WO1999001710A1

WO1999001710A1 - Electric furnace with eccentric taphole and process of steel making in this furnace

Info

Publication number: WO1999001710A1
Application number: PCT/EP1998/003815
Authority: WO
Inventors: François-Noël RICHARD; Tonino CECI
Original assignee: Vesuvius France SA
Current assignee: Vesuvius France SA
Priority date: 1997-06-30
Filing date: 1998-06-23
Publication date: 1999-01-14
Anticipated expiration: 1999-12-30
Also published as: ZA985596B; FR2765316B1; AU8852798A; FR2765316A1

Abstract

The invention concerns an electric-arc furnace with an off-center taphole (26) through the bottom, designed for steel making and known under the name of an EBT furnace. It is comprised of a vat (4) for receiving the metal to be melted, and a protuberance (6) that prolongs the vat (4) laterally. A roof covers the protuberance (6). An upper wall prolongs the vertical wall of the vat (4). An arch (12) covers the upper walls of the vat (4), this arch (12) capable of being retracted laterally in a sweep zone, a taphole (26) being provided in the bottom of the provided protuberance (6). A closure device for the taphole (26) is comprised of a stopper (32), handling means for displacing the stopper (32) between a rest position, a waiting position in which the lower end of the stopper (32) is close to the taphole (26), and a closure position in which the stopper (32) closes off the entrance to the taphole (26), at least partially.

Description

ELECTRIC FURNACE WITH ECCENTRIC TAPHOLE AND PROCESS OF STEEL MAKING IN THIS FURNACE

The invention concerns an electric furnace with an off-centered taphole through the bottom, intended for steel making, and known under the name of furnace EBT, comprised of a vat for receiving the metal to be smelted, and a protuberance that prolongs the vat laterally, the vat and the protuberance having a bottom and a side wall lined with a refractory covering, a roof covering the protuberance, an upper wall that prolongs the vertical wall of the vat, arch covering the upper walls of the vat, this arch capable of being retracted laterally in a sweep zone for furnishing an access for charging the furnace, with a taphole in the bottom of the protuberance, a passage being formed in the roof of the protuberance, a device for closing the taphole that consists of a stopper at least partially of a refractory material, handling means for displacing the stopper between a rest position into which the stopper is placed outside of the bath, a waiting position in which its lower end is close to the taphole and a closure position, in which the stopper closes at least partially the entrance to the taphole, and fixation means for fixing the stopper on the handling means, these means being comprised of at least a stopper support mounted at the end of the handling means.

The taphole is preliminarily closed in the melting of the metal charge. This closure is effected by shutting off the exit of the taphole by means of a sand retailing device located outside of the furnace such as those, for example, that are described in the U.S. Patents Nos. 4,523,747 and 4,799,287. Then a operator situated on a catwalk that overhangs the roof of the protuberance fills the taphole with an adequate sealing mass of the sand denoted below. When this is completed, a metal charge is introduced into the vat, the arch having been retracted to permit this filling. When the filling is complete, the arch is put back in place and the electrodes are lowered to melt the charge. When the charge has been melted, a ladle is placed under the taphole and the sand closure device is open. The sand runs out of the taphole and the molten metal flows into the ladle.

CCTJ IRMATION COPY During the pouring, the furnace is inclined progressively toward the protuberance to preserve a maximum height of molten metal above the taphole. By the end of the pouring, the inclination reaches 15° to 20°. The furnace is raised to stop the pouring. However, this type of furnace has two major shortcomings.

By the end of pouring, the height of the molten steel above the taphole is less than at the beginning of pouring. A vortex has had time to be established, unless a quite substantial heel is preserved. This vortex entrains a substantial amount of slag into the ladle. For this reason, among others, the metallurgist is forced to leave a heel in the furnace, which, when it is quite substantial, leads to a decrease in the useful capacity of the furnace and thus a decrease in the steel production.

On the other hand, about five seconds is required to regulate the furnace sufficiently to interrupt the pouring as soon as slag is detected in the stream of steel.

Assuming that the diameter of the taphole is as large as possible for reducing the pouring time and optimizing the production, a substantial amount of slag has time to pass into the ladle during this delay.

Although they improve the performance of classic furnaces, electric furnaces with an off-center taphole do not permit a reduction in the amount of slag sufficiently for a good many applications.

A metallurgical vessel comprised of a vat on which a protuberance is formed, a tap hole being provided in the bottom of the protuberance, is also known from the DE 34 37 810. A closure device for the tap hole includes a stopper which may be displaced vertically between a closure position, in which the lower end of stopper seals the tap hole off, and an upper position in which said lower end of the stopper is situated above the level of the metal bath in the vessel. The stopper itself is formed of a metal tube covered with a coating of a refractory material, at least in its submerged portion.

