EP0462478A1 - Device for teeming a liquid metal - Google Patents

Abstract

Improvement to devices used for causing molten metal to flow, characterised in that the effective section of the device, consisting of the available conduit surface for the flow of the metal upon progressive opening of the passage, is firstly kept small, and in that at least over a part of its height the flow conduit possesses an active profiled passage section, the profile extending as far as the limiting end of the conduit and having a shape comparable to that of a three- to four-pointed star, the bases of the points of the profile extending axially in the direction of the flow and used for guiding the flowing metal. …<IMAGE>…

Description

The present invention relates to an improvement made to the devices used for flowing liquid metal from any container, either into another treatment or transport container, or into an enclosure used for solidifying the metal. The containers thus targeted can be, inter alia, converters and electric furnaces for processing or processing, transfer or nuance bags, tundish distributors. The devices used for flowing liquid metal can be conventional shutters with cattails, but they are more and more nozzles with sliding or rotary drawers in the forms and in the most varied executions, these devices being designated below sometimes , regardless of type, simply by the name of the shutters.

In metallurgy there is a fairly considerable importance to the devices used to temporarily delimit the flow of molten masses during their flow or transfer. The importance is not only ergonomic and economic in that it is imperative to be able to execute these operations with the least possible danger for the operators and to manage to obtain in addition the greatest possible output of the operation; but the metallurgical incidences, which arise from the way of a nozzle or a shutter to behave in operation, that is to say how will be shaped and how will flow the metal jet which originates in the channel the shutter, are also considerable. Unless there are very special treatments to be carried out on a burst jet, the aim is generally to obtain the most compact flow jets, the smoother and more laminar as possible. However, this is not so easy and we very often obtain, either at the start of the casting, or during the casting itself, a burst, open and turbulent jet which we also often qualify as 'umbrella jet' . If up to now we did not know well the real causes of the formation of the umbrella jet, we did however know well the negative metallurgical effects of this phenomenon.

Thus, in the steel industry, whether in the field of conventional casting in a mold or in that of continuous casting, a turbulent jet causes considerable amounts of air. The metal, in this case steel, therefore undergoes oxidation on the surface, which leads to an increase in inclusions on the finished product which negatively affects the degree of internal cleanliness, that is to say on the properties of the product obtained and on its processability. In casting in a mold, a turbulent jet causes in addition inside the mold a foaming and splashes which solidify prematurely against the wall of the mold and lead to a quality of surface of the bars which is insufficient and which will be responsible for the surface defects during rolling. If the pouring stream continues, free and unprotected or immersed or protected, flows into the upper part of the mold in a turbulent manner, it becomes impossible to hold a meniscus more or less constant and immobile. Uncontrolled slag entrainments result therefrom and surface defects as well as inclusions of oxides in the steel are the consequence.

These uncontrolled appearances of the turbulent flow regime lead metallurgists to take a whole series of empirical preventive measures. Among these, we can generally cite the use of nozzles having very variable 'nozzle length' to 'diameter of the orifice' ratios, the modification of the geometry or of the composition of certain parts - especially moving parts of the nozzle, as well as the more frequent exchange of the parts of the nozzle which are subject to wear.

More specifically, Canadian patent No. 1,200,384 describes a sliding assembly for closing the orifice of a steelworks pocket. This set is distinguished by the fact that the fixed upper part is provided with a round section orifice and the movable lower part with a section orifice in the shape of an equilateral triangle. The base of the triangle is narrow and has approximately the same dimension as the diameter of the round upper orifice and the bisector of the triangle covers itself with the opening and closing path. For closing the movement of the movable lower nozzle part is done in the direction of the angle of the apex of the equilateral triangle, i.e. that both in the case of closure and in that of opening, both the final jet in the process of throttling and the initial jet in the process of formation will be significant, given that halfway through the stroke of the movable part in front of the orifice of the fixed upper part is half of the total flow section which is exposed. The declining or nascent jets are oriented in the gutter formed by the angle of the top of the section and flow therefrom, not without forming vortices and without projecting fairly large splashes.

Also, to avoid all these drawbacks, both practical and also metallurgical, the aim of the present invention is to provide the nozzles with the improvements necessary so that they give laminar flow jets under all operating conditions, discontinuous or continuous, and that they therefore make it possible to reduce the frequency and the importance of internal and surface defects of the product obtained by casting.

