WO2018092090A1

WO2018092090A1 - Continuous casting device for thin slabs

Info

Publication number: WO2018092090A1
Application number: PCT/IB2017/057226
Authority: WO
Inventors: Alfredo Poloni
Original assignee: Danieli & C. Officine Meccaniche S.P.A.
Priority date: 2016-11-18
Filing date: 2017-11-17
Publication date: 2018-05-24
Also published as: IT201600116859A1; EP3541548A1; CN110198795A; CN110198795B; EP3541548B1

Abstract

The device for casting thin slabs consists of a crystallizer with two curved mobile walls (5, 6) made of material with a good heat conductivity, cooled and opposite the casting plane (Z), and two fixed lateral walls (7, 8), perpendicular to the first, refractory so as to form a cross section with a funnel shape.

Description

"CONTINOUS CASTING DEVICE FOR THIN SLABS"

FIELD OF THE INVENTION

The present invention concerns a continuous casting device, in particular to cast slabs with an extremely limited thickness, for example comprised between 5 and 40 mm.

BACKGROUND OF THE INVENTION

In the steel industry, a type of cast product with a parallelepiped geometry and a width predominant over thickness is identified by the term "slab".

Currently, the state of the art in casting devices is represented by two different families of machines: continuous casting with a crystallizer mounted on an oscillating bench with adjacent cooled plates, and that of twin roll strip casters. One such twin roll strip caster is described, for example, in US-A-2016/023268. The first category of casting machines comprises a vertically oscillating crystallizer consisting of plates made of material with a high coefficient of heat conductivity, cooled by cooling devices, such as pipes made in the thickness of the plates, to remove heat from the molten metal that is made to pass through the crystallizer. Thus, the liquid metal is solidified by the contact of the liquid metal with the cooled walls of the crystallizer.

When the product exits from the crystallizer, it has an external solidified skin, and a liquid or semi-liquid core inside.

At the exit from the crystallizer, to prevent the ferrostatic thrust of the liquid component inside the skins from generating bulging of the external surface, it is necessary to contain the slab with groups of rolls disposed under the mold and guide it to complete solidification.

Rolling stands are disposed downstream of the casting line, to reduce the thickness of the cast slab.

There are many disadvantages of this first category of machines, which affect different aspects, primarily those related to the product which, due to its slow solidification, has internal defects such as segregation, dendrites, and porosities. Moreover, with current technologies, plate-type crystallizers allow to cast thicknesses of less than 60mm. Added to these aspects is the complexity of the machine which has a high number of components, high bulk and high maintenance costs.

This first category also has disadvantages connected to the conformation of the crystallizer. For example, to make the heat exchange with the molten material efficient, continuous contact between the crystallizer walls and the metal must be guaranteed. The crystallizer walls, for this purpose, have a taper such as to follow the heat shrinkage of the metal that gradually solidifies and such as to prevent re- melting or sticking. Furthermore, these types of crystallizers provide to insert a discharger into the central upper part of the crystallizer to introduce molten metal between the walls.

The presence of the discharger, however, requires crystallizers with particular profiles, which increase both the construction difficulties and the control of the product during the casting step.

All of these aspects contribute to increasing the design complexity of the casting device and, with it, of the costs and controls during the operating steps. The other type of casting machines is represented by the so-called "Twin roll strip caster" technology, in which the machines comprise two counter-rotating rolls cooled internally, and the casting channel formed by them is closed laterally by refractory plates. With this type of machine, it is possible to obtain a high slab casting speed, even more than 60 m/min, and to obtain very thin products, formed by the union of two skins generated by the contact with the two cooled rolls and the liquid steel.

However, this type of casting machine has a very high cost of maintaining the components. In fact, in order to guarantee sufficient heat exchange, the rolls must be of considerable size, with high construction and maintenance costs.

Furthermore, in this type of machine, the cooled rolls allow to generate a skin of the metal that is cast which, as it exits from the roll, is just a few millimeters thick. The cast slab can have a maximum thickness of 3-4mm overall.

It is therefore a purpose of the present invention to obtain a continuous casting device for very thin slabs that is an alternative to the state of the art, which is simple, economical and less bulky.

Another purpose of the present invention is to obtain a continuous casting device able to produce a high quality cast product.

