CRYSTALLIZER FOR THE CONTINUOUS CASTING OF A METAL PRODUCT, AND CORRESPONDING CASTING METHOD

Abstract

Crystallizer for the continuous high-speed casting of a metal product (P), which has a casting cavity (13) defined by walls (14) connected to each other in correspondence with edges (15) and provided with cooling means (16).

Claims

1. A crystallizer for continuous casting of a metal product, configured to cast the product at a casting speed between 6 m/min and 15 m/min, the crystallizer having a casting cavity, into which liquid metal is cast, defined by walls connected to each other in correspondence with edges and being provided with primary cooling means associated with said walls, wherein an upper level of the liquid metal defines a meniscus wherein said cavity has an octagonal-shaped cross-section with a distance between two opposite walls comprised between 110 mm and 220 mm, and length comprised between 500 mm and 1500 mm, and has a taper converging downward of the single type comprised between 0.8%/m and 1.5%/m or of the multiple or parabolic type comprised between 2%/m and 4%/m in the meniscus zone, and between 0.2%/m and 1.0%/m in a lower part of the crystallizer, and wherein said primary cooling means are configured to produce a thermal flow in correspondence with the meniscus greater than approximately 6 MW/m.sup.2 and up to 14 MW/m.sup.2, and with an average value comprised between 3 MW/m.sup.2 and 5.5 MW/m.sup.2.

2. The crystallizer as in claim 1, wherein said walls have a thickness comprised between 12 mm and 30 mm and are connected by edges with a connection radius comprised between 5 mm and 25 mm.

3. The crystallizer as in claim 1, wherein the walls have equal sizes and all angles between said walls have a value of 135°.

4. The crystallizer as in claim 1, wherein only opposite walls have equal sizes, there being at least one long wall of greater length and at least one short wall of shorter length, and all the angles between said walls have a value of 135°.

5. The crystallizer as in claim 4, wherein a difference in length between the longest and the shortest wall ranges from 5% to 20%.

6. The crystallizer as in claim 1, wherein the length is comprised between 600 mm and 1200 mm.

7. The crystallizer as in claim 1, wherein said edges have a connection radius comprised between 10 mm and 15 mm.

8. The crystallizer as in claim 1, further comprising, on an external surface, a plurality of grooves open toward an outside and parallel to a longitudinal development of the crystallizer, configured to receive cooling liquid.

9. The crystallizer as in claim 8, further comprising, on an external surface, a coating layer suitable to close said grooves with respect to the outside and define cooling channels.

10. The crystallizer as in claim 9, wherein said coating layer is made with bands of fibers impregnated with a polymeric resin.

11. The crystallizer as in claim 1, further comprising cooling channels made in a thickness of the walls of the crystallizer and configured to receive a cooling fluid.

12. Continuous casting apparatus, comprising a mold and a crystallizer as in claim 1, in which the mold comprises foot rolls, disposed at an exit of the crystallizer, for a guide length comprised between 150 mm and 800 mm, in which the cast product at exit from said mold follows a casting line with aid of a plurality of guide rolls disposed directly downstream of said foot rolls following a machine radius comprised between 5 m and 25 m.

13. The apparatus as in claim 12, wherein lubrication of the walls of the crystallizer occurs using a powdered lubricant.

14. The apparatus as in claim 13, wherein said powdered lubricant is a mechanical mixture of silicates and/or aluminum-silicates of alkaline and/or alkaline earth metals with addition of elemental carbon selected from amorphous graphite, coke or carbon black.

15. The apparatus as in claim 12, wherein a distance between a cooling fluid and the walls in contact with the liquid metal has a value comprised between 8 mm and 10 mm.

16. The apparatus as in claim 12, wherein a pressure of a cooling fluid corresponding to a vicinity of a meniscus is comprised between 6 bar and 20 bar, and in a zone corresponding to an end part is comprised between 2 bar and 10 bar.

17. The apparatus as in claim 12, wherein the cooling device is configured to exchange thermal flows comprised between 6 MW/m.sup.2 and 10 MW/m.sup.2.

18. The apparatus as in claim 12, wherein a machine radius has a value comprised between 7 m and 20 m.

19. The apparatus as in claim 12, wherein the guide length is comprised between 200 mm and 500 mm.

20. A steel plant comprising a continuous casting apparatus as in claim 12, and a rolling line connected in line to said continuous casting apparatus.

