WO2016092526A1

WO2016092526A1 - Mold for continuous casting and relating continuous casting method

Info

Publication number: WO2016092526A1
Application number: PCT/IB2015/059582
Authority: WO
Inventors: Milorad Pavlicevic
Original assignee: Milorad Pavlicevic
Priority date: 2014-12-01
Filing date: 2015-12-14
Publication date: 2016-06-16

Abstract

Mold for continuous casting of a liquid metal comprising a tubular body (21) provided with a transit cavity (25) and an entrance edge (23) into which, during use, the liquid metal is introduced up to a meniscus level (M), and comprising at least one heating device (27) cooperating with the tubular body (21).

Description

"MOLD FOR CONTINUOUS CASTING AND RELATING CONTINUOUS CASTING METHOD"

FIELD OF THE INVENTION

The present invention relates to a mold for continuous casting used in the steel industry to cast billets or blooms of any type and section, round or polygonal with at least three sides, such as square, rectangular, double T-shaped of the type called beam blanks, U-shaped, sheet piles or similar or comparable sections.

The present invention also relates to the crystallizer function present in a mold.

Furthermore, the present invention relates to molds for the continuous casting of steels or its alloys, but with the natural adaptations, the teachings of the invention can also be applied for the continuous casting of alloys of copper, brass or other metals.

The present invention also relates to a continuous casting method that provides to use a crystallizer that has a tubular shape and a vertical, curved or sub-vertical development.

BACKGROUND OF THE INVENTION

In the field of continuous casting it is known that it is necessary to reach high casting speeds in order to increase the overall production capacity of a steel plant. It is also known that reaching high casting speeds is correlated to the optimization of a plurality of technical and technological parameters thanks to which the liquid metal is partly solidified.

These parameters affect the capacities of the crystallizer function to support the high heat and mechanical stresses and wear to which it is subjected during use.

We use the expression "crystallizer function" in that a mold has the crystallizer function associated with its internal wall, for example in the single- body mold, or has the crystallizer function that is obtained in the removable body present in the mold or part of it, which cooperates with the liquid metal that has to be solidified.

Hereafter we use the word crystallizer both to indicate the internal wall of the monobloc mold, and also to indicate the removable body inside which the liquid metal is cast to be solidified. Solutions are also known, of molds having at least the internal surface, with the crystallizer function, having a conical shape with the taper for example comprised between 0.8%/m and 5%/m, opening toward the edge where the metal material is introduced.

It is also known that in a mold, during casting, the heat flux along the longitudinal extension has a peak around the zone of the meniscus, i.e. in correspondence with the zone where, during casting, the level of the liquid metal is positioned.

It is also known that during the design and production of a crystallizer, in the tubular body that defines the crystallizer, a zone is defined in which, during use, the meniscus of the liquid metal will be positioned.

The high heat flux present in the zone of the meniscus M generates an unwanted deformation of the mold, which causes problems that will be described with reference to figs. 1 , 2a, 2b and 2c and which the present invention intends to overcome.

The deformation profile of the wall can be different, depending on how the mold is made, and the deformation can also vary inside an integral mold and a mold with a replaceable wall with crystallizer function.

The deformation profile can be almost uniform over the whole periphery of the crystallizer function, or different on one or more specific parts of the periphery, mainly depending on the general geometry of the mold.

In fact, the deformation can be different along the periphery of the crystallizer depending on the different behaviors that the point-to-point shape of the geometry allows.

Furthermore, the deformation profile can assume different values both depending on the geometric configuration of the section of the wall, and also depending on other factors, such as heat exchange, temperature of the molten metal, etc.

In particular, fig. 1 relates to a mold 10 suitable to cast round products and is shown in a condition excessively and deliberately deformed, to give a clearer understanding of the negative phenomena that occur and that prevent an increase in the casting speed in known solutions. The liquid metal 12 is discharged continuously into the mold 10 until a determinate level or meniscus M is reached and, above it, lubricating materials 16 are distributed, such as lubricating powders or oils which, on contact with the liquid metal 12, become liquid and define a layer of lubricating liquid 17 that is interposed between the liquid metal 12 and the lubricating materials 16.

The solidification of the liquid metal 12 begins, in a known manner, in correspondence with the meniscus M and the internal surface 1 1, with the formation of a solid layer or skin 14, which progressively increases in thickness.

Due to the high heat flux around the meniscus M, the internal surface 11 of the mold 10 deforms to define a concave portion 15 practically under the level of the meniscus M, and a portion with negative taper 13 near and above the meniscus M.

Both the concave portion 15 and the portion with negative taper 13 develop over the entire periphery of the mold 10 in the case of round products, even if in certain conditions, in particular determined by the physical structure, not uniformly.

Merely by way of example, the concave portion 15 can be subject to a deformation that can even reach around 0.25 mm and more, compared with its non-deformed condition.

By negative taper we mean that the internal surface 11 has an inclination, indicated in fig. 1 by -a, that faces toward the inside of the casting cavity of the mold 10.

Negative taper also occurs, during use, in the case where the internal surface 1 1 , when the mold is cold, is made with positive taper as described above.

During casting, the mold 10 is made to oscillate, with a defined and desired motion, also in terms of values, in a direction (indicated in the drawings by the arrow F) substantially parallel to its longitudinal extension, both to prevent the cast liquid metal 12 from welding with the internal surface 1 1, and also to facilitate the descent of the cast product with its layer of skin 14 in formation. The speed of oscillation of the mold 10 is usually much higher than the casting speed of the liquid metal 12.

During the upward movement of the mold 10 the internal surface 11 of the mold 10 is wet by the lubricating liquid 17 over all its perimeter. During the downward movement of the mold 10, also called "negative strip", the mold 10 transports the lubricating liquid 17 downward but, due to the presence of the portion with negative taper 13, the mold 10 impacts on the solidified first skin 14, thinning the lubricating liquid 17 and interrupting it if the inclination -a is high. This effect, which occurs in the state of the art, heretofore has not allowed to exceed casting speeds higher than 2.5m/min for casting round pieces and 7m/min for casting billets.

