United States Patent Cooper METHODS OF PRODUCING LARGE STEEL INGOTS [75] Inventor: Lloyd R. Cooper, Pittsburgh. Pa.

[73] Assignee: Heppenstall Company, Pittsburgh,

[22] Filed: Oct. 9, 1973 [21] Appl. No.: 404,246

52 us. Cl. 164/52 511 7 1m. (1 B22D 27 02 [58] Field of Search 164/52, 252; 29/5265 [56] References Cited UNITED STATES PATENTS 3 603374 9/1971 Cooper 164/52 Sept. 30, 1975 3,610.318 l0/l97l Simmons 164/252 Primary E.\'aminerFrancis S. Husar Assistant E.\'um'iner.lohn E. Roethel Attorney. Agent, or Firm-Buell, Blenko & Ziesenhcim [57] ABSTRACT A method is provided for producing large ingots by the steps of casting an ingot, forming an axial hole through the ingot and progressively melting and refining the central zone of the ingot at the axial hole by electroslag remelting technique and progressively resolidifying the metal in the hole to form a solid ingot.

4 Claims, 3 Drawing Figures U.S. Patent Sept. 30,1975

Fig.3.

Fig.2.

This invention relates to new methods of producing large steel ingots free of central voids and cavities and particularly to methods of producing large steel ingots with refined central zones free from excessive heterogeniety, pipes, cavities and .shriiikagefvfoids that are indigenous to teemed and solidified steelingots of largev crosssections.

In the industrial production of large steel ingots, the liquid steel is teemed into a mold (usually of cast iron). in the case of fully deoxidized (killed) steel, a reservoir of additional liquid metal, in 'a refractory lined, or

otherwise insulated shell, is provided above the main body of the ingot, to retain sufficient steel in a liquid state while the ingot body is becoming solidified, so the liquid steel in the reservoir may feed into the ingot body and satisfy the volume change incurred by the,

shrinkage of the solidifying steel.

The purpose of the reservoir, or sinkhead, or hot-top, is to prevent pipe or'shrinkage cavities or voids within the ingot body, and to reduce the heterogeneous characteristics in the inner portion of the ingot body. Unfortunatel y, this purpose is not always served, particularly "in those large, massive, steelin gots that may still be partially liquid in the central portion after two, three or even four days. The natural laws of selective freezing of steels dictate that some of the named defects and heterogenities will occur.

The invention described here makes use of the well known electroslag remelting and refining process to progressively 'remelt, refine, and resolidify the major central zone of the conventionally teemedand solidithe necessary metal to complete the remelting and refining process herein described. The upper extension of the mold may be refractory-lined, as with a conventional sinkhead, although for the execution of this process the upper extention may also be cast-iron; In any event the upper extension should be cylindrical, without flutes or corrugations, so that it may fit within a jacket as described hereinafter.

After the ingot is solid, a round central axial hole is formed, as by forge punching, or trepanning, or by boring or trepanning on a lathe, or by other convenient means. The size of this axial hole will normally be about per cent of the diameter of the ingot body, although larger or smaller diameters may be used provided there is enough weight of metal available to fill the axial hole, and provided there is sufficient room in which to operate the process.

The resulting ingot with the central axial hole is then positioned in an electroslag remelting fixture, on a stool which is preferably water cooled. The stool may contain a recess or pocket which contains the starting materials for-melting the slag and a portion of the slag to be used in the melting process. The diameter of the recess should be large enough to permit melting a major cross section of the ingot body in the ensuing process. A water cooled jacket is attached over the top extensionand shoulder of the ingot and sealed at the shoulde'r or ingot body to prevent leakage of slag and metal as the process proceeds.

An electrode is located in the center of the axial hole. This may be a consumable electrode of a chemical composition common to that of the ingot, or as hereaft er described, the electrode may be non-consumable. The elec,trode, if non-consumable, should be watercooled, preferably made of copper, with a water-cooled head, faced with tungsten, molybdenum, or other electricallyconductive material that will resist attack by the liquid slag.

g The electroslag melting process is started and the slag in the pocket of the stool is melted and thelwalls of the axial hole are melted and flow into the recessed stool.

,Thisraises the slag level, progressively melting more steel from the ingot walls around' the axial hole. The melting is continued with the slag rising until it melts most if not all of the metal in the upper body extension and all of the displaced metal from the ingot walls and extension has satisfied the needs of the axial hole and all of the ingot body has solidified with a remelted and refined central zone. i

Carefully weighed amounts of slag must be used during this process, so that the volume of slag is always known (initially and-after any additions). From the volume of slag, and the thickness of the slag layer, which can be measured while the melting is in process, it is possible to calculate the penetration of the slag into the ingot body at all times, and thereby the diameter ofthe central zone that is being remelted and refined. Experimental measurements of this type of remelting have shown that the outer curve, where the liquid slag contacts the ingot wall, is most nearly represented by an ellipse, and that the volume, V of the slag maybe determined by the equation:

rrpr 2 where r is the radius of the axial hole, I is the depth of 'slag,'V isthe volume of slag and p is the penetration of melted zone. The electrical power can be increased or decreased, and the rate of raising the electrode controlled, to produce the desired depth of slag, and thereby the desired size of the remelted and refined central zone.

