EP0727500B1 - Method and installation for producing metal ingots - Google Patents

Abstract

Metal blocks, in particular, those of steel and alloys on nickel and cobalt basis, are produced by melting of self-consuming electrodes under an electrically conductive slag in an atmosphere of controlled composition. Melting of the electrode takes place in a gas-tight space bounded by the slag bath surface, the wall of a mould (20), and a mould hood (24). Electrode change takes place by removing the hood (24) from the mould and replacing it with another hood (24a). The electrode rod (26) has a gas-tight fit with the hood opening (28).

Description

The invention relates to a method for producing Blocks of metal - especially of steel as well Nickel and cobalt-based alloys - by remelting self-consumable electrodes under electrically conductive Slag in an atmosphere of controlled composition. In addition, the invention covers systems for this Commitment.

To produce high-quality blocks with a good block structure and The so-called electro-slag remelting process has a high degree of purity especially proven. At the time the introduction of the procedure were proportionate used simple systems in which a single Melting electrode in a water-cooled stand mold Air was melted. The block length that could be produced was here limited by the length of generation of the electrode. Such systems were therefore mostly with a high Fill factor, i.e. a high ratio of cross-sectional area of the melting electrode to the cross-sectional area of the water-cooled mold, operated.

The advantage of this approach was a simple one Arrangement of the plant, but a number of disadvantages compared to low flexibility, high cost of long standing crucibles and in the manufacture of electrodes as well large overall height for the blocks. Another disadvantage was that Necessity to use high melt current due to the comparatively large fill factor a diameter ratio between the electrode and Mold of over 0.7.

The disadvantages described here led early to introduce a number of different procedures with short molds and the possibility of remelting of several electrodes in succession through the application of electrode change technology. It made it possible, too long blocks with block lengths of more than 6 m from one larger number of significantly shorter consumable electrodes Lengths to make about 2 m, there is also a free Choice of fill factor offered.

The fact, the diameter ratio between Electrode and mold over the entire technically possible and choose a reasonable range from 0.4 - 0.8, led to the manufacture of blocks of larger diameter with a more favorable ratio of tension to Amperage than this due to the standing crucible systems whose geometric limitations would have been achievable.

By using short dies either as lifting dies or as fixed molds in combination with lowerable ones Floor plates, the mold costs became significant reduced, and together with the electrode change technology the flexibility of the systems was increased.

Systems with lifting molds with the middle are known Speed at which the on the Base plate standing block grows, and two in horizontal Direction swiveling or moving, in vertical Electrode carriage with high-current clamps that can be delivered, through which the consumable electrode is clamped to the circuit and which one alternately, once the other electrode carriage is in use.

Instead of a lifting mold, there are also short ones in a work platform permanently installed molds in combination with lowerable ones Base plate used, the base plate in Is lowered at a rate equal to the Block assembly speed corresponds. These plants will again either with two swiveling in the horizontal direction or movable electrode trolleys equipped with high-current terminals or with just one vertical only movable electrode trolley with current clamp in combination with two swiveling auxiliary arms for loading and unloading Electrode and electrode remnant.

In all these system variants, the remelting takes place at Atmospheric pressure more or less in air, but occasionally the gap between the electrode may also have been tried and mold to seal with a cover and protective gas or dried Air in the gap between the electrode and the mold initiate. These efforts remain due to the irregular Naturally, the surface of the cast electrodes unsuccessful, the effort being essentially on one Avoiding hydrogen uptake.

In addition to the system and process variants described above there are also ESU plants, in which - in the same As with the standing crucible systems - from a single one long melting electrode or a bundle of electrodes a block is made in a stand mold. This Systems naturally have all the disadvantages described above of electrode systems with stand molds, however accepted this, since this is the first large-scale Production of large blocks weighing over 10 t from steels with nitrogen contents far above the Solubility at atmospheric pressure became possible.

The constantly increasing demands on the properties of use of steels and alloys lead to demands after ever lower levels of oxygen and non-metallic Inclusions especially in the case of remelted ones Steels, so that the implementation of electroslag remelting in a perfectly controllable atmosphere gaining interest. Every metallurgical Possibility of an oxygen-free gas phase in the gap between the electrode and the mold, can be exploited can. In particular, this is intended to form the scale on the hot electrode surface just before immersing the Electrode in the slag bath can be avoided because of this in the case of open melting, oxygen is constantly in the slag bath - and thus into the remelted metal - is transported.

