A SUBMERGED ENTRY NOZZLE Field of the invention

The present invention relates to a submerged entry nozzle to pour molten steel in the ingot moulds for slabs. State of the art

In the common industrial practice, submerged entry nozzles, hereinafter also designated as SENs, having a simply tubular shape with two ports obtained on the two sides of the lower end, are used in the continuous casting of slabs. Very often this solution is coupled to the slide gate regulation system, in other cases to the stopper system, in the inlet part for the molten steel.

Fluid dynamics and process problems occur in these entry nozzles. For the types of entry nozzles in which a slide gate is used, an asymmetry is produced in order for the flow to be forced to preferably occupy one side of the SEN. If, instead, a stopper is used as a regulation system, an asymmetry problem does not occur. When the flow descends to the level of the side ports, it starts to flow through them so that the circular section of the SEN becomes too wide with respect to the remaining vertically passing flow rate.

This results in a rotation of the flow being induced in the lower part of the SEN, the flow rotation possibly being: - always in the same rotation direction in the case the slide gate regulation system is used, because of the asymmetry induced by it;

- continuously alternating in an unstable manner between the two opposite rotation directions in the case the stopper is used, the latter not introducing any asymmetry on a specific side. The rotation produced inside the entry nozzle, it being either stable or unstable depending on the regulation system, as described, results in the molten steel flow not uniformly flowing out from the ports but rather tending to preferably occupy a side thereof. Furthermore, the impact of the descending current in the SEN on the completely closed and flat lower base makes the distribution of the flow rate on the two ports of the SEN continuously unstable and not balanced.

Disadvantageous consequences of this fluid-dynamical behaviour of the SENs of the known art are the following:

- an unstable motion field is generated within the crystalliser, with subsequent un-

stable conditions for the first solidification and the formation of longitudinal cracks,

- the jets produced by the ports of the SEN tend to impact on one of the broad sides of the crystalliser, which is always the same in the case of the slide gate, whereas in the case of the stopper it is both sides on which the jets impact in a random and unstable manner;

- a subsequent non-uniformity of the thermal flow extracted from the crystalliser, of the thickness and temperature of the forming solid skin is caused;

- a surface waviness of the meniscus is produced and thus non-uniform conditions occur during the course of the first solidification of the casting product; - the surface speed of the meniscus and the vorticity of the surface flow around the SEN may not be controlled, with the trapping of the covering dusts in the product;

- in case of possible injection of argon in the SEN, a non uniform distribution of the bubbling is produced within the crystalliser, with the subsequent possibility of the formation of pin-holes in the casting product.

Many attempts have been made to improve the problems set forth above. For instance, from document JP7232247A there is known a submerged entry nozzle for the continuous casting of steel products which aims to reduce the inclusion defects of the slabs. It suggests a tapering of the end section of the entry nozzle from which the steel flows out. At the terminal tip of the tube, for the passage of the molten steel there are a slot extending for the entire width of the tube and two circular holes on the two sides of the tube having symmetry axes parallel to the longitudinal symmetry plane of the slot. The results obtained in virtue of this entry nozzle are better, however, in the course of the casting, steel flows which are not completely satisfactory are generated within the ingot mould.

The present invention thus intends to overcome the drawbacks discussed above displayed by the known submerged entry nozzles, by means of an entry nozzle that produces a higher quality casting product. Summary of the invention It is the primary object of the present invention to provide a submerged entry nozzle that solves all of the fluid dynamics and process problems listed above, in a simple and effective manner. This object is achieved by an entry nozzle for molten steel, adapted to be used

submerged in an ingot mould for slabs, comprising an elongated tubular body, defining a longitudinal axis and a vertical symmetry plane passing through the longitudinal axis, having

- a central hole with an axis parallel to the longitudinal axis, - a first opening at a first end thereof for the inlet of the molten steel,

- a pair of side ports at a second end for the outlet of the molten steel, defining respective average outlet directions for the flow, each direction forming a first angle in the range between 20° and 90° with the longitudinal axis,

- a second opening at the second end of the tubular body for the outlet of the molten steel, having an outlet direction substantially parallel to the longitudinal axis and a passage section having a surface such as to allow a steel flow rate much lower than the flow rate of the side ports, wherein there are provided, in proximity of the second end of the tubular body and within the latter, two surfaces tilted with respect to the longitudinal axis and converging towards the second end, the ideal or actual intersection line of said tilted surfaces laying on a vertical symmetry plane.

As the fluid dynamical problems depend on the design of the SEN, in virtue of the specific configuration of the inner conduit, of the outlet ports, and of the opening on the bottom, the SEN according to the present invention ensures a stable and balanced flow and is capable of ensuring an optimal process that does not display the problems previously set forth.

