EP1253986B1 - Method and arrangement for producing casting moulds from metal - Google Patents

Description

The invention relates to a method and an arrangement for the production of castings according to the preamble of Claim 1.

There is a trend towards use in energy machine construction today stationary gas turbines with high specific power as Alternative to those that have often been rejected for environmental reasons Nuclear power plants, where using steam turbines is enough could be found. The higher working temperatures gas turbines require the use of high-alloy Iron and especially nickel-based alloys that are used to achieve of the required properties Ti, Al, B, Nb, Ta, W, etc. have. So far, gas turbines have been preferably used as aircraft engines, for those with comparatively small turbine shafts could be found. For their manufacture relatively small blocks with a diameter of 500 mm and among them, needed by means of remelting self-sufficient electrodes with sufficient Quality standard could be produced. With sufficient A raw block in particular becomes the quality standard understood, which is largely free of macroscopic structures and structural inhomogeneities such as segregations and other error phenomena called "Freckles" and "White Spots "are known.

White spots are imperfections compared to the rest Material on alloy elements are depleted. This Fault induction is only from the vacuum arc process known with self-consuming electrodes ago and it will assumed that this error symptom is caused by the Electrode tip falling down, not melted in the melting sump Dendrite branches is caused. In the electro-slag remelting process with self-consumable This fault phenomenon has not been observed to date with electrodes.

Freckles are occasional point or spot-shaped segregations that occur during solidification high alloy blocks occur along the dendrites can, if the alloy contains elements whose Density differs significantly from the density of the base alloy different. So there are iron or nickel based alloys, the high levels of specifically light elements, such as Ti or Al, but also specifically heavy elements such as W, Nb, Ta, included, particularly susceptible to this error. While for blocks of smaller dimensions up to 400 to 500 mm block diameter of these defects only sporadically and only occurs under unfavorable remelting conditions, is the production of flawless blocks with larger diameter even with the best control of the remelting conditions almost impossible. This is on it attributed to that in the manufacture of large remelting blocks inevitable long solidification times and large swamp volumes on the one hand a rough solidification structure result in segregation processes favor.

Now, however, for the construction of stationary gas turbines sufficiently high specific power large turbine shafts needed for their production again accordingly large ingots with diameters of well over 500 mm, preferably up to 1000 mm, are required. To the current state of the art of remelting with self-consumable Electrodes can produce sufficient flawless raw blocks not made from the alloys required for this become.

DE 196 14 182 C discloses a short water-cooled, mold open at the bottom for the production of blocks or strands, in which a casting mirror is made by an electrically conductive Slag is covered, in which the block or strand in the shaped lower part and either by lifting the Mold or withdrawn by lowering the block or strand becomes. In the formed from water-cooled elements The mold wall is at least one that is not directly water-cooled, current-conducting element installed so that it on the one hand comes into contact with the slag bath as well on the other hand, not to the level of the liquid metal enough; over this completely below the surface element of the slag bath is a contact a power source.

From US-A-5 799 721 is a process for remelting especially of steels as well as Ni and Co base alloys, to one strand by melting off at least one self-consuming Electrode in an electrically conductive Slag bath, the ratio of the cross-sectional area one or more consumable electrodes to the cross-sectional area of the strand to be produced as a casting cross section selected greater than 0.5 and a melting rate in kg / h is set which is 1.5 times to 30 times the Strand diameter corresponds; the one deviating from the round cross-section equivalent strand diameter becomes the circumference of the casting cross section. In a funnel mold the melting rate in kg / h corresponds to 5 to 15 times the equivalent calculated from the scope of the pouring cross-section Strand diameter, and the ratio of the cross-sectional area / n the melting electrode / s to the cross-sectional area the casting cross-section is equal to or greater than 1.0, with the strand in the lower, narrow part of the funnel mold is formed as well as the slag bath to expand into it upper part is enough.

In a process for making castings from metal according to GB-A-1 568 746, the ESU mold with electrical insulated water-cooled current-conducting elements. Solid metal is continuously added to the slag bath Form of granules fed to a bloom in any Pour length. Such granules have solid Condition a higher density than both the slag bath and also the liquid metal of the melting sump, with the result that such granules very quickly through the hot Slag and fall there due to the short dwell time do not melt or only partially melt. Get unmelted Particles in the liquid metal sump, so they also sink there due to their higher density the phase boundary solid-liquid. But there they melt no longer because there is not enough in this area There is more heat available. With progressive solidification such particles are then unmelted Condition as natural foreign metal inclusions in the solidification structure locked in. Because such unmelted Particles have a different structure - and therefore different ones Properties - than the remelted metal are them in remelting blocks, of which a uniform, fine and error-free solidification structure is required, undesirable.