21. Steel plant as in claim 20, wherein said continuous casting apparatus and said rolling line are configured to operate in endless mode.

22. A method for continuous casting of a metal product, to obtain a productivity between 50 ton/h and 150 ton/h, comprising: supplying a crystallizer as in claim 1; providing foot rolls at an exit of the crystallizer; providing guide rolls directly downstream of the foot rolls in order to define a machine radius comprised between 5 m and 25 m; providing a primary cooling in the crystallizer with a thermal flow value in correspondence with the meniscus greater than 6 MW/m.sup.2 and up to 14 MW/m.sup.2 and with an average value comprised between 3 MW/m.sup.2 and 5.5 MW/m.sup.2; casting at a casting speed between 6 m/min and 15 m/min.

23. The method as in claim 22, wherein a pressure of the cooling fluid in a segment corresponding to an upper zone of the crystallizer, which corresponds to the vicinity of a meniscus, is comprised between 6 and 20 bar, while in a lower zone of the crystallizer, which substantially corresponds to an end part of the crystallizer, the pressure is comprised between 2 and 10 bar.

24. The method as in claim 22, wherein said product, continuously cast, is then subjected to rolling in a rolling line in endless mode without interruptions between continuous casting and rolling.

25. A cast product P obtained with a continuous casting apparatus as in claim 12, wherein a deformation deflection of each side of the cross-section, due to a bulging effect outside the crystallizer, is less than 5% with respect to the length W of said side.

Description

DESCRIPTION OF THE DRAWINGS

[0077] These and other characteristics of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

[0078] FIG. 1 is a schematic lateral view of a continuous casting apparatus, in which a crystallizer in accordance with the present invention can be used;

[0079] FIG. 2 is a cross-sectional view, along the section line II-II, of FIG. 1;

[0080] FIG. 3 is a variant of FIG. 2

[0081] FIG. 4 is another variant of FIG. 2;

[0082] FIGS. 5a-5d schematically show possible cross-section shapes of the crystallizer according to the present invention;

[0083] FIG. 6 is a schematic graph of the trend of the deformation deflection of one side of the octagon;

[0084] FIG. 7 is a schematic illustration of a casting line;

[0085] FIG. 8 is a schematic illustration of a possible application of the present invention;

[0086] FIGS. 9a, 9b and 9c show a table and two corresponding graphs comparing a casting that uses a crystallizer with square section and a casting that uses a crystallizer with an equivalent octagonal section.

[0087] 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.

DESCRIPTION OF EMBODIMENTS

[0088] We will now refer in detail to the various embodiments of the invention, of which one or more examples are shown in the attached drawings. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one embodiment can be adopted on, or in association with, other embodiments to produce another embodiment. It is understood that the present invention shall include all such modifications and variants.

[0089] Before describing these embodiments, we must also clarify that the present description is not limited in its application to details of the construction and disposition of the components as described in the following description using the attached drawings. The present description can provide other embodiments and can be obtained or executed in various other ways. We must also clarify that the phraseology and terminology used here is for the purposes of description only, and cannot be considered as limitative.

[0090] Embodiments of the present invention concern a tubular type crystallizer for continuous casting indicated by reference number 12, and configured to solidify the liquid metal which is introduced inside it and produce a cast product P at exit.

[0091] In accordance with FIG. 1, a continuous casting apparatus is schematized, indicated as a whole with reference number 10, in which the crystallizer 12 is associated in a known manner with a mold 11 and defines a casting line Z along which the product P in the process of being solidified transits.

[0092] The crystallizer 12 has a crystallizer length LM, determined along the casting line Z. Such crystallizer length LM can be comprised between 500 mm and 1500 mm, preferably between 600 mm and 1200 mm and more preferably between 780 mm and 1100 mm.

[0093] The crystallizer 12 (FIG. 2 and subsequent) has a casting cavity 13 with a substantially octagonal-shaped cross-section, defined by eight walls 14 connected to each other in correspondence with as many edges 15.

[0094] The cross-section of the casting cavity 13 will therefore define the shape of the cross-section of the cast product P at exit from the crystallizer 12. For this reason, in particular linked to a uniformity of cooling, it is preferable, although not strictly binding, that the shape of the octagon is symmetrical with respect to two axes orthogonal to each other.

