Method and device for continuous casting of molten materials

Abstract: Device and corresponding method for the continuous casting of molten materials, comprising a vertical duct (1') having the shape of a funnel (1) made of refractory material for receiving molten material, and a second duct (13') where the molten material is cooled off, the two ducts being set one on top of the other and being axially aligned. The molten material is injected into the channel, around a stretch of which electromagnetic means (4-9) are set for generating magnetic forces on the molten material, the said means consisting of a plurality of windings (4-8) of electrically conductive material and of a ferromagnetic core (9) and may be electrically supplied to produce a magnetic flux along the direction of the channel, thus producing a set of forces acting on the molten material (2), which are directed orthogonally with respect to the direction of the said magnetic flux so as to maintain detachment of the outer surface of the molten material (2) from the walls of the channel.

Description: Field of the invention The present invention refers to a method for the continuous casting of molten materials, and to the corresponding device for carrying out the said method, in particular usable in plants for the continuous casting of billets, blooms, slabs and the like, to improve the surface and internal quality of the cast product. State of the art Continuous casting is a technique which is today extensively used in the production of metallic bodies having various shapes and sizes, such as blooms, slabs, billets, and the like. The aim is to achieve increasingly high speeds of vertical casting, which, however, leads to the accentuation of certain problems, such as the difficulty of obtaining a uniform distribution of the speeds throughout the cross section of the product being cast. In addition, the use of a discharging device immersed in the molten material does not facilitate the attainment of a uniform distribution of the said speeds on account of the turbulence caused in the metallic mass. These elements may give rise to certain phenomena that damage the surface and internal quality of the finished products. Among other things, it is possible to cite as an example the lack of uniformity in the flow of liquid metal in the crystallizer, with consequent non-uniform solidification and possible tearing of the shell being formed. This danger is more accentuated, in particular, in the case of high rates of casting, at which the said shell is formed with a smaller thickness and/or a larger number of cracks is present on the surface or within the cast product, thus possibly causing disastrous leakage of liquid metal from the solidified shell of the ingot. Another unfavourable phenomenon that may occur regards disturbances of the stability of the meniscus which may cause poor lateral lubrication and leads to the solidifying metal sticking to certain points of the crystallizer, so increasing the risk of tearing of the shell and the production of defects on its surface. Another problem that may arise in this type of plant is linked to the longitudinal oscillation of the crystallizer, which is necessary to prevent the solidifying metal from sticking to the walls of the inner duct of the crystallizer and to facilitate flow of particular lubricants in the gap between the crystallizer and the solidifying shell. However, this oscillation may, in the presence of a disturbed meniscus, bring about deep and irregular markings on the surface of the shell of the cast product. The drawbacks listed above are of considerable importance both for the final quality of the cast product obtained and for the optimization of the production achievable from the casting system and for the cost of the subsequent transformation of the finished products. In fact, to verify the defectiveness of the billets, blooms, slabs or similar cast pieces it is necessary to inspect them and possibly subject them to additional surface-conditioning treatments, this resulting in an increase in production costs and/or in a poorer quality of the end product. Numerous solutions have been proposed in the past to overcome such problems. For example, particular covering powders are used for limiting oxidation of the molten bath and for lubricating the interface between the solidifying metal and the walls of the crystallizer in a more stable way, with consequent positive effects on the surface and internal quality of the cast product. Another known solution consists of using particular discharging devices, set between the tundish and the crystallizer, whereby it is possible to control the flow of the liquid metal entering the crystallizer, so as to reduce the non-metallic inclusions in the cast product, at the same time favouring flotation of gases on the surface. In this way, there is a reduction in the disturbance of the meniscus, where the initial solidification occurs, and direct "hot" flows of molten metal are avoided, which could lead to the partial re-melting of some areas of the shell that is forming. Attempts to improve the fluid-dynamic conditions in the crystallizer regard the use of particular "ducts or tanks" made of refractory material and set immediately upstream of the crystallizer with the purpose of removing the meniscus of the liquid metal from the area of start of solidification, thus limiting the possibility of drawing particles of refractory material or dross into the solidifying metal, and favouring uniformity of the rate of flow of the metal and of heat exchange between the cast product and the crystallizer, especially in the area of initial solidification, which normally takes place in the area of joining between the refractory material of the walls of the "tank" and the contiguous edge of the cooled metal crystallizer, referred to as "triple point". The situation in this area proves very delicate because the molten metal tends to adhere to the