The dependent claims describe preferred embodiments of the invention. Brief description of the drawings Further features and advantages of the invention will become more apparent in light of the detailed description of preferred, though not exclusive, embodiments of a submerged entry nozzle shown by way of non-limitative example with the aid of the accompanying figures in which:

Figure 1 shows a view of the entry nozzle according to the invention; Figure 2 shows a side view of the entry nozzle in Fig. 1 ; Figure 3 shows an axial section of the entry nozzle in Fig. 1 along a vertical plane; Figure 4 shows an axial section along plane C-C of a first embodiment of the entry nozzle in Fig. 3; Figure 5 shows an axial section along plane C-C of a second embodiment of the

entry nozzle according to the invention;

Figure 6 shows a section which is perpendicular to the longitudinal axis, along plane A-A, of the entry nozzle in Figure 5;

Figure 7 shows a section of the casting product in the ingot mould with the profile of the thickness of the skin during solidification with an entry nozzle of the state of the art;

Figure 8 shows a section of the casting product in the ingot mould with the profile of the thickness of the skin during solidification with an entry nozzle according to the invention. Detailed description of preferred embodiments of the invention

With reference to the Figures, a first embodiment of the entry nozzle or SEN, which is the object of the present invention, is shown, globally indicated with reference numeral 1. The SEN has an elongated body having a tubular form 2 that preferably displays a longitudinal symmetry axis X. Such an elongated tubular body 2 is provided with a first opening 3 at a first end, that forms the inlet for the molten steel, cooperating, for instance, with a tundish or an undertundish or another appropriate container, depending on the continuous casting plant in which the entry nozzle is used. This passage may be opened or closed, according to the operative requirements, di- rectly or indirectly by means of a stopping and/or control device for the flow, as for instance a slide gate system or a system incorporating a stopper. The tubular body 2 is provided with a central hole 4, having a preferably but not exclusively circular section, the axis of which is parallel to the longitudinal axis X or, preferably, coincident therewith. Two outlet side ports or mouths 5, 6 for the steel which is poured in the ingot mould are provided at the end of the tubular body 2, opposite to that end where the first opening 3 for the inlet of steel is positioned. During the casting process, these ports remain submerged within the molten steel bath under the level of the meniscus. The ports 5, 6, having the same shape and section, are obtained in the side walls of the tubular body 2. These two ports 5, 6 have their side walls, which determine the outlet flow of the steel, slightly diverging towards the outside of the tubular body, as shown in Fig. 1 and in Fig. 6. Arrows F1 and F2 indicate the average

outlet direction of the steel flow respectively from the ports 5 and 6 and substantially lay on a same vertical symmetry plane of the tubular body 2 passing through the longitudinal symmetry axis X of the tubular body itself. The side ports 5 and 6 have a trapezoidal shape with the shorter base of the tra- pezium placed downwards, in order to insert themselves in the tapered shape of the tip of the tubular body 2 of the entry nozzle 1.

The ports 5 and 6 are divergent along the outlet direction of the flow from the ports themselves both in vertical section (Figure 4) and in horizontal section (Figure 6), maintaining a trapezoidal section having a progressively increasing surface from the axis X of the tubular body towards the outer surface of the tubular body 2.

The upper connector 8 of the ports 5 and 6, corresponding to the longer base of the trapezoid the ports display the shape of, is configured and dimensioned so as to exploit the so-called "Coanda effect" and therefore ensure a uniform and stable fluxing of the molten steel also in the upper part of the ports themselves. With reference to Figure 4, the following angles are defined:

- "Y" is the angle that the tangent to the external edge of the upper connector 8 of the port 5, 6 forms with the axis X;

- "β" is the angle that the tangent to the lower outer edge 5', 6' of the mouths 5, 6 forms with the axis X; - "α" is the angle that the average direction of the outlet flow F ₁ , F ₂ forms with the axis X.

According to the present invention said angles may advantageously take values included in the following ranges:

20° < α < 90° (α -15°) ≤ β ≤ α α ≤ γ < (α + 15°)

In proximity of the second end or tip of the tubular body 2, the inside of the tube is defined by two planes 9, 10 tilted with respect to the longitudinal axis X and converging towards the tip of the tubular body 2. The ideal or actual intersection line of these two planes 9, 10 lays on the symmetry plane Y of the tubular body 2.

These two planes 9, 10 form a narrowing of the inside of the tip of the tubular body associated to the trapezoidal shape of the ports 5, 6, and the global geometry reduces the passage section of the steel flow in proportion to the flow rate out-

flowing from the side ports.