DE-B-24 25 032 describes a method for manufacturing of cast blocks with good ductility and fine-grained Solidification texture made of high-melting iron and metal alloys - In particular from austenitic chrome-nickel steels or nickel and cobalt based alloys - through Melting off at least one self-consuming electrode according to the electroslag remelting process (ESR). The one used Slag should - of low nitrogen solubility - do not react with the gas above; not in this atmosphere with the liquid slag reacting gas is melted under an overpressure of at least 20 at in a gas-tight pressure chamber, that from the slag surface in a mold, one that Pressure vessel comprising mold and one on the pressure vessel cylindrical bell with hat-like lid is limited. The latter passes through an axial electrode rod, that can be moved relative to the lid. In In one embodiment, the mold replaces the pressure vessel. The hat-like Lid is available in both configurations for changing electrodes by the extending one Electrode rod carried in a gas-tight Implementation stores. This gas tightness is essential because a sudden release of pressure under excess pressure standing gas is a source of danger for operation would represent.

Knowing this state of the art Inventor set the goal to remedy the identified shortcomings.

The teaching of the independent leads to the solution of this task Claim 1 with which a surprisingly simple and technically feasible way is shown, an electric slag remelting using short sliding molds and electrode change technology under a controllable To enable atmosphere at approximately atmospheric pressure. A special embodiment even allows electro-slag remelting with short slide mold and electrode change under considerably increased compared to atmospheric pressure or lowered pressures in the room above the Slag bath.

The invention also relates to a system according to Claim 7 for performing the method.

Favorable further education indicate the subclaims.

According to the invention, the electroslag is remelted under controlled protective gas atmosphere with almost atmospheric Pressure in sliding crucibles using the electrode change the melting of the self-consumable electrode carried out in a gas-tight room, through the slag surface, the wall of the water-cooled Chill mold and the wall of one on the water-cooled Mold - gas-tight - seated hood is limited and in the a gas line to adjust the atmosphere flows; the hood contains a bushing in which for the power supply is a smooth electrode rod with a clamping mechanism can be moved by suitable sealing elements. This locked room for the electrode change is opened by the gas-tight Connection between lower hood flange and mold flange separately and the hood - depending on the Conception of the plant - to the extent that the Electrode remnant removed from the melting area and a new electrode is brought into the melting position can. After replacing the electrode, the Remelting process continued immediately and on the other hand the hood is immediately replaced on the mold flange and sealed gastight, immediately afterwards through suitable measures in the closed room again the desired protective gas atmosphere is set.

An embodiment is also within the scope of the invention in which the remelting of the self-consuming electrode in the gas-tight closed space takes place under a pressure, which is significantly lower than atmospheric pressure for example below 500 mbar; the block is in a chamber is built up in which the same pressure as in the room above the slag bath and at which is the pressure in the chambers before changing electrodes is first brought to atmospheric pressure before the gas-tight Connection between hood and mold flange for the execution of the electrode change is opened.

In a further variant of the method according to the invention there is melting of the self-consuming Electrode in a gas-tight room under an over Atmospheric pressure, for example above 2.0 bar; the block is also built up in a chamber here which is the same pressure as in the room above the Slag bath prevails and during which Electrode change the pressure above the slag bath is maintained that after withdrawing the electrode remnant in the hood the room above of the slag bath at the level of the mold flange a gas-tight built in between the mold and the hood Slider closed, then the pressure in the hood lowered to atmospheric pressure and only then gas-tight Connection between hood and slide flange for the purpose the electrode change is opened. To Remove the electrode remnant and insert one The new electrode in the melting position is the first Hood placed on the sealing flange and gas and sealed pressure-tight, the pressure in the hood to the same value as the pressure above the slag bath set, the gas-tight slide above the Slag bath opened and then the new electrode to continue the remelting process in the slag bath lowered.

Helpful for the invention is the fact that the Electroslag remelting the liquid metal sump a slag bath is covered, which the direct Contact the atmosphere with the surface of the liquid Prevented swamp. As already mentioned, when remelting in air oxygen especially because of that Slag bath and further introduced into the metal sump, because the one immersed in the overheated slag bath Melting electrode immediately above the slag bath heated to high temperatures above 1000 ° to 1200 ° C is so that this part of the electrode in contact with the Oxygen in the air forms scale, which then continues Course of the melting of the electrode in the slag bath is entered.

To the scale formation on the heated above the slag Electrode surface - as the cause of one possible oxygen uptake - it is to prevent required melting the electrode in one to allow an oxygen-free atmosphere to take place.

On the other hand, the melting process is interrupted Electrode no oxygen transfer into the metal sump because this - as already mentioned - through the slag bath is shielded from the atmosphere and a direct one Transition of oxygen through the slag in a highly basic, practically no heavy metal ion-free slag can take place. The slag can only then Transport oxygen when they change heavy metal ions Contains valence, such as ions of Iron, manganese, chromium or the like.