Another purpose of the present invention is to obtain a continuous casting device for slabs which allows to increase the casting speed with respect to traditional plate-type crystallizers for slabs.

Another purpose of the present invention is to obtain a continuous casting device able to cast slabs which are thicker than those that can be cast with twin roll crystallizers.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

These and other purposes, which will appear clear from reading the description of the invention, are obtained by means of a device for casting metal slabs that, in conformity with claim 1, comprises a crystallizer provided with two first walls facing each other with respect to a casting plane, and two second walls facing each other, with a smaller surface extension than the surface extension of the first walls, and associated with the first walls to define a concavity for containing liquid metal.

In accordance with one aspect of the present invention, the two first walls are defined by plates having a convex shape with a convexity facing toward the inside of the concavity. Moreover, the device comprises at least a pair of presser rolls disposed parallel to the casting plane and positioned at the exit end of the crystallizer.

Thanks to the solution of the device of the invention, advantages in quality, simplicity and economy are achieved, reducing the number of components used and their complexity. In this way it is possible to make thin slabs of quality with thicknesses comprised in particular between 5 and 40mm and preferably between 14 and 20mm.

In accordance with some embodiments of the present invention, at least one of the first walls is associated with at least one actuator configured to move at least one of the first walls along a curved path lying on a plane orthogonal to the casting plane. Moreover, by using a normal hydraulic movement the functioning is simplified, thus reducing the complexity of the device of the invention with respect to continuous casting devices for slabs in the state of the art.

The present invention also concerns a wall for a crystallizer for casting slabs, in which the wall is provided with an entrance edge through which, during use, the liquid metal is introduced, and with an exit edge through which, during use, the slab exits at least partly solidified. The wall, according to the invention, is defined by a plate with a convex shape, with a convexity that extends from the entrance edge to the exit edge.

The dependent claims concern preferred embodiments of the invention disclosed in the independent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention will become more apparent in the light of the detailed description of some preferred but non- exclusive embodiments, shown by way of non-restrictive example, with the aid of the attached drawings wherein:

- fig. 1 shows a lateral section of the casting device of the invention;

- fig. 2 is a three-dimensional view of the crystallizer belonging to the casting device of fig. 1 ;

- fig. 3 is another three-dimensional view of the crystallizer of fig. 2,

- fig. 4 shows a lateral section of the crystallizer of fig. 2;

- fig. 5 is a plan view of the sealing rolls of the casting device of the invention;

- fig. 6 is a schematic representation of the oscillatory motion of the crystallizer of the casting device of fig. 1.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

With reference to the attached drawings, a casting device for thin slabs, indicated overall by the reference number 100, comprises, in a preferential but non-exclusive configuration, a mold 1 or casting apparatus provided with a crystallizer 2 configured to solidify the liquid metal that is introduced into it. The crystallizer 2 comprises two first walls 5, 6 with a conformation with plates that are located facing each other, and two second walls 7, 8 which also have a conformation with plates facing each other and associated with the first walls 5, 6 to define together a concavity 17 containing liquid metal that is cast. The first walls 5, 6 substantially define the flat surfaces of the slab while the second walls 7, 8 substantially define the thickness of the slab.

The first walls 5, 6 have a width substantially corresponding to the width of the slab 18, much larger than the width of the second walls 7, 8. By way of example only, the first walls 5, 6 have a width equal to or greater than 3 times the width of the second walls 7, 8.

The first walls 5, 6 are disposed facing each other and with respect to a casting plane Z interposed between the first walls 5, 6. The casting plane Z, vertical during use, identifies the plane according to which the slab 18 is cast; and the first walls 5, 6 are disposed on one side and on the other of the casting plane Z. The first walls 5, 6 are each provided with an entrance edge 23 through which the liquid metal is introduced, and an exit edge, or lower surfaces 9, 10, in correspondence with which the slab 18 comes out of the crystallizer 2. The entrance edge 23 is opposite the exit edge 9, 10.

Since the first walls 5, 6 are disposed symmetrically with respect to the casting plane Z, the lower surfaces or exit edges 9, 10 of the first walls 5, 6 are disposed parallel to each other and determine the exit thickness of the slab 18 from the mold.

