Refractory nozzle for the continous casting of steel

Abstract

 Invention relates to a submerged entry nozzle (21) for the thin slab casting comprising a divider (14) having a geometry of the ogival type, continuous contours and an angle θ at the vertex comprised between 30° and 60°. The divider in its lower portion is narrowing symmetrically with its sides (15,15') towards the median vertical axis (13).

Description

[0001] The present invention relates to nozzles for the continuous casting of molten metal or alloy and in particular to a submerged entry nozzle that improves the flow behavior associated with the introduction of liquid metal into a mold through a casting nozzle. More particularly, the present invention relates to submerged entry nozzles for the continuous casting of conventional slabs, thin slabs and so-called semi-thin slabs. Conventional slabs are generally 120 to 450 mm thick, semi-thin slabs 70 to 120 mm thick and thin slabs are less than 70 mm thick. Thin slabs are generally cast at high speed (>5 m/min) with steel flows up to 8 ton/min so as to line-up continuous casting with rolling-finishing processes.

[0002] Submerged entry nozzles (SEN in the following) also called dip pipes are used:

• to carry molten metal from an upper metallurgical vessel into a mold, in particular from a tundish into a casting mold, without exposing the molten metal to air,
• to evenly distribute the molten metal in the mold so that heat extraction and solidified shell formation are uniform, and
• to deliver molten metal to the mold in a quiescent and smooth manner, without excessive turbulence particularly at the meniscus, so as to allow good lubrication, and minimize the potential for surface defect.

[0003] A SEN is commonly a pipe with a single inlet on one end and one or more exit ports located at or near the other end. The inner bore of the SEN between the inlet region and the exit region is often simply a cylindrical axially symmetric pipe section.

[0004] The nozzles for conventional slab casting have typical inner bore dimensions of 60 to 80 mm diameter and 600 to 900 mm length. The exit region of the nozzle may simply be an open end. The nozzle may also incorporate two oppositely directed outlet ports in the sidewalls of the nozzle where the end of the pipe is closed. The opposite directed outlet ports deflect molten metal stream with respect to the vertical. The nozzle inlet is connected to the source of molten metal, the upper metallurgical vessel or the tundish. The rate of molten metal flow through the SEN is regulated at or upstream of the SEN inlet by means of a valve device. Commonly, the valve device is a stopper rod having a tip that is positioned proximate to the nozzle inlet.

[0005] The tundish is filled with molten metal and head pressure in the tundish is nearly constant during casting.

[0006] Molds for slabs are formed of four walls extending vertically with horizontal cross-section having two sides with larger length than the other two sides. The submerged part of the nozzle is designed to fit the internal size of the mold and so as to keep an adequate distance from the walls. The cross section of the exit region of a submerged entry nozzle for slab casting can be rectangular, polygonal or elliptical with outlet ports directed towards the narrow sides and/or the lower part of the mold.

[0007] The submerged entry nozzle for thin slab casting may comprise a first portion having an essentially circular cross section, a second portion or transition portion in which the geometry changes from an essentially radial symmetry to an essentially planar symmetry and a third portion, the diffuser or diffusion portion. The SEN may also comprise a divider or baffle positioned in the diffusion portion or exit region of the pipe to divide and deflect the stream of molten metal exiting the pipe.

[0008] Such nozzles are known in the art and are for instance disclosed in US 6,464,154 patent.

[0009] Submerged entry nozzles have to meet various requirements depending on the problem addressed. Typical problems of this technology are widely described in the pertinent literature and imputable to various causes. In particular when the casting rate is high, the stream of molten metal discharged from the SEN has the tendency to cause the mass (or bath) of molten metal within the heart of the slab (which is solidified in its upper portion on the outside only) to strongly circulate within the upper region of the mold. Thereby, it is likely that some portion of the circulating mass reaches the top surface of the bath of molten metal in the mold with excessive dynamic pressure, kinetic energy, and turbulence so as to produce deleterious waves on the bath surface. As a consequence, the liquid lubricating slag that is artificially produced by the melting of a special powder on the bath top surface (or 'meniscus') of the slab tends to collect in the most depressed parts of the wavy meniscus, resulting in poor distribution of lubricant, increased wearing of the mold, poor surface quality of the slab and non-uniform heat exchange between the slab being formed and the mold, which is a possible cause of cracks in the slab. The amplitude (or height) of the waves on the meniscus can be used to estimate the extent of the problem and give an idea of the minimum thickness of liquid slag required on the meniscus, increased by a fraction depending on the vertical oscillatory movement of the mold.