The inventor was aware of this state of the art set the goal of a technically feasible process for the production of largely segregation and in particular Freckel-free metal castings, in particular made of high-alloy steels as well as Ni and Co base alloys large size after an electroslag or smelting Casting method using a known per se short, current-carrying, water-cooled mold, to create in the wall there are current-conducting, not directly water-cooled Elements electrically insulated from the cast body forming part of the mold are installed.

The teaching of the independent leads to the solution of this task claim; the subclaims give favorable further training on. In addition, all combinations fall within the scope of the invention from at least two of those in the description, the Drawing and / or the features disclosed in the claims.

According to the invention is a largely segregation and Freckel-free bloom, the cross-sectional area at most 90% of the part of the mold that forms the cast body, arranged in this and using one by the Current passage heated up in the area of the current-carrying Elements of the mold slag bath continued metered pouring of liquid metal connected to the supplied metal; by a Relative movement between mold and block becomes the level of the slag level in the mold for almost as long kept constant until the bloom in the desired Radially doubled in length, therefore, surrounded by a cast coat is this process possible with the double block with a larger mold to be repeated one or more times as long until the desired final dimension of the cast body is reached is. The procedure is basically for any Cross-sectional shapes suitable. However, if raw blocks are required, which are processed by forging, so round blocks are best made.

When performing the procedure, it is important that the casting or melting speed is set in this way is that the resulting swamp depth is one after segregation-free solidification directed above. As It has proven to be cheap, the average casting or Melting speed in kg / h to adjust that they are between 0.25 times and 5 times the sum from equivalent bloom diameter and mold diameter in mm, with the equivalent diameter for shapes deviating from round cross sections by the quotient Scope / π is determined. In the case of extremely sensitive to segregation Alloys get the best results, if the casting or melting speed in the range between 0.8 and 1.5 times the sum of the equivalents Diameter according to the above Connection set becomes.

The preliminary block required to carry out the procedure is preferably by a remelting process with self-consumable Electrode made, here a block dimension is selected, which ensures a fine-grained structure and with which an occurrence of Freckling and segregations can be avoided. in principle can the bloom block also by an electric slag or other casting process can be made as long as one sufficient freedom from freckles and segregation ensured is.

With crack-sensitive alloys - and also for a good one and flawless binding between the bloom and the doubled Layer - it may be appropriate to the bloom preheat to a temperature of up to 800 ° C. For the production of homogeneous freckles and segregation-free Blocks and castings for the production of forgings or the like. the block is doubled with an alloy, which has the same chemical composition as the bloom having. For special applications - for example the production of composite rollers, the one tough core and wear-resistant surface must - the bloom can be completely with an alloy different composition can be doubled.

To carry out the procedure, it is necessary that the liquid slag bath is always at the level of the Mold wall built-in, not directly water-cooled and electrically insulated from the rest of the mold Elements is because there is a power supply line into the slag bath. The return line the current then takes place via the bloom or Base plate on which the bloom is resting. By the Passage of electricity becomes the slag bath in a liquid state held and heated to the extent that on the one hand the metal of the Is melted on the surface and that on the other hand, even with slow pouring of liquid metal for the purpose of doubling a premature solidification on the Place where the meniscus of the liquid metal mirror with the water-cooled wall of the mold is avoided becomes.

The pre-prepared block is used to start the process on one - in the clear mold opening - Chair put on, either water-cooled - and thus reusable - be or even from the same Material like the bloom can exist. The chair with that the pre-block sitting on it is initially in the mold positioned that its top edge is straight with the Upper edge of the lower, water-cooled, the new block surface forming part of the mold. Well now Voltage applied, but initially no current flows because there is no conductive connection between the current-carrying Elements and the bloom or the floor slab. Then a pre-melted slag desired composition in the gap between the bloom and poured mold wall, allowing a current to flow begins as soon as the slag level in the area of current-conducting elements in the mold wall. Now we the desired electrical power, according to the dimensions set from bloom and mold and after short time, which is sufficient for those exposed to the slag bath Melt the surface of the bloom with the Pouring of the metal to be doubled started. To the Slag level always in the area of the current-carrying elements to hold, is according to the system configuration - For example, with a fixed chair - the mold raised continuously or in steps approximately in the way like by adding metal to the slag mirror increases. However, the system has a fixed one Chill mold, so the chair is made from the The mold was pulled down so that it was almost as good constant level of slag level in relation to the ensure current-conducting elements. Pouring in liquid metal can be according to the desired Build-up speed continuously or discontinuously done in steps. With a gradual supply may but the volume of the amount of metal in a single step do not exceed the volume of the slag bath. Either with continuous as well as gradual metal supply however, care should be taken to ensure that the above average pouring rate is not exceeded.

