US3682622A

US3682622A - Method of electroslag remelting of consumable electrodes

Info

Publication number: US3682622A
Application number: US64825A
Authority: US
Inventors: Boris Evgenievich Paton; Boris Izrailevich Medovar; Oleg Petrovich Bondarenko; Jury Vadimovich Latash; Vitaly Mikhailovich Baglai; Vladimir Grigorievich Shirshov
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-08-18
Filing date: 1970-08-18
Publication date: 1972-08-08
Anticipated expiration: 1989-08-08

Abstract

A METHOD OF ELECTROSLAG REMELTING OF CONSUMABLE METAL ELECTRODES IN A COOLED MOLD, WHEREIN SHORT IMPULSES OF CURRENT ARE IMPRESSED UPON THE WORKING CURRENT OF AN ELECTRODE, THE AMPLITUDE AND DURATION OF SAID PULSES BEING SUFFICIENT TO EFFECT POSITIVE BREAKING OFF OF DROPS OF THE ELECTRODE METAL DURING THE FORMATION OF THE DROP PRIOR TO THE TIME AT WHICH IT WOULD NATURALLY DROP.

Description

METHOD OF ELECTROSLAG REMELTING 0F CONSUMABLE ELECTRODES Original Filed June 8, 1967 I I I I I I I I I I I I I I 1972 a. E. PATON E L 2 Sheets-Sheet 1 I l I 4 y fi- J FIG] Aug. 8, 1972 a. a. PATON EI'AL 3,682,622

METHOD OF ELECTROSLAG REMELTING 0F CONSUHABLE ELECTRODES Original Filed June 8. 1967 2 Sheets-Sheet a 4 9/ 2 F163 )7 l l United States Patent US. CI. 75-10 18 Claims ABSTRACT OF THE DISCLOSURE A method of electroslag remelting of consumable metal electrodes in a cooled mold, wherein short impulses of current are impressed upon the working current of an electrode, the amplitude and duration of said pulses being sufiicient to etfect positive breaking off of drops of the electrode metal during the formation of the drop prior to the time at which it would naturally drop.

This is a continuation of application Ser. No. 644,719, filed June 8, 1967, now abandoned.

The present invention relates to methods of electroslag remelting of consumable electrodes and to installations for effecting said methods.

The invention is intended to be used in electrometallurgy, in particular when making high-quality steels and alloys by the method of electroslag remelting.

Known in the prior art are methods of electroslag remelting of electrodes in a cooled mould with the use of a direct and alternating current, and installations for eflecting said methods, in which the metal of the electrode, when passing dropwise through the layer of molten slag, is cleaned from non-metallic inclusions, the latter being absorbed by the slag. Besides, the metal drops, when forming and dropping, are subjected to a partial cleaning from sulphur, phosphorus and gases (see the book Electroslag Remelting, by B. I. Medovar, Yu. V. Iatash, Kiev, 1963, pp. 48 and 49.

It has been established that smaller drops of metal when passing through the layer of molten slag are likely to be cleaned quicker and more completely from non-metallic inclusions and other impurities than it is possible to achieve with larger drops.

In the existing methods of electroslag remelting and in the installations .for effecting same, the size of drops is determined by natural conditions of the melting process, the size of drops being sufiiciently large, with the diameter ranging from to 12 mm. For this reason, the exchange reactions occurring between the metal of the drop and the slag are insufficiently vigorous, and the possibilities of the process of electroslag remelting are thus not completely used to advantage.

An object of the present invention is to eliminate the above-said disadvantage.

The principal object of the present invention is to develop such a method of electroslag remelting of consumable electrodes and to provide such an installation for efiecting same, which would provide for a more complete cleaning of the metal being remelted from undesirable impurities owing to a considerable decrease in the size of drops of the electrode metal by way of positive breaking off of the drops prior to the moment of their being naturally broken ofi.

This object is accomplished by superimposing short pulses of current on the working current of the electrode,

the amplitude and duration of said pulses being fairly sufficient for the positive breaking off of the drops of electrode metal during the formation of the drop up to the moment of its being naturally broken off, which is taking place in the process of the electroslag remelting of consumable electrodes in a cooled mould.