These two tilted planes 9, 10 may advantageously be combined to a corresponding outer narrowing of the walls 9', 10' of the tubular body 2. A further opening, provided on the bottom 4' of the entry nozzle 1 , has an outlet direction of the steel flow substantially aligned with the longitudinal axis X of the tubular body 2. It may have a passage section, which is advantageously circular, preferably obtained in the form of a calibrated hole 1 1 , with a much smaller surface than the passage section surface of the central hole 4 of the tubular body 2 and of the side mouths 5, 6. This further opening serves to stabilise the pressure cell which is generated by the flow when it impacts the bottom of the SEN, thus ensuring the equivalence of the flow rate of the flow outflowing from the side ports 5, 6 and avoiding the transversal huntings of the flow within the SEN. A minimum flow rate of steel flows out through said calibrated hole 1 1 , in virtue of the small size thereof. Preferably, but not necessarily, the outer lower edge 5', 6' of the mouths 5, 6 is placed beyond the inner bottom line 4' of the tubular body in the direction of the tip.

In virtue of the synergy of all these constructive elements this SEN is capable of giving a complete and stable fluxing in the terminal part of the SEN. The flow through the ports 5, 6 is balanced, stable and uniform.

This behaviour of the flow in the terminal part of the SEN and through the ports allows to obtain a series of features of the fluid dynamics in the crystalliser, and accordingly the following beneficial advantages in terms of the process:

- an extremely stable motion field within the crystalliser, with optimal conditions for the formation of the first solidification;

- a jet distribution within the crystalliser which is well centred and does not tend to impact on one of the broad sides of the crystalliser;

- a subsequent optimum uniformity of the thermal flow extracted from the crystalliser, of the thickness and of the temperature of the forming solid skin; - the minimisation of the surface waviness of the meniscus with subsequent conditions of good uniformity for the first solidification of the product;

- a uniform distribution of the surface speed of the meniscus and the absence of vorticity of the meniscus around the SEN, thus avoiding the trapping of the cover-

ing dusts in the product;

- a better uniformity in the distribution of the bubbling within the crystalliser (in the eventuality of argon injection in the SEN).

In a second alternative embodiment, shown in Figures 5 and 6, the opening made in the lower closure or bottom of the SEN may also have the shape of a longitudinal slot or eye 12, instead of a circular hole, but having a longitudinal extension not exceeding the inner diameter of the hole 4 of the tubular body 2. In both cases, anyway, said opening (calibrated hole 1 1 or longitudinal slot 12) is not a supply port for the flow in the crystalliser having a flow rate comparable to that of the side mouths 5, 6, but rather serves to carry out the outlet of a minimum amount of flow rate which does not change the flow course within the crystalliser but serves to give stability to the flow within the SEN. Advantageously in order to obtain this effect it is provided that:

- the percent ratio between the passage section surface of said opening and the passage section surface of the central hole 4 of the tubular body 2 is comprised between 1 and 20%;

- the percent ratio between the passage section surface of said opening and the sum of the outer trapezoidal section surfaces of the side mouths 5, 6 is comprised between 1 and 25%; - and the percent ratio between the flow rate outflowing through said opening and the flow rate through the central hole 4 is comprised between 1 and 30%. In particular, in the embodiment wherein said opening is a calibrated hole 1 1 :

- the percent ratio between the passage section surface of the calibrated hole 1 1 and the passage section surface of the central hole 4 of the tubular body 2 is comprised between 1 and 5%, preferably equal to about 2,5%;

- the percent ratio between the passage section surface of the calibrated hole 1 1 and the sum of the outer trapezoidal section surfaces of the side mouths 5, 6 is comprised between 1 and 6%, preferably equal to about 3%;

- and the percent ratio between the flow rate outflowing through the calibrated hole 11 and the flow rate through the central hole 4 is comprised between 1 and 4%, preferably equal to about 2%.

Instead, in the embodiment wherein said opening is a longitudinal slot 12:

- the percent ratio between the passage section surface of the longitudinal slot 12

and the passage section surface of the central hole 4 of the tubular body 2 is comprised between 10 and 20%, preferably equal to about 16%;

- the percent ratio between the passage section surface of the longitudinal slot 12 and the sum of the outer trapezoidal section surfaces of the side mouths 5, 6 is comprised between 15 and 25%, preferably equal to about 20%.

- and the percent ratio between the flow rate outflowing through the longitudinal slot 12 and the flow rate through the central hole 4 is comprised between 10 and 30%, preferably equal to about 12%.