The loading of the vessel must preferably take place when the stopper is in its upper position so that the charge will not damage it when rolling on the bottom of the vessel. It is also desirable in not compulsory that, during the fusion of the charge, the stopper is raised above the bath to avoid a premature erosion of by slag and steel. In said upper position, the upper end of the stopper interferes with the sweep zone of the arch for most existing furnaces, which prevents the opening of said arch.

In addition, since the stopper is tubular, the flow of liquid steel produces a strong depression inside the stopper. It is consequently necessary that the walls of the stopper are perfectly gas tight to avoid that important quantities of gas are sucked up through them, in the disclosed installation. Such a stopper is in addition difficult to manufacture, in particular because of the problems which would arise from the difference of thermal dilation between the refractory material and the metal of the tube.

The object of the invention is an electric arc furnace that remedies these shortcomings. The purpose of this furnace is to control the passage of slag into the ladle by stopping the flow more rapidly than in the known type of EBT furnaces, without the stopper interfering with the sweep zone of the arch. It also proposes to reduce and perhaps eliminate the formation of a vortex without requiring a stopper that is difficult to manufacture.

The invention achieves these goals through the fact that the means for displacing the stopper between its rest position and its closure position make it possible to avoid the sweep zone of the arch. A device for closing the taphole that consists of a stopper at least partially of a refractory material, handling means for displacing the stopper between a rest position into which the stopper is placed outside of the bath, a waiting position in which its lower end is close to the taphole and a closure position, in which the stopper closes at least partially the entrance to the taphole, and fixation means for fixing the stopper on the handling means, these means being comprised of at least a stopper support mounted at the end of the handling means.

As soon as this is required, for example, as soon as slag is detected, the handling means displace the stopper from its waiting position to its closure position, which is very rapid (e.g., 0.5 second). The furnace is thus regulated with a taphole totally or partially closed. The amount of slag that passes into the ladle is thus reduced quite substantially.

To remove the stopper from the molten metal, one could imagine a handling arm similar to that in a ladle or distributor that assures a vertical linear raising and lowering movement of a stopper rod above a nozzle, the principal difference being that the travel of the handling means has to be sufficient to remove the stopper completely from the molten steel, i.e. , much more than the movement of a stopper above a nozzle. The space available above the roof of the protuberance is insufficient for the placement of such a handling means without interfering with the sweep zone of the arch. This zone should be released at certain times to permit the opening of the furnace cover. This is why a simple raising and lowering mechanism is not sufficient and thus has to be accompanied by supplementary means for retracting the stopper and its handling arm out of the sweep zone of the arch.

This is why, in one variant, the invention has a handling arm that facilitates effecting a vertical upward displacement of the stopper, followed by a backward movement of the arm, or a pivoting movement around a vertical axis, then a lowering movement of the stopper in the vertical position.

With this variant, however, the stopper and its arm encroach temporarily in the sweep zone of the arch. For reasons of safety, it is desirable that the trajectory of the arm and the stopper does not interfere with the sweep zone of the arch at any time.

This is why, according to a preferred variant, the invention is designed so that the means for displacing the stopper between its rest position and its closure position facilitate avoiding the sweep zone of the arch. Thus, the safety devices are simplified.

In an advantageous implementation, the means for displacing the stopper between its rest position, its waiting position and eventually its closure position, without interference with the sweep zone of the arch are comprised of a rotating arm turning around an axis external to the protuberance and solid with the furnace. This axis is preferably situated essentially at mid-height of the taphole and the roof of the protuberance to permit retracting the assembly through a hole in the roof of the protuberance of reduced dimension and without interfering with the vertical wall of the protuberance and the upper wall of the vat.

In the cases where the available space is particularly reduced, a curved stopper that decreases the space required for its displacement can be used. However, a curved stopper is difficult to produce. Furthermore, it has to be oriented angularly to the assembly, which necessitates the provision of a positioning device.

A rotating arm permits only a circular displacement of the end of the stopper. At the end of travel a rectilinear movement in the axis of the taphole is desirable to assure a closure, whatever the level of erosion of the refractory lining around the taphole. This is why in a preferred variant the means for displacing the stopper, also having means for displacing the stopper linearly in a direction essentially parallel to the axis of the taphole. This linear movement should not be compared with the linear movement that would be necessary for removing the stopper from the casting bath and requires only a reduced travel for bringing the lower end of the stopper in the waiting position close to the taphole to the closure position. This linear movement is generally limited to 200 or 300 mm.