Raising the mold or lowering the base plate with the chair can be continuous again in a manner known per se or in steps, the middle Lifting or pulling speed again accordingly on the Metal feed speed must be matched. at step by step it must be ensured that the Single step must not be greater than the height of the in the mold wall built current-conducting elements. On every lifting step is followed by a pause until the Slag level again close to the original level has reached. If you work step by step, you can continue A return stroke is switched on between the trigger stroke step and the pause be, then trigger stroke, return stroke and pause so on each other must be matched to that of the middle Metal feed rate.

In contrast, with a continuous take-off speed worked so it can be good for training Surface can be helpful if the mold, as from Continuous casting known, executes an oscillating movement.

Another important requirement is that the a lowerable cast iron plate in the Is withdrawn from the permanently installed mold that approximately the level of the slag bath in the mold remains constant. In addition, the liquid is poured Metal or melting of the added solid Metal parts preferably under a protective gas atmosphere controlled composition and pressure; the controlled vacuum is in the range between 1 and 600 mbar is set, in case of overpressure more preferred as 2 bar.

Instead of pouring liquid metal, too solid metal in the form of bars, chips or granules in introduced the slag bath and melted in it to be brought. The metal feed in the gap between The bloom and mold is made in the manner described above continue until the entire bloom is doubled is. Then the energy supply to the slag bath is switched off and the doubled bloom block after complete Solidification of the doubled layer removed from the system.

Basically, the procedure can be carried out in open air be because the liquid slag bath the underneath protects lying metal mirrors against atmospheric oxygen. It is recommended for the production of high quality alloys himself, the process under a controlled Perform protective gas atmosphere, in which case depending also worked under negative or positive pressure according to the requirements can be.

In an arrangement for manufacturing according to the invention cast-iron and in particular low-freckel cast body Metal - in particular from steels as well as Ni and Co base alloys after an electroslag or smelting Casting process - with a short, conductive, water-cooled mold, not directly in the wall water-cooled current-conducting elements are electrically insulated opposite the part of the mold that forms the cast body are installed and the one with the mold down assigned base plate is to be carried out of the described method, a casting gap for receiving liquid metal through one on the bottom plate posed block and the mold limited. Prefers two chill molds connected in series; the The inner diameter of the downstream mold should be be larger than that of the previous mold.

Further advantages, features and details of the invention emerge from the description below more preferred Exemplary embodiments and with reference to the drawing; this shows in each of its three figures a longitudinal section through a casting device with mold.

In a water-cooled mold 10 with an annular hollow body According to FIG. 1, 12 is one from below - in turn hollow - bottom plate 20 inserted, which is part of a chair 22 is. The outer diameter of this bottom plate 20 is slightly shorter than the inner diameter d of the mold 10; the base plate 16 can start up the system as far into the mold opening or the mold interior 11 of the Height h be inserted until it is just below the upper edge 13 of the mold hollow body 12 is arranged.

A ring-like insulating element 14 rests on the upper edge 13 and on this a current-conducting element 16, which is also ring-shaped; the latter is separated from the top by an upper insulating element 14 _a from a water-cooled hollow ring 18.

For example, by a remelting process with self-consumable Electrode-manufactured bloom 24 rests the bottom plate 20 of the chair 22 in one position near the top edge 12 of the lower water-cooled portion of the Mold 10 was brought for the starting process. The apron 24 defines a casting gap 25 with the mold 10 Width b.

The current conducting element 16 and the chair 22 are via high current lines 26, 26a with one pole each of an equal or AC power source 28 connected. To start the process is liquid slag from a vessel in the Mold 10 and the ingot 24 limited mold gap poured, until a slag level 32 of an emerging Slag bath 31 of height e approximately the upper edge of the current conducting element 16 has reached.

Then - as shown schematically in Fig. 2 - Continuous liquid metal 34 through the slag bath 31 poured at the intended casting speed, wherein the latter on the one hand welded to the bloom 24 and on the other hand in contact with this and the lower water-cooled Part of the mold 10 solidifies and one with the Vorblock 24 firmly connected double cladding layer 36 forms.

FIG. 3 schematically shows the repeated radial doubling of a pre-block 24 that has already been doubled once by applying a further doubled layer 38 _a in a mold 10 with a larger inner diameter d.

