cross section through a multi-part Kokilleneinheit with an improved over the prior art mold, which is particularly suitable for carrying out the method according to the invention.

A lower base part of a sliding mold as a multi-part mold unit 10 represents a water-cooled solidification or casting mold 12, the inner wall preferably consists of an insert 14 made of copper; this is installed in a water jacket or water tank not separately shown in the drawing. In this insert 14, metal dripping off in the mold longitudinal axis A is collected in a metal sump 4 and solidified into a remelt block 6. The metal mirror 5 of the metal sump 4 is superimposed by a slag bath 8.

The remelting block 6 is withdrawn from the casting mold 12 by a lowerable base plate 16, which is only indicated here, in such a way that the metal mirror 5 always remains below an upper, water-cooled flange surface 18 of the casting mold 12; above which there is an upper mold part 20, which is constructed in several parts and a water-cooled support structure 22 in the form of a tubular ring rectangular cross section and a likewise annular insert 24 includes. Preferably, of graphite or a refractory metal - such as tungsten or molybdenum - existing inner layer 26 is in contact with the slag bath 8 of height h. The inner layer 26 rectangular radial section has an inner diameter which corresponds approximately to the inner diameter d of the mold 12.

Between the inner layer 26 and the support structure 22, an intermediate layer 28 is also arranged rectangular radial section, which takes over the function of thermal insulation. This intermediate layer 28 is preferably made of a heat-resistant, refractory, ceramic material resistant to thermal shock, for example, a high-temperature resistant ceramic fiber mat, refractory bricks or other ceramic high-temperature resistant material such as ramming masses or granules.

Basically, the support structure 22 of the upper, heat-insulating Kokillenteils 20 may also be formed as an extension of the water jacket of the lower mold part 12, in which then the inner layer 26 and the intermediate layer 28 are installed. Furthermore, the upper die part consisting of the support structure 22, the inner layer 26 and the intermediate layer 28 can be held down by a rectangular radial section which is also water-cooled cover ring 30 here in the form of a tubular ring, for which purpose the cover ring 30 does not pass through here shown elements with the water jacket of the lower, water-cooled mold part 12 can be clamped together.

Instead of a block deduction from the mold 12 of the remelting block 6 can be constructed on a fixed base plate. In this case, it is necessary to mold unit 10 according to the block building speed - in an analogous manner, as described above for the block deduction - either gradually or continuously increase, which in Fig. 1 is indicated by a bracketed, upward arrow.

In a particular embodiment of the mold unit 10 can - as shown here - standing in contact with the slag 8 insert 24 of the upper Kokillenteils 20 via a corresponding high-current line 32 with a return line 34 of the melt stream to the melt power supply 35 connected, so that the Insert 24 at the same potential as the bottom plate 16 is located.

But there is also the possibility - not shown here - to connect the insert 24 of the upper Kokillenteils 20 with the high-current lead 36 to Abschmelzelektrode 2, if between the water-cooled lower mold part 12 and at least the inner layer 26 and, if necessary , the support structure 22 of the upper Kokillenteils 20 is a non-conductive electrical current, also not shown here intermediate layer of, for example, refractory ceramic material is installed. In this case, the inner layer 26 of the upper mold part is at the potential of the consumable electrode 2.

Claims (11)

1. Method for electro slag remelting of metals, in particular of iron- and nickel-based alloys, for the production of remelting blocks (6) formed of at least one fusible arc-welding electrode (2) in a short, two-part sliding ingot mould (10), of which the lower ingot mould part (12) forming the casting cross-section is water-cooled, and of which the upper ingot mould part (20) is insulated either completely or in part against thermal dissipation, characterised in that, except for in the starting phase, during normal block construction, by controlling the relative movement between the ingot mould (10) and the remelting block (6), the metal layer (5) is maintained in the lower, water-cooled part (12) of the ingot mould, i.e. below the line of separation (18) between the water-cooled and insulated parts of said ingot mould, in such a way that the distance between the surface of the metal layer (5) on the one hand and the plane determined by the line of separation (18) between the cooled and insulated parts on the other is at least at 5 mm, but at the most 100 mm, and in that the slag bath (8) floating on the metal layer (5) is situated at least up to 75 % of the height (h) thereof in the region of the insulated part.
2. Method according to claim 1, characterised in that the relative movement between the ingot mould (10) and remelting block (6) is carried out in a step-wise manner, the speed of the movement corresponding at least to twice the speed of block construction.
3. Method according to claim 1, characterised in that the relative movement between the ingot mould (10) and the remelting block (6) is carried out continuously.
4. Method according to either claim 1 or claim 2, characterised in that, with step-wise movement, each relative movement step is followed by a step in the opposite direction, of which the stroke length is at most 30 % of the stroke length of the original step.
5. Method according to claims 1 and 3, characterised in that an oscillation is superimposed on the continuous relative movement between the ingot mould (10) and the remelting block (6).
6. Ingot mould for electro slag remelting, which mould is divided along the longitudinal axis, is water-cooled at least in two parts, the lower part (12) thereof forming the casting cross-section being water-cooled and the upper part (20) thereof connected to said lower part being insulated against thermal dissipation, for carrying out the method according to at least one of claims 1 to 5, characterised in that the part (20) insulated against thermal dissipation is constructed in a multi-layered manner, a heat-insulating intermediate layer (28) being arranged between a water-cooled supporting construction (22) representing the outer layer and an inner layer (26) which is resistant to high temperatures and in contact with the slag bath, which intermediate layer consists of a refractory ceramic material which is resistant to temperature change, preferably of insulation bricks, fibrous mats, ceramic wool, granulated metal, fine-grain material or the like, and in that the metal layer (5) is always maintained beneath the line of separation (18) in the water-cooled region by a lowerable base plate (16) or by a liftable ingot mould unit (10), except for in the starting phase.
7. Ingot mould according to claim 6, characterised in that the water-cooled supporting construction (22) of the upper ingot mould part (20) is part of a water box of the lower ingot mould part (12) and is thus connected thereto.
8. Ingot mould according to claim 6, characterised in that the upper ingot mould part (20) which is constructed in a multi-layered manner and insulated against thermal dissipation is fixed between a water-cooled cover ring (30) arranged thereabove and the lower water-cooled ingot mould part (12).
9. Ingot mould according to any one of claims 6 to 8, characterised in that the layer (26) of the upper ingot mould part (20) which is in contact with the slag bath (12) is made of graphite or a metal with a high melting point, preferably of tungsten or molybdenum.
10. Ingot mould according to claim 9, characterised in that contact with a power source (35) can be produced via the layer (26), consisting of graphite or a metal with a high melting point, of the upper ingot mould part (20) through a high-power line (32).
11. Ingot mould according to claim 9, characterised in that the layer (26), consisting of graphite or a metal with a high melting point, of the upper ingot mould part (20) is electrically insulated from the lower, water-cooled part (12), forming the remelting block, of the ingot mould (10).

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