What is claimed is
1. A process for producing refined metallic ingots of improved metallurgical properties by electroslag remelting a consumable metallic electrode, said process being also characterized by an improved fusing rate, comprising the steps of
2. The process for electro-slag-remelting as set forth in claim 1, wherein said converted current flows in one direction during a period T1, and in an other direction during a period T2, said electrode forming respectively during said periods the positive and the negative pole in an electric circuit; and selectively adjusting said basic frequency (f).
3. The process for electro-slag-remelting as set forth in claim 1, in which the basic frequency (f) is in the range of 5-10 cycles and is substantially below the frequency of said power supply current.
4. The process for electro-slag-remelting as set forth in claim 2, wherein the relationship of the periods T1 and T2 is different than 1.
5. An apparatus for electro-slag-remelting of metallic electrodes by means of a three - phase power supply current, comprising in combination,
6. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 5, wherein said convertor means comprising three selenium rectifiers each of which is respectively connected to a different phase of said three-phase power supply current, on the one hand, and to the neutral point of said three-phase power supply current, on the other hand, thereby converting said three-phase current into a weakly pulsating direct current.
7. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 5, wherein said convertor means comprise thyristor means which are operatively connected to said selenium rectifiers and which periodically change the direction of said direct current.
8. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 7, including auxiliary electrode means mounted in said apparatus, said electrode and said auxiliary electrode means extending into said mold,
9. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 5, including electronic control means for adjusting the operation thereof.
10. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 9, wherein said electronic control means comprise a set of thyristors.
11. A process for producing refined metallic ingots of improved metallurgical properties by electroslag remelting, said process being also characterized by an improved fusing rate, comprising the steps of
12. The apparatus for electro-slag-remelting of metallic electrodes as set forth in claim 5, that a three or six phase thyristor group is connected to said three phase transformer, which convert the three phase current into a uni-phase current, the direction of which changes periodically, said consumable electrode being connected to said thyristor group.
BACKGROUND OF THE INVENTION
The known installations for electro-slag-remelting metals by fusing down an electrode can be powered by direct or alternating current having the normal power supply frequency of 50 cycles respectively 60 cycles.
When direct current is used, however, the fusing rate(in kilograms of metal per hour)and the energy consumption (in kilowatt-hours per ton) of the electrode depend, all other conditions remaining constant, on the one hand, and the metallurgical properties of the metal ingot produced by the remelting process, on the other hand, substantially on the direction of the current flowing through the electrode. Thus, for example, if the to be remelted electrode represents the positive pole and the formed metal ingot represents the negative pole the fusing rate is high, whereas the metallurgical properties of the metal ingot are poor (for example the metal ingot has a high oxygen content). When, in contradistinction thereto, the to be remelted eletrode represents the negative pole and the metal ingot represents the positive pole, the metallurgical properties of the metal ingot are substantially better (for example the metal ingot has a low oxygen content) but the fusing rate is substantially lower than in an arrangement of opposite polarity as is described above.
When alternating current of normal power supply frequency of 50 cycles, respectively 60 cycles, is used, the fusing rate is almost as high as in the first aforedescribed example but the metallurgical properties of the formed metal ingots are similar to those of the second aforedescribed example. The use of alternating current has, however, the drawback of substantial inductive resistance occurring in the feed lines. Since the remelting of metallic electrodes requires a very large electric power input, an unduly high unbalance in the power supply frequently occurs. In order to avoid these drawbacks there has already been proposed an arrangement in which a three-phase electric power supply is used for electro-slag-remelting of electrodes in which the base plate, on which the formed metal ingot rests, is connected to the neutral point of the three phase system and three electrodes are each connected to one phase of the three-phase system. Such an arrangement is, structurally speaking, very complex because three separate electrodes must be simultaneously remelted in one mold. Such an arrangement is not only expensive to construct but involves also substantial production costs for the electrodes themselves. The alternating phase-displaced currents, which flow through the three electrodes, produce a rotating magnetic field which causes the electrically conductive slag layer, which is superposed on the molten metal, to rotate. This rotational movement causes the slag layer to form a rotating paraboloid which increases the surface through which a heat exchange between the slag layer and the cooled walls of the mold may take place. In addition thereto, the specific heat transfer characteristics between the slag and the cooled walls fo the mold are also increased. Consequently, the alternating current three-phase power supply produces substantially higher heat losses in the electro-slag-remelting arrangement than the installations operating with ordinary alternating current or direct current.
