GB2036617A

GB2036617A - Electroslag remelting process

Info

Publication number: GB2036617A
Application number: GB7934678A
Authority: GB
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1978-10-06
Filing date: 1979-10-05
Publication date: 1980-07-02
Also published as: GB2036617B

Abstract

A process for producing metal ingots by the electroslag remelting process in which a metal tube 4 containing a mixture of additive(s) (eg alloying ingredients) and binder material is attached to a lengthwise extending face of an electrode 3 to be remelted. The additive may be Fe-Mo, Ni, Fe-C powder, Fe-Va, stainless steel, or calcium silicide, or may be a deoxidant such as Ca. The binder may be a slag- former, eg glass. <IMAGE>

Description

SPECIFICATION Electroslag remelting process This invention relates to the remelting of metals by the electroslag remelting process.

In the electroslag remelting process a slag within a mould is maintained in a molten state and at a temperature above the melting point of metal to be refined. Unrefined metal is introduced into the mould and is refined as it passes in droplet form through a bath of the molten slag, refined droplets collecting to form a pool beneath the slag bath. The mould walls are cooled by the circulation of a coolant, normally water, and a solidified ingot is built up beneath the molten metal pool. The metal to be refined is conveniently introduced into the mould in the form of a consumable electrode and the slag maintained molten by passage of an electric current from the electrode to the base plate on which the mould stands.

During the course of the process, it is known to make alloy additions to the metal pool in order to produce an ingot having a composition different from that of the alloy or metal constituting the consumable electrode. Similarly, it is known to introduce reactants, such as deoxidants, to the metal pool to promote chemical reactions to, for example, minimise the oxygen content of the refined metal.

Thus, it has been proposed to feed manually or mechanically such additives onto the surface of the molten slag in sufficient quantity for the intended alloy enrichment of the refined metal or to produce the required chemical reaction. These known methods of making alloy additions suffer from the disadvantage that neither provides the necessary matching of additive and electrode consumption rates thereby leading to a non-uniform distribution of alloy within the refined ingot. Additionally, as many alloying ingredients have a higher melting point than that of the electrode metal and the operating temperature of the slag bath, these are unable to melt completely during their passage through the slag bath. Again, the result is a nonhomogeneous distribution of the alloying elements in the metal bath and, consequently, in the ingot produced.Furthermore, there is a limit to the amount of additive which can be charged in this manner since its addition extracts heat from the slag bath.

An alternative approach has been to encapsulate the required alloy constituents within a tubular electrode of the metal to be refined. This electrode may comprise the only electrode used in the process or may be secondary to and separate from the main melting electrode. Both approaches suffer from the disadvantage of high electrode preparation costs.

Additionally, it is difficult to ensure that a separate auxillary tubular electrode would melt back at a rate commensurate with the principal electrode thereby not achieving the required matching of alloy addition and electrode consumption rates.

According to the present invention in one aspect, a process for producing metal ingots by the electroslag, remelting process which comprises attaching to a lengthwise extending face of an electrode of the metal to be refined, a metal tube containing a mixture of additive(s) and binder material, and immersing the lower end of the electrode with tube attached into a quantity of molten, electricallyconductive slag present in a mould whereby, during the course of the process, the lower ends of the electrode and the attached tube together prog ressively melt to produce below the molten slag a pool of refined metal and additive which solidifies to form a metal ingot.

According to the invention in another aspect, there is provided an electroslag remelting process which uses, in combination, a consumable metallic electrode to one lengthwise extending face of which is secured a metal tube containing a compacted mixture of additive(s) and a binder. The additive is preferably in granular, particulate (e.g. wire chippings), or powder form.

The term "additive" as used herein is intended to include alloying components (e.g. metallic elements and their oxides) used to enrich or change the composition of the ingot and reactants, for purposes such as deoxidisation and scavenging, which react chemically with impurity contained in the metal pool. The binder preferably comprises a slag making material having a composition suitable for use in the electroslag refining process. Alternatively, it may comprise a refractory material such as glass.

One tube may extend along the entire length of the electrode length to be melted. Alternatively, several tubes may be provided, their total length being not less than the length of electrode to be melted. In a further alternative arrangement, the tube(s) extends over only a portion of the electrode length to be melted to produce an ingot which varies in composition over its length.

According to the present invention in a still further aspect, there is provided in a method of electroslag remelting the improvement which comprises encapsulating within a metal tube a compacted mixture of additive(s) and binder, and securing the tube to one lengthwise extending face of a metallic electrode to be consumed in the process.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 shows a side elevation partly in section of apparatus in accordance with the invention; and Figure 2 is a section taken along line ll-ll of Figure 1.

The apparatus illustrated in the drawings includes an open ended elongate mould 1 containing a quantity of molten, electrically-conductive refining slag 2; three steel electrodes 3 of rectangular section are each supported from a conventional electrode carrier (not shown) so that their lower ends are immersed in the molten slag. Tack welded to one lengthwise extending side face of each electrode 3 are three rectangular section mild steel tubes 4, each filled with a homogeneous mix of powdered alloy constituents in quantity sufficient to enrich the metal of the electrode to produce an ingot having the required alloying content, and slag making powder of composition identical to or compatible with the molten slag 2. The powdered mix is compacted within the metal tube by, for example, applying vibrations to the tubes.A series of vent holes 5 is formed in one side of each tube 4, each hole being plugged with ceramic consumable fibres to prevent loss of powder from the tube. In addition, the lower open end of each tube is initially closed by a steel plug (not shown).