In conformity with the present invention, the installation for eliecting the method proposed herein is provided with a pickup determining the moment of formation of the electrode metal drops, the input of said pickup being connected to a current measuring instrument, while its output is connected to a time relay actuating a control unit of the device setting up current pulses in the circuit of the consumable electrode.

In one of the embodiments of the installation for effecting the herein-proposed method, an ignitron contactor may be employed as the device for setting up current pulses in the circuit of the consumable electrode, said ignitron contactor being connected in series with a suitable source of current adapted to feed the circuit of the eonsumable electrode.

The nature of the present invention will become more fully apparent from a consideration of the following description of a preferred embodiment of the method of electroslag remelting of consumable electrodes, taken in conjunction with the accompanying drawings illustrating one possible embodiment of the installation for efiecting the proposed method, in which:

FIG. 1 represents a consumable electrode at the moment of a molten metal drop naturally breaking off;

FIG. 2 shows the electrode at the moment when the drop of molten metal is positively broken off;

FIG. 3 represents an oscillogram of alternating current of the furnace with the natural breaking olf of metal drops from the end of the consumable electrode;

FIG. 4 shows the oscillogram with the positive breaking oil of metal drops from the end of the consumable electrode;

FIG. 5 is a schematic diagram of the installation for the electroslag remelting effected according to the method of the invention; and

FIG. 6 represents an oscillogram of current, passing through the ignitron contactor of the proposed installation. The essence of the proposed method is as follows. In the process of the electroslag remelting of consumable electrodes, the metal to be remelted is cleaned from undesirable impurities by making it interact with the molten slag. Among other factors, the degree of cleaning of the metal being remelted is determined by the magnitude of the surface of contact between the liquid metal and slag. A decrease in the size of drops signifies an increase in this surface, and is likely to speed up the processes of interaction of metal of the drop with the slag.

The liquid metal is retained at the electrode end by the force of surface tension F (FIG. 1) and hydrostatic lifting force F The force of gravity P and electrodynamic force F tend to separate the metal to be melted in the form of drops from the electrode end (FIG. 1). In the process of the electroslag remelting up to the moment of the natural breaking off of the metal drop, there is observed an inequality.

At the moment when the drop is naturally broken off, the forces tending to separate the drop from the end of the electrode (P-l-F exceed the forces retaining the drop at the end of the electrode Under the natural conditions of the melting process, this inequality is observed when the size of the drop is rather considerable (up to 10 to 12 mm. in diameter), mainly on account of an increase in the weight, and hence in the size of the drop.

The component of the electrodynamic force F compressing the electrode and tending to separate the molten metal therefrom, is directed downwards and proportional to the square of the current. When remelting with the use of an alternating current, F varies with time according to the following law:

where c is the coefficient of proportionality,

'r is the amplitude of alternating current,

w is the angular frequency of the current and t is the current moment of time.

From Equation 3 for the force F it is evident that when remelting with the use of an alternating current of the angular frequency w, the force F is likely to preserve constant the direction of action, sin-cc cos 2 wt=l, and varies from zero to F =C-r In order to decrease the size of drops of the electrode metal, i.e. to break off positively drops of small size, it is necessary, as it becomes evident from Equation 2, to increase for a short time the electrodynamic force F up to the value F (FIG. 2) which must be sufficient for effecting positive breaking ofl of the drop of electrode metal during the formation of the drop up to the moment of its natural breaking 05.

An increase in the force F is achieved due to the superimposing of short pulses of current upon the working current of the furnace, the amplitude and duration of said current pulses being suflicient to effect the positive breaking off of small drops which is illustrated in FIGS. 3 and 4.

FIG. 3 shows an oscillogram of the alternating working current with the natural breaking off of drops of molten metal from the end of a consumable electrode. The current of frequency A equal to 50 cycles per second, when passing through the electrode and melting down the slag, is modulated by the low frequency B. This is caused by the fact that the drop growing at the end of the electrode shunts the interelectrode gap, whose resistance determines the current of the furnace. Hence, on the oscillogram (FIG. 3) the minimum value of the amplitude of the working current T1 in each period T corresponds to the absence of the drop at the end of the electrode, while the maximum value of the amplitude of the working current 1-; corresponds to the moment immediately preceding the natural breaking off of the drop.

Thus, at each moment of time of each period the increment of the amplitude of the working current A-r is determined from the equation:

Where 1- is the value of the working current amplitude at a certain moment of time of the period under consideration;

r; is the minimum value of current amplitude in the period under consideration, characterizing the size of the metal drop growing at the electrode end.