According to possible solutions, the sizing of the exit thickness of the slab exiting from the crystallizer 2 is regulated by the action of moving at least one of the first walls 5, 6 of the crystallizer nearer to or away from the other, actuated for example by means of an actuator unit 19. In this way, the exit gap of the product can be widened or narrowed according to needs, or the thickness of the cast slab 18. In particular, it can be provided that the actuator unit 19 is connected to at least one of the first two walls 5, 6 to move, in a direction incident to the casting plane Z, a first wall 6 nearer to the other.

In accordance with one aspect of the present invention, the first walls 5, 6 have a convex shape with the convexity facing toward the inside of the concavity 17. In particular, it is provided that the surfaces of the first walls 5, 6 facing toward the concavity 17, have said convex shape.

Specifically, it is provided that this convexity extends along a transverse plane of the first walls 5, 6 intersecting the entrance edge 23 and the exit edge 9, 10 thereof.

In other words, the first walls 5, 6 disposed on the opposite sides of the casting plane Z are curved with the convexity facing toward the inside of the concavity 17 so that the latter has a section that tapers from the upper zone towards the bottom, where the exit of the product is disposed.

In accordance with a possible solution, the first walls 5, 6 have a convexity that extends for their whole height, determined in a direction parallel to the casting plane Z, that is, from the entrance edge 23 to the exit edge 9, 10.

In accordance with a possible solution, the convexity of the first walls 5, 6 is defined by an arc of a circle. The arc of a circle can have a radius of curvature R comprised between 2 and 10 meters.

According to a possible solution, in correspondence with the exit edges 9, 10, the first walls 5, 6 have a tangent plane that develops in a direction parallel to the casting plane Z and in which the tangent planes of the edges of the exit edges 9, 10 are parallel to each other.

However, it is not excluded that in possible embodiments, not shown, the first walls 5, 6 also have portions of wall, for example positioned in correspondence with the entrance edge 23 and exit edge 9, 10, having a flat conformation.

In accordance with possible solutions, the exit edges 9, 10 are distanced from each other by a first distance El, while the entrance edges 23 of the first walls 5, 6 are distanced from each other by a second distance E2, bigger than the first distance El.

The first walls 5, 6 are made of at least a heat conducting material, that is, a material with high heat conductivity, typically copper or copper alloys.

The first walls 5, 6 can be cooled by devices, that is, cooling elements or systems. By way of example only, it can be provided that distribution channels of a cooling fluid are associated with the first walls 5, 6. The channels can be integrated into the thickness of the walls 5, 6 or outside the walls, or defined by interspaces in which the cooling fluid flows, provided on the surface of the first walls 5, 6 that is external during use. The cooling devices allow the solidification of the liquid metal that comes into contact with the first walls 5, 6, and the formation of two independent skins, one on each first wall 5, 6.

The casting speed of the slab 18 can be controlled so that, in correspondence with the exit edge 9, 10 of the first walls 5, 6, the solidified skins generated by contact with each first wall 5, 6 are not yet joined together.

Moreover, it is provided that the first walls 5, 6 have a bigger surface extension than the second walls 7, 8.

The second walls 7, 8, on the contrary, have a substantially flat conformation and are associated with the first walls 5, 6 in correspondence with the lateral edges of the latter.

The second side walls 7, 8, therefore, close the concavity 17 on the sides.

In accordance with some embodiments of the invention, the second walls 7, 8 are made of refractory material and allow to maintain a high temperature of the liquid metal, preventing the steel from solidifying on contact with them.

The concavity 17 defined by the first walls 5, 6 and the second walls 7, 8 therefore has a cross section, from the entrance edge 23 to the exit edges 9, 10, which reduces toward the latter.

This conformation allows to have, in the upper part of the crystallizer 2, a width such that it is possible to insert a discharger 22 to pour the molten metal, for example from a tundish, and at the same time to allow to reduce the turbulence, while in the lower part the distance between the first walls 5, 6 defines the exit width of the slab 18, delimited by the second walls 7, 8, on the narrow sides.

In accordance with another aspect of the present invention, the slab casting device 100 comprises a sealing unit 3 disposed directly and immediately under the crystallizer 2 in order to close the slab 18, and in particular the edges 21.