[0010] In addition, the regions of the bath surface where the circulating liquid metal reenters into the liquid bath show high meniscus curvature and in these regions the particles of powder and lubricating slag are easily entrapped in the slab being formed, this being a further cause of cracks and other surface flaws (defects) in the slab. The speed of the circulating liquid metal flows on the bath surface is related to the formation of vortexes and surface depressions at the re-entering regions of the circulating liquid metal flow. Vortexing and surface depression increase the potential for the entrainment of powder and slag particles within the circulating liquid metal and subsequent entrapment of particles in the solidified slab. The maximum speed of flow on the bath surface and the differences between speeds on the bath surface when comparing similar locations on either side of the SEN can also be used to characterize the extent of the problem.

[0011] Furthermore, the circulating liquid metal mass, when reaching the bath surface, introduces turbulent fluctuations in the meniscus area and in the liquid metal flow on and near the meniscus. The turbulence existing at the meniscus is an important cause of the unstable control of the molten metal level in the mold, as well as of the chemical and physical wear (or slag erosion) of the refractory material that forms the SEN, limiting its service life.

[0012] Another value characterizing the extent of the problem is the horizontal velocity of recirculating flows measured at given positions and at a certain depth (about 30-60 mm) under the meniscus (submeniscus velocity). This value should not exceed a given upper limit, otherwise deleterious waves, particle entrainment and entrapment, and excessive bath surface turbulence can be expected.

[0013] On the contrary, an excessively stagnant meniscus without a continuous supply of novel metal at higher temperature could lead to premature freezing (solidification) of the meniscus and a poor melting of the artificial powders which generate the liquid slag. For this reason, a lower limit of said value of submeniscus velocity has also to be settled.

[0014] With regard to the solidification occurring within the slab, possible unsteady swirls and recirculations of the liquid mass flowing from the SEN affect adversely the process. To avoid adverse effects, the flow must be as stable and homogeneous as possible. It would be advisable to have a feed stability without varying flows within the hardening (solidifying) slab and a distribution of the fluid mass as much symmetrical as possible with respect to the longitudinal axis of the slab, with wide and foreseeable recirculations without dead zones, in order to have the maximum thermal homogeneity in the horizontal sections. From this standpoint, it appears to be advantageous to have a so-called "double-loop" flow pattern within the liquid mass in the upper portion of the slab, with the outgoing stream from each of both ports of the SEN splitting into two flow loops as the stream approaches a narrow face of the slab: (1) one flow loop comprising an eddy that circulates upward near the narrow face, then turning toward the SEN (the SEN being positioned near the central longitudinal axis of the slab) near and along the meniscus,and then turning downward toward the bottom of the SEN and the SEN ports near the outside of the SEN, and (2) the other flow loop comprising an eddy that circulates downward near the narrow face, then turning toward the central longitudinal axis of the slab, then turning upward toward the bottom of the SEN and the SEN ports. The double-loop flow pattern is mirrored on each side of the SEN, which is to say on each side of the central longitudinal axis of the slab, since the SEN has two ports that are also mirrors of each other on each side of said axis, and ideally the mirrored flow pattern would be symmetric about the central longitudinal axis. However even if achieving the double-loop flow pattern, upward circulation of the liquid metal mass with excessive speed, dynamic pressure, kinetic energy or turbulence can be associated with the problems mentioned above.

[0015] To overcome the above-mentioned inconveniences it is known to use electromagnetic braking devices as described in US 2004/0244942. The casting streams exiting the SEN are braked by a magnetic field which is applied between the broad sides of the continuous casting mold to reduce the turbulences and to slow the circulating mass, ensuring a more uniform casting of the steel strand. However, such equipment is hard to control. When the magnetic field intensity is too high, it causes either an undesirable reflecting effect on the streams oriented towards the braked zone increasing turbulence, or generates an excessively stagnant meniscus leading to the problems mentioned above, and on the contrary when the magnetic field intensity is too low, it may have either little effect or influence on the flow or alter the flow pattern in a deleterious manner. In addition, electromagnetic braking devices are expensive equipments that require adapted molds which are also very expensive.