refractory material, which is colder on account of the vicinity of the copper cooled by forced circulation. Consequently, in this area surface defects or failures arise in the bodies produced. To overcome such a problem, from the US patents Nos. 5 027 887, 5 045 276, and 4 130 423 it is known that gases may be used, such as nitrogen or argon, or solid lubricants, which are injected at the said joint to form a protective layer. However, in this type of solution, the liquid metal, which is considerably heavier than the lubricant and the gas, frequently manages to tear the protective layer and to come into contact even so with the walls of the "tank", which are made of refractory material. The US patents Nos. 5 494 095 and 5 379 828 propose the solution of setting, between the "tank" made of refractory material and the crystallizer, an insert consisting of a material having a thermal and electrical conductivity lower than that of the material of which the crystallizer is built, so that the molten metal will start to solidify at the insert itself. The joint between the insert and the refractory material of the tank is heated by means of an alternating electromagnetic field. A similar solution, but without heating of the joint between the refractory material and the intermediate insert is proposed by the US patent No. 4 773 469. In continuous casting, in particular of in the casting of thin slabs (i.e., just a few centimetres thick) or of strip (just a few millimetres thick), it has been proposed to use electromagnetic fields in order to obtain confinement of the molten metal (see, for example, the US patents Nos. 4 353 408 and 5 513 692). Up to now, the systems referred to above have not yielded satisfactory results or have proven too costly to implement. For this reason, the present invention proposes to overcome the drawbacks discussed above presented by the known systems of the state of the art. Summary A primary purpose of the present invention is to overcome the problems referred to above by providing a method of continuous casting presenting high efficiency, productivity and reliability. One aim of the present invention is to improve the surface quality of continuously cast products, combining this improvement with an increase in the casting speed to obtain a consequent increase in productivity. A further purpose of the present invention is to protect the triple point from direct contact with the liquid metal, preventing cooling of the metal in that area. A further purpose of the present invention is to reduce the surface wear of the refractory material with which the tank or duct upstream of the crystallizer is lined. These purposes are achieved by a device for the continuous casting of molten materials which, in accordance with Claim 1, comprises a first duct, designed to receive molten material, set in a substantially vertical position, a second duct designed to cool the molten material, set in a position lower than that of the said first duct, the said first and second ducts being axially aligned and connected operatively to define a channel designed to enable passage of said molten material, means of injection of the molten material into said channel, and electromagnetic means arranged around at least one stretch of said channel and coaxially thereto and designed to generate magnetic forces operating on said molten material, the said device being characterized in that the said electromagnetic means are made up of a plurality of windings of electrically conductive material and a ferromagnetic core, which can be electrically supplied and are designed to produce a magnetic flux in a direction longitudinal to the channel itself, producing a set of forces acting on said molten material directed orthogonally to the direction of said magnetic flux to keep the outer surface of said molten material detached from the walls of said channel for a substantial stretch of its length. Thanks to this arrangement, the productivity of the machine increases considerably, producing a more homogeneous cast product having high surface finish, with a consequent reduction in production costs as compared to known continuous-casting devices of the past. According to a further aspect of the invention, a method is envisaged for continuous casting of molten materials, in particular of metallic bodies such as blooms, slabs, billets, and the like, which, in accordance with Claim 13, comprises the following steps: a) continuous pouring of the molten material into a funnel by means of a discharging device until a level corresponding to the covering of the said discharging device is reached; b) formation of a protective layer of dross on the top surface of the molten material; c) excitation, with alternating current, of a plurality of electromagnetic windings, generating a longitudinal magnetic flux inside the duct of the funnel made of refractory material containing the molten material; d) detachment of the external surface of the molten material from the inner wall of said duct so as to form a free space on the entire perimeter of the molten material; e) advance of the molten material along the duct and in the direction of a second duct of the crystallizer, keeping the thickness of said free space constant along the entire perimeter of the molten material; f) start of the process of solidification of the molten material just below the area of joining between the funnel and crystallizer equipped with a forced cooling system, with start of formation of a solid superficial layer of the cast material; g) continuation of solidification of the material during advance of the piece inside the duct of said crystallizer; and h) extraction of the solidified material from the casting device by appropriate means of extraction. Thanks to this method, any phenomena of sticking of the cast material to the walls of the duct or crystallizer are prevented, and protection of the surface of the refractory material lining the inside