As observed above, the regulation of the furnace for stopping the pouring takes about five seconds. It is not necessary to close the taphole completely during this brief lapse of time, but it is sufficient to reduce the stream significantly. Contrary to other known uses of a stopper rod on a nozzle in the case where the container does not rock, and where consequently the tightness should be perfect, one can be content with a partial tightness. In one variant, however, the invention has means that also assure a complete tightness. For this, the lower end of the stopper is covered with an elastic material such as a cap of refractory fibers, which forms a joint capable of absorbing the irregularities of the taphole.

As observed, the diameter of the taphole is substantial. Furthermore, it assumes a flared shape due to erosion, which further enlarges its diameter. The outside diameter of the stopper should be sufficient to cover it completely. A cylindrical stopper that would completely cover such a taphole would have a weight and a cost that would not justify the needs of its mechanical strength. This is why, in one variant, the cylindrical part of the stopper has a diameter smaller than that of the taphole and the stopper has an enlarged lower end, whose diameter is larger than that of the taphole.

The fact of retracting the stopper from its access zone above the taphole, in particular with a rotating arm, offers the additional advantage of being able to providing several closure devices, one of which can be brought alternatively into the closure position at will.

According to one embodiment, this is why the furnace has at least two pivoting arms located outside of the furnace, any one of these arms capable of being used at will for closing off the taphole. This arrangement avoids a maintenance intervention for replacing the stopper.

As explained previously, the simple presence of the stopper above the taphole significantly inhibits the vortex effect. In order to improve this result even further, the stopper has a non cylindrical form in its immersed part. This non cylindrical form could be obtained in various ways, notably by providing it with one or more fins, at least in the immersed part. The presence of these fins impedes the rotation of the molten metal and thus the appearance of the vortex. However, the final part of the stopper remains cylindrical in order to adapt to the shape of the taphole. In a preferred mode of implementation, the handling means of the stopper have means for entraining the stopper in a rotational movement.

It is conceivable that a rotary movement of adequate velocity in the clockwise direction in the southern hemisphere and in the counterclockwise direction in the northern-hemisphere would cancel the rotation effects engendered by coriolis acceleration and thus totally eliminate the vortex. However, such a system is difficult to regulate.

In a preferred form of the invention, an alternative movement will be used to agitate the steel bath in the zone of the taphole and substantially perturb the rotational movements that generate the vortex. The rotational movement of the stopper is preferably begun as soon as the stopper is brought into its waiting position and it will be preferably stopped before bringing the stopper into its closure position so as not to damage the end of the stopper. It is thus possible to continue the pouring longer. The volume of the heel can be reduced. The useful capacity of the furnace is thus increased.

According to one variant, the stopper is supported as a cantilever by its upper end. In addition, the movements of the furnace and the liquid bath can increase the forces perpendicular to its axis.

It is thus necessary to have a sturdy embedding of the stopper on the stopper support. According to one variant of the invention, an external fixation of the stopper support on the external part of the stopper is preferred.

The stopper is preferably produced by isostatic pressing through the interior. This process makes it possible to obtain a more precise external shape than pressing on the exterior, such that the cone can be produced directly by pressing, without further machining.

According to another variant, a metallic insert is placed inside of the stopper. In a preferred variant, the plug of the stopper is placed on the stopper support that is itself perforated by an orifice. In this configuration it is necessary that the junction between the stopper and the stopper support be tight.

In a preferred embodiment, the assembly is actualized by two complementary cones that assure both a sturdy rigid hold and a good tightness. According to this configuration, protuberances or hollows will be provided on the stopper, in which the retention mean can be engaged so as to draw one cone onto the other.

The stopper should be changed periodically. Its position in front of the furnace does not permit of easy access. This is why the invention provides a method of fixing the stopper on the handling means without manual manipulation. The fixation means have automatic means of opening and closing the retention means.

In a preferred variant, the stopper is comprised of a tube having a longitudinal hole open at both ends of the stopper and a diameter greater than 50 mm so that when the stopper is in the closure position after pouring, this channel can serve as a guide for filling the taphole with sand.

However, the tubular form of the stopper has the disadvantage that this latter takes in slag in the interior of the hole when the stopper is introduced into the liquid bath. According to a preferred variant, this is why the lower end of the stopper is closed off by a fusible cover, e.g., a sheet of fine steel fixed at each pouring to the end of the stopper.