SUMMARY OF THE INVENTION
It is a principal object of this invention to provide an electro-slag remelting process and apparatus which substantially eliminates the afore-described disadvantages and drawbacks.
It is a more specific object of this invention to provide an apparatus for an electro-slag-remelting process which renders particularly advantageous results insofar as the fusing rate and the metallurgical properties of the produced metal ingot are concerned.
The apparatus of this invention uses as a power supply a three-phase alternating or ordinary two-phase alternating current. This power supply current is converted into a current the direction of which periodically changes and which has a basic frequency (f) that is substantially different from the frequency of the three-phase or two-phase alternating power supply current. The converted current flows through the electrode which is to be remelted. It has been found to be advantageous if in the apparatus of this invention the relationship of times (T1, T2) during which the to be remelted electrode alternately forms the positive, respectively the negative pole as well as the basic frequency (f) of the current flowing through the to be remelted electrode can be adjusted. It has been furthermore found to be advantageous that the basic frequency (f) of the current which flows through the to be remelted electrode is substantially lower than the frequency of the power supply current and amounts to, for example, 5 - 10 cycles.
It has also been found advantageous in order to attain optimum results if the relationship of times (T1, T2) during which the to be remelted electrode alternately forms the positive, respectively negative pole is different from 1.
An apparatus in accordance with this invention for carrying out the process of the invention is fed by a three - phase power supply and has rectifiers, such as, for example, dry selenium rectifiers or adjustable rectifiers such as thyristors, and a convertor which is preferably a thyristor. Each phase of the three-phase power supply system is provided with a rectifier; one of the input terminals of each rectifier is connected to the neutral point of the three-phase power supply system and the other terminal to the convertor.
The inductive resistances can advantageously be reduced in the apparatus of this invention by positioning the switch of the converter which is connected to the to be remelted electrode as closely as possible to the electrode and position the switching elements of the convertor which are connected to the base plate supporting the mold as closely as possible to said base plate.
A particularly simple and efficient embodiment of this invention comprises a main power supply line and a line leading from the neutral point of, for example, a three-phase or six-phase power supply which have a set of thyristors connected thereto for converting the power supply current into a uni-phase current the direction of which periodically changes. The to be remelted electrode is connected to the uni-phase current.
In a preferred refinement of the aforedescribed embodiment adjustable electronic or mechanical controlling means cooperate with the set of thyristors for adjusting the uni-phase input current for the electrode.
BRIEF DESCRIPTION OF THE DRAWING
The invention is illustrated by way of example in the accompanying drawing which forms part of this application and in which
FIG. 1 is a schematic illustration of a first of many embodiments for carrying out the process of this invention;
FIG. 2 is a detailed circuit diagram of the convertor illustrated in FIG. 1;
FIG. 3 is a graph showing how the voltage U in the to be remelted electrode varies as a function of the time t; and
FIG. 4 is a second one of many embodiments for carrying out the process of this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Before referring in detail to the figures it should be noted that the switches S1, S2, S3 and S4 of the convertor 5 may not only be one of the many well-known mechanical types but may also be electronic type switches such as electronic switches which operate with changing control voltages (blocking voltages) as are, for example, present in the operation of a thyristor.