of the duct or tank is guaranteed. Brief description of the Drawings Further characteristics and advantages of the invention will emerge more clearly in the light of the detailed description of a preferred, but not exclusive, embodiment of a device for the continuous casting of molten materials, for the production of blooms, slabs, billets, and the like, illustrated to provide a non-limiting example with the aid of the annexed tables of drawings in which: Figure 1 is a sectional view of the continuous-casting device according to the invention; Figure 2 is a sectional view of a second embodiment of the continuous- casting device according to the invention; Figure 3 is an enlargement of a detail of the device of Figure 1; Figure 4 is an enlargement of another detail of the device of the Figure 1; Figure 5 is a sectional view of a detail of the device according to the invention; Figure 6 is a perspective view of a detail of the device according to the invention in a variant embodiment; and Figure 7 is a perspective view of a detail of the device according to the invention in a further variant embodiment. Detailed description of the invention With reference to the figures cited, a device for carrying out continuous casting according to the invention, indicated globally by C, comprises a funnel 1 made of refractory material defining inside it a duct 1', in which a feed discharging device 3 is immersed, which conveys the metal or other material 2 in the molten state from the tundish into the funnel 1. The dimensions of the duct 1' of said funnel are appropriately selected according to the dimensions of the section of the material to be made by casting. A device, which is in itself known and not illustrated in the figures, for regulating the level of the metal inside the duct 1' of the funnel 1 ensures that the level always remains at an optimal height for operation of the casting device. A layer of dross 10 forms a plug which prevents contact of the metal in the top part of the duct 1' with the environmental air and thus prevents oxidation of the metal 2 itself. The bottom part of the funnel 1 is surrounded by an electromagnetic device which comprises a plurality of magnetic windings 4, 5, 6, 7, 8, made of a material with high electrical and thermal conductivity, in the form of a ring, and a magnetic core 9. The number of windings is appropriately selected according to the height of the column of molten metal which is to be maintained in operation above the triple point, that is, the point of contact between the bottom part of the duct 1' of the funnel 1 and the top part of another duct 13' made inside the crystallizer 13, and coaxial with the first duct 1'. The respective ducts 1' and 13' of the funnel 1 and of the crystallizer 13 define a channel through which the cast pieces are guided in a continuous way. Each of the said windings is provided with a cooling system 11, advantageously a water cooling system, but it is also possible to use other cooling systems of a known type. The funnel 1 and the crystallizer 13 are separated from one another by one 8 of the said windings, which is set coaxially to both of the said elements, the said winding having the shape and dimensions of its internal section corresponding to those of the funnel 1 and hence of the section of the piece to be cast. The internal surfaces of this winding are coated with a deposited protective layer 14, of appropriate thickness, to insulate the winding both electrically and thermally from the element produced by casting. This layer of insulating coating may be deposited for example by laser-welding techniques. The winding 8 advantageously includes a lubrication system comprising ports 12 fed with a lubricating material to favour sliding of the cast product within the duct, according to the type of casting to be made. This winding 8, in its bottom part, is in contact with a metal crystallizer 13, for example made of copper, which is cooled by means of a forced-circulation water cooling system of its own, not illustrated in the figures. At the joint between the winding 8 and the crystallizer 13, and between the windings themselves are set electrically insulating inserts 16. Each winding consists of two turns insulated from one another by a layer of insulating material 15. The details of the areas of contact of the windings are represented in Figures 3 and 4. In conformance with the invention, it is possible to avoid the use of lubricant in the area corresponding to the surface of contact between the external surface of the cast product and the internal walls of the ducts 1' and 13'. For this purpose supplying the windings with alternating current is envisaged to induce, inside the funnel 1, in the longitudinal direction, a magnetic field and parasitic currents in the liquid metal 2, which are directed orthogonally to the axis of the ducts 1' and 13'. The useful frequency of the alternating current is comprised within an interval ranging between 50 and 25 000 Hz and the intensity is comprised within an interval ranging between 100 and 10 000 A. The parameters of the current are appropriately defined according to the type of cast metal and to the maximum limit of overheating allowed in the casting process. The electromagnetic forces which arise in the liquid metal have a direction orthogonal to the line of magnetic induction and also perpendicular to the parasitic currents. Consequently, the electromagnetic forces are also perpendicular to the internal surfaces of the windings and are indicated with the reference number 20 in Figure 5. The electromagnetic pressure is such as to repel the liquid metal along the entire internal surface of the casting device starting from the level of the top free surface of the liquid as far as the area of the triple point and a circular free space 19 or 21 around the piece 2 of cast metal. Consequently, the electromagnetic pressure also promotes and maintains detachment of the solidifying "skin" 