[0095] In particular, such axes define respectively the right-left symmetry and the intrados-extrados symmetry of the section.

[0096] FIGS. 5a-5d show possible embodiments of the octagonal section of the casting cavity 13 of a crystallizer 12.

[0097] According to possible embodiments, the section of the crystallizer can be a regular octagon with the sides, that is, the walls, all equal to each other, of a length W and the angles (α) between the sides which are also equal to each other and equal to 135 degrees (FIG. 5a).

[0098] In accordance with other possible embodiments, it is provided that the sides may have different lengths, wherein the difference in length between the longest side (W.sub.L) and the shortest side (W.sub.S) of the crystallizer can vary from 5% to 20%, preferably from 5% to 10%.

[0099] In these embodiments, the section of the crystallizer can therefore have 6 sides, opposite each other, of a shorter length W.sub.S and 2 sides, opposite each other, of a greater length W.sub.L, wherein the angles (α) between the adjacent sides are all equal to each other, of a value of 135 degrees, in order to respect the symmetry of the section around the respective axes, as in the example shown in FIG. 5b.

[0100] According, to other possible variants, of which a possible example is shown in FIG. 5c, the section of the crystallizer can have sides all of equal length (W) and disposed so as to form angles of different width (α.sub.1 and α.sub.2), wherein the opposite angles as above are equal to each other with a value comprised between about 125 degrees and about 145 degrees, preferably between about 130 degrees and 140 degrees.

[0101] FIG. 5d shows a variant of the section represented in FIG. 5c, in which the section of the crystallizer is rotated by 90°, consequently, the cast product P will have intrados and extrados sides that are different from those of the cast product P generated by the section in FIG. 5c.

[0102] The edges 15 are advantageously connected with a connection radius comprised between 5 mm and 25 mm, preferably between 10 mm and 15 mm. The connection radius defines an area in which the heat exchange is much greater than the median of the walls. This exchange tends to create the detachment of the solid skin formed by contact of the liquid metal on the walls of the crystallizer and therefore causes the lack of a correct heat exchange, with a consequent localized reduction of the thickness of the skin and the risk of formation of longitudinal cracks which can also lead to breakage of the skin and leakage of liquid metal (breakout).

[0103] The choice of connecting the walls of the crystallizer, obtaining a corresponding section shape of the cast billet, on the other hand benefits the subsequent rolling operations, where a more rounded angle reduces or prevents the phenomenon of laps. Higher connection radius values, on the other hand, are more sensitive to the formation of longitudinal cracks that can be prevented by carefully choosing the connection radius as a function of the section and taper, in order to maintain the contact between the skin and the crystallizer wall sufficient for a uniform distribution of the heat exchange even in the corner region.

[0104] In accordance with possible embodiments (FIG. 2), the walls 14 can be distinct elements separate from each other and connected in correspondence with the edges 15 by connection means, for example threaded.

[0105] According to possible variants (FIGS. 3 and 4), the walls 14 can be made, or connected together, in a single body to define a monolithic body.

[0106] The walls 14 of the crystallizer 12 can have the same thickness to ensure uniformity of cooling of the cast product P and advantageously have a reduced thickness, comprised between 12 and 30 mm such as to ensure an adequate rigidity of the crystallizer.

[0107] The crystallizer 12 is provided with cooling devices 16, also called primary cooling devices, configured to cool the liquid metal in contact with the walls 14. Such primary cooling devices are advantageously high efficiency ones, in order to achieve a high heat exchange.

[0108] According to one possible variant (FIG. 2), the cooling devices 16 comprise an external jacket 29 into which the crystallizer 12 is inserted. Between the external jacket 29 and the crystallizer 12 a hollow space 30 is defined, which externally surrounds the entire crystallizer 12 and in which, during use, the cooling fluid is circulated.

[0109] According to possible solutions of the invention, the cooling devices 16 (FIGS. 3 and 4) comprise cooling channels 17 associated with the crystallizer 12 and in which a cooling fluid is circulated.

[0110] In particular, according to one possible variant (FIG. 3), the crystallizer 12 can be provided, in the thickness of the walls 14, with a plurality of cooling channels 17 which develop in a direction substantially parallel to the longitudinal development of the crystallizer.

[0111] According to another variant (FIG. 4), the crystallizer 12 is provided on its external surface with a plurality of grooves 19 open toward the outside and parallel to the longitudinal development of the crystallizer 12 itself.