Furthermore, the flow of molten metal generates the entrainment of air from the longitudinal hole toward the taphole, which could be detrimental to the quality of the steel. To avoid such aspiration, the invention preferably has closure means that close off both the upper part of the stopper and the fixation means of the stopper, the connection between these fixation means and the stopper being effected in a tight manner. In order to impede the aspiration of air even more, the invention provides means for injecting a neutral gas into an axial hole of the stopper. This injection is made either in the stopper itself or in the fixation means of the stopper, the connection between these fixation means and the stopper being made tight, or even in the closure means.

The upper part of the stopper will be designed to permit an easy pouring of the sand through the longitudinal hole, e.g., by means of a funnel or hopper. The sand can be poured manually or by means of an automatic filling device.

The invention also concerns a process of steel making in an electric arc furnace.

According to this process: the outlet of the taphole is closed off by means of a sand retention device located outside of the furnace; the taphole is filled with sand; a metal charge is introduced into the furnace; the metal charge is melted by supplying electrical energy; the exit of the taphole is unstopped so that the sand runs out of the taphole and the molten metal flows into a ladle; the furnace is progressively inclined toward the taphole; - the furnace is regulated as soon as the presence of slag is detected in the molten metal that flows into the ladle.

The process is characterized in that: at the same time as the furnace is regulated, the entrance of the taphole is closed off by means of a closure device comprised of a stopper and means for displacing the stopper during a time interval less than that necessary for interrupting the flow by regulating the furnace.

The stopper is preferably placed outside of the furnace during the charging of the latter with a metal charge and the melting of this charge and the stopper is introduced into the steel bath during the pouring. _{l ()}

The stopper is preferably introduced into the furnace until the lower end of the stopper reaches a point located a short distance above the taphole, and a complete closure of the taphole is obtained by a linear displacement movement in a direction essentially parallel to the axis of the taphole.

A stopper with a non cylindrical external form is preferably used; an alternative rotational movement is impressed on the stopper, preferably prior to the appearance of the vortex in order to inhibit its formation and development. As soon as this is required, the alternative rotation of the stopper is preferentially stopped, the taphole of the stopper is closed and the furnace is simultaneously regulated.

Preferably prior to the beginning of pouring, the upper end of the stopper is closed by means of a plug and, after the pouring is stopped, the taphole is cleaned, its outlet is closed off by means of a sand retention device. The plug that closes the upper end of the stopper is opened and the taphole is filled with sand by passing this sand through the longitudinal hole of the stopper. The stopper is brought into its rest position.

A fusible plug is preferably placed in the lower end of the stopper prior to its introduction into the steel bath.

An elastic joint, notably a joint of refractory fibers, is preferably placed at the lower end of the stopper to assure a tightness between this end and the taphole.

Other characteristics and advantages of the invention will become evident from a reading of the implementation examples, given by means of illustration with reference to the attached Figures:

Figures 1-3 are schematic cross-sectional views of a furnace according to the invention, the arm being in three different positions; Figures 4 and 5 are cross-sectional views of an enlarged scale of the end of the arm of the furnace of Figures 1-3. Figure 6 is a magnified view of the manipulation means of the furnace represented in the position of Figure 3, the retention and clamping means being open.

Figure 7 is an enlarged cross-sectional view of a stopper that is equipping the furnace of Figures 1-6. Figure 8 is an enlarged cross-sectional view of the lower end of a variant of the stopper.

Figure 9 is an enlarged cross-sectional view of a variant of the upper end of a stopper.

Figures 1-3 show cross-sectional views of an electric-arc furnace with an off- center taphole according to the invention and showing different positions of the stopper 32. This furnace, designated by the general reference 2, is comprised of a cylindrical vat 4 and a protuberance 6 that laterally prolongs the vat 4.

The vat 4 has a bottom 7 and a vertical wall 8 prolonged by a water-cooled upper wall 10. An arch 12 closes the upper part of the vat 4. This arch 12 carries one or more electrodes 14 that supply the electric power necessary for melting the charge 15 of the furnace. The arch 12 is mounted on an articulated arm 34 (not shown) that permits displacing it horizontally to permit charging the furnace with scrap iron. The arch 12 is displaced in a horizontal plane represented by the line 16. The zone situated above the line 16 in which the arch 12 is displaced will be called the "sweep zone of the arch 1211 in the following.

The protuberance 6 has a bottom 20 and a vertical wall 22. The zone above the protuberance 6 is closed by a roof 24. The bottom of the vat 4, its vertical wall 8, the bottom 20 of the protuberance 6 and its vertical wall 22 are provided with a thick layer of refractory lining.