[0016] It is finally to be mentioned a further inconvenience due to the fact that within the molten metal are present oxides which tend to settle on the inner surfaces of the SEN, changing its internal geometry, resctricting or clogging the flow passage and adversely affecting the flow characteristics in the mold.

[0017] Except for this last inconvenience which get worse in the case of low flow rates in the various passage cross-sections and in case of low turbulence (Reynolds number under certain values), all the other previously mentioned drawbacks get worse at high flow rate (higher casting speed) and for larger slabs.

[0018] A direct consequence of the above mentioned drawbacks is the poor quality of the cast slab.

[0019] The document EP 0925132 addressed some of the drawbacks hereby mentioned and discloses a submerged entry nozzle that provides a good slowing down of the flow and reduced clogging phenomenon due to the oxides, through the accomplishment of the double stream diffusion. However, at high casting rates or steel flow greater than 2,5 ton/min, instability and detachment of the flow occurs along the contours of the flow divider. Vortexes are generated, flow is altered along the contours of the flow divider and a vein partition (flow separation) phenomenon arises. These vortexes have the tendency to be dragged by the stream within the mold and to cause an excessive fluid friction (turbulent interaction) between the opposed narrow surfaces of both obtained exiting flows with consequences of instability, asymmetry, and oscillation of the mold flow pattern, as well as excessively rapid circulation of flows towards the meniscus (bath surface) without the proper penetration of the liquid mass.

[0020] In addition, there are limits bound to a pressure drop in the region of the flow regulating valve, down to so low absolute values as to jeopardize, in consequence of oscillatory phenomena, the stability of regulation and to bring about a rapid dissolution (vaporization) of the flow regulating surfaces formed of refractory material. In theoretically stagnant conditions with closed regulating valve and gas-tight SEN first filled with molten steel then dipped in a mould, if the distance between the closed valve and the free surface of the metal bath in the mould exceeds 1.4 m, then high vacuum conditions would be realized and a meniscus formed within the upper portion of the SEN (as if it were a liquid steel-based barometer). The dynamic casting process by gravity with open control valve differs somewhat from this static condition, but the pressure in the upper portion of the SEN is still substantially controlled by the total ferro-static head resulting from the elevation difference between the valve closure and bath surface in the mould. The most critical section of the controlling valve is the variable and reduced one in which a sudden flow acceleration takes place with a consequent increase of turbulence and an important dissipative phenomenon. The result is a sudden pressure drop just in such regulation region. The magnitude of the sudden pressure drop increases as the length of the SEN increases in consequence of the elevation difference mentioned above. In the case of a SEN as described in EP 0925132A operating at high casting rates, the sudden pressure drop causes flow instabilities and oscillations, thereby worsening the aforementioned problem of instability and detachment of the flow occuring along contours of the flow divider and in turn causing instabilities and oscillations in the streams outgoing from the SEN ports.

[0021] Ideally to increase pressure in the upper portions of the SEN, particularly just downstream of the regulation region, and thereby alleviate the problems mentioned above, the SEN should be as short as possible. However, this requirement is difficult to meet in particular for big tundishes, with steel flow rates up to 8 ton/min. Mounting requirements of the SEN under the tundish makes it difficult to limit the length of a SEN and lengths in the range of 1000 to 1300 mm can be required.

[0022] In addition, the increase of the kinetic term of Bernoulli's equation as flow rate increases reduces further the pressure realized in the SEN.

[0023] It is therefore an object of the present invention to provide a SEN that avoids the above mentioned drawbacks and this also at high flow rates, up to 8 tons of steel per minute.