The impact of the mold 10 against the skin 14 also causes deformations or oscillation marks, in which traces of the lubricating liquid 17 can be deposited. A lack of or insufficient lubrication causes possible welding, temporary and localized, of the skin 14 on the internal surface 1 1, and also axial tensions and transverse cracks of the skin 14, with consequent breakages, also called "bleeding".

During the downward movement of the mold 10, the portion with negative taper 13 ensures a sure contact of the skin 14 with the internal surface 11 and therefore an optimal heat exchange.

This region of the mold 10 with sure contact can extend for a distance P which, in the case of continuous casting of round sections, can vary, merely by way of example, between 10mm and 20mm depending on the casting speed.

In the region located under the portion with negative taper 13, in correspondence with the concave portion 15, between the internal surface 11 and the skin 14, the thickness of which is defined by the two continuous lines 14e and 14i, also because of the shrinkage of the cast product, a large interspace or gap 18 is generated, consisting of air and solid lubricant 19 that is deposited on the internal surface 11 of the mold 10.

The layer of air and solid lubricant generates a high heat barrier that prevents the mold 10 from removing heat from the skin 14 which is forming; this can lead to localized fusions of the forming skin 14 with a consequent reduction in its thickness.

With reference to figs. 2a, 2b and 2c, the negative phenomena are described that occur and block an increase in the casting speed in known molds 10 for casting square products. It is quite clear that similar problems also occur in molds 10 configured to cast products with a polygonal section.

Fig. 2a shows the development of the internal surface 1 1 of the mold 10 in correspondence with the meniscus M, with a line of dots and dashes in its non- operating or cold condition, and a line of dashes in its operating or hot condition.

As can be seen, the internal surface 11 in proximity to the flat walls is subjected to a radial dilation whereas in correspondence with the rounded connection portions, and for a region of the flat walls comprised between 10mm and 15mm from the rounded connection regions, is subjected to a more accentuated deformation toward the outside.

Figs. 2b and 2c are views in a longitudinal section along the section line B-B and respectively C-C of the mold 10 in fig. 2a respectively in a zone in correspondence with one of the flat walls and in correspondence with one of the rounded connection portions.

In fig. 2b we can see how the internal surface 1 1, in its flat region, has a positive taper, i.e., open toward the entrance end of the liquid metal 12 for the whole longitudinal development.

This condition ensures a safe contact between the skin 14 that is generated and the internal surface 11, guaranteeing an optimal heat exchange and a homogeneous supply of lubricating liquid 17 between the skin 14 and the internal surface 11 of the mold 10.

In correspondence with the flat portions of the internal surface 11 therefore, the disadvantages of insufficient supply of lubricating liquid are extremely limited.

On the contrary, in fig. 2c, which shows the behavior in correspondence with the rounded connection portion, we can see how the deformation progress of the internal surface 11, in proximity to the rounded connection zones, is comparable to the one shown in fig. 1, and has the portion with negative taper 13 and the concave portion 15 as described above.

In the rounded connection zones the same problems occur as those described previously with reference to fig. 1.

In the portion with negative taper 13, the skin 14 is in contact with the internal surface 1 1 for a height of about 20mm-50mm from the meniscus M, whereas in correspondence with the concave portion 15 the skin 14 detaches from the internal surface 1 1 after about 20mm, with a consequent deterioration in its capacity to remove heat and difficulties in the solidification of the liquid metal.

This can cause localized welding of the skin 14 to the internal surface 1 1 in the zone comprised between the flat wall and the rounded connection portion of the internal surface 1 1 of the mold 10.

For connection radii of the edges of the cast section with sizes smaller than 12mm, for example for radii of curvature of 3 -6mm, there is a further disadvantage, i.e. cracks on the edges, also called "off-corner cracks".

In particular, as shown in fig. 2a, during solidification, the portion of skin 14 in proximity to the flat walls has a much bigger thickness than that near the rounded connection portions.

The portion of skin 14 located in correspondence with the flat walls exerts traction on the skin 14 located in the edge regions, entailing a thinning thereof and a further detachment from the internal surface 1 1 of the mold 10.

In proximity to the edge, the skin 14 is therefore subjected to localized microfusions and to a deterioration in the heat fluxes which the internal surface 1 1 is no longer able to remove due to the detachment of the skin 14.

Following this, in the zones of the edges there is a thinning of the skin 14 and cracks in the cast product, with a consequent deterioration in quality.

Another disadvantage that limits the increase in casting speed is connected to the stresses to which the zone of the edges is subjected where the material is deformed plastically, causing a rhomboidal shape of the mold 10 and consequently also of the cast product.

These phenomena occur in all those products that have flat angled walls connected by rounded connection zones with rounded connection radii of limited sizes.

All these disadvantages considerably limit the casting speeds obtainable and drastically reduce the working life of a wall with crystallizer function.

The state of the art has not found a satisfactory solution to all these problems.

For example, in an attempt to increase the casting speed, an unsatisfactory cooling is obtained, and hence an insufficient thickness of the skin at exit from the mold 10, with consequent problems of breakage of the skin. On the contrary, when trying to obtain an optimal cooling of the product, the casting speed has to be reduced and hence productivity is also reduced.

The present invention therefore proposes to give an answer to the problems indicated above by way of example, supplying a solution that allows both to increase the casting speeds, to increase the working life of the walls and also to obtain continuously cast products with optimum surface quality, eliminating internal cracks in the zone of the edge, called "off-corner cracks", and minimizing the depth of the oscillation marks.

In the field of continuous casting, molds are also known that are used for the continuous casting of large metal products, by way of example ingots with diameters of more than 600mm.

Examples of these known molds are described in documents KR-A- 201 1.0121975, GB-A-1.353.927, GB-A-2.017.551, US-A-5.375.648 and WO-A- 99/21670, and comprise induction heating devices configured to heat the portion of the liquid metal contained in the mold and located near the free level of the molten metal.

The heating action, in this case, is intended to maintain, in correspondence with the free level, a determinate quantity of molten metal, preventing it from solidifying. To prevent this disadvantage occurring, reduced casting speeds must be maintained, for example less than 0.15m/min.