A taphole 26 is located in the bottom 20 of the protuberance 6. A sand retention device 28, located outside of the furnace 2, permits closure of the exit of the taphole 26.

According to the invention, the furnace is equipped with a closure device for the taphole 26. This device is comprised of a stopper 32 that has a lower end 32a and an upper end 32b (in the position shown in Figure 1), handling means for the stopper 32 and fix xaattiioonn mmeeaannss for mounting the stopper 32 on the handling means.

The handling means are comprised notably, but not exclusively of an arm 34 that is L-shaped. The arm 34 is articulated at its end 34a around a horizontal axis 36 mounted on a support 37 connected to the structure of the furnace. At its end 34b it has means for mounting the stopper 32 as well as supplementary handling means for this stopper. These means will be described in more detail below.

The handling means permit a displacement of the stopper 32 between different positions, i.e., a rest position that can constitute a deposition and removal position of the stopper 32, a waiting position and a closure position.

The rest position of the stopper 32 is a position in which the latter is situated completely outside of the steel bath 25. In all the known systems using stopper rods, such as distributors, the stoppers are mounted prior to the introduction of the molten metal into the container. In the case of d e invention furnace the stopper 32 is not used to regulate the flow nor to completely close off the taphole 26 as in the classic applications of stopper rods because the pouring is done with a full opening. It is thus possible to keep it outside of the furnace in a position called the rest position. Keeping the stopper 32 inside the furnace permanently would be very detrimental because the furnace operates continuously for weeks. The slag foams up and attains a quite substantial thickness that would attack the stopper 32 chemically. During charging, the blocks of scrap iron to be melted cover the protuberance 6 of the furnace and would damage the stopper 32. Finally, the off-centered zone of the furnace is heated by burners that give rise to an atmosphere corrosive for the stopper 32. This is why the stopper 32 remains outside of the furnace during the filling stage of the furnace with scrap iron so as not to expose it unnecessarily to the aggressive atmosphere of the furnace.

Figures 2 and 3 represent two possible rest positions. In Figure 2 the arm 34 has pivoted by 90° relative to its closure position. Figure 3 shows another position of the arm 34. In this position the arm 34 has pivoted by 180° relative to its closure position. Due to the 180° rotation of the arm 34, the stopper 32 is in an opposite position relative to the one that it occupies in the furnace. There could be other rest positions, e.g. , a position in which the stopper 32 would be partially inside of the furnace, the important factor being that its lower end is above the level of the steel bath.

The positions of Figures 2 and 3 could also constitute deposition and removal positions of the stopper 32. In particular, in the position shown in Figure 3 the stopper 32 is in an opposite vertical position. It can be manipulated easily, without undergoing a cantilever or unstable force as when it is in a horizontal position.

During pouring, the stopper 32 is brought into a waiting position in which its lower end 32a is situated in the immediate proximity of the taphole 26 in order to be able to close the latter as rapidly as possible when desired. The stopper 32 is brought into its waiting position as late as possible during pouring, but early enough to be ready to close the orifice rapidly, and also to prevent the formation of the vortex.

In the closure position the lower end 32a of the stopper 32 is applied on the taphole 26 and closes off the latter at least partially.

A passage 39 is formed in the roof 24 of the protuberance 6 to permit die entrance and exit of the stopper 32.

As can be seen in Figures 1-3, the axis 36 of the arm 34 is placed essentially at mid-height between the entrance of the taphole 26 and the roof 24 of the protuberance 6. This position permits d e circle 38 described by the lower end 32a of die stopper 32 to be as small as possible, and for it to avoid both the upper wall 10 of the vat 4 and the vertical wall 22 of the protuberance 6. The stopper 32 also avoids the sweep zone of the arch 12. In this manner, there is no interference between the movement of the arm 34 and that of the arch 12. In the implementation example described the stopper 32 is rectilinear, but another form could be conceivable, notably a curved stopper 32 that would allow a reduction in the space occupied by the circle 38.

Figure 7 shows an enlarged view of the stopper 32. Its upper end 32a has the form of a male cone. The conical form 32a of the upper part of the stopper 32 is preferably produced by isostatic pressing through the interior, which permits obtaining it directly without any other machining. Its lower end 32b becomes enlarged in the form of a cone 51. The flared lower conical form 51 of the stopper 32 allows it to cover a taphole 26 of large diameter, particularly when it is eroded, while preserving a smaller diameter 32c, in particular smaller than the diameter of the taphole, for the conventional cylindrical part of the stopper 32. In effect, if the stopper 32 presented a constant section sufficient to cover the taphole 26, its weight and cost, would be substantially increased.