[0112] In accordance with one possible solution (FIG. 4), a coating layer 18 is applied on the external surface in order to close the grooves 19 with respect to the outside and define the cooling channels 17. The coating layer 18 can be made with bands of fibers, for example of carbon, wrapped around the axis of the casting line Z and impregnated with a polymeric resin.

[0113] According to other solutions, the grooves 19 can be closed to define the cooling channels 17 according to one and/or the other of the embodiments described in WO-A-2014/207729 in the name of the Applicant.

[0114] Advantageously, for all these variants, in order to maximize the heat exchange, the distance between the cooling fluid and the internal walls of the crystallizer in direct contact with the liquid metal is minimized. This distance is measured in a direction orthogonal to the axis of the crystallizer and has a value comprised between 8 mm and 10 mm. According to one possible solution, the cooling devices 16 can comprise feeding and evacuation members, not shown in the drawings, and configured to circulate the cooling fluid along the cooling channels 17.

[0115] According to the invention, the pressure of the cooling fluid in the segment corresponding to the upper zone of the crystallizer 12, which corresponds to the vicinity of the meniscus, is comprised between 6 and 20 bar, while in the lower zone of the crystallizer, which corresponds substantially to the end part of the crystallizer, it is comprised between 2 and 10 bar.

[0116] Inside it, the crystallizer has a substantially conical development gradually narrowing downward from the meniscus zone to the exit zone of the crystallizer, in order to follow the progressive shrinkage of the billet as it gradually cools down along the crystallizer, thus defining a slope of the internal walls with respect to the longitudinal axis of the crystallizer.

[0117] The typical unit of measurement of the taper is expressed in%/meter.

[0118] As is known, a crystallizer can have a single taper for its entire height (“single” taper) or it can have different tracts or segments with decreasing taper values from the entry section to the exit section (“multiple” taper), which varies stepwise from one segment to the next, thus defining a line broken at several points between the consecutive segments. The multiplicity of the multiple taper is normally double, triple, quadruple. Above the quadruple, it is usual to define the multiple taper as a “parabolic” taper, since the broken line has tens of points and is such as to approximate a continuous variation of the taper, within the working tolerances of the internal walls of the crystallizer.

[0119] According to the invention, the internal taper of the crystallizer can be of the single type or even of the multiple or parabolic type.

[0120] If of the single type, the taper has values comprised between 0.8%/m and 1.5%/m.

[0121] If of the multiple or parabolic type, the taper has values comprised between 2.0 and 4.0%/m in the meniscus zone and between 0.2 and 1.0%/m in the lower part of the crystallizer, with an average value comprised between 0.8 and 1.5%/m.

[0122] Thanks to the internal conical configuration of the crystallizer, it is possible to limit the detachment of the billet from the walls of the crystallizer to a minimum, since the shrinkage of the billet is compensated by the narrowing of the section of the central cavity.

[0123] A billet with an octagonal-shaped section, compared to a square-shaped one with an equivalent section (area), has the advantage of having a higher and also more uniform average distribution of the temperature on the external surface of the cross-section, with particular regard to the zones of the edges. The temperature delta between the edges and the face center is very low, in the order of 8-15° C. compared to an equivalent square section in which the difference is in the order of 40-65° C. Furthermore, the internal zone (or core) of the octagonal-shaped section is on average warmer than a square section, therefore it has a more favorable enthalpy average.

[0124] The octagonal billet also has advantages in the rolling process: in fact, since the obtuse angle between the sides of the section is more open, it allows for a greater analogy with the round section and therefore a lower risk of the so-called “laps” during the rolling step and therefore fewer defects on the rolled products.

[0125] In addition, since as explained above the obtuse angle of the octagonal billet has a higher temperature, it entails less wear of the channels of the rolling cylinders.

[0126] The octagonal shape also allows advantageously to have greater uniformity of heat exchange in the crystallizer, in particular in the zone immediately below the meniscus, that is, the instance there is the greatest heat exchange and coinciding with the formation of the first skin. This greater uniformity translates into a thickness of the skin that is more homogeneous on the perimeter, both between one side of the product and the other, and also along the same side.

[0127] A skin with homogeneous thickness is less subject to the formation of cracks under the skin which can lead to breakouts.