This object is fully achieved by a device serving to make flow of metal, which is characterized in that the effective section of the device, constituted by the surface of the conduit available for the flow of metal during the progressive opening of the passage, is first kept small and in that at least over part of its height the flow conduit has a profiled active passage section, the profile extending to the last end of the conduit and having a shape comparable to that of a star with three to four branches, the bottoms of the branches of the profile extending axially in the direction of flow and serving to guide the metal during the flow of the latter.

According to a possible embodiment of the invention, the bottoms of the branches of the star profile, that is to say the radial ends of the rays of the star, extend in a way like grooves axially along of the metal flow duct. Apart from a part which is possibly not profiled and occasionally flared at the top of said duct, the profile covers a very significant part of the length of the duct and in any case it extends to its lower end. The above-mentioned spokes of the star profile form an acute angle between them for the three-branched form and a right angle (or with arms with square bases or rectangular bases resp.) For the four-branched form, that is to say say that the sections are actually substantially triangular and cross-shaped. In operation these shapes round off by themselves due to erosion, but we can also shape them from the start so that the bottoms of the axial grooves of three- or four-pointed stars are rounded and that at their intersections the walls of the branches form obtuse angles for the triangular profiles and right angles for the cruciform profiles. If a material that is too weak is used and the cruciform profile erodes to an already rather square shape, the beneficial effect of the proposed measures on the jet is completely lost.

The best effect is obtained if the cross-section of the flow duct, arranged in the lower plate of the obturator -or integral with the latter-, is oriented in such a way in or vis-à-vis the movable, sliding or rotary shutter, that a pointed part of the section is open in the first place during the relative movement of the respective parts of the drawer during the opening thereof. This tip is therefore oriented substantially in the direction of movement if the lower plate, carrying the nozzle portion with profiled conduit, is mobile, while it is in the opposite direction to the movement if an upper mobile plate is used for the shutter. In the in the case of the cross, the axis of the latter can be rotated by a given angle relative to the direction of opening - closing. We can quite easily realize, by the corresponding projections one on the other of the two profiles of the upper and lower parts of a sliding shutter, that if the movable part has moved to the middle of the opening path, the profiled section is still quite far from being half-discovered and the same is true of the round section of the other part of the shutter. Contrary to this, the passage section is at least 50% cleared halfway through the opening path in the case of the assembly according to the aforementioned Canadian patent 1,200,384.

According to the invention, care will still be taken to ensure that the section at the inlet of the flow conduit is at least equal, if not preferably 10% to 15% greater than the section at the level where the jet leaves the conduit. . The flare, apart from a possible enlarged and rounded seat, intended to receive the head of a stopper device, will preferably be located in the upper part of the duct, the lower part thereof being of diameter constant. In the case of a sliding shutter the ratio between the section at the outlet of the upper part and the middle section of the lower part will be roughly similar to what has been said about the upper and lower sections of the profiled duct itself. even, that is to say that if the sections do not remain substantially constant, they decrease from top to bottom.

• au début de l'ouverture progressive du tiroir de la busette le jet naissant n'est que très inhomogène et asymétrique, alors que la busette n'est que partiellement remplie. Il s'ensuit une forte production d'éclaboussures et de gouttes de métal, mais pas encore à proprement parler de jet et/ou de parapluie,
• lorsque le tiroir est suffisamment ( environ 50% ) ouvert, il en sort un jet qui est plus ou moins centré, mais qui, pour l'exécution classique de la busette avec section ronde ou ovale, est invariablement en rotation rapide. Une fois que la tension de surface du métal n'est plus suffisante pour tenir le jet fermé et cohérent, la rotation se transforme en mouvement rectiligne suivant les tangentes et il y a apparition d'un parapluie à environ 15 à 20 cm en dessous du nez de la busette à écoulement libre.