Fins are present on the cylindrical part of the body of the stopper 32 and on the lower end in the form of a cone. An axial channel 41 traverses the stopper 32 from one end to the other.

The simple presence of the stopper 32 in the waiting position above the taphole

26 during pouring significantly inhibits the appearance and development of the vortex, in particular due to the fins 50. However, as will be explained with reference to Figures 4 and 5, the implementation example described also has means that permit a rotation of the stopper 32, which improves its efficacy even more.

Figure 4 shows an enlarged view of the end 34b of the arm 34. A carriage 60 is mounted on the end of the arm 34. A stopper support 68 is mounted on the carriage 60. This carriage 60 can be displaced linearly relative to the arm 34 by means of rollers 62 that roll on rails 64 solid with the arm 34. A jack 66 is mounted at one of the ends to the arm 34 and at its other end to the carriage 60. This jack assures the displacement of the carriage 60 in both directions. In the example shown, the stopper 32 is mounted on the stopper support 68 by a conical pressing-on. The upper end 32a of the stopper 32 engages in a female cone 78 with the same angle of conicity as the stopper support 68.

Means assure the retention and clamping of the stopper 32 on the stopper support

68. These means are comprised of retention hooks 86 articulated at the lower end of the rod 82. These hooks engage under a protuberance 88 of the upper part of the stopper 32. A rod 82 can slide in a bore of the support. A spring 84 causes the rod 82 to be displaced upward. In this manner, the hooks clamp the two cones firmly to each odier. A sturdy embedding of the stopper 32 on the stopper support 68 is thus assured.

Means are provided to permit an easy, perhaps automatic removal of a worn stopper 32 and its replacement with a new stopper 32.

A lug 90 solid with the hook 86 can slide in a guide groove 92 formed in a solid part of the stopper support 68. The upper end 82a of the rod 82 project clearly beyond the support. When no force is exerted on the rod 82, the spring applies the hook on the protuberance 88 of the stopper 32 and holds the two cones firmly applied, one against the odier. On the other hand, when a force is exerted on the rod to displace it relative to the stopper support 68 by over compressing the spring 84, as shown in Figure 5, the lug 90 is displaced in the guide groove and the hook is removed from the protuberance. The stopper 32 can then be removed from the mounting means and dien a new stopper 32 put in place.

Several methods can be used to actuate the rod 82 and free the stopper 32 from the holding and clamping means.

According to a variant shown in Figure 5, the carriage 60 can be displaced relative to the arm 34 so that the rod 82 comes to rest on a stop 100 solid with the arm

34 that removes the hooks 86. This operation is carried out, of course, when the arm 34 is in the charging/discharging position (Figure 2 or 3, for example). The stopper 32 can then be withdrawn by means of a manipulator or a crane, and a new stopper 32 can be put in place.

According to another variant (Figure 6), the support 37 of the arm 34 is prolonged by a stop 98. Through a rotation of the arm 34, the rod 82 comes to rest on the stop 98, which removes the hooks 86 in a manner identical to that described above.

Because of the carriage 60, the stopper 32 can be displaced rectilinearly over a short distance. In effect, the pivoting arm 34 permits only the displacement of die end 32a of die stopper 32 along the circle 38. Now the entrance of the taphole 26 is eroded during the pouring. So that the stopper 32 can be applied as tightly as possible on the taphole 26, it is necessary to make it penetrate into the taphole 26 over a short distance corresponding to this wear. It is thus desirable to be able to displace the stopper 32 linearly, essentially in the direction of the axis of the taphole. It is precisely this function that the carriage 60 performs.

The stopper support 68 is mounted rotationally on the carriage 60 by means of two bearings 70. A jack 72 having one end fixed to the carriage 60 and the odier end fixed to an arm (not shown) of the stopper support 68 facilitates entrainment of the stopper support 68 in a movement of alternative rotation. This movement agitates the steel bath in the taphole 26 zone and significantly perturbs the rotational movements diat generate the vortex.

In the implementation example shown, the invention has means that facilitate filling the taphole with sand. For this purpose, the stopper support 68 is traversed from one end to the other by a longitudinal hole 74 at the upper part of which a funnel 76 is provided.

The longitudinal hole 74 is prolonged by the axial channel 41, which traverses the stopper from one end to the other. When the stopper 32 is in the closure position, its lower end is applied on the taphole. If sand is introduced into the longitudinal hole, it will be conveyed into the taphole 26.

However, the presence of the axial channel 41 involves the aspiration of air caused by the flow of the molten metal.