Referring now specifically to the embodiment illustrated in FIGS. 1 and 2 of the drawing, a normal three-phase power supply (having the usual frequency of 50 cycles) feeds current through the terminals R, S and T and transforms the current by means of a three-phase current transformer 1 into a current of considerably lower voltage. The current is then converted into a direct current having minor pulsations by means of the plurality of rectifiers 2. The current is then conducted by means of the lines 3 and 4 to the converter 5. The operation of the convertor 5 is controlled by means of mechanically or electronically operated means 6, such as, for example, a thyristor described in greater detail hereinafter. The convertor 5 causes the current at the output thereof to periodically change direction. The basic frequency (f) as well as the time periods T1 , T2 during which the current flows in a positive respectively negative direction can be adjusted by means of the control means 6. The positive direction of the current is indicated by the arrow Z in FIG.1. The basic frequency (f) can be determined by the following formula
f = 1: (T1 + T2)
One of the output leads 7 of the convertor 5 is connected to the to be remelted electrode 9 and the output leads 8 is connected to the base plate 10 which preferably is made of copper. If steel is to be remelted with the apparatus then the electrode 9 consists of steel which consumes itself by being molten down. The lower end of the electrode 9 axially extends into a water-cooled mold 11 wherein there is present the liquid layer of slag 12 which forms above the liquid layer of molten steel 13. A solidified ingot of steel 10 eventually forms due to the cooling effect produced by the walls of the mold 11 and comes to rest on the base plate 10. After the electrode 9 has been fuesd with the exception of a small remainder, the latter is pulled upwardly out of the slag layer 12 and is replaced with a new electrode 9. An auxiliary electrode 15 is provided which prevents the slag layer 12 from solidifying while the exchange of electrodes 9 is effected by passing the current through the auxiliary electrode 15. The auxiliary electrode comprises a plurality of rods 15' which are mounted in a support ring 16. The lower ends of the rods 15' also extend into the liquid slag layer 12. A line 18 leads from the positive pole of the rectified direct current to one contact of a switch 17, whereas a line 19 leads from the negative pole of the rectified direct current to the other contact of the switch 17. The auxiliary electrode 15 can thus be connected either to the posivite or negative pole of the rectified direct current by selectively adjusting the switch 17. Therefore, if, for example, the auxiliary pole 15 is connected with the positive pole of the rectified direct current (see FIGS. 1 and 2), the latter will have positive pulsating voltages with respect to the base plate 10 and the electrode 9. Consequently, the base plate 10 as well as the electrode 9 only have negative potentials with respect to the auxiliary electrode 15. The significance of this is that the auxiliary electrode 15 represents the anode of the electrolyte bath which is constituted by the liquid electric slag in which there are present negatively charged ions, such as, for example, oxygen ions, which separate out. On the other hand, if the switch 17 assumes the position indicated by the broken line in FIG.1 (and thus the auxiliary electrode 15 is connected to the negative pole of the rectified direct current) the auxiliary electrode 15 represents the cathode and there are present in the electrolytic bath (that is the liquid slag layer 12) positively charged ions, such as, for example, metal ions which are separated out. It is therefore possible to decisively influence the metallurgical reaction by means of the auxiliary electrode 15. Thus, the undesirable impurities, such as the so-called "steel-impurities" can be separated out at the auxiliary electrode 15 and the transfer of certain substances present in the metal ingot into the slag layer can be avoided.
Referring now specifically to the embodiment illustrated in FIG.4, there is shown a normal three-phase power supply of normal frequency, for example, 50 cycles, which has the usual main power lines R, S, T. A three-phase current transformer 21 transforms the power supply current into a current having a substantially lower voltage and the neutral point lead 23. Either three or six leads 22 (depending on the construction of the device) conduct the current from the secondary of the transformer 21 to the set of thyristors 24. The three-phase, respectively six-phase,current is converted by the set of thyristors into a uni-phase current, the direction of which periodically changes. An electronic or mechanical control means 25 are connected to the set of thyristors 24 and are adapted to adjust the relationship of the times T1, T2 during which the current flows in a positive, respectively negative direction, as well as to adjust the basic frequency (f). A line 26 leads from the set of thyristors 24 to the self-consuming electrode 28, and a second line 28 leads from the set of thyristors to the base plate 29 which is preferably made of copper. The electrode 28 which consists of to be molten down steel is concentrically mounted in a water-cooled mold 30 and extends with its lower end into the liquid slag layer 31 which forms on top of the molten metal layer 32. The solidified metal 33 comes to rest on the base plate 29. The voltage U, which the electrode 28 has relative to the solidified metal 33, varies in accordance with the time period t as indicated in the graph of FIG. 3.
In contradistinction to the known electro-slag-remelting operations, the process of this invention achieves the optimum operating conditions for all types of electro-slag-remelting processes insofar as the fusing rate and the metallurgical properties of the formed metal ingot are concerned. The construction costs for the apparatus of the invention are relatively low. A further advantage resides in the fact that the apparatus of the invention may be powered with three-phase current of ordinary frequency; all three phases of the power-supply current are completely symmetrically loaded. Furthermore, the significantly higher output factor cos φ is attained with the apparatus of the invention which is a significant advantage when compared to the apparatuses operating with ordinary alternating current or three-phase current. This output factor is arrived at by virtue of the fact that the self-consuming electrode is connected to a uni-phase current, in which the inductive voltage drop is relatively very small when basic frequencies are used in the circuit, for example, 5 - 10 cycles, which are significantly smaller than the normal circuit frequencies.