[0024] This object is achieved by a SEN having a new divider shape. The divider has an ogival or rocket-shaped geometry and its contour (outline) is continuous and smooth without the presence of angular points according to the analytical definition (non-derivable points of the plane function describing the contour because the first right and left derivatives do exist and are distinct). Preferably, the ogival type divider is combined with a second portion and a third diffusion portion respecting defined ratios and heights. The angle θ (theta) at the vertex of the divider is comprised between 30° and 60°, most preferably 45°. Angle θ is the angle formed between straight lines, drawn on each side of the vertical centerline of the nozzle, tangent to the contour of the upper part of the flow divider at the points of least curvature of said contour.

[0025] Another advantage of the SEN is that it renders superfluous or not strictly necessary the use of electromagnetic brakes, while having shown an optimal working capability even in combination with such apparatus and with different arrangements thereof. With a SEN according to the invention, the steel flow velocity is reduced progressively in the section approaching the outlet ports. This way, two slow and steady fluid streams are obtained of nearly identical flow rates, without breaking down of the fluid veins (without detachment or separation of flow) and without vortexes. The residual kinetic energy is more easily dispersable within the body of the liquid mass of the slab being formed, thus giving rise to non-swinging (non-oscillating) circulation loops, symmetrical with respect to the vertical axis (central longitudinal slab axis, with minimal wave amplitude at meniscus, with respect to both its center and the more depressed regions, to submeniscus velocities within an optimal range, and to reduction of turbulent fluctuations of meniscus level allowing improved mold level control.

[0026] The dependent claims relate to preferred and alternative embodiments of the SEN according to particular aspects of the present invention.

[0027] Another object of the invention is a SEN according to claim 12. This SEN has a restriction located under the flow regulation region. The resulting head restriction increases the absolute pressure level in the region of the regulation valve in order to stabilize the control thereof and reduce the erosion phenomenon of the refractory surfaces controlling the stream.

[0028] These and other objects, advantages and characteristics of the SEN according to the invention will be better understood by those skilled in the art when reading the following description of a preferred non limiting embodiment of the invention, given with reference to the drawings in which:

Fig. 1 shows a longitudinal cross sectional view of a nozzle according to the prior art.

Fig. 2 shows en enlargement of the diffusion portion of the SEN of Fig. 1

Fig. 3 shows a longitudinal sectional view of a submerged entry nozzle (SEN) according to the invention, while Figs. 4, 5 and 6 show cross sectional views of cross-sections 10, 20 and 11 of the nozzle of Fig. 3.

Fig, 7 shows a longitudinal sectional view of a preferred embodiment of a nozzle according to the invention.

[0029] Fig. 1 shows a nozzle 1 according to the prior art as shown in EP 0925132. Molten metal flows by gravity from an upper vessel or tundish 3 into a slab casting mold 5. Tundish is filled with molten metal 2 and the level of metal in the tundish is maintained nearly constant, except at the beginning and at the end of the casting. Head is thus also nearly constant. The mold 5 comprises four vertical walls, two sides in horizontal section being longer than the other two.

[0030] The SEN comprises a first portion 6 having an essentially circular cross section, a second portion or transition portion 8 and a third portion, the diffuser or diffusion portion 18. The upper part of first portion 6 of the nozzle 1 is attached in a known manner to the tundish 3. The flow regulation in this case is made by the help of a stopper 19. In the transition portion 8, the geometry changes from an essentially radial symmetry to an essentially planar symmetry. In the transition portion 8, the nozzle 1 dimension is lowered in one direction perpendicular to the mold larger dimension and is enlarged in the other direction parallel to the mold larger dimension. The diffusion portion 18 also called flat portion or diffuser 18 has bottom outlet ports 9, 9'. The outlet ports are located under the level of molten metal 17 during use.

[0031] The diffusion portion 18 comprises a central flow divider 4 integral with both wider walls of the diffuser and adapted to divide the flow into two separate channels 16, 16' terminated at the bottom by two outlet ports 9, 9' discharging downwards. Divider 4 is shown in greater detail in Fig.2. The divider is bulged and comprises linear sections that meet at distinct angles to form distinct angular points of intersection. The meeting of the linear sections at said points results in discontinuities in the smoothness of divider 4 at said points and these discontinuities originate detachments of the flow, thereby generating vortexes that can be dragged into the molten bath in the mold by the streams flowing within channels 16 16'.