In particular, the sealing unit 3 can comprise two presser rolls 11, 12 disposed parallel to the casting plane Z and positioned at the exit end defined by the exit edges 9, 10 of the crystallizer 2.

The presser rolls 1 1, 12 can be preferably idle, that is, free to rotate around the respective axes of rotation.

The presser rolls 11, 12 can be associated with cooling members provided to cool the presser rolls 11, 12 internally.

The presser rolls 11, 12 extend for a length at least equal to or greater than the width of the first walls 5, 6.

As described above, the exit edges 9, 10 are distanced from each other by said first distance El, and the presser rolls 11, 12 define between them a passage gap 26 with a width G equal to or less than the first distance El . This condition allows to exert on the solidified skins exiting from the crystallizer 2, a pressure action such as to define the slab 18.

The presser rolls 11, 12 have end portions 24 between which a central portion 25 is interposed, having a smaller diameter than that of the end portions 24.

In particular, the central portion 25 has a first diameter Dl while the end portions 24 have a second diameter D2 greater than the first diameter Dl.

The distance between the respective surfaces of the two presser rolls 11, 12 is therefore minimal at the ends and greater in the central segment, so as to laterally join the two skins formed upstream by the contact along the first walls 5, 6 and then to form an edge 21 closed on each side of the slab 18, at the same time allowing a still liquid or semi-liquid core to remain inside the forming slab.

During the casting of the slab 18, the crystallizer 2 is subjected to an alternate motion with a vector component parallel to the casting direction.

In accordance with one aspect of the present invention, at least the first walls 5, 6 are associated with at least one actuator 15 configured to move at least the first walls 5, 6 along respective curved paths 20 lying on a plane orthogonal to the casting plane Z and intersecting the entrance edge 23 and exit edge 9, 10.

According to possible solutions, the first walls 5, 6 can also be connected to guide means and/or articulated mechanisms configured to define the movement along the curved paths 20 of the first walls 5, 6.

The movement is alternated along the curved paths 20 and, as well as promoting the advance of the slab 18 towards the exit from the crystallizer 2, also prevents the latter from sticking, that is, welding to the first walls 5, 6 causing the so-called sticking phenomenon.

The sealing unit 3, that is, the presser rolls 1 1, 12, are connected to the first walls 5, 6 and oscillate with them, that is, they are mobile together with the first walls 5, 6 by the at least one actuator 15. In accordance with a possible solution, the presser rolls 11, 12 and the first walls 5, 6 are installed on at least one common support structure 26, and the at least one actuator 15 is connected to the at least one support structure 26 to move together with each other the first walls 5, 6 and the presser rolls 11, 12 along the curved paths 20.

In accordance with another embodiment of the present invention, the movement, that is, the curved paths 20, shown schematically in fig. 6, are obtained by moving the first walls 5 and 6, together with the relative and integral presser rolls 11, 12 of the sealing unit 3, by means of the actuator 15, along two arcs of a circle.

The arcs of a circle can have their respective virtual centers disposed in symmetrical positions with respect to the casting plane Z.

In accordance with a possible solution, the virtual centers can substantially correspond to the centers of the radiuses of curvature of the first walls 5, 6. This solution allows to limit the mechanical stresses and perturbations to which the metal product is subjected as it forms in the crystallizer 2.

Given that the two independent skins are formed in contact with the first walls 5, 6, the oscillatory movement along the two arcs of a circle allows the gradual descent of the skins along the crystallizer 2 toward the zone where they are joined to form the slab 18.

In accordance with possible solutions, the oscillation of the first walls 5, 6 is such as to maintain the exit edges 9, 10 always distanced by the same first distance El. This allows to guarantee that the slab 18 exits always with the same thickness.

The sealing unit 3 is followed by a drawing unit 4 provided to facilitate the discharge of the slab 18 from the crystallizer 2.

The sealing unit 3 and the drawing unit 4 are disposed in succession along the casting direction.

The drawing unit 4, on the contrary, is integral with the mold 1, that is, it is installed in a fixed position with respect to the first walls 5, 6 and the presser rolls 1 1, 12.

The drawing unit 4 can comprise motorized rolls 13, 14 located downstream of the presser rolls 11, 12 and having their own axes parallel to each other and parallel to the axes of the presser rolls 1 1, 12.