All the solutions described above are not suitable for casting metal products such as billets, round pieces, beam-blanks with reduced cross section sizes and high casting speeds, i.e. casting speeds higher than 2.5m/min and required for example for casting carbon steels, alloyed steels and other metals that are less sensitive to the action of quick solidification.

One purpose of the present invention is to obtain a mold for continuous casting, and the connected crystallizer of whatever section it may be, which allows to reach much higher casting speeds than current ones, and hence allows to increase the productivity of a steel plant, obtaining, for example for tubular molds, casting speeds of at least 20m/min.

Another purpose of the present invention is to obtain a mold and the connected crystallizer of whatever section it may be, for continuous casting, which allows to obtain cast products with a high quality of the surface and the internal structure. Another purpose of the present invention is to obtain a mold for continuous casting which allows to increase the duration of the working life of the wall of the crystallizer, of whatever section it may be, before it has to be replaced or restored.

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.

In accordance with the above purposes, a mold for the continuous casting of liquid metal comprises a tubular body that defines the crystallizer function with its internal part.

The tubular body is provided with an entrance edge through which the liquid metal is introduced, a transit cavity, an exit edge and a zone in which, during use, the meniscus of the liquid metal is positioned.

According to a first feature of the present invention, the mold comprises at least one heating device configured to heat a portion of the tubular body located between the entrance edge and a zone located above and distanced from the zone of the tubular body in which, during use, the meniscus is positioned. The heating device, during use, deforms at least the internal surface of the tubular body.

A heating of the mold that does not affect the zone of the meniscus and a zone under the meniscus prevents supplying additional heat where the heat flux is already high due to the molten metal present. In this way it is possible to optimize the mechanical resistance of the tubular body, and also to further limit the taper effect of the internal surface of the mold with crystallizer function. By zone located above the meniscus we mean a zone located at least at 15mm, preferably at least 20mm, more preferably at least 30mm, above the zone of the tubular body where the meniscus is positioned during use.

Therefore, according to one solution of the present invention, the heating device is located above the level of the meniscus. This prevents the further supply of heat energy to the mold in the zone where the heat flux generated by the solidifying liquid metal are already extremely high, and promotes a rapid start of the solidification process in correspondence with the level of the meniscus, which allows to increase the casting speeds.

Heating this portion of the tubular body allows to make the portion of the material of the tubular body maintain the required geometrical shape of the internal surface located at least above the meniscus.

Heating the portion located above the meniscus allows to induce in the internal wall, which has the crystallizer function, a desired and controlled deformation and in any case such as to at least limit the formation of the concave zone around the zone of the meniscus.

This desired deformation allows at least to linearize the internal wall as much as possible, or to increase the extension of the concave zone so as to at least reduce the negative effect.

By linearization we mean that the internal wall of the mold, at least for a zone located around the meniscus, during casting, has a linear development, i.e. substantially parallel to the axis of longitudinal development of the mold or a development with taper that opens toward the entrance edge, or positive taper.

It comes within the field of the present invention to provide that the linearization of the internal wall is obtained for a region located substantially around the meniscus or around where the meniscus is positioned on each occasion during the oscillation of the mold. A variant of the present invention provides that the heated portion extends longitudinally for a height comprised between 5mm and 120mm, preferably comprised between 5mm and 50mm.

According to variants of the invention, the at least one heating device is chosen from a group comprising a Joule effect heating device, ultrasound or fuel, or which uses a possible combination of these technologies. By way of example, the heating device is chosen from a group comprising at least one of either a resistance, a burner, an ultrasound exciter, an infrared lamp, a lamp with a source in the near infrared (NIR), or a laser source.

According to a variant, the heating device is chosen from a group comprising an electromagnetic wave device, magnetic induction, or which uses a possible combination of these technologies. According to the present invention, and depending on the material that makes up the wall which has the crystallizer function, the heating of the portions can be comprised between 60°C and 450°C, by way of example comprised between about 200°C and 300°C.

However, it comes within the field of the present invention to obtain a heating temperature that adapts the physical-chemical requirements and characteristics of the wall to the need to obtain the deformation profile that is beneficial for the desired result and that comes within the spirit of the invention.

According to a possible form of embodiment of the invention, the at least one heating device is configured to heat the at least one portion of the wall that makes up the tubular body by acting on the external surface of the wall. This allows to dispose the heating devices in a non-challenging environment, unlike the one where the liquid metal is cast.

According to another form of embodiment, the at least one heating device is configured to heat the at least one portion of the wall that makes up the tubular body by acting on the internal surface of the wall. This solution allows to dispose the heating devices in positions that do not interfere with the equipment that has to cooperate with the external wall of the mold, such as support and oscillation means and/or cooling means, or other means.

According to another variant, the at least one heating device is integrated or partly integrated in the thickness of the wall that makes up the tubular body.

According to another variant, and if the transit cavity of the tubular body has a substantially polygonal shape with at least three sides, for example square, hexagonal, T-shaped, double T-shaped, U-shaped etc., or other shape defined by walls connected to each other by rounded connection portions, the mold comprises, or can also comprise, a plurality of heating devices configured specifically to heat a desired and defined part of the walls of the tubular body such as the connections or zones contiguous to them. This allows to limit the occurrence of cracks near the rounded edges of the cast product.

It comes within the spirit of the invention to regulate the heating action, in the portion where the heating device is operating, in a direction substantially parallel to the longitudinal development of the tubular body. A variant of the invention regulates the heating action, in the portion where the heating device is operating, in a perimeter direction.

It comes within the spirit of the invention to connect the heating action to the deformation obtained in the wall of the tubular body.

A variant of the invention connects the point-by-point heating action to the local deformation which occurs on each occasion in the wall of the tubular body, or in defined perimeter positions of the wall of the crystallizer.

Another variant of the invention provides that the mold comprises cooling means configured to cool the tubular body along its longitudinal extension, for at least most of its longitudinal extension, which can comprise for example at least 70% of its length.