The effectiveness of the method according to the invention should be based on of the following example:

Contains the high-temperature Ni base alloy Inconel 718 in addition to the alloying elements Cr and Mo each 0.9% Ti and Al as well as over 5% Nb and also B. The alloy is extremely sensitive to Freckel, so that high quality Blocks both after the electroslag and after the Vacuum arc remelting process only up to block diameters of about 450 mm can be produced with certainty. Blocks would be used for the construction of stationary gas turbines made of this alloy with a weight of 12 to 18 tons and one Block diameters from 900 to 1000 mm are required.

For the production of a raw block with a weight of 16.5 t and a block diameter of 950 mm, a melting electrode with a diameter of 340 mm and a length of 4 m was used in an electroslag remelting system with a standing crucible in a standing mold with a 420 mm inside diameter with a melting rate of 350 kg / h remelted into a block with a length of 2.6 m. The chemical composition of the consumable electrode was 0.03% C, 0.18% Si, 0.21% Mn, 19.15% Cr, 2.97% Mo, 52.83% Ni, 0.89% Ti, 0, 92% Al, 5.26% Nb, 0.0042% B and 17.85% Fe. A slag with 70% CaF ₂ and 15% Al ₂ O ₃ and CaO was used for the remelting. The slag bath height e was set to 14 cm and the power supply to the slag bath 31 was kept in the range from 350 to 380 kW at a melt current between 9.0 and 10.0 kA. The remelting time was just under 10 hours, of which about 25 minutes. for starting, about 8 h 45 min. for the block construction and almost 50 min. for the hottopping. After remelting, the block was cooled and removed from the mold. The block had a smooth surface and had a diameter of 404 mm when cold. The weight was 3060 kg.

This pre-block 24 thus produced was then placed in an electro-slag remelting plant with a lowerable base plate 20 on the water-cooled base plate chair 22 of an electrically conductive mold 10 with an inner diameter d of 700 mm and the upper edge of the chair 22 to just below the upper edge of the lower water-cooled part 12 permanently installed mold 10 in the work platform. The main switch of the power supply was then switched on and the voltage was set to 70 V, with no current being measured yet. About 70 kg of liquid slag with the composition 70% CaF ₂ and 15% Al ₂ O ₃ and CaO each were then poured into the gap between bloom 24 and mold 10, whereupon a current of initially about 2.5 to 3.0 kA was added began to flow, which then increased to 9.0 to 9.8 kA within about 10 minutes, which also achieved an output of about 690 kW. A first portion of liquid metal 34 was now poured into the gap between bloom 24 and mold 20, which had been melted in a vacuum induction furnace with 6 t capacity from cast sticks, which came from the same melt as the melting electrode of bloom 24 and thus the same had chemical composition. The amount of the first portion was 15 kg, causing the slag level 32 to rise by less than 7 mm. This triggered a first withdrawal step of 3.5 mm, which was followed by a further withdrawal step of 3.5 mm after a 30-second pause. One minute after the first portion, a further portion of 15 kg was poured in, a 3.5 mm pull-off step being triggered by the rising of the bath level, which was followed by another after 30 seconds. This process continued while maintaining electrical power until the top of the bloom 24 after 6 hours and 25 minutes. was reached. After a cooling period of 25 min. the doubled bloom block 24 was removed from the plant. This had a flawless, smooth surface with little slag buildup and now had a weight of 8810 kg with a diameter of 690 mm.

This doubled block 24 was then in a current-conducting mold 10 with a working diameter of 965 mm inserted and the process of Repeated doubling or armoring (cladding) again, the following parameters were set. It became 95 kg of liquid slag of the same composition used. At a voltage of 60 V, the current rose again pouring in the slag within 12 min. of 3.5 kA to 17.0 kA, with an output of 1020 kW. Portions of liquid metal were added every minute 34 of the same chemical composition of 24 each kg poured in, the lowering took place again every 30 seconds, the individual lifting steps were 3.9 mm. This The process was continued until - after 5 h 35 min. -the entire length was doubled. After a cooling period from again 25 min. the block was lifted out of the system. It had a diameter of just over 9050 mm and weighed 16,740 kg. The block surface was sufficient to process the block directly by forging to be able to.

The block was therefore forged in a bogie hearth furnace heated and then on a 4500 t - Forging press pre-forged to a diameter of 600 mm. The forging blank was of this size let it cool down and go crazy. During an ultrasound test after over-revving, no error messages were found become. However, the ultrasound test showed Indications of coarse grain, which did not affect the use, because further hot working was planned.