18 from the walls of the duct 13' of the crystallizer 13, thus reducing friction between the latter and the cast piece. The thickness of the cavity 19 or 21 around the cast piece may be regulated by varying the current in the turns of the windings 4 to 8. For this purpose, each winding has an independent electrical supply and is connected in parallel with a capacitor, so creating an oscillating circuit. The parameters of inductance of the turns and capacitance of the capacitor are selected so that the oscillating RLC circuit works close to the resonance value of the current. As the molten metal approaches the turn, the current increases by 7 to 10 times, and consequently the electromagnetic forces increase, thus preventing contact between the turn and the liquid metal 2. In fact, in continuous-casting systems, generally the molten metal 2 fed by the discharging device 3 does not have a regular motion, and this causes lack of uniformity of temperature and the possibility of drawing of non-metallic inclusions, such as dross or fragments of refractory material, within the bath and as far as the start-of-solidification area. The molten metal, with a flow rendered uniform by the fact that the turbulence of immission remains confined at a distance from the crystallizer 13, proceeds down the funnel, until it reaches the boundary area with the crystallizer 13, where it cools, and the solidified "skin" 18 of the piece of casting starts forming in a homogeneous way. In this way, the risk is avoided of the "skin" 18, in some point of the surface of the cast piece, having a non-uniform thickness that gives rise to cracks, which, during the casting process, may open on account of the thermomechanical stresses and thus cause the molten metal to leak out. In addition, the fact that there is a cavity 21 which surrounds the liquid metal also upstream of the triple point prevents the whole cast piece 2 from coming into contact with the internal walls of the duct and consequently prevents its solidification in contact with the refractory material of the funnel 1 and with that of the induction coil 8. At the level of the triple point, it may be advisable to inject a lubricant into the said cavity, which may advantageously contain ferromagnetic particles that help to concentrate the said electromagnetic forces, so enabling the formation of a free space 19 or 21 that is larger and more stable. In this way, a superficial film 17 is formed, which covers the cast piece 2, beginning from the portion where its solidification starts. In the channel made up of the duct 1' of the funnel 1 and of the duct 13' of the crystallizer 13, casting is started by initially closing the duct in the bottom part with a dummy bar, after which the duct is filled with molten metal by means of a discharging device 3. After activation of the system of windings from 4 to 8, the liquid metal is detached from the wall of the duct under the action of the electromagnetic forces 20. During normal full operating conditions, the molten metal 2 is protected from oxidation according to three different procedures. The first solution consists in forming a floating plug of covering powders and dross 10 in the top part, and a layer of dross in the free space around the surface of the cast piece. The second solution consists in creating a plug of dross 10 which closes the top part of the duct that is full of molten metal, to which the formation of the free space 19 around the lateral surface of the cast piece 2 is associated, the said space being filled by an inert gas, for example argon. The third solution, instead, envisages that both the plug 10 for closing the cast metal on top and the free space 19 around the cast piece should be filled with inert gas. In these latter two cases, the casting device is provided with a series of ducts (not illustrated in the figures), the outlets of which are arranged along the entire internal space of the device in order to allow the inert gas to flow in the most appropriate way. The process according to the present invention also envisages the possibility of introducing lubricant at the height of the said triple point, with the purpose of favouring sliding of the skin being formed against the walls of the duct 13' of the crystallizer 13, should this prove necessary. Should the crystallizer 13, in its operation as element for cooling the cast bar, contemplate the possibility of moving with oscillations in a vertical direction, there is the risk that the position of the triple point will thus vary. In this case, it is possible to stabilize the position of the triple point by varying the current in the windings, since by so doing the length of the duct changes according to the oscillation, so that the bottom point of the duct 1' does not change its position in space. To facilitate penetration of the electromagnetic forces within the crystallizer 13 and to enable start of solidification of the liquid metal below the joining point between the said crystallizer 13 and the area above it made up of coils alone, or else of coils and refractory material, the crystallizer 13 is appropriately made up of a plurality of vertical segments 22 which are electrically insulated from one another. As an alternative, the said vertical segments may advantageously simply be provided with incisions in the top portion of the crystallizer that have a predetermined length L of between 50 and 100 mm from the top of the crystallizer, as represented in Figure 6. Into the incisions 22', an electrically insulating material is inserted. In a further advantageous variant embodiment of the crystallizer 13, the latter is made up of four plates 23-26 joined together along their respective longitudinal edges. Also in this case, the plates are electrically insulated from one another in the longitudinal direction in the area of mutual contact. In this way, a simplified structure is obtained, as well as a consequent reduction in fabrication costs.