According to a variant, the cooling means are configured to cool a region that also extends above the meniscus. This ensures that the solidification of the liquid metal begins immediately in the zone of contact with the internal surface of the mold, preventing the generation of layers of liquid metal above the meniscus and which, in contact with the mold, would prevent drawing the lubricating material between the walls and the forming skin.

BRIEF DESCRIPTION OF THE DRAWINGS

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

- fig 1 shows, by way of example, in cross section, a mold for continuous casting in accordance with the state of the art,

- fig. 2a is a cross section view of a mold for cast products with a square section; - fig. 2b is a view in section from B to B of fig. 2a;

- fig. 2c is a view in section from C to C of fig. 2a;

- fig. 3 is a graph that shows developments of the profile of the deformation line of the internal surface of the wall with crystallizer function in relation to different heating conditions of the crystallizer;

- figs. 4a, 5a, 6a, 7a, 8a and 9a show possible variants of the present invention;

- figs. 4b, 5b, 6b, 7b, 8b and 9b show possible deformation developments of the upper part of the internal surface, with crystallizer function, in accordance with the state of the art (line of dashes), and in modes relating to the solutions in figs. 4a, 5a, 6a, 7a, 8a and 9a;

- figs. 10-14 show possible variants of the present invention;

- fig. 15 is a cross section of a mold with the internal wall, with crystallizer function, replaceable;

- figs. 16 and 17 are perspective views of the internal wall, with crystallizer function, in accordance with possible variant forms;

- figs. 18 and 19 are cross section views of the internal wall of figs. 16 and 17, in accordance with their possible variant forms;

- fig. 20 is a section view of the internal wall, with crystallizer function, with a double T shape;

- fig. 21 is an enlarged detail of fig. 20 in accordance with a possible variant form;

- fig. 22 is a schematized and partial view in longitudinal section of an integral mold for continuous casting;

- figs. 23 and 24 are perspective views of possible integral molds for continuous casting;

- figs. 25 and 26 are enlarged details, in section, of portions of integral molds in accordance with possible variant forms;

- figs. 27-29 show some possible cross sections of integral molds with flat walls angled with respect to each other and roundly connected.

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 form of embodiment can conveniently be incorporated into other forms of embodiment without further clarifications.

DETAILED DESCRIPTION OF SOME FORMS OF EMBODIMENT

Forms of embodiment of the present invention relates to molds for continuous casting indicated generically by the reference number 10.

The mold 10 for continuous casting comprises a tubular body 21 defined by at least one wall 20 provided with an internal surface 22 with crystallizer function.

The shape of the cross section of the internal surface 22 is coherent with the shape of the cast product exiting from the mold 10. The tubular body 21 has a development along a longitudinal axis Z, vertical, sub- vertical, curved or partly curved.

The tubular body 21 is made of a material with high heat conductivity, for example it can be made of copper or its alloys, such as a copper-silver alloy, or a copper-chromium-zirconium alloy, to allow a rapid solidification of the liquid metal introduced.

A variant provides that the tubular body 21 is made of a single material along its longitudinal extension, to ensure that the tubular body 21 has a uniform and substantially continuous development, for example heat development, i.e. correlated to its deformation.

The tubular body 21 is provided with an entrance edge 23 for the liquid metal material, and an exit edge 24 of the at least partly solidified metal product.

The tubular body 21 can be made substantially in a single body or as several components, in which case the wall with crystallizer function is normally provided replaceable in a known manner.

The internal surface 22 defines the transit cavity 25 for the passage of the metal material.

The transit cavity 25 can have, in a known manner, a progressive reduction in sizes from the entrance edge 23 to the exit edge 24, also called "positive taper". During use, liquid metal material is introduced into the transit cavity 25 until a determinate level of the meniscus "M" is reached, and then the level of the meniscus M is maintained for the whole casting time. The tubular body 21 therefore has a zone 26 in which, during use, the meniscus M is positioned. The zone 26 can be directly identified by the tubular body 21 and is connected to the design parameters of the specific tubular body 21.

The meniscus M is positioned at a known height, normally comprised between 70mm and 150mm, preferably between 80mm and 140mm, or between 90mm and 130mm with respect to the entrance edge 23 of the tubular body 21, although it is not excluded that, in other forms of embodiment, or for particular needs, the meniscus M is positioned at a different height, for example less than 70mm or higher than 150mm. Fig. 3 shows schematically the behavior of the internal surface 22 with crystallizer function in current conditions of the state of the art and in two application conditions of the invention.

Line A shows the behavior in the state of the art, while lines B and C describe behaviors obtainable with the present invention in different application modes and according to different types of mold.

As will appear obvious hereafter in the description, Applicant has found that the deformations identified by lines A, B and C have a substantially identical progression over the whole circumferential development of the internal surface 22 in the case of molds 10 for the casting of round products, or in correspondence with the connection portions between walls 20 in the case of casting of square products or polygonal in general.

According to one aspect of the present invention, and as shown in figs. 4a, 5a, 6a, 7a, 8a and 9a, at least one heating device 27 is associated with the tubular body 21, and is configured to heat a portion 28 of the tubular body 21 which is comprised between the entrance edge 23 and the zone 26 of the tubular body 21 around which, during operations, the meniscus M is located.

In particular, it can be provided that the portion 28 is disposed completely above and distanced from the zone 26 where the meniscus M is positioned during use.

Figs. 4b, 5b, 6b, 7b, 8b and 9b show comparisons between the deformation development of the internal surface 22, with crystallizer function, according to the state of the art (line of dashes), and modes of the specific form of heating as applied according to the present invention (continuous line) and relating to the solutions as per figs. 4a, 5a, 6a, 7a, 8a and 9a.