When Ar=A =(1- n), there occurs natural breaking off of the drop from the end of the consumable electrode.

When remelting with the use of a direct current, the working current of the electrode is also likely to vary periodically as a result of shunting of the interelectrode gap by the drops of molten metal, and these variations of the working current during each period also characterize the size of the metal drop growing at the electrode end.

To effect positive breaking off of small drops of the electrode metal, pulses C of current (FIG. 4) are to be superimposed upon the working current, the current having the amplitude 'r 1' at the moments when (FIG. 4). An abrupt increase in the electrodynamic force F acting upon the molten metal at the electrode end, results in the positive breaking off of drops of molten metal. The magnitude of the pulse amplitude and duration T are to be selected for each particular case, on account of the complicated calculations required, so that said values be sufficient to effect positive breaking off of metal drops.

When comparing oscillograms represented in FIGS. 3 and 4, it becomes evident that the frequency of breaking off of metal drops when operating according to the proposed method is likely to increase, though the speed of the melting process remains constant, on account of which the surface of contact between the molten metal and liquid slag increases, which contributes to a more complete elimination of undesirable impurities from the metal being remelted.

One of the possible embodiments of the installation for effecting the proposed method of electroslag remelting of consumable electrodes is comprised of a furance 2 (FIG. 5) for electroslag remelting with the use of an electrode 1 immersed into molten slag 3, a power transformer 4, an ignitron contactor 5, a unit 6 for controlling and igniting the ignitron of the ignitron contactor, a pickup 7 indicating the moment of formation of drops of the electrode metal, a current transformer 8, and a contactless time relay 9.

The installation represented in the drawing operates in the following manner: The ignitron contactor 5 is switched on and is operated by the unit 6 for controlling and igniting the ignitrons of the ignitron contactor in a continuous mode the ignition angle a. (FIG. 6) equalling, for example, 70 to and ensuring the working current of the furnace having the amplitude D. A signal from the current transformer 8 is supplied to the pickup 7 indicating the moment of formation of the electrode metal drops.

At the moment 0 of beginning of the formation of the molten metal drop at the end of the electrode 1, as determined by the pickup 7 according to the value of the increment of the working current amplitude A=(r1- (see Equation 4), a pulse C is produced at the output of the pickup 7, which is fed to the contactless time relay 9, synchronized with the network. The time relay 9 produces a rectangular pulse of a duration equal to one or several periods of the supply network current, said pulse being fed to the unit 6 adapted for controlling and igniting the ignitrons of the ignitron contactor 5.

During the supply of the pulse C there occurs an abrupt variation of the ignition angle a igniting the ignitrons from the working (a=70 to 90) to the full-phase current (a=0), on account of which an abrupt increase in the current up to the intensity 1- with the amplitude E takes place in the secondary circuit of the power transformer 4 supplying the furnace. The duration of the current pulse C is equal to the duration of the pulse fed to the control unit 6 from the time relay 9.

This pulse causes an increase in the electrodynamic force F which breaks off from the end of the electrode 1 a portion of molten metal in the form of a small drop.

There are also possible other embodiments of the installations for effecting the proposed method of electroslag remelting of consumable electrodes with the use of either alternating or direct current.

When carrying out the electroslag remelting according to the proposed method on the installation of the invention, there is achieved a considerable increase in the efficiency of treating metal with slag, owing to which the remelted metal possesses a lower percentage of sulphur, phosphorus, gases, non-metallic inclusions and other harmful impurities than it is possible to obtain by means of other known methods of electroslag remelting on the existing installations.

What is claimed and desired to be secured by Letters Patent is:

1. A method of electroslag remelting of consumable metal electrodes, said method comprising immersing a consumable electrode into molten slag, supplying a working current to said electrode to cause the formation of drops of the material of the electrode at the lower end thereof, and intermittently impressing on said working current impulses of increased amplitude and of short duration relative to the intervals between said impulses, said impulses being of an amplitude and of a duration to produce removal of the drops at a time earlier than that at which they would naturally fall from the electrode and of a size smaller than that at which they would naturally fall from the electrode.