[0032] Fig. 3 shows a nozzle 21 according to the present invention. The nozzle 21 comprises a first portion 6 having an essentially circular cross section, a second portion or transition and division portion 23 and a third portion, the diffuser or diffusion portion 24. Nozzle 21 also comprises an ogival flow divider 14 having two parts, an upper part enlarging downward and a lower part narrowing downward. The first portion 6 of nozzle 21 has an entrance section 7 adapted to the stopper nose shape to regulate the flow of molten metal and an exit having a circular cross-section 11. The second portion 23 entrance section corresponds to the first portion 6 exit section 11. The second portion 23 has an intermediate cross-section 10 and an exit cross-section 20, both cross-sections being of oval, elongated and of planar symmetry. The transition and division portion 23 of nozzle 21 comprises the upper part of central flow divider 14. Cross-section 20 is coincident with the widest part of flow divider 14. The entrance section of the diffusion portion 24 corresponds to the exit section 20 of second portion 23. The intermediate cross-section 10 is coincident with the top of flow divider 14. The area of said cross-section 10 for the passage of flow is preferably lower than area of cross-section 11 for the flow passage. The area of cross-section 20 for the passage of flow is preferably lower than the area of cross-section 10 for the passage of flow. The vertical axes passing through the medium point of each of the two cross-sections of the channels 16, 16' of the diffuser at the level of section 20 are indicated as 22 and 22'.

[0033] According to the present invention, the ratio between cross-section areas 10 and 11 is in the range from 0.6 to 0.8. Cross-sections 11, 10 and 20 are shown in Figs. 4, 5 and 6, respectively. Between cross-sections 11 and 10, the side walls of nozzle 21 are divergent downwards in a direction parallel to the larger dimension of the mold, in all other directions, walls are convergent, thus causing a reduction of cross-section area downwards. According to the present invention, the reduction of the cross-section area between section 11 and section 10 is accomplished over a short length comprised between 4 and 6 times the hydraulic radius of the exit section 11 of first portion 6: the hydraulic radius is equal to the ratio between the cross-section area and the relevant wet periphery (for circular sections as it is the case here, the hydraulic radius is equal to 1/4 of the diameter). Basically, from the fluid-dynamic point of view, the SEN according to the invention provides a length of strong acceleration of the flow of molten material between cross-section 11 and cross-section 10 as well as a further acceleration between section 10 and lower section 20. The maximum flow velocity takes place at section 20 and gradually decreases afterwards along both channels 16 and 16' while maintaining the contact with the walls of the nozzle. Section 20 is below the upper vertex of flow divider 14 and corresponds to the largest width of the divider 14. The high flow rate and turbulence in section 20 reduces oxides deposition in the flow dividing zone.

[0034] In a preferable embodiment, the passage cross-section area of both channels 16 and 16' becomes narrower between cross-section 10 and cross-section 20. This is achieved by a suitable shape of the flow divider 14. The flow is then slowed down in the parts of the diffusion channels 16 and 16' located under cross-section 20. Ideally, the velocity at outlets ports 9, 9' should not exceed 1.2 m/sec for any working condition.

[0035] Preferably, the outlet ports 9, 9' are generally rectangular in shape with a so-called "aspect" ratio between the long side and short side at that cross-section comprised between 3 and 10. The inner sidewalls 12, 12' of diffusion portion diverge symmetrically and downwards with respect to vertical axis 13. In a particular embodiment as shown in Fig.7, walls can diverge with a curvature that is increasing from the top to the bottom and is a function of the maximum possible flow diffusion without vein detachment at the operation rates, the diffusion portion being bell shape. Through an analytical definition, when the curve pertains to plane (x, y) with x and y referring to horizontal and vertical positional coordinates relative to the start of curvature at cross-section 20 and y'(x) and y"(x) referring to the first order and second order derivatives of the function y(x) that describes the curved portion of inner side walls 12, 12', then the curvature is expressed by the formula: c=y''(x)/(1+y'²(x))^3/2.