According to the present invention, depending on the results to be obtained and on the sections of the transit cavity 25, the heating devices 27 can cooperate with the whole perimeter development, internal and/or external, of the tubular body 21, or with a large part thereof, for example in the case of round or annular sections or similar, in order to obtain that the deformation line around the meniscus M (see fig. 3) generates an internal surface, with crystallizer function, as much as possible with an open taper toward the entrance edge 23 as per line B or C. In the case of flat walls 20, connected and angled with respect to each other, to define a section by way of example with a polygonal shape, square and more or less rounded, or U-shaped, T-shaped, double T-shaped (fig. 20), the heated zone can also affect only, or in particular, the connection zones between the walls 20 to prevent at least the phenomena of off-corner cracks.

According to another variant, the entity of heating of the tubular body 21 can be modulated continuously or discretely, during the casting process, for example in relation to heat fluxes detected on the walls 20 or in relation to the deformations detected in the latter.

According to a possible variant, the heating device 27 can be configured to carry out a differentiated heating along the axial extension of the tubular body 21. The differentiated heating along the axial extension of the tubular body 21 can be determined inversely proportionate to the heat flux that is to be removed.

According to another variant of the invention, it can be provided that the at least one heating device 27 is configured to carry out a differentiated heating, in a controlled manner, along the perimeter development of the wall or walls 20 of the tubular body 21.

The differentiated heating along the axial extension and/or along the perimeter development can be carried out depending on detections of temperature or of deformation carried out on the walls 20 of the tubular body 21.

According to a possible variant, the heating of the tubular body 21 is started before the start of the casting process, so that when the discharge of the liquid material into the tubular body 21 is started, the latter is already deformed in a desired manner, and suitable to overcome the disadvantages of the state of the art. Figs. 4a, 5a and 6a show possible applications of the invention, suitable to heat replaceable tubular bodies 21 , according to specific and particular requirements, which have the crystallizer function inside them as will be described hereafter in the description.

According to the form of embodiment in fig. 4a, the heating device 27 is installed on the internal surface 22 of the wall 20 and extends for a height H comprised between 5mm and 30mm, in the case shown here about 20mm and for the height H is configured to carry out a heating of the portion 28 of wall 20. According to this solution, the heating device 27 is disposed at a distance L from the entrance edge 23 which is comprised between 30mm and 80mm, preferably around 60mm.

According to possible solutions, the heating device 27 is configured to heat a zone which is located at least at 15mm, preferably at least 20mm, still more preferably at least 30mm above the zone 26 where the meniscus M is positioned.

It can therefore be provided, in a possible variant, that the heating device 27 is associated with the tubular body 21 in a zone comprised between the entrance edge 23 and about 80mm from the entrance edge 23 of the tubular body 21, and does not affect a more inner zone of the transit cavity 25. This position ensures that the heating device 27 does not also heat a zone of the tubular body 21 located under the level of the meniscus M.

Fig. 4b shows the deformation development of the internal surface 22 to which a heating is applied of the portion 28 with a position and configuration of the heating device 27 as in fig. 4a. As can be seen, thanks to the heating, the internal surface 22 around the meniscus M tends to open toward the outside for at least a portion of 15mm located above the meniscus M and which extends also below it, in the region where the liquid metal first solidifies. This opening toward the outside of the internal surface 22, also called positive taper, allows to obtain a homogenous lubrication over the whole perimeter of the meniscus M.

According to the variant in fig. 5a, the heating device 27 is associated with the internal surface 22 of the wall 20 and extends from the entrance edge 23 toward the inside of the transit cavity 25 for a height H comprised between 30mm and 100mm, in the case shown here for about 80mm.

Fig. 5b shows the deformation development of the internal surface 22 with a heating of the portion 28 to a temperature Tl and a temperature T2, higher than Tl . As can be seen, thanks to the heating, the internal surface 22 around the meniscus M has a taper that opens toward the entrance edge 23. In particular, from fig. 5b it is possible to appreciate how the amplitude of the positive taper is closely correlated to the heating action of the wall 20 and increases progressively as the heating temperature increases. With this solution, it is therefore possible to ensure that the supply of a sufficient quantity of lubricating material is maintained during casting, at least with regard to the casting of round products, and to prevent off-corner cracks occurring, at least with regard to casting square products, or polygonal in general.

Fig. 6a shows another variant in which two heating devices 27 are installed on the internal surface 22 of the tubular body 21. In particular, a first heating device 27 is located in direct proximity to the entrance edge 23, and extends for a height H comprised between 5mm and 30mm, in this case for a height H of about 25mm, and a second heating device 27 located in an intermediate position between the entrance edge 23 and a zone located above the zone 26 in which, during use, the meniscus M is positioned and which extends for a height H which is comprised between 5mm and 40mm, in this case about 30mm. The second heating device 27 is located, in the case shown here, at a distance L from the entrance edge 23 of about 50mm.

Fig. 6b shows the deformation development of the internal surface 22 with a heating of the portion 28 with the configuration shown in fig. 6a. In this variant too, it can be seen how the internal surface 22, around the meniscus M, opens toward the outside, linearizing the surface region located around the meniscus M.

It is quite obvious that, even if the solutions shown in figs. 4a-6a provide the at least one heating device 27 associated with the internal surface 22 of the tubular body 21, variants are not excluded in which the heating device 27 is associated with the external surface 29 of the wall 20. According to these variants, and as will become clear hereafter in the description, the portion of the heating device 27 facing toward the outside, and which during use is in cooperation with the cooling liquid, is heat insulated to preserve the latter's capacity to cool, whereas the other part of the heating device 27 is located in direct contact with the wall 20 of the tubular body 21 , to ensure heat exchange.

As can be appreciated from the variants shown, by playing on the position, sizes or temperatures of the heated portion 28, it is possible to obtain the desired and suitable internal profile of the internal surface 22 with crystallizer function so as to prevent the onset of the problems that exist in the state of the art.

Figs. 7a, 8a and 9a show variants of the invention in molds 10 of the integral type, i.e. that have integrated cooling means as will be described hereafter.

According to the form of embodiment in fig. 7a, the heating device 27 is installed on the external surface 29 of the wall 20 and extends for a height H comprised between 5mm and 30mm, in the case shown here about 15mm and for the height H is configured to carry out a heating of the portion 28 of the wall 20.