[0128] The cooling device according to the present invention is configured so as to allow to exchange high thermal flows in a relatively small distance, that is, within the length of the crystallizer defined above. These thermal flows are greater than about 6 MW/m.sup.2 and can reach up to 14 MW/m.sup.2 in correspondence with the meniscus, for casting speeds comprised between 6 m/min and 15 m/min.

[0129] Considering the average values, the thermal flow is comprised between 3 MW/m.sup.2 and 5.5 MW/m.sup.2.

[0130] In accordance with possible solutions, the octagonal-shaped crystallizer according to the present invention is configured for high productivity, that is, higher than 50 tons/h and up to about 150 tons/h, also in accordance with the method described in WO-A-2018/229808 in the name of the Applicant.

[0131] As is known, the liquid metal produced in the melting furnace of the steel mill is discharged from the ladle to a tundish below, and from there it is continuously discharged inside the crystallizer until a determinate upper level, or meniscus M, is reached.

[0132] One of the fundamental conditions in the casting process is to work as much as possible in stationary conditions, in particular in the meniscus zone. In fact, meniscus perturbations are responsible for most of the defects found downstream, from cracks to the rhomboid shape.

[0133] Furthermore, the reduction of the friction force between the cast product and the internal wall of the crystallizer constitutes another important condition for increasing the casting speed and improving the quality of the product itself.

[0134] For this purpose, as is known, lubricating materials such as powders or lubricating oils are distributed above the meniscus to minimize the friction between the skin being formed and the internal walls of the crystallizer.

[0135] The lubricating materials in contact with the liquid metal become liquid or vapor and create a layer of lubricant which is interposed between the liquid metal 12 and the internal walls of the crystallizer.

[0136] It is also known that the liquid metal can be discharged from the tundish to the crystallizer through an unguided free jet or through a discharger , the exit end of which is located below the level of the meniscus M (submerged discharger or SES).

[0137] The Applicant has experimented that in order to cast at high speeds in stationary conditions and obtain a good quality of the product (also on the rolled product) it is advantageous to use powdered lubricant as a lubrication system in the crystallizer and to discharge the liquid metal from the tundish to the crystallizer through a submerged discharger or SES.

[0138] The lubrication powders allow a beneficial insulating effect and a more homogeneous distribution on the meniscus. In particular, the powders are scattered on the metal bath in a suitable quantity, where they melt in contact with the liquid metal forming a surface slag that infiltrates the interstice between the casting metal and the copper of the crystallizer, ensuring the lubrication necessary for sliding.

[0139] Such powders are a mechanical mixture of silicates and/or aluminum-silicates of alkaline and/or alkaline-earth metals with the addition of elemental carbon chosen from amorphous graphite, coke or carbon black.

[0140] According to one aspect of the present invention, downstream of the mold 11, advantageously, there is no containing device to contain the deformation toward the outside of the faces of the cast product P. The Applicant, in fact, has experimented that, thanks to the sizing and to the appropriate design of the crystallizer 12 as indicated above, it is possible to cast an octagonal-shaped section at high speed without the aid of containing sectors and at the same time prevent the phenomenon of bulging or, worse, break-out of the skin of the cast product P.

[0141] By eliminating the containing devices of the state of the art, it is therefore possible to eliminate the periodic adjustment/alignment actions that they require which, as known, are expensive in terms of time and costs.

[0142] In accordance with possible implementations of the invention, the mold 11 comprises a plurality of guide rolls, also called foot rolls 25, disposed at the exit end of the crystallizer 12 and which are an integral part of the mold 11.

[0143] The foot rolls 25 guide the exit of the cast product P and have the function of keeping it centered in the crystallizer 12 so that the walls of the cast product P are all in contact with the respective internal surfaces of the crystallizer 12 and therefore the heat exchange is uniform on all faces as a result.

[0144] In possible implementations of the invention, the foot rolls 25 are connected to, and integrally mobile with, the mold 11.

[0145] For this purpose, the foot rolls 25 can be installed on a common support element 26 attached to the mold 11.

[0146] In accordance with possible solutions, the foot rolls 25 can be grouped into at least one group of foot rolls, in the case shown in FIG. 1 two groups of foot rolls 25, spaced along the casting line Z. Each group of foot rolls 25 at least partly surrounds, during use, a cross-section of the cast product P.