As can be seen from the example of iron, only these ions can be oxidized at the slag-gas phase phase boundary - if the gas phase contains oxygen - according to the reaction: 2 (FE ²⁺ ) + 1 / 2O 2 _Gas  2 (FE ³⁺ ) + (O ^2- ), with which an additional oxygen ion is also absorbed into the slag. At the phase boundary between the metal sump and the slag, the trivalent iron is reduced again to the divalent iron, oxygen being simultaneously released to the liquid metal in accordance with the reaction: 2 (Fe ³⁺ ) + (O ^2- )  2 (Fe ²⁺ ) + [O] _metal .

Does the slag contain heavy metal ions with changing valence, can therefore continuously transfer oxygen from the gas phase the slag bath can be transported into the metal bath. However, ESR slags are as they are offered by retailers become extremely low in heavy metal oxide. Only during the remelting due to the constant entry of tinder - despite ongoing slag deoxidation - the heavy metal oxide content the slag, with which the direct Oxygen transfer from the gas phase via the slag in the metal sump gets going.

If it succeeds through appropriate measures, a Scale formation on the electrode surface and thus To prevent the introduction of oxygen ions into the slag bath, so during the short period of electrode change an oxygen-containing atmosphere above the Slag bath can be tolerated.

Furthermore, it should be noted that in the case of electroslag remelting in sliding crucibles (lifting mold or short mold and lowerable floor plate) the surrounding atmosphere penetrates into the gap between the block surface and the mold wall, but not in contact with the liquid metal there comes; this solidifies on contact with the water-cooled Mold wall immediately and is there - even if it is superficially oxidized - no longer able to To transport oxygen into the liquid metal sump.

Fig. 1 bis 3:

mit Hebekokille sowie zwei schwenkbaren Säulen bei unterschiedlichen Verfahrensschritten;

Fig. 4, 5:

eine andere Ausführung mit absenkbarer Bodenplatte und fester Säule in zwei verschiedenen Betriebszuständen.

Further advantages, features and details of the invention result from the following description of two preferred exemplary embodiments, which are also discussed with regard to the implementation of the method sequence, and from the drawing; each shows a sketchy vertical section of an inert gas ESU system in

1 to 3:

with lifting mold and two swiveling columns in different process steps;

4, 5:

another version with a lowerable base plate and a fixed column in two different operating states.

On both sides of the vertical axis A of an inert gas ESU system 10, two electrode pits 16, 16 _a for electrodes 18, 18 _{a are} provided outside two pivotable columns 12, 12 _a in their adjusting platform 14. In the vertical axis A, between the columns 12, 12 _a connected by a yoke 13 _, a mold 20 of height a can be seen, which rests on the adjusting platform 14 in FIG. 1.

On each column 12 or 12 _a two carriages movable thereon are arranged one above the other, the upper one being referred to as the electrode carriage 22 and the lower one as the hood carriage 23. On the latter, a hood 24, 24 _{a is} fixed, which extends coaxially to an electrode 18, 18 _a hanging on an electrode rod 26. The electrode rod 26 is attached at one end to the electrode carriage 22 by means of a clamping mechanism, with the aid of which a melt current is connected to the electrode 18, 18 _a , the electrode rod 26 being inserted into the interior 25 of the hood 24, 24 _a through a gas-tight axial passage 28 guided and can be moved with the electrode carriage 22 relative to the column 12, 12 _a .

The electrode rod 26 can move the electrode 18, 18 _a relative to the hood 24, 24 _a . Thus, the electrode 18 _{a of} the column 12 _a on the right in FIG. 1 rests _, for example, in the electrode pit 16 _a , ie below its hood 24 _a at a distance b from it.

After preparing the mold 20 for starting the first electrode 18 in the clamping mechanism of its electrode rod 26 fixed and by starting the electrode carriage 22 introduced into the hood 24. The latter will now pivoted over the mold 20 and on the mold flange 21 attached to form a gas-tight connection. After setting a suitable protective gas atmosphere becomes the electrode 18 by lowering the electrode carriage 22 lowered until it is on a Base plate 30 or an ignition plate arranged there or Detonator sits on. The base plate 30 can be on a not recognizable block car with which a finished ESU block 32 moved out of the plant area can be.

Now the melting current is switched on and after melting either in the mold 20 or slowly via a metering device, not shown added slag the remelting process initiated. The Electrode 18 is one of the difference from the electrode melting rate and block build speed corresponding dimension in an emerging slag bath 34 followed up.