Fig. 7b shows deformation developments of the internal surface 22 with heating of the portion 28 to a temperature Tl, a temperature T2, a temperature T3 and a temperature T4, progressively increasing. As can be seen in this drawing, thanks to the heating, the internal surface 22 around the meniscus M deforms so as to generate a taper that opens toward the entrance edge 23. In particular, it can be seen how, as the heating temperature increases, the internal surface 22 of the wall 20, in the region around the meniscus M, assumes a positive taper, i.e. it opens progressively toward the entrance edge 23.

In the variant shown in fig. 8a, the heating device 27 is installed on the external surface 29 of the wall 20 and extends for a height H comprised between 30mm and 120mm, in the case shown here about 45mm and, for this height H, carries out a heating of the portion 28.

From fig. 8b it can be seen how a greater deformation capacity is conferred on the zone of the wall 20 located in correspondence with the entrance edge 23, such as to allow the internal surface 22 to limit, if not eliminate, the generation of a concave zone around the meniscus M.

Fig. 9a shows another variant in which the heating device 27 is installed on the internal surface 22 of the wall 20 and extends from the entrance edge 23 for a height H comprised between 10mm and 90mm, in the case shown here about 30mm and for this height H is configured to carry out a heating of the portion 28 of the wall 20.

In this form of embodiment too, it can be seen from fig. 9b that the development of the deformation line of the internal surface 22 is subjected to a positive deviation around the meniscus M to overcome the disadvantages described above.

It comes within the spirit of the invention that, in order to be used in other configurations of the mold 10, the variants described above with reference to figs. 4a-9a can be conveniently combined with each other to obtain the desired result of eliminating the disadvantages of the state of the art. According to the variants in figs. 10 to 14, we shall now describe other variants of the present invention in which the heating device 27 is partly or completely integrated in the thickness of the wall 20.

In particular, it can be provided that the heating device 27 is positioned in a seating 30 provided in the thickness of the wall 20.

According to a variant, the seating 30 can be defined by one or more grooves, open toward the internal surface 22 (fig. 10), toward the external surface 29 (fig. 11), or toward the entrance edge 23.

The seating 30 can have a desired extended development along the perimeter of the tubular body 21 , or can extend for only a determinate surface extension.

In the variant in fig. 12, the seating 30 is a hole with a section suitable to contain the heating devices 27, made in the thickness of the wall 20 and open at least toward the entrance edge 23. The seating 30 can have an orientation that can be parallel or inclined with respect to the longitudinal extension of the tubular body 21. According to a variant of fig. 12, the seating 30 is a groove made in the thickness of the wall 20, which extends on the perimeter of the tubular body 21 and is open toward the entrance edge 23.

In the variant in fig. 13, the heating device 27 comprises at least one heating element 31 associated with the wall 20 to heat it.

According to a possible solution, the heating element 31 can be provided with a protruding contact portion 32 suitable to heat the portion 28 of the tubular body 21. In particular, the heating element 31 can be provided with several contact portions 32 provided to heat also the entrance edge 23 of the tubular body 21.

The heating element 31 can have a tubular conformation substantially mating with the shape and size of the transit cavity 25 of the tubular body 21, and can be associated with the latter, for example by interference, so as to generate a pre- tensioning of the portion 28 of the tubular body 21 such as to accentuate the positive taper toward the outside.

The heating element 31 can have heaters integrated in its thickness which heat the heating element 31 and transfer the heat to the portion 28 of the tubular body

21, or can be associated with one or more external heaters, for example burners or magnetic inductors, provided to heat the heating element 31. The heating of the heating element 31 determines a heat dilation of the latter which, being inserted in the transit cavity 25, dilates the portion 28 toward the outside, thus obtaining a positive taper of the transit cavity 25. The heat dilation also ensures a reciprocal contact between the heating element 31 and the tubular body 21, also during the heat dilations of the latter. Although the form of embodiment shown in fig. 13 refers to an application of the heating element 31 for replaceable molds, a similar application for an integral mold is not excluded.

The heating element 31 also confers greater structural rigidity on the portion 28 of the tubular body 21, ensuring a homogenous behavior of the latter in its cross section.

The heating element 31 can be positioned on the internal surface 22, as shown in fig. 13, or on the external surface 29 of the wall 20.

In particular, with regard to the specific variant of fig. 13, the heating element 31 extends during use from the entrance edge 23 toward the exit edge 24, so as to position the contact portion 32 in an internal zone of the wall 20 with respect to the entrance edge 23.

The heating element 31 is also provided with an abutment portion 33 which during use is positioned resting against the entrance edge 23 and which allows to define a predefined position of the heating element 31.

According to a variant, with its contact portion 32, the heating element 31 can extend substantially in a closed ring to heat the whole perimeter, internal or external, of the wall 20, or can heat only a limited region of the wall 20.

Fig. 14 shows a possible application of a heating device 27 to a gripping portion 46 usually provided in the wall 20 and which, in a known manner, allows to connect the tubular body 21 to auxiliary means such as support and oscillation means of the mold 10. According to this solution, the heating device 27 can therefore also have a support and oscillation function.

With reference to figs. 15-21, we will now describe some forms of embodiment according to the invention of the mold 10 with tubular body 21 of the replaceable type which has the cry stallizer function.

In fig. 15, cooling means 34, outside the tubular body 21, are associated with the replaceable tubular body 21, and are not replaced during the life of the mold 10. In this variant, the cooling means 34 comprise one or more external walls 35 that surround the wall 20 of the tubular body 21 externally and which define with the latter one or more interspaces 36 in which the cooling fluid is made to flow.

The cooling means 34 are connected to introduction devices 37 and discharge devices 38, to make the cooling fluid flow in the interspace 36.

The cooling fluid can be fed to the interspace 36 in equicurrent (fig. 15) or in counter-current with respect to the discharge direction of the cast product.

The cooling means 34 are configured to cool the tubular body 21, along its longitudinal extension, for a region that also extends above the zone 26 where the meniscus M is positioned during use.

Furthermore, the cooling means 34 ensure that the solidification of the liquid metal introduced into the mold 10 starts immediately in the zone of contact with the internal surface 22 of the mold 10.