This is why a tight plug 44 closes the upper part of the channel 41. This plug can be placed either directly on the stopper 32 or on the stopper support 68. A gas supply channelization 46 permits the injection of a neutral gas into the channel 41 of the stopper 32. This injection avoids the aspiration of air into the steel dirough the channel 41. The channelization can be located in d e stopper support 32, in the plug 44, or even directly on the stopper 32. When the plug is mounted directly on die stopper 32 and the gas supply channelization is mounted on this plug, it is not necessary for the connection between the stopper 32 and die arm 54 to be tight. On the other hand, when the plug is mounted on the arm 34, it is necessary that it be tight.

Another shortcoming of the presence of the channel 41 resides in the fact that this latter takes slag inside of the hole when the stopper 32 is immersed in the steel. Figure 8 shows a stopper 32 whose lower end is closed off by a fusible cover 54. This cover 54 has the function of protecting the axial channel 41 against the entrance of slag when the stopper 32 traverses the latter. As soon as the stopper 32 penetrates into the steel, the cover 54 melts and the axial channel 41 is disengaged.

Furthermore, in this variant the lower end of die stopper 32 is covered widi an elastic material, such as a cap of refractory fibers 58, which forms a joint capable of absorbing the irregularities of the taphole 26. It should also be noted that the stopper 32 of Figure 8 has only a single fin 50.

After a pouring, the taphole is cleaned and then the sand retention device is again closed. The plug 44 that closes the upper end of the stopper 32 is opened and the taphole 26 is filled with sand by passing this sand through the axial channel 41 of the stopper 32. The stopper 32 is raised by an upward translation movement and it is brought into its rest position. The lower end is furnished widi a new joint and a new fusible cover 54.

As explained, it is preferable to keep the stopper 32 outside so as to better take up the embedding stresses that its upper end undergoes during manipulations, notably when it is in the horizontal position. These stresses are substantial given the length and weight of the stopper 32.

However, die stopper 32 can also be fixed through the interior, as shown in

Figure 9. In this case, the stopper support 68 has a male cone 69 that engages in a female cone of the stopper 32. In this case, the stopper 32 is preferably reinforced by an insert 106, e.g. , a metallic one, that resists the tensile stresses better. If the assembly is self-clamping, the two cones can be separated by means of an expelling cone 108 that engages in a groove 110 of die cone 69 of die stopper support 68.

Claims

I. Electric-arc furnace with off-centered taphole (26) through the bottom, designed for steel production, and known under the name of an EBT furnace, comprised of a vat (4) designed to receive the metal to be melted, and a protuberance (6) that prolongs the vat (4) laterally, the vat (4) and d e protuberance (6) having a bottom and a lateral wall furnished with a refractory lining, a roof (24) covering the protuberance (6), an upper wall prolonging the vertical wall of the vat (4), an arch (12) covering the upper walls of the vat (4), this arch (12) being capable of being retracted laterally in a sweep zone for providing an access for charging the furnace, a taphole (26) being effected in the bottom of die protuberance (6), a passage (39) being formed in the roof (24) of the protuberance (6) a closure device for the taphole (26) comprised of a stopper (32) at least partially of a refractory material, handling means for displacing die stopper (32) between a rest position in which the stopper (32) is placed outside of the bath, a waiting position in which the lower end of the stopper (32) is close to the taphole (26), a closure position, in which the stopper (32) closes off at least partially the entrance of the taphole (26), and fixation means for mounting the stopper (32) on the handling means, these means being comprised of at least one stopper support (68) mounted at the end of the handling means characterized in that the means for displacing the stopper (32) between its rest position and its closure position make it possible to avoid the sweep zone of die arch (12).

2. Furnace according to claim 1, characterized in that the means for displacing the stopper (32) between its rest position and its closure position are comprised of a rotary arm (34) mounted to turn around an axis (36) external to the protuberance (6) and solid with die furnace, said axis (36) being situated essentially at mid-height of the taphole (26) and the roof of the protuberance (6).

3. Furnace according to any one of claim I or 2, characterized in that the handling means for displacing die stopper (32) also have means for displacing die stopper (32) linearly in a direction essentially parallel to the axis of the taphole (26) over a travel path of nearly 300 mm, means comprised notably of a carriage 60 mounted in a sliding manner at the end of the rotating arm (34).

4. Furnace according to any one of claims 1-3, characterized in that the stopper (32) has a cylindrical part having a diameter less than that of die taphole (26) and in that it has an enlarged lower end whose diameter is greater dian that of the taphole (26).