2. A method of electroslag remelting comprising forming a bath of molten slag, immersing a consumable electrode in said bath of molten slag, passing electric working current between said electrode and said bath of molten slag to cause the immersed end of said electrode to melt and to form drops of liquid metal on said electrode, impressing on said working current impulses of increased amplitude and of short duration relative to the intervals between said impulses, the amplitude and duration of said impulses being sulficient to effect positive breaking off of said drops of liquid metal from said electrode during formation of each drop prior to the time such drop would naturally fall from said electrode.

3. A method of electroslag remelting as recited in claim 2, wherein said working current is alternating current.

4. A method of electroslag remelting as recited in claim 3, wherein said impulses are alternating current impulses.

5. A method of electroslag remelting as recited in claim 4, further comprising connecting in the path of said working current a circuit element which is controllable to permit conduction only during a predetermined portion of each cycle of said working current and wherein said impulses are impressed on said working current by increasing the predetermined portion of each cycle that conduction is permitted by said circuit element.

6. A method of electroslag remelting as recited in claim 2, further comprising detecting when the working current increases to a predetermined value, and impressing said impulses on said working current upon detection that said working current has increased to said predetermined value.

7. A method of electroslag remelting comprising: forming a bath of molten slag; immersing at least an end portion of a consumable electrode in the bath of molten slag; supplying workable electrical power to the electrode so that it passes from the immersed portion of the electrode through molten slag to cause the end portion of the electrode to melt and to form drops of liquid material from the electrode on the immersed end portions; impressing on said working electrical power, increments of electrical power additive to that of said working electrical power, the electrical power of said increments being sufiicient to elfect positive separation of said drops of liquid material from the immersed part of the electrode prior to the time which is normally required for a liquid material drop to form and to normally fall from the electrode as a result of normal working electrical power.

8. A method of electroslag remelting as set forth in claim 7, wherein the working electrical power is alternating current.

9. A method of electroslag remelting as set forth in claim 8, wherein the increments of added electrical power are alternating current impulses.

10. A method of electroslag remelting as set forth in claim 9, further comprising: connecting in the path of the working electrical power a circuit element which is controllable to permit working alternating current conduction only during a predetermined conduction angle of the working current and wherein the impulses are impressed on the working current by increasing the conduction angle permitted by the circuit element.

11. A method of electroslag remelting as set forth in claim 7, further comprising detecting when the working electrical power increases to a predetermined value, and impressing the increments upon detection that the working electrical power has increased to the predetermined value.

12. A method of electroslag remelting as set forth in claim 8, further comprising making the impulses of additive electrical power of increased amplitude over that of the working electrical power and of short duration relative to the time period of the impulses.

13. A method of electroslag remelting comprising: forming a bath of molten slag; immersing at least an end portion of a consumable electrode in the bath of molten slag; supplying workable electrical power to the electrode so that it passes from the immersed portion of the electrode through molten slag to cause the end portion of the electrode to melt and to form drops of liquid material from the electrode on the immersed end portion; impressing on said working electrical power, increments of electrical power additive to that of said working electrical power, the electrical power of said increments being sufficient to effect positive separation of said drops of liquid material of less than 10 mm. diameter from the immersed part of the electrode.

14. A method of electroslag remelting as set forth in claim 13, wherein the working electrical power is alternating current.

15. A method of electroslag remelting as set forth in claim 14, wherein the increments of added electrical power are alternating current impulses.

16. A method of electroslag remelting as set forth in claim 15, further comprising: connecting in the path of the working electrical power a circuit element which is controllable to permit working alternating current conduction only during a predetermined conduction angle of the working current and wherein the impulses are impressed on the working current by increasing the conduction angle permitted by the circuit element.

17. A method of electroslag remelting as set forth in claim 13, further comprising detecting when the working electrical power increases to a predetermined value, and impressing the increments upon detection that the working electrical power has increased to the predetermined value.

18. A method of electroslag remelting as set forth in claim 14, further comprising making the impulses of additive electrical power of increased amplitude over that of the working electrical power and of short duration relative to the time period of the impulses.

References Cited UNITED STATES PATENTS 1,967,872 7/1934 Doan -123 R X 2,504,868 4/1950 Muller 219l37 X 3,047,708 7/1962 Stark 219-146 X L. DEWAYNE RUTLEDGE, Primary Examiner I E. LEGRU, Assistant Examiner U.S. Cl. X.R. 219-73