The tubular body 21 in fig. 15 can have a circular cross section shape as in fig. 16, square as in fig. 17, double T-shaped as in fig. 20, or other cross section shapes as previously defined and of a known type in continuous casting.

According to the form of embodiment in fig. 16, the tubular body 21 has a circular cross section and the heating device 27 is configured to heat a portion 28, on the whole perimeter of the wall 20, in at least one zone comprised between the entrance edge 23 and a zone located above the zone 26 where the meniscus M is positioned during use.

This solution allows to overcome at least the disadvantages of insufficient supply of lubricating material and the occurrence of surface cracks in the product that are generated during the casting of round products.

The heating device 27 can be associated with the internal surface 22 of the tubular body 21 (figs. 15 and 16) or the external surface 29.

In fig. 16, the heating device 27 has a closed ring shape, to cover the perimeter development of the tubular body 21.

In variant forms, the heating device 27 comprises a plurality of heating sectors located one as a prosecution of the other, to occupy the substantial part of the perimeter of the tubular body 21 to obtain a substantially uniform heating along said perimeter portion 28. According to the forms of embodiment in figs. 17, 18 and 19, the tubular body 21 has a cross section of the transit cavity 25 with a substantially square shape, or in general rectangular, defined by four walls 20 connected to each other by rounded connection portions 39 or connecting portions.

The walls 20 can be connected in a single body with the rounded connection portions 39, to obtain a monolithic tubular body 21.

The heating devices 27 are installed and configured to heat portions 28 located in correspondence with the rounded connection portions 39, in the region comprised between the entrance edge 23 and a zone located above the zone where, during use, the meniscus M is positioned. This solution allows to overcome at least the disadvantages connected to the formation of off-corner cracks.

However, it is within the spirit of the invention that also the flat walls 20 can have, or are affected by, heating devices according to one or another of the variants already shown here.

The heating devices 27 are configured to heat at least an angular region of the rounded connection portion 39 which extends for an angular amplitude a comprised between 30° and 90°, preferably between 60° and 90°, even more preferably about 90° as shown in figs. 18 and 19.

In correspondence with the rounded connection portions 39, the internal surface 22 has a rounded connection R. According to the forms of embodiment shown in figs. 17 and 18, the rounded connection R has sizes comprised between 6mm and 10mm, for example 8mm, and the present invention provides a solution to the disadvantages described with reference to fig. 2d.

In fig. 19, the rounded connection R has sizes comprised between 15mm and 60mm, preferably comprised between about 25mm and 50mm, and the present invention provides a solution at least to the disadvantages of insufficient supply of lubricating material which have limited over time the production of internal surfaces 22 for square products with rounded connection portions with high rounded connection radii, for example higher than 12mm.

Cast products with a high connection radius are particularly advantageous to simplify, accelerate and reduce the subsequent rolling steps to which the product is usually subjected after casting. Fig. 20 shows the cross section of a tubular body 21 with a double T shape, also called beam blanks.

In the variant in fig. 20, the disadvantage of off-corner cracks occurs in the rounded connection portions 43, between the walls 20, which are located more external to the casting volume of the shape of the product. The heating devices 27 are therefore provided at least near the rounded connection portions 43 to heat portions 28 of the tubular body 21 comprised between the entrance edge 23 and a zone located above the zone 26 in which, during use, the meniscus M is located.

The rounded connection portions 43 are suitably rounded so that the internal surface 22 of the tubular body 21, in this zone, has a rounded connection R which, merely by way of example, is comprised between 2mm and 5mm.

The heating devices 27 can be provided in correspondence with the internal surface 22 (fig. 20) or the external surface 29 (fig. 21) of the tubular body 21.

According to variants of figs. 16-21, the heating devices 27 described above can also consist of a plurality of burners 45, of a known type, disposed in a coordinated manner with respect to each other and each configured to heat the desired portions 28 of the tubular body 21.

Figs. 22-29 show examples of molds 10 of the integral type. By integral molds we mean known molds of the type provided with cooling means 34 defined by channels 47 made in the thickness of the wall 20 and which cooperate with the internal surfaces 22 of the wall 20 to keep them at the desired temperature.

The channels 47 are connected to introduction 37 and discharge 38 devices to make the cooling fluid circulate as described above.

With regard to this form of embodiment too, the cooling means 34 or channels 47 are configured to cool the mold 10, along its longitudinal extension, for a region that extends above the meniscus M, with the same functions as described above with reference to fig. 15.

Fig. 23 shows a mold 10 in which the transit cavity 25 of the tubular body 21 has a substantially cylindrical shape and is suitable to cast billets or blooms with a circular section, and the heating device 27 is configured to heat a portion 28 on the whole perimeter of the wall 20 according to one or another of the variants described above. In fig. 24 the mold 10 has the transit cavity 25 with a substantially square shape defined by walls 20 connected in correspondence with rounded connection portions 39.

The walls 20 of the mold 10 can be reciprocally connected to define a single monolithic body.

The heating devices 27 are installed and configured to heat portions 28 located in correspondence with the rounded connection portions 39 according to one or another of the variants described above.

However, it is not excluded that in variants of fig. 24, the heating devices 27 heat the whole perimeter development of the mold 10.

Figs. 25 and 26 show possible variants of heating devices 27 installed on tubular bodies 21 with a square section.

The present description describes by way of example square sections, including rectangular or polygonal sections.

In particular, in fig. 25 the heating devices 27 comprise a plurality of burners 45, configured to heat the rounded connection portions 39 according to one or another of the variants described above.

In fig. 26, the heating devices 27, one or more in number, are positioned in respective blind holes 30 made in the thickness of the rounded connection portions 39 according to one or another of the variants described above.

As an alternative to or in combination with the burners, it is possible to use heating devices 27 which comprise one or more electric resistances.

In figs. 24-27 it is provided that the substantial part of the rounded connection portions 39 are not affected by the cooling means 34. This allows to keep the zone in correspondence with the rounded connection between the walls 20 that are hotter than the walls 20, and allows to optimize the heating by the heating devices 27.