[0036] The sidewalls 15, 15' of flow divider 14 face towards inner sidewalls 12, 12', respectively. Flow divider 14 narrows in its part located below cross-section 20 as sidewalls 15, 15' approach each other to form two angles β with vertical axis 13. Preferably each angle P is ≤8°. Side walls 15,15' can consist of straight lines or curved contours. Angle β is the half angle formed between straight lines, drawn on each side of the vertical centerline of the nozzle (axis 13), tangent to the contour of the lower part of the flow divider at the points of least curvature of said contour. The contour of the flow divider 14 has to be continuous and derivable without the presence of distinct angular points of the intersection of linear sections, so as to avoid any discontinuities of smoothness. This way, detachments of the flow passing in the vicinity of the flow divider 14 (that would otherwise unavoidably give rise to vortexes which, being then dragged by the stream, interfere with and take away firmness from the flow within the mold) within flow channels 16, 16' are avoided.

[0037] Preferably, the side walls 12,15 and 12', 15' are symmetrical to the vertical plane respectively comprising axes 22 and 22' and perpendicular to the large walls of the diffuser below cross-section 20. Fig. 7 shows the preferred embodiment of flow divider 14 in which the geometry is of the ogival type, with an angle θ at the vertex of 45° wherein the line segment forming the intersection between the upper part of the flow divider with a vertical half-plane, having its origin in axis 13 and parallel to the large walls of the diffuser, and symmetrical with respect to the vertical plane passing through axis 13 perpendicular to the large walls of the diffuser, is a well connected sequence of circle arcs and straight segments, without discontinuities or angular points. Angular points are avoided by maintaining tangency at the joining points (or intersections) of segments. Furthermore the radii of said circle arcs are increasing from the top downwards and the lowest arc is well connected towards the bottom without any discontinuity or angular points with the segment of one of the two converging straight lines 15, 15' according to said angle β.

[0038] In order to increase the pressure in the regulation region, a throttle X is located in the first portion 6 under the regulation region 7 and at a distance from the latter comprised between 4 and 8 times the hydraulic radius of said first portion. The restriction has the form of a disk provided with a circular gauged hole whose hydraulic radius with respect to the hydraulic radius of the first portion 6 is in the range of 0.4 to 0.6 and whose thickness with respect to the diameter of the gauged hole is in the range of 0.3 to 0.7. Throttle X restricts the magnitude of the sudden pressure drop in regulation region 7 and thereby alleviates previously described flow instabilities and oscillations and thus increases the stability of flow regulation and lessens the damage to the refractory materials of flow regulating surfaces.

[0039] Possible additions and/or changes may be introduced by those skilled in the art into the above described and explained embodiment of the SEN according to the present invention without departing from the scope of the invention. In particular, the submerged entry nozzle 21 instead of being provided with regulation region 7 could be directly connected through a flange in a known manner with the bottom of container 3, whereas the regulation could be achieved through another element placed in container 3. In an alternative embodiment, the SEN 21 could also be secured through a flange, still in a known manner, below a flow regulating sliding gate and placed on the bottom of vessel 3, to operate in a known manner by selectively shutting the passage port formed between two perforated and opposed plates sliding over each other.

1. Nozzle according to the prior art.

2. Molten metal.

3. Upper vessel (tundish).

4. Divider according to the prior art.

5. Slab casting mold.

6. First portion of SEN.

7. Entrance section of first portion/regulation region.

8. Second portion of prior art SEN (transition portion).

9. 9'. Bottom outlet ports.

10. Exit of second portion.

11. Exit of first portion.

12. 12' Inner sidewalls.

13. Nozzle vertical axis.

14. Divider according to the invention.

15. 15'. Sidewalls of divider lower portion.

16. 16'. Channels in diffuser.