The motorized rolls 13, 14 can have a constant diameter along their axis.

By means of an actuator 16, the motorized rolls 13, 14 can be moved reciprocally nearer to or away from each other, to adapt to the thickness of the cast product, that is, the slab 18.

We will now describe the functioning of the device according to the preferential, but non-exclusive, configuration shown in the drawings and the description.

The discharger 22, shown schematically to comprise the functioning of the casting device 100, during casting operations, pours the cast metal from a container, for example from the tundish, into the crystallizer 2.

The molten metal goes into contact with the first walls 5, 6 and the second walls 7, 8.

The first walls 5, 6, as described above, can have different heat conductivity properties, in fact the molten metal gives up heat to form a solid thickness along the first walls 5, 6, while it remains in its liquid state in the interface with the second walls 7, 8 which do not transmit heat or do it only to a negligible degree.

Therefore, two solid surfaces, called "skins", are formed which, because of the presence of the two refractory walls, are not connected by other solidified surfaces in correspondence with the short sides of the slab 18, given that the refractory material that makes up the walls keeps the material in a molten condition when in contact with them. In this way, the skins remain independent of each other and are not subjected to heat stresses as far as the exit from the crystallizer 2.

Moreover, as the material does not solidify along the short side, no heat shrinkages of the slab 18 are generated and the skins, thrust by the ferrostatic action, remain adherent to the curved surfaces of the first walls 5, 6, continuing the heat exchange action and thickening as they gradually approach the exit from the crystallizer 2.

This leads to a regular and rapid growth of the solidified layer while the material descends, preventing the formation of segregations, dendrites and defects in the product.

The oscillation, as described above, can take place according to an arc of a circle indicated by the arrows 20, determined by the casting radius along which the first two opposite walls 5, 6 slide.

This continuous and regular movement performed by the actuator 15 takes place from bottom to top and vice versa, helping the material to continue its vertical sliding until it reaches the tapered exit section. The second refractory walls 7, 8, on the contrary, remain stationary, causing the closure of the concavity 17 through their precise contact with the first walls 5, 6.

Near the exit from the crystallizer 2, the slab 18, seen from a cross section view, has three layers forming its thickness: the external layers are the two solidified skins whereas the central layer consists of the liquid or semi-liquid core. The sides of the product are closed by contact with the second walls 7, 8, before exit from the crystallizer 2, then by the presser rolls 11, 12.

The next step consists of the passage of the slab 18 into the sealing unit 3 which, as described above, follows the oscillation of the crystallizer 2 and has the two presser rolls 11, 12, having a larger diameter D2 at the ends than the diameter Dl in the central zone. Through the continuous pressing, performed by the presser rolls 11, 12 in the zone with the larger diameter D2, the skins are joined in the lateral zones, forming the closed edges 21 and preventing the liquid core from escaping when the slab ceases to be in contact with the second walls 7, 8. The subsequent passage in the drawing unit 4 is essential for an optimal casting speed and extraction of the product, which at the same time can be calibrated in tolerance by a compression of the thickness of the slab, for example it can be taken to parity with the thickness of the sides sealed by the sealing unit 3. With this operation, we therefore obtain the forced closure of the two skins and the liquid cone present at the center of the slab 18, obtaining known advantages, deriving from the effect of "soft reduction" treatments, well-known in the steel industry. The reciprocal distance between the motorized rolls 13, 14 can be adjusted through the actuator 16 to obtain products of the desired thickness.

Furthermore, it should be noted that in a possible alternative configuration of the device, the drawing group 4, by suitably choosing the distance between the presser rolls 1 1, 12, can also perform a simple extraction of the material without reducing its thickness.

The casting device 100 described here allows to produce very thin and high quality slabs, with a reduction in complexity and plant costs.

The high casting speed, for example 6-7 m/min, and the particular constitution of the crystallizer 2, with long walls of regular and curved geometry, without concavities, forming a cross-section similar to a V on the curved sides, and with the short lateral walls made of refractory material, allows to obtain extremely homogeneous cast products, with a considerable reduction in defects and without the problem of the skin re-melting in the crystallizer, resulting in a high quality of the finished product.