In possible variants, not shown in the drawings and possibly combinable with the variants described here, the rounded connection portions 39 can have a reduction in the thickness of the wall 20, at least on its external surface 29. This increases the deformation capacity of the rounded connection portion 39 and reduces the energy required for heating this zone. According to the variants in figs. 27, 28 and 29, the internal surface 22 of the tubular body 21 is connected in correspondence with the rounded connection portions 39 with a rounded connection radius R with sizes comprised between 2mm and 5mm, for example 3mm, and for these sizes of the rounded connection, the invention allows to solve the disadvantages of cracks in the zones of the edges.

In figs. 28 and 29, the rounded connection R has sizes comprised, merely by way of example, between 30mm and 60mm, for example about 50mm. For these values of the rounded connection R, the present invention supplies a solution to the disadvantages of insufficient supply of lubricating material and allows to optimize the subsequent rolling process as described above.

According to the forms of embodiment shown in figs. 28 and 29, it can be provided that the heating devices 27 are configured to heat a portion 28 that extends for an angular amplitude a of the rounded connection portion 39, even only about 45° or more.

In the variant shown in fig. 29, the heating devices 27 comprise first heating elements 48 installed in correspondence with the rounded connection portions 39, and second heating elements 49 installed in correspondence with the walls 20 and suitable to carry out a differentiated heating with respect to the first heating elements 48. This solution allows to differentiate the heating along the perimeter of the tubular body 21 depending on the deformation of the internal surface 22 to be obtained.

The first heating elements 48 and the second heating elements 49 can be installed on the internal surface 22 or on the external surface 29 of the tubular body 21.

However, it is not excluded that in other variants, combinable with the forms of embodiment described here, the heating devices 27 heat the entire perimeter development at least of the portion of the tubular body 21 comprised between the entrance edge 23 and a zone below as identified in the forms of embodiment described above.

Claims

1. Mold for continuous casting of a liquid metal comprising a tubular body (21) that defines at least part of a crystallizer, said tubular body (21) being provided with a transit cavity (25), with an entrance edge (23) and with a zone (26) in which, during use, the meniscus (M), of the liquid metal is positioned, cooling means (34) being associated with said tubular body (21), characterized in that it comprises at least one heating device (27) configured to heat a portion (28) of said tubular body (21) located between the entrance edge (23) and a zone located above and distanced from said zone (26) in which, during use, the meniscus (M) is positioned, said heating device (27), during use, deforming at least the internal surface (22) of said tubular body (21).

2. Mold as in claim 1 , characterized in that said at least one heating device (27) is configured to heat a zone of the tubular body (21) located between the entrance edge (23) and at least 15mm or more above said zone (26) in which the meniscus (M) is positioned during use.

3. Mold as in claim 1 or 2, characterized in that said at least one heating device (27) is associated with said tubular body (21) in a zone comprised between said entrance edge (23) and about 80mm from said entrance edge (23), and does not affect a more internal zone of said transit cavity (25).

4. Mold as in any claim hereinbefore, characterized in that said tubular body (21) is made in a single body.

5. Mold as any claim hereinbefore, characterized in that said portion (28) extends longitudinally for a height (H) comprised between 5mm and 120mm, preferably comprised between 5mm and 50mm.

6. Mold as any claim hereinbefore, wherein the transit cavity (25) of said tubular body (21) has a circular shape, characterized in that said heating device (27) is configured to heat a portion (28) of the tubular body (21) with a circumferential shape.

7. Mold as any claim from 1 to 5, wherein the transit cavity (25) of said tubular body (21) has a polygonal shape, said transit cavity (25) being defined by at least three walls (20) connected to each other by rounded connection portions (39; 43), characterized in that it comprises a plurality of said heating devices (27) configured to heat a plurality of said portions (28) of the tubular body (21), provided in proximity to at least some of said rounded connection portions (39; 43).

8. Mold as any claim from 1 to 6, characterized in that said heating device (27) comprises a heating element (31) with a tubular conformation substantially mating with the shape and size of said transit cavity (25), and is associated by interference with said tubular body (21) so as to generate a pre-tensioning of said portion (28), heaters being associated and/or integrated with said heating element (31) in order to heat and dilate said heating element (31) and said portion (28).

9. Mold as in any claim hereinbefore, characterized in that the at least one heating device (27) is configured to carry out a differentiated heating along the axial extension of the tubular body (21).

10. Mold as in any claim hereinbefore, characterized in that the at least one heating device (27) is configured to carry out a differentiated heating along the perimeter development of the tubular body (21).

11. Crystallizer for a mold (10) as in any claim hereinbefore.

12. Continuous casting method that provides to discharge a liquid metal through an entrance edge (23) into a transit cavity (25) of a tubular body (21), keeping a meniscus (M) level of said liquid metal in said transit cavity (25), and to cool said tubular body (21) along at least most of its longitudinal extension, characterized in that it provides to heat, by means of at least one heating device

(27), a portion (28) of said tubular body (21) located between the entrance edge (23) and a zone located above and distanced from a zone (26) in which the meniscus (M) is positioned, said heating deforming at least the internal surface (22) of said tubular body (21).

13. Method as in claim 12, characterized in that it provides to heat a zone of the tubular body (21) located between the entrance edge and at least 15mm or more, above said zone (26) in which said meniscus (M) is positioned.

14. Method as in claim 12 or 13, wherein the transit cavity (25) of said tubular body (21) has a circular shape, characterized in that a heating is carried out on a portion (28) of the tubular body (21 ) with a circumferential shape.

15. Method as in claim 12 or 13, wherein the transit cavity (25) of said tubular body (21) has a polygonal shape with at least three sides, said transit cavity (25) being defined by walls (20) connected to each other by rounded connecting portions (39), characterized in that the heating is carried out of a plurality of said portions (28) of the tubular body (21), provided in proximity to said rounded connecting portions (39).

16. Method as in any of the claims from 12 to 15, characterized in that said at least one portion (28) of the tubular body (21) is heated to a temperature comprised between 60° and 450°C.