This process has a significant advantage over the well-known process of electro slag remelting in a watercooled crucible. In the conventional electroslag remelting process, particularly in the large ingot sizes, there is considerable absorption of hydrogen from the atmosphere, through the unprotected surface of the liquid slag. Here, however, this new process is self-shielding from the absorption of hydrogen. The only surface of slag that is not covered is the annular space between the electrode, and wall of the axial hole in the ingot. This surface, usually less than 20 per cent and often as low as 10 per cent of the cross-sectional area of the remelted central zone, is actually not influenced by the atmosphere until it approaches the top of the ingot. When the process is nearly complete, and the slag surface opens to the atmosphere, there is only a limited opportunity for absorption of hydrogen, in the upper end of the ingot. Fortunately, because of progressive resolidification, the metal that is liquid at this time does not extend deeply into the ingot, and any hydrogen that is absorbed, has the opportunity to diffuse from the steel in subsequent processing.

In the foregoing general explanation of my invention, I have set out certain objects, purposes and advantages of this invention. Other objects, purposes and advantages will be apparent from the following description and accompanying drawings in which:

FIG. 1 is a vertical section through an ingot, stool and jacket at the start of remelting according to this invention.

FIG. 2 is a vertical section of the ingot, stool and jacket of FIG. 1 partway through the melting process of this invention; and

FIG. 3 is a vertical section of the ingot, stool and jacket on completion of the remelt process of this invention.

Referring to the drawings, I have illustrated an ingot having an upper body extension 11 and a central axial hole 12 from which metal has been removed by any known process. The ingot is seated on a watercooled stool 13 having a recess or pocket 14 beneath the axial hole 12. Starting chips and slag 15 are placed in recess 14 beneath axial hole 12 and a water-cooled copper electrode 16 is inserted in the hole 12 to contact the starting mix 15. A water-cooled jacket 17 is placed around the body extension 11 and the upper ingot 10 and sealed to the ingot with a sealing material 18.

The reference line AA indicates the intended top surface of the ingot after completion of the electroslag remelting process. In the original teeming and solidification of the ingot, allowance must be provided for more than sufficient metal above this reference line to completely fill the central axial hole, and the pocket in the recessed stool. The weight of this upper extension of the ingot body will thus be governed by the planned size of the axial hole and stool recess.

The process is started, the slag is first melted, and the heated slag in turn melts steel from the side wall of the ingot 10 within axial hole 12. The liquid steel flows into the recessed stool, raising the slag level, and progressively melts more steel from the ingot wall around the axial hole.

As the metal melts from the ingot wall, it raises the level of slag and forms a liquid metal pool 20 beneath the molten slag 21 (See FIG. 2) within an enlarged void 23 around the axial hole 12. As the slag rises the metal at the bottom of the pool solidifies in the arc identified as S in FIG. 2. This melting and solidification progresses upwardly until it melts most if not all of the upper body extension 11 and the axial hole 12 has been filled with remelted metal 24 and the ingot top is covered with slag 21 as shown in FIG. 3.

The resulting ingot is free from voids, shrinkage cavities and the undesirable heterogeniety characteristics of conventional large ingots.

In the foregoing specification I have set out certain preferred practices and embodiments of my invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. The method of producing large steel ingots with a refined central zone comprising the steps of:

a. Casting a steel ingot of the required final size having an excess of metal over that finally required;

b. Forming an axial central cavity through the full length of the ingot, having a minor cross sectional area of the ingot;

c. Progressively melting and refining the central zone of the ingot around the central axial hole by passing an electrical current through a fused refining slag in the central hole by way of an electrode positioned within the central axial hole to the ingot to melt a major cross sectional area of the ingot, and wherein the electrode is not consumed during the remelting process, and the metal to complete the filling of the axial central hole is obtained totally from the initial ingot; and

d. Progressively resolidifying the melted metal within the central zone to form a solid ingot mass.

2. The method of claim 1 wherein the steel ingot is provided with a main body and a top extension of smaller cross section than the main body, said top extension containing sufficient metal to completely fill the axial cavity when melted.

3. The method of claim 2 wherein said top extension is free from external flutes or corrugations.

4. The method of claim 2 wherein the top extension is surrounded by a cooling jacket during the melting operation.