Claims: Device for the continuous casting of molten materials, comprising a first duct (1') designed to receive molten material, set substantially in a vertical position, a second duct (13') designed to cool the molten material, set in a position below the first duct (1'), said first and second ducts (1', 13') being axially aligned and operatively connected so as to define a channel designed to allow passage of the said molten material, means (3) for injection of the molten material into the said channel, electromagnetic means (4-9) set around at least one portion of said channel and coaxially to the latter, and designed to generate magnetic forces operating on said molten material, the said device being characterized in that the said electromagnetic means are made up of plurality of windings (4-8) of electrically conductive material and of a ferromagnetic core (9), the said electromagnetic means (4-9) being able to be electrically supplied and being designed to produce a magnetic flux in the longitudinal direction of the channel itself, so producing a set of forces acting on the said molten material (2) which are directed orthogonally with respect to the direction of the said magnetic flux, in order to maintain detachment of the outer surface of the said molten material (2) from the walls of the said channel for a substantial portion of its length. Device according to Claim 1, wherein the said first duct is a funnel (1) made of refractory material. Device according to Claim 2, wherein the said electromagnetic means are set around and outside the said funnel (1). Device according to Claim 3, wherein the said electromagnetic means are supplied with alternating current at a frequency of between 50 and 25 000 Hz and at an intensity of between 100 and 10 000 A. Device according to Claim 2, wherein at least one (8) of the said windings, which is in a position lower than that of the other windings, is set between the said first duct (1') and the said second duct (13') and is provided with an internal wall facing towards the inside of the said duct. Device according to Claim 1, wherein the said bottom winding (8) is provided with ports (12) designed to introduce lubricant (12) into the said first duct. Device according to Claim 1, wherein the said windings (4-8) are provided with a plurality of channels (11) designed to convey a coolant fluid. Device according to Claim 7, wherein the said bottom winding (8) is provided, on its surface that is in contact with the liquid metal, with an electrically insulating coating (14). Device according to any one of the foregoing claims, wherein each winding (4-8) is connected in parallel to capacitors with which it forms an electrical resonance circuit, and in that each winding (4-8) can be electrically activated separately from the other windings or in combination with them. Device according to any one of the foregoing claims, wherein a peripheral wall of the said second duct (13) is made up of a plurality of vertical segments (22) that are electrically insulated from one another. Device according to any one of Claims 1-9, wherein the peripheral wall of the said second duct (13) is provided with incisions in its top portion which have a predetermined axial length L. Device according to any one of Claims 1-9, wherein the peripheral wall of the said second duct (13) is made up of four plates (23-26) connected together along their respective longitudinal edges. Method for continuous casting of molten materials, in particular of metallic bodies, such as blooms, slabs, billets and the like, using a device according to any one of Claims 1-9, which comprises the following steps: a) continuous pouring of the molten material into a funnel (1) by means of injection means (3) until a level corresponding to the covering of the outlet mouth of the said injection means (3) is reached; b) formation of a protective layer (10) of dross on the top surface of the molten material; c) excitation, with alternating current, of a plurality of electromagnetic windings generating a longitudinal magnetic flux inside the first duct (1') made of refractory material containing the molten material; d) detachment of the outer surface of the molten material from the internal wall of said duct (1') so as to form a free peripheral space (19, 21) on the entire perimeter of the molten material; e) advance of the molten material along the first duct (1') and in the direction of a second duct (13') keeping the thickness of the said free space constant along the entire perimeter of the molten material; f) start of the process of solidification of the molten material at the height of the joining area between the first (1) and second (13') ducts defined as triple point, with start of formation of a solid superficial layer of the cast material; g) continuation of solidification of the material during advance of the piece inside the second duct (13') and h) extraction of the solidified material (2) from the casting device by appropriate means of extraction. Process according to Claim 13, wherein said protective layer (10) on the top surface of the molten material consists of dross or inert gas. Process according to Claim 14, wherein the said free peripheral space (19) is filled with an inert gas. Process according to Claim 14, wherein the said free peripheral space (21) is filled with dross. Process according to Claim 15 or Claim 16, wherein at the height of the said triple point lubricant is injected in order to favour sliding of the solidifying skin along the walls of the second duct. Process according to Claim 15 or Claim 16, wherein the current in the windings (4-8) is varied selectively according to the vertical oscillations of the second duct (13') to maintain constant the size of the free peripheral space (19, 21) at the triple point and the relative position of the triple point with respect to the cast piece.

Inventors: CODUTTI ANDREA, KOLESNICHENKO ANATOLY, PAVLICEVIC MILORAD, POLONI ALFREDO, CODUTTI, ANDREA, KOLESNICHENKO, ANATOLY, PAVLICEVIC, MILORAD, POLONI, ALFREDO
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 2001
Main classification: B22D11
IPC codes: B22D11