The detailed analysis of the positive results obtained with nozzles of this particular geometry according to the invention, as well as simulation studies carried out for conventional nozzles with drawers, enabled us to understand a little better the burst phenomena of the jet of casting and umbrella formation, phenomena which can now be reconstructed in simulation. Thus, the bursting of the jet in two successive phases can be explained briefly as follows:

• at the start of the progressive opening of the nozzle drawer the nascent jet is only very inhomogeneous and asymmetrical, while the nozzle is only partially fulfilled. It follows a strong production of splashes and drops of metal, but not yet strictly speaking jet and / or umbrella,
• when the drawer is sufficiently open (approximately 50%), a jet comes out which is more or less centered, but which, for the conventional execution of the nozzle with round or oval section, is invariably in rapid rotation. Once the surface tension of the metal is no longer sufficient to keep the jet closed and coherent, the rotation becomes a rectilinear movement along the tangents and there is the appearance of an umbrella about 15 to 20 cm below the nose of the free-flowing nozzle.

While we cannot almost avoid the first phenomenon above, resulting from the asymmetric entry of metal into the nozzle, we can reduce it according to the present invention by giving a tip of the particular section of the conduit flow according to the invention a specific orientation relative to the opening movements of the shutter plates, so that - compared to conventional casting - there is at the start of casting a smaller cross section and that 'then the passage section becomes progressively, and not suddenly, larger. The most dramatic improvement, however, is achieved where the disturbances were most noticeable and were clearly responsible for most of the external and internal faults on the product, namely the uncontrolled instability of the jet flowing under an umbrella. The abovementioned suppression of the rotation of the steel leads to an axial guiding effect such that now the steel exits in a straight line from the nozzle. This results from the fact that the metal entering the duct is concentrated in corners which are oriented differently than the opening through which the steel is returned. The jet is therefore subdivided into several parts or flows and its rotation is prevented. It is also understood that too pronounced wear of the profile can again lead to a stage where rotation is no longer suppressed so effectively again. This also explains why between a section of substantially cruciform shape or a triangular shape, the triangular shape has more advantages. Indeed the grooving at axial extension can be more angular and also deeper with progressive narrowing towards the bottom of the spokes, while it does not fade so much through wear. It is a little different from the cruciform section. This leads to a substantially square section, if significant or rapid wear wears out the prominent corners of the cross. Towards the end of the casting the jet is therefore less well guided in a channel becoming more and more square, and the suppression of the rotation is less pronounced. For this form the resistance to erosion of the material is therefore greater because it conditions efficiency.

In the light of these explanations, it appears that a nozzle need not necessarily have the particular geometry of its section over the entire length of the flow channel. It is however essential that the end part of the flow channel of the body of a nozzle or of the lower part of a shutter has the particular geometry. In general, the profile covers an extent greater than at least 50%, preferably greater than 75% of the total length of the channel. It is also important that the shape stability of the nozzle is good and changes little by erosion, so that even at the end of casting the profile remains sufficiently pronounced to prevent the jet from starting to turn and leading to the formation of an umbrella jet.

On the other hand, it is also recommended to keep the extent of the effective section in a certain relation to that of the standard section of a completely conventional nozzle. Thus, this section should not be less than about 40% of the standard section of a circular section nozzle and not more than about 120% of this reference section.

It has also been found that the wear of parts other than that of the channel used for guiding the jet according to the teachings of the present invention, such as for example the wear of the upper edge of the movable plate of a shutter, does not is not critical. Indeed, if the section increases at this level, while the length of the nozzle obviously remains constant, the slightly conical opening of the jet (i.e. its flare) will be a little larger, without there being any formation of an umbrella spray. This is favorable because due to the turbulence at the level of the passage of the metal from the circular opening plate to the 3 - 4-ray section below the nozzle, the wear of the mobile plates is greater than before. This wear can be greatly reduced if the initial part of the nozzle passage channel remains circular, which, as already mentioned, in no way diminishes the intrinsic efficiency manifested in preventing rotation and in suppressing the formation of the umbrella.

Whereas for the jets coming from conventional nozzles the bursting is permanently underlying, because there is always rotation of the jet in a round or polygonal flow channel and therefore latent risk of the appearance of the bursting in umbrella, this is no longer the case for the nozzle showing the configuration according to the invention. The first orientation tests in a steelworks with falling casting in ingot molds showed a 50% reduction in surface defects such as 'straws' on the first two ingots of each steel casting, while for each full load the reduction in surface defects were greater than 20%.

• en figure 1: les profils de la section triangulaire, ainsi que de la section en étoile à trois branches;
• en figure 2: une vue en projection horizontale d'une plaque inférieure mobile avec section ronde à l'entrée et à section en étoile à un niveau un peu plus bas;
• en figure 3: une vue pareille à celle de la fig. 2, sauf que la section profilée du chenal d'écoulement est cruciforme. La section de la partie supérieure 4, généralement immobile, de l'obturateur présente une section ronde 5.