The rod then became approximately the same length in two Pieces divided so that the possibility arose from the To take a pane in the middle of the block for further examinations This disc was ground and subjected to a hot etch test subjected. It was shown that the Washer was free across the entire block cross-section of block segregations and in particular Freckel segregations. The bond of the doubled layers was on everyone Make flawless. The structure in the doubled Layers were significantly finer than that in the original Bloom. All in all, one disc showed an excellent one Structure and appeared for an application for rotating turbine parts of large dimensions are quite suitable.

Claims (14)

1. Method of producing castings with little segregation and, in particular, few freckles, from metal, in particular steels and Ni and Co base alloys by an electroslag melting or casting process using a short, electrically conductive, water-cooled die, in the wall of which electrically conductive elements which are not directly water-cooled are installed in such a manner that they are electrically insulated from the part of the die forming the casting, characterised in that a substantially segregation-free and freckle-free bloom having a surface cross section of no more than 90 % of the part of the die forming the casting is joined therein to the metal supplied using a slag bath situated in the region of the electrically conductive elements of the die and heated by the passage of current by the continued metered pouring-in of liquid metal and that the level of the slag in the die is kept approximately constant by means of relative movement between the die and the bloom until the bloom of predetermined length is surrounded by a cast-on covering layer, the casting-on of the covering layer, i.e. what is referred to as doubling of the bloom, being carried out at least once more in a die having a larger diameter until the predetermined final diameter of the casting is achieved.
2. Method according to claim 1, characterised in that the average casting or melting rate in kg/hr is set in such a manner that it is between 0.20 times and 5 times the sum of the equivalent bloom diameter and die diameter in mm, the equivalent diameter being determined by the quotient of the circumference/π.
3. Method according to claim 2, characterised by a casting or melting rate in kg/hr of between 0.80 times and 1.5 times the sum of the equivalent bloom diameter and die diameter.
4. Method according to one of claims 1 to 3, characterised in that the substantially freckle-free bloom with little segregation is produced by a remelting process with a consumable electrode or that the substantially freckle-free bloom with little segregation is produced by electroslag casting.
5. Method according to one of claims 1 to 4, characterised in that the bloom to be doubled is preheated to a temperature of no more than 800°C.
6. Method according to one of claims 1 to 5, characterised by circles with a concentric arrangement as the geometric shape of the cross sections of the bloom and the die(s) or characterised in that the geometric shapes of the bloom and the dies are polygonal, in particular rectangular or square.
7. Method according to one of claims 1 to 6, characterised in that the chemical composition of the bloom and the chemical composition of the one or more doubled layers correspond with one another.
8. Method according to one of claims 1 to 7, characterised in that the chemical compositions of the bloom and of the one or more doubled layers are different.
9. Method according to one of claims 1 to 8, characterised in that the relative movement between the casting and the die is produced by lifting the die in accordance with the removal of slag along the casting resting on a stationary bottom plate.
10. Method according to one of claims 1 to 8, characterised in that the casting resting on a lowerable bottom plate is removed from the fixedly installed die in such a manner that the level of the slag bath in the die remains approximately constant.
11. Method according to one of claims 1 to 10, characterised in that the pouring-in of the liquid metal or the fusion of the fixed metal parts added is carried out in an inert-gas atmosphere of controlled composition under controlled pressure, the controlled pressure possibly being set in the region of between 1 and 600 mbar or more than 2 bar.
12. Method according to claim 9 or claim 10, characterised in that the lifting of the die or the lowering of the bottom plate is carried out in individual steps followed by a pause, a smaller counter step preferably being carried out between the step and the pause.
13. Method according to claim 9 or claim 10, characterised in that, given constant relative movement between the casting and the die, the latter is moved in an oscillating manner.
14. Arrangement for producing castings with little segregation and, in particular, few freckles, from metal, in particular steels and Ni and Co base alloys by an electroslag melting or casting process, comprising a short, electrically conductive, water-cooled die (10), in the wall of which electrically conductive elements (16) which are not directly water-cooled are installed in such a manner that they are electrically insulated from the part (12) of the die (10) forming the casting, and comprising a bottom plate (20) associated with the die at the bottom, for carrying out the method according to at least one of the preceding claims, characterised in that a casting gap (25) for receiving liquid metal (34) is delimited by a bloom (24) placed on the bottom plate (20) and the die (10), at least two dies (10) being arranged one behind the other and the inner diameter (d) of the downstream die being larger than that of the preceding die.

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