Before the first electrode 18 is completely consumed, the second electrode 18 _{a is} clamped to the second electrode rod 26 in the loading position and moved into the second hood 24 _a , the latter already being brought into a position which enables a safe pivoting into the melting position .

2 has almost melted, the melt stream is switched off at the same time Retract the remaining piece of the electrode 18 into the hood 24, the hood / mold connection opened, hood 24 slightly raised and then by swiveling the column 12 from the melting position to the loading / unloading position swung out, in which the electrode remnant is removed becomes.

As soon as the melting position is free, the second column 12 _{a is} pivoted and the hood 24 _a with the second electrode 18 _{a is brought} over the melting position. Now, on the one hand, the hood 24 _{a is} lowered and placed on the mold flange 21 and, on the other hand, the melt flow is switched on; the electrode 18 _a is shut down until it touches the slag bath surface and the remelting process thus continues. After the hood 24 _{a has been placed} on the mold flange 21, the atmosphere in the now closed melting chamber is immediately replaced (FIG. 3).

After the melting of the second electrode 18 _a of the above-described process is repeated and a third electrode 18 _b remelted - this repetition is performed several times with additional electrodes, until the desired block length is reached.

When the method is carried out in the system 10 described above with a mold 20 which can be lifted, it is possible to produce long ESU blocks 32 from a plurality of comparatively short melting electrodes 18, 18 _a , 18 _b in sliding crucibles under a controllable protective gas atmosphere.

In another embodiment of a system 10 _k according to FIGS. 4, 5 suitable for carrying out the method, a short mold 20 _{k is} permanently installed in a work platform 36 and the ESU block 32 formed in the mold 20 _k with a lowerable base plate 30 _k subtracted downwards at the same speed as the block assembly speed. This inert gas ESU system 10 _k is equipped with a fixed column 38, along which a hood carriage 23 and an electrode carriage 22 can be moved in the vertical direction.

The electrode carriage 23 keeps the electrode rod 26 a clamping cylinder 40, thanks to which the connection of the Melt current to the electrode 18 is established; the Electrode rod 26 is also gas-tight here Implementation 28 led into the interior of the hood 24.

In order to remove the electrode remnant from the melting position and to transport a new electrode 18 _a into the melting position, two pivotable auxiliary arms - neglected in the drawing - are provided as loading and unloading arms.

After preparing the mold 20 _k for starting, the first electrode 18 is pivoted into the melting position, this is accepted and clamped by the electrode clamp, and the hood 24 and electrode 18 are lowered until on the one hand the latter on the lowerable base plate 30 _k or the ignition plate or on the other hand Hood 24 sits gas-tight on the mold flange 21.

After setting the desired atmosphere, the power is switched on and after the slag has melted, the actual remelting begins. During the remelting of the first electrode 18, the second electrode 18 _{a is} prepared and suspended in the loading arm mentioned. When the first electrode 18 has melted down to a small disk, the melt flow is switched off, the hood / mold connection is opened, hood 24 and electrode rod 26 are moved into the change position. There the unloading arm picks up the remaining electrode and swivels it out of the melting position. If this is free, the new electrode 18 _{a is} pivoted into it by means of the loading arm and clamped by the electrode rod clamp. The loading arm is swung out, the melt flow is switched on, and the electrode 18 _a and the hood 24 are simultaneously lowered until, on the one hand, the electrode 18 _a touches the surface of the slag bath 34 or, on the other hand, the hood 24 sits gas-tight on the mold flange 21. The protective gas atmosphere is then reestablished in the closed space above the slag. Now the remelting process is continued until the second electrode 18 _{a is also} consumed. This can now be changed again in the manner described above. Several electrodes are remelted one after the other until the desired block length is reached.

The system 10 _k shown in FIGS. 4, 5 with a lowerable base plate 30 _k can alternatively also be equipped with two pivotable columns, each with an electrode and hood trolley. In this case, the swiveling loading and unloading arms can be omitted.

This system 10 _k of FIGS. 4, 5 with its lowerable base plate 30 _k , fixed column 38 and protective gas hood may also be designed in a relatively simple manner as a vacuum and / or overpressure system. In this case, the base plate 30 _k with the block 32 built thereon is lowered into a lower vessel connected to the lower mold flange in a gas-tight and pressure-tight manner, the pressures between the hood 24 and the lower vessel being connected via a pressure compensation line. In order to be able to maintain an overpressure above the slag bath 34 in particular while performing the electrode change, a gate valve is preferably installed between the upper mold flange 21 and the hood flange and is closed before the hood 24 is relieved of pressure. The gate valve is only opened when the hood 24 has been placed gas and pressure-tight after receiving the new electrode 18 _a and the pressure in the hood 24 has been set to the same pressure as in the space above the slag bath 34.