The refined ingot is indicated by reference numeral 6 and is shown supported on a steel base plate 7 which initially closes the lower open end of the mould 1. A conventional lifting device (not shown) is provided to raise progressively the mould during operation of the process to enable an ingot of greater length than the height of the mould to be produced.

In one example of a process carried out in accordance with the invention, the mild steel tubes 4 were each filled with a mix of vibratory compacted stainless steel powder of AISI 430 composition and slag making powder. The bottom open end of each tube was closed with a steel plug and the mild steel tubes then tack welded to one side face of each of three carbon steel electrodes in the manner illustrated in the drawings.

The electrodes with tubes attached were then supported with their lower ends immersed in the molten slag in the manner illustrated and electrical current passed through them in the conventional manner to maintain the molten slag at a temperature at or above the melting points of the electrodes, the tubes and their contents. As the electrodes and tubes melted a pool 8 of liquid metal enriched with chromium formed below the slag bath, this pool progressively solidifying to produce the refined ingot. The vent holes 5 formed in the tubes 4 enabled evolved expanding gases to pass from the tubes to the atmosphere.Heat radiated and conducted from the molten slag to the powdered contents of the tubes caused the powdered slag within the parts of the tubes immediately above the molten slag to frit to provide a fused bond to prevent irregular discharge of the powdered contents of the tubes to the slag. The consumable steel plugs retained the powder initially within the tubes. Throughout the melt, uniform melting of the tubes, their contents and the electrodes occurred.

It is considered that the molten alloy leaving the tubes 3 mixes with the liquid metal coating on the electrode tip to produce droplets which comprises a homogeneous mixture of the metal and alloy. Further mixing probably occurs within the metal pool 8.

The quantity of alloy powder present in the tubes is, obviously, dependent upon the required enrichment of the metal to be refined. The powdered slag making material is used merely to fill the remaining space of each tube and to bond the contents to prevent irregular discharge from the tubes.

While the invention has been described with reference to the addition of chromium alloy it is to be understood that other alloying constituents (e.g.

Ferro Molybdenum, nickel, calcium silicide, an iron carbon powder and Ferro Vanadium) may be em pioyed. In addition, the additive may consist of a reactant such as a deoxidant or scavenging agent such as calcium or calcium silicide. Furthermore, the binding material mixed with the additive may comprise a material other than a slag making material.

Thus a refractory material such as glass may be employed.

In an alternative unillustrated embodiment, discreet lengths of tube are tack welded to one or more side faces of an electrode. These discreet lengths may extend along the full length of the electrode or may extend over only partially this length. In the latter case the invention may be employed to produce an ingot which has a differing chemical composition over its length. Additionally, such tubes may be welded to one side face of electrodes of cross-section other than rectangular, e.g. square, oval or round.

Claims

1. A process for producing metal ingots by the electroslag remelting process which comprises attaching to a length-wise extending face of an electrode of the metal to be refined, a metal tube containing a mixture of additive(s) and binder material, and immersing the lower end of the electrode with tube attached into a quantity of molten, electrically-conductive slag present in a mould whereby, during the course of the process, the lower ends of the electrode and the attached tube together progressively meltto produce below the molten slag a pool of refined metal and additive which solidifies to form a metal ingot.

2. A process as claimed in claim 1 wherein the additive comprises an alloying component in granular, particulate or powder form.

3. A process as claimed in claim 1 wherein the additive comprises a reactant in granular, particulate or powder form which reacts chemically with impurities contained in the pool of refined metal.

4. A process as claimed in any one of claims 1 to 3 wherein the binder comprises a slag making material.

5. A process as claimed in any one of claims 1 to 3, wherein the binder comprises a refractory material.

6. A process as claimed in any one of the preceding claims wherein the metal tube is so attached to the electrode that it extends along the entire length of the electrode length to be melted.

7. A process as claimed in any one of claims 1 to 6 wherein the metal tube is attached to the electrode such that it extends over only a portion of the electrode length to be melted.

8. A process as claimed in any one of claims 1 to 5 wherein one or more additional metal tubes containing a mixture of additive(s) and binder material are attached to the said one or another lengthwise extending face of the electrode, the total length of the metal tubes being not less than the electrode length to be melted.

9. An eiectroslag remelting process which uses, in combination, a consumable metallic electrode to one lengthwise extending face of which is secured a metal tube containing a compacted mixture of additive(s) and a binder.

10. A method of electroslag remelting the improvement which comprises encapsulating within a metal tube a compacted mixture of additive(s) and binder, and securing the tube to one lengthwise extending face of a metallic electrode to be consumed in the process.

11. A process for reducing metal ingots by the electrslag remelting process substantially as herein described with reference to Figures 1 and 2 or the accompanying drawings.