17. Molten metal level in mold.

18. Third portion of prior art SEN (diffusion portion).

19. Stopper.

20. Cross-section coincident with transition section of divider.

21. Nozzle according to the invention.

22. 22' Axes of the channels 16,16'

23. Second portion of SEN according to the invention (transition and division portion)

24. Third portion of SEN according to the invention (diffusion portion)

Claims

1. A submerged entry nozzle (21) for casting molten metal from an upper vessel (3) into a mold (5), the nozzle (21) comprising:

- a first portion having an essentially circular cross section (6); the upper end of said first portion being connected to the upper vessel (3);

- a transition and division portion (23) in which the symmetry of the nozzle passes from an essentially circular to a planar symmetry; and

- a diffusion portion (24) comprising a divider (14) and having outlet ports (9, 9') that correspond to two separate passages (16, 16') with respect to the vertical axes of the nozzle (13), characterized in that:

- the divider has a geometry of the ogival type and is symmetrical with respect to the vertical axis of the nozzle (13) and comprises two portions, a first upper portion enlarging downward and a lower second portion narrowing downward;

- the divider has continuous contours without the presence of angular points;

- the angle at the vertex θ (theta) of the divider (14) is comprised between 30° and 60°, preferably 45°; and

- the divider (14) in its lower portion narrows symmetrically with its sides (15, 15') towards the median vertical axis (13).

2. A nozzle according to claim 1, characterized in that said narrowing sides (15, 15') of said divider (14) comprise straight contours that form with said vertical axis (13) two angles β≤8°.

3. A nozzle according to claim 2, characterized in that:

- the line segment forming the intersection between the upper part of the flow divider (14) with a vertical half-plane, having its origin in the axis (13) and parallel to the large walls of the diffuser, and symmetrical with respect to the vertical plane passing through axis (13) perpendicular to the large walls of the diffuser, is a well connected sequence of circle arcs and straight segments, without any discontinuity or angular points;

- the radii of said circle arcs are increasing from the top downwards; and

- the lowest arc is connected downwards, without any discontinuity or angular points, to the segment of one of said two convergent straight lines (15, 15').

4. A nozzle according to claim 1, characterized in that said narrowing sides (15, 15') of said divider (14) comprise curved contours whose curvature is increasing from the top to the bottom.

5. A nozzle according to anyone of claims 1 to 4, characterized in that:

- the ratio between the cross section area of diffusion portion (24) in its higher transversal passage section area (10) and the cross-section area (11) at the end of first portion (6) is in the range of between 0.6 and 0.8; and

- the divider (14) at the level of its transition section (20) between its first and second portion forms two passage areas for the stream, whose addition results in a total area being narrower than the cross section area (10).

6. A nozzle according to any one of claims 1 to 5, characterized in that the reduction in cross section area between sections (11) and (10) is carried out in a total length with respect to the hydraulic radius of the first portion (6) between 4 and 6

7. A nozzle according to any one of claims 1 to 6, characterized in that the outflow cross sections of outlets (9, 9') of each separate passages (16, 16') are nearly rectangular in shape with an "aspect" ratio between the long side and the short side comprised between 3 and 10.

8. A nozzle according to any one of claims 1 to 7, characterized in that said diffusion portion (24) has lateral inner walls (12, 12') that symmetrically diverge from a median vertical axis (13) and with respect to the latter are diverging from the top downwards.

9. A nozzle according to claim 8, characterized in that said inner side walls (12, 12') of the diffuser (24) are straight and form an angle α≤8° with vertical axis (13).

10. A nozzle according to claim 8, characterized in that the inner side walls (12, 12') of the diffusion portion (24) comprise curved contours whose curvature is increasing from the top to the bottom.

11. A nozzle according to any one of claims 1 to 10, characterized in that said straight or curved walls (12, 15) and (12', 15') are symmetrical with respect to the vertical plane respectively comprising axes (22) and (22') and perpendicular to the large walls of the diffuser (24) below cross-section (20).

12. A nozzle according to any one of the previous claims, characterized in that:

- the total length of the nozzle (21) is comprised between 1400 and 1000 mm;

- said nozzle comprises a throttle (X) located in the first portion (6) and under the flow regulation region (7);

- the ratio of the hydraulic radius of the passage of said throttle with respect to the hydraulic radius of the first portion (6) is comprised between 0.4 and 0.6; and

- said throttle (X) is spaced apart from the flow regulation region at a distance comprised between 4 and 8 times the hydraulic radius of the first portion (6).

13. A nozzle according to claim 11 characterized in that said throttle (X) has the form of a disk provided with a gauged circular hole.

Drawing