[0147] The foot rolls 25 of each group are located on a same lying plane parallel to the cross-section of the cast product P.

[0148] The foot rolls 25 are installed directly downstream of the exit of the crystallizer 12.

[0149] In accordance with one possible implementation of the invention, the mold 11 can comprise a number of groups of four foot rolls 25 comprised between 1 and 4, preferably 2.

[0150] In accordance with possible solutions, the foot rolls 25 are installed in a longitudinal portion of the casting line Z that has a guide length LG.

[0151] The guide length LG can be comprised between 150 mm and 800 mm, preferably between 200 mm and 500 mm.

[0152] According to possible implementations, the casting speed V.sub.C is greater than 6 m/min, preferably greater than 6.5 m/min, and can reach up to 15 m/min.

[0153] In particular, for an octagonal-shaped section with sizes equivalent to those of a square section with a side comprised between 150 and 200 mm, speeds comprised between 6 and 8 m/min can be reached, while for a side comprised between 100 and 150 mm, speeds comprised between 8 and 15 m/min can be reached.

[0154] Such a setting of the casting speed V.sub.C allows to reach high productivity of the steel plant.

[0155] In some embodiments, the machine radius Rm, that is, the radius of curvature of the casting line Z, can be a value comprised between 5 m and 25 m, preferably between 7 m and 20 m, even more preferably between 10 m and 15 m, even more preferably between 9 m 12 m.

[0156] The skin of the cast product P, at exit from the crystallizer, has to have a thickness such that, under the action of the head of liquid metal, the sides of the cross-section of the cast product are deformed at most by a predefined deflection “f”.

[0157] Specifically, the sides of the cast product P behave in a manner that is reasonably close to that of a beam that has its ends embedded and is subjected to a uniformly distributed load which is ferrostatic pressure, as shown in FIG. 6. The section of this beam has a rectangular shape with a smaller side “b” and a larger side “h”. The latter represents the thickness of the solidified skin in the flexion plane of the beam.

[0158] The deflection “f” can therefore be determined by the relation:

[00001]$f=\frac{p.Math.b.Math.W^4}{384.Math.E.Math.I}$

in which: [0159] “p” is the distributed load acting on the skin of the cast product P at exit from the foot rollers 25 and which can be determined by the relation: p=ρ.Math.g.Math.H in which H (FIG. 7) is the height of the head of liquid metal that acts on the skin of cast product P at exit from the foot rollers 25.

[0160] H can also be determined as

H=Rm.Math.sin(θ)=Rm.Math.sin(L/Rm) [0161] W is the length of the side of the regular octagon [0162] E is the elasticity modulus, or the Young modulus, of the cast material [0163] “I” is the surface quadratic moment of the resistant section defined by the relation

[00002]$I=\frac{b.Math.h^3}{12},$

in which “h” represents the thickness of solidified skin, which can also be expressed by the empirical formula

h=K√{square root over ((L/V.sub.C))}.

[0164] The solidification constant K can be determined from literature and is a variable value in relation to the size and type of cast product P and therefore the casting process that is carried out.

[0165] In accordance with one possible solution, the admissible deformation deflection “f” of each side of the octagon, that is, the deformation allowed and due to the bulging effect, is less than 5%, preferably less than 3%, even more preferably less 1.5% of the length W of the side of the regular octagon.

[0166] The deformation deflection can be expressed in absolute terms and in this case it is indicated with “F”, it is measured in millimeters and is obtained as follows: F=f*W.

[0167] In accordance with another embodiment of the invention, the apparatus 10 comprises at least one guide mean 27, in this specific case two guide means 27, configured to guide the cast product P along the casting line Z.

[0168] In accordance with one possible embodiment of the invention, each guide mean 27 comprises at least one, in this specific case only one, pair of guide rollers 28 positioned respectively on the intrados and extrados side of the cast product P.

[0169] The guide means 27 are installed in a fixed position and are configured to guide, the cast product P along the casting line Z.

[0170] A plurality of cooling members 32 are also provided, installed downstream of the mold 11 and configured to cool the cast product P. Such cooling carried out on the product at exit from the mold 11 is called secondary cooling and serves to condition the solidification process of the still liquid core of the cast product. The cooling members 32 can comprise a plurality of delivery nozzles 34, interposed between the foot rolls 25 and the guide rolls 28, and configured to deliver a liquid for cooling the cast product P, for example water, or a mixed fluid air-water (air-mist).