5. Furnace according to any one of claims 2-4, characterized in that it has at least two pivoting arms (34) located outside of the furnace, any one of these arms (34) capable of being used at will for closing off die taphole (26).

6. Furnace according to one of claims 1-5, characterized in that the stopper (32) has a non cylindrical external shape designed to reduce the effects of the vortex.

7. Furnace according to claim 6, characterized in diat d e stopper (32) has at least one fin (50) on an enlarged lower end (51) of the stopper, whose diameter is greater than that of the taphole (26).

8. Furnace according to any one of claims 1-7, characterized in that the final portion of the stopper (32) remains cylindrical in order to adapt to die shape of die taphole (26) and in that the non cylindrical shape is present at least in the immersed part of the stopper (32).

9. Furnace according to any one of claims 1-8, characterized in d at die handling means involve means for imparting an alternative rotational movement to the stopper (32).

10. Furnace according to any one of claims 1-9, characterized in that the upper end of the stopper (32) has an internal or external cone that fits on a supplementary cone of the stopper support (68).

11. Furnace according to any one of claims 1-10, characterized in that the fixation means are comprised of retention and clamping means, and in that the upper end of the stopper (32) has protuberances (88) or hollows in which the retention means can engage.

12. Furnace according to claim 11 , characterized in that the fixation means are fitted with comprised of automatic opening and closing means of the retention means.

13. Furnace according to any of claims 1-12, characterized in that the stopper (32) has an insert, in particular, a metal insert, at its upper part.

14. Furnace according to claim 13, characterized in that the lower end of the stopper (32) is closed off by a fusible cover.

15. Furnace according to any of claims 1-14, characterized in that the stopper (32) or the stopper support (68) has means for closing the upper part, of the axial hole of the stopper (32) in order to prevent the aspiration of air due to the section caused by the stream of molten metal.

16. Furnace according to any of claims 1-15, characterized in that it has a tight connection between the stopper (32) and the stopper support (68).

17. Furnace according to any one of claims 1-16, characterized in that the stopper (32) is produced by isostatic pressing through the interior.

8. Process for steel making in an electric-arc furnace having a vat (4) designed to receive the metal to be melted and a protuberance (6) diat prolongs the vat (4) laterally, the vat (4) and die protuberance (6) having a bottom and a lateral wall furnished with a refractory lining, a roof covering the protuberance (6), an upper wall that prolongs the vertical wall of the vat (4), and an arch (12) covering d e upper walls of die vat (4), this arch (12) capable of being retracted laterally in the sweep zone to provide an access for charging the furnace, a taphole (26) in the bottom of the protuberance (6), a process according to which: the exit of the taphole (26) is closed off by means of a sand retention device situated outside of the furnace; die taphole (26) is filled widi sand; a metal charge is introduced into the furnace; the metal charge is melted by supplying electrical energy; the outlet of die taphole (26) is unstopped so that the sand runs out of the taphole (26) and die molten metal flows into a ladle; the furnace is progressively inclined toward the taphole (26); the furnace is regulated as soon as the presence of slag is detected in the molten metal that flows into the ladle, characterized in that: at d e same time as the furnace is regulated, the entrance of die taphole (26) is closed by means of a closure device comprised of a stopper (32) and means for displacing the stopper (32) during a time lapse less than that necessary for interrupting the pouring in regulating the furnace, in that die stopper (32) is placed outside of the furnace during the charging of d e latter with a metal charge and the melting of this charge, and in that the stopper (32) is introduced into the steel badi during the pouring.

19. Process according to claim 18, characterized in that the stopper (32) is introduced into the furnace until the lower end of die stopper (32) reaches a point situated at a short distance above the taphole (26), and in that a complete closure of the taphole (26) is obtained by a linear displacement movement in a direction essentially parallel to the axis of the taphole (26).

20. Process according to any one of claim 18 or 19, characterized in that: a stopper (32) having a non cylindrical external form is used; an alternative rotational movement is impressed on the stopper (32), preferably prior to the appearance of the vortex, in order to inhibit its formation and development; as soon as required, d e alternative rotational movement of die stopper (32) is preferentially stopped, the taphole (26) of the stopper (32) is closed and simultaneously the furnace is regulated.

21. A stopper for an electric arc furnace having a tap hole (26) said stopper having a cylindrical portion having a diameter (32c) smaller than die diameter of the taphole of die furnace and an enlarged lower end portion (32b), having a diameter greater than the diameter of the taphole (26) of the furnace.

22. A stopper according to claim 21 characterized in diat it has at least one fin (50) on said enlarged lower end portion (32b).