Claims (15)

1. Method for the production of a remelt ingot made of metals, in particular steels, as well as nickel and cobalt base alloys, by successively remelting at least two consumable electrodes under an electrically conductive slag in an atmosphere of controlled composition, in which each of the electrodes used to produce the remelt ingot is itself melted in a space delimited in a gastight manner by the slag bath surface provided in a mould, the wall of the mould and a hood for each electrode sitting thereon, the gastight connection between the hood and the mould being released in order to open the sealed space in order to change the electrodes, the residual portion of the electrode being removed from the melting region when the hood is raised, a new electrode being brought into the melting position and the hood being placed on the mould and connected thereto in a gastight manner, after which the prescribed inert-gas atmosphere is established once again in the closed space and the remelting process is continued.
2. Method according to claim 1, characterised in that the consumable electrode is remelted in the space sealed in a gastight manner at a pressure which is much lower than atmospheric pressure, that the ingot is formed in a space in which the same pressure is maintained as that prevailing in the space above the slag bath, the pressure in the spaces being brought to atmospheric pressure before the gastight connection between the hood and the mould is opened in order to change the electrodes.
3. Method according to claim 2, characterised in that remelting is carried out in the space sealed in a gastight manner at a pressure of less than 500 mbar.
4. Method according to claim 2, characterised by a pressure of less than 200 mbar during remelting.
5. Method according to claim 1, characterised in that the consumable electrode is remelted in the space sealed in a gastight manner at a pressure above atmospheric pressure and the ingot is formed in a space in which the same pressure is maintained as that prevailing in the space above the slag bath, the pressure above the slag bath being maintained while the electrodes are changed in that, once the residual portion of the electrode has been retracted into the hood, the space above the slag bath level with the mould region connected to the hood is first sealed by means of a gastight closing member provided between the mould and the hood, then the pressure in the hood is reduced to atmospheric pressure and then the gastight connection between the hood and the closing member is opened in order to change the electrodes, remelting optionally being carried out in the gastight space at a pressure of more than 2.0 bar.
6. Method according to claim 5, characterised in that, once the electrode portion has been removed and a new electrode has been introduced into the melting position, the hood is placed on the closing member and is sealed in a gastight and pressure-tight manner therewith, that the pressure in the hood is adjusted to the value of the pressure above the slag bath, that the gastight closing member is opened above the slag bath and then the new electrode is lowered into the slag bath in order to continue the remelting process.
7. Installation for carrying out the method according to one of the preceding claims, characterised by a hood (24, 24_a) for each electrode movable relative to a mould (20, 20_k) in the region of a mould flange (21) thereof and to be releasably connected thereto in a gastight manner in order to form a gastight space, at least one gas line opening into this space and an electrode (18, 18_a) to be arranged therein, to which end the hood is traversed in the region of a gastight passage (28) by an electrode rod (26) which can be raised and lowered relative to/within the hood.
8. Installation according to claim 7, characterised in that the gas line opens into the hood (24, 24_a).
9. Installation according to claim 7 or claim 8, characterised in that the electrode rod (26) is supported at one end in a current-conducting manner in a clamping means (40) of an at least vertically adjustable electrode carriage (22) coaxial with the hood (24, 24_a).
10. Installation according to claim 7 or claim 9, characterised by at least two electrode rods (26) each with an associated hood (24, 24_a) vertically adjustably supported on a hood carriage (23).
11. Installation according to claim 9 or claim 10, characterised in that the hood carriage (23) and/or the electrode carriage (22) can pivot about an axis (12, 12_a).
12. Installation according to one of claims 7 to 11, characterised in that the water-cooled mould (20) of small height (a) is arranged in such a manner that it can be raised relative to a base plate (30).
13. Installation according to claim 7, characterised in that the water-cooled mould (20_k) of small height (a) is secured in a plane (36) and a lowerable base plate (30_k) is associated therewith, an electrode carriage (22) and a hood carriage (23) for vertically adjusting a hood (24) traversed by the electrode rod (26) possibly being associated with the mould (30_k).
14. Installation according to claim 13, characterised in that pivotable auxiliary arms are provided for transporting the electrode (18) into the melting position and the residual portion of the electrode out of this melting position and/or that a vessel for receiving the base plate with the ingot (32) formed thereon connected in a pressure-tight manner to the mould (20_k) is arranged below the lowerable base plate (30_k), the ingot being connected to the hood (24) by a pressure-compensating means.
15. Installation according to claim 14, characterised by a gate valve between the upper mould flange (21) and the hood (24).

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