Possible embodiments of the invention are illustrated in the accompanying drawings which show respectively:

• in FIG. 1: the profiles of the triangular section, as well as of the star section with three branches;
• in FIG. 2: a view in horizontal projection of a movable lower plate with a round section at the entrance and a star section at a slightly lower level;
• in FIG. 3: a view similar to that of FIG. 2, except that the profiled section of the flow channel is cruciform. The section of the generally stationary upper part 4 of the shutter has a round section 5.

As illustrated in Figure 1, the star section presents preferably the ends of branches 21, that is to say the tips of the spokes, rounded. These spokes are moreover also easier and more economical to produce by refractorists than the sharp points 11 of the strictly triangular section, represented by the thinnest line 1. The three lateral sides of the section represented by the thicker line 2 are re-entrant. The lateral necking 23, as shown, is for example between 3 and 4 mm relative to the corresponding side wall of the equivalent triangular section 1. In both cases the radius of a branch of the profile is covered with the axis relative displacement between the parts of a sliding shutter with at least one movable part and it points in the direction of movement of the movable lower part 3 which, in view of the opening of the casting passage, meets the orifice of round section in the upper plate.

In FIG. 2, the projection of the lower plate 3 to the left of the figure shows that over at least a few millimeters the section of the flow duct is circular 5, but that the underlying part has the star section 2 which extends To the bottom. In this case the guidance of the jet therefore does not start directly under the opening plate but slightly lower and it continues in the profiled end part of the nozzle.

Figure 3 shows a view similar to that of Figure 2, except that the profiled section 6 of the flow channel is cruciform. As illustrated, the ends of the arms of the cross can be rounded instead of being at right angles. The completely upstream part of the channel is also of round section 5, while further down the section has the profile 6. The radius of the profile illustrated remains constant from top to bottom, but the surface of the section at the entrance is slightly larger than on exit.

This new design according to the invention also has an additional advantage which plays especially during the rapid emptying of metallurgical vessels containing metal and slag, as is the case for steel plant converters, or when the level is left in a large surface container, such as tundish, lower very low. Indeed the anti-rotation action of the profile according to the invention is opposed to the formation of a too large flow vortex. Thus, as the vortex cannot really develop, there is no slag cone and / or air which is entrained by the center of the vortex and which generally requires a premature stop of the emptying with a loss material and productivity.

Claims (9)

Improved device for flowing molten metal, characterized in that the effective cross-section of the device, constituted by the surface of the conduit available for the flow of metal during the progressive opening of the passage, is first kept small and in that at least over part of its height the flow conduit has an active profiled passage section, the profile extending to the last end of the conduit and having a shape comparable to that of a star with three to four branches, the bottoms of the branches of the profile extending axially in the direction of flow and serving to guide the flowing metal. Device according to claim 1, characterized in that the bottoms of the branches of the three-pointed star profile are at an acute angle, the section therefore being substantially in a triangle. Device according to claim 1, characterized in that the bottoms of the branches of the four-pointed star profile are at right angles, the section therefore being substantially cross. Device according to one of Claims 1 to 3, characterized in that the active profile of the cross-section of the duct, with its channels extending axially, extends over the greater part of the length of the nozzle and occupies the lower part of that -this. Device according to one of claims 1 to 4, characterized in that the bottoms of the axial channels are rounded and in that at their intersections the spokes of the branches form angles between an obtuse angle and a right angle. Device according to one of Claims 1 to 5, characterized in that the profile of the cross-section of the flow conduit in the lower part of the nozzle is oriented in such a way towards the other upper part than a part pointed or narrow of the section is opened in the first place during the relative movement of the parts relative to each other. Device according to one of Claims 1 to 6, characterized in that the cross-section of the flow conduit is between 40% and 120% of the standard cross-section of a conventional nozzle with circular cross-section. Device according to one of Claims 1 to 7, characterized in that the ratio of the sections to the inlet and to the outlet of the flow conduit is between 1 and 1.15. Device according to one of Claims 1 to 7, characterized in that the ratio of the sections between the upper and lower parts of an assembly with relatively movable parts is between 1 and 1.15.

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