[0171] The delivery pressure at exit from the nozzles can advantageously be comprised between 0.5 and 12 bar, preferably between 1 and 10 bar, even more preferably between 1.5 and 9.5 bar, in order to guarantee a correct cooling and therefore a correct solidification of the cast product P in the speed range from 6 to 15 m/min.

[0172] As regards the intensity of the secondary cooling, suitable specific water flow rates have to be guaranteed, for example quantifiable in 1.2-2.5 liters per kg of cast steel, preferably 1.7-2.1 l/kg, while the cooling density (1/min per m.sup.2) has to be higher in the upper part of the casting machine, where the temperatures of the cast product are higher, the vaporization of the cooling water is stronger and the skin is still relatively thin, and therefore the transmission of heat with the liquid core is facilitated.

[0173] The homogeneity of temperature on the perimeter of the cross-section can be obtained by appropriately choosing the number of nozzles and the trend of their emission of cooling liquid. It is also advantageous to provide a selective control of the emission of the nozzles between the front and rear side of the cast product P, increasing the emission on the rear side in order to compensate for the lack of stagnation phenomena in the concave zone on the front side.

[0174] In order to obtain the homogeneity of the temperatures of the cast product P in the longitudinal direction along the casting line, a dynamic control of the total emission and/or distribution of the cooling density along the casting machine is carried out, in order to keep the surface temperature of the cast product P substantially constant, at a value comprised in the range 900-1200° C., preferably 1,000-1,100° C. The temperature is influenced by a number of parameters such as the size of the cross-section of the cast product, the casting speed, the overheating temperature of the liquid steel, the order of magnitude of the heat exchanges in the mold and the chemical composition of the molten steel.

[0175] The surface temperatures of the cast product P are calculated by means of suitable solidification models which take into account: [0176] the chemical composition of the steel; [0177] the sensitivity of the steel to thermal gradients (possible internal or surface cracks in the transverse or longitudinal direction); [0178] geometric characteristics of the casting machine; [0179] expected casting speed; [0180] expected metallurgical lengths.

[0181] For this purpose, the secondary cooling system is formed with various nozzle zones comanded by sectoral valves for water and/or water-air in the case of “air-mist”, which in the upper part of the casting machine can comprise nozzles both on the front and also the rear side, while in the lower part they can be differentiated between front and rear side. These valves can only control some of the nozzles, so as to have more than one active cooling command.

[0182] The crystallizer described so far can be advantageously installed in a steel plant in which a casting line feeds the rolling line directly, for example in endless mode, greatly reducing, or even eliminating, the need for intermediate heating, thanks to the greater casting speed and therefore the higher temperature of the cast product.

[0183] In accordance with possible implementations (FIG. 8), the crystallizer described above can also be installed in a steel plant 100 provided with several casting lines for the production of billets.

[0184] The plant 100 can comprise a first rolling line 101 located directly in line with a first casting line and configured to roll the cast product for example in endless mode (co-rolling).

[0185] The plant can also comprise additional casting lines, parallel to the first, which feed a second rolling line 103 in direct hot charge mode, by means of a common transfer plate 102 located downstream of the casting lines.

[0186] An induction heating device 104 for the rapid heating of the billets can be interposed directly upstream of the first rolling line 101 and/or the second rolling line 103.

[0187] To highlight the advantages obtained by using a crystallizer that has the characteristics indicated above, FIGS. 9a, 9b and 9c respectively show a comparison table, and two graphs in which the main casting parameters are compared, without containment, respectively of a square and an octagon with an equivalent section (area).

[0188] The length of the crystallizer has been set to 1000 mm with a useful cooling length of 880 mm.

[0189] For the side of the equivalent square, lengths comprised between 100 mm and 200 mm have been considered.

[0190] As can be seen, the absence of containment makes it necessary to use, in the case of squares, much lower casting speeds, with consequent lower productivity. It is significant to note that the permanence time, in relation to the different casting speed, is much shorter in the case of an octagon compared to that of the equivalent square. The thermal flow is also much greater if casting an octagonal-shaped billet based on the characteristics of the crystallizer described above.

[0191] It is clear that modifications and/or additions of parts may be made to the crystallizer as described heretofore, without departing from the field and scope of the present invention.

[0192] It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of crystallizer 10 and method, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.