The geometry of the concavity 17, similar to a funnel, also allows to adjust the height of the meniscus of the molten metal, which makes it possible to obtain thicker or thinner skins in relation to the time of contact with the first walls 5, 6 which is defined when controlling the casting device. This makes it possible to cast different thicknesses or special metals with different structural and chemical characteristics that require different casting speeds.

Finally, by means of the secondary actuation system, or actuator 16, it is also possible to intervene in a simple manner on the final thickness of the product.

Claims

1. Casting device for metal slabs, comprising a crystallizer (2) provided with two first walls (5, 6) facing each other with respect to a casting plane (Z), and two second walls (7, 8) facing each other, with a smaller surface extension than the surface extension of the first walls (5, 6), and associated with said first walls (5, 6) to define a concavity (17) for containing liquid metal, characterized in that said two first walls (5,6) are defined by plates having a convex shape with a convexity facing toward the inside of the concavity (17).

2. Device as in claim 1, characterized in that it comprises at least a pair of presser rolls (11, 12) disposed parallel to said casting plane (Z) and positioned at the exit end of said crystallizer (2).

3. Device as in claim 1 or 2, characterized in that at least one of said first walls (5, 6) is associated with at least one actuator (15) configured to move at least one of said first walls (5, 6) along a curved path (20) lying on a plane orthogonal to the casting plane (Z).

4. Device as in any claim hereinbefore, characterized in that said first walls (5, 6) are each provided with a respective entrance edge (23) through which, during use, the liquid metal is introduced, and with an exit edge (9, 10) through which, during use, said slab (18) exits at least partly solidified, and in that said convexity extends from the entrance edge (23) to the exit edge (9, 10).

5. Device as in claim 1, 2 or 3, characterized in that said first walls (5, 6) are each provided with a respective exit edge (9, 10) through which, during use, said slab (18) exits at least partly solidified, in that said exit edges (9, 10) are distanced from each other by a distance (El), and in that said presser rolls (1 1, 12) define with respect to each other a passage gap (26) with a width (G) equal to or less than said first distance (El).

6. Device as in any claim hereinbefore, characterized in that said presser rolls (11, 12) have end portions (24) between which a central portion (25) is interposed, having a diameter less than that of said end portions (24).

7. Device as in any claim hereinbefore, characterized in that said presser rolls (11, 12) can be moved together with said first walls (5, 6) by said at least one actuator (15).

8. Device as in any claim hereinbefore, characterized in that said presser rolls (11, 12) and said first walls (5, 6) are installed on at least a support structure (27), and in that said actuator (15) is connected to said at least one support structure (27), in order to move together said first walls (5, 6) and said presser rolls (1 1, 12).

9. Device as in any claim hereinbefore, characterized in that said curved paths (20) are shaped like arcs of a circle with their respective virtual centers of rotation disposed in symmetrical positions with respect to the casting plane (Z).

10. Device as in any claim hereinbefore, characterized in that said presser rolls (1 1, 12) are free to rotate around respective axes of rotation.

1 1. Device as in any claim hereinbefore, characterized in that said first walls (5, 6) are made of a heat conductor material, and in that cooling devices to cool said first walls (5, 6) are associated with said first walls (5, 6).

12. Device as in any claim hereinbefore, characterized in that said second walls (7, 8) are made of a heat refractory material.

13. Device as in any claim hereinbefore, characterized in that said a drawing unit (4) is provided, downstream of said presser rolls (11, 12), comprising two motorized rolls (13, 14) to extract the slab (18) from the crystallizer (2).

14. Device as in claim 13, characterized in that it comprises at least an actuator (16) connected to said motorized rolls (13, 14) and provided to adjust the reciprocal distance between the latter in order to adjust the thickness of the slab (18).

15. Device as in any claim hereinbefore, characterized in that it comprises an actuator unit (19) connected to at least one of said first walls (5, 6), in order to move, in a direction incident to the casting plane (Z), one of said first walls (6) nearer to the other.

16. Wall for a crystallizer (2) for casting slabs, said wall being provided with an entrance edge (23) through which, during use, the liquid metal is introduced, and an exit edge (9, 10) through which, during use, said slab (18) exits at least partly solidified, characterized in that it is defined by a plate having a convex shape, with a convexity that extends from the entrance edge (23) to the exit edge (9, 10).