Vertical semi-continuous casting device and method for manufacturing aluminium alloys

Abstract

The invention relates to a vertical semi-continuous casting method with direct cooling of a raw form (5) made of aluminium alloy, such as a rolling plate or an extrusion billet, using a device comprising: an open-ended mould (1) made from at least one hollow element defining at least one peripheral chamber (12) having a solid inner wall (13) which is intended to come into contact with the aluminium alloy and allowing the circulation of a first cooling liquid capable of performing primary cooling; a plurality of spray nozzles (21) integrated into at least one spray bar (2) which is independent of the mould and is capable of performing secondary cooling, independent of the primary cooling, by spraying a second cooling liquid; a false bottom (3), capable of closing the lower portion of the mould at the start of the casting, having an upper portion (31) intended to come into contact with the aluminium alloy and a lower portion (32); and a descender (4) capable of gradually lowering the false bottom during the solidification of the raw form. The method and the device according to the invention solve the problem of distortion when starting the casting process.

Description

[0001] Description

[0002] Title of the invention: Vertical semi-continuous casting device and method for manufacturing aluminum alloys

[0003] Field of invention

[0004] The invention relates to the field of manufacturing semi-finished products such as rolling plates and extrusion billets made of aluminum alloys by vertical semi-continuous casting.

[0005] State of the art

[0006] Aluminum alloy rolling slabs and extrusion billets are typically manufactured by casting into a vertical mold, or ingot, positioned on a casting table above a casting pit or well.

[0007] The mold has an essentially rectangular section in the case of plates or circular in the case of billets, with open ends, the lower end being closed at the start of casting by a false bottom which moves downwards thanks to a descender during the casting of the plate or billet, the upper end being intended for the supply of metal.

[0008] The mold and false bottom define the cavity into which the metal is poured during the filling phase, before the descent phase begins.

[0009] At the start of the casting process, the false bottom is in its highest position in the mold. As soon as the poured liquid metal has cooled sufficiently to form a solidified wall, the false bottom is lowered at a predetermined speed. The solidified metal then flows out of the lower part of the mold and is constantly cooled, thus forming the plate or billet.

[0010] This type of casting process in which the metal extracted from the mold is cooled directly by the impact of a coolant is known as semi-continuous, vertical, direct-cooled casting (DC-casting).

[0011] A major problem encountered in vertical semi-continuous casting of aluminum alloys is to achieve the transition from the zero descent rate at the start of product formation to the steady-state descent rate, which is typically of the order of mm per second. During this transitional stage, the initial absence of marsh, which only gradually forms, deprives the product of a degree of freedom to accommodate the volumetric contraction that accompanies solidification. This results, under the effect of the thermal gradient between the surface cooled directly by the water and the core of the product, in a deformation of the plate foot, known to those skilled in the art as butt curl.If it is too pronounced, which occurs when the foot is cooled too violently or when the cast format is large or when the cast alloy is relatively poor at conducting heat and becomes the site of a significant thermal gradient (this is the case, among others, with alloys rich in Mg or Mn), the camber can cause several types of problems. "Cracks" or "breakthroughs" result from the loss of thermal contact between the metal and the mold or between the metal and the false bottom. Excessive camber can also cause "hanging", that is to say, one of the small edges of the plate rises in the mold in such a way that it can get stuck there; when it falls back down, sooner or later, it is accompanied by dangerous projections of liquid metal.

[0012] Patent EP3439813 B1 thus proposes a control system for detecting this problem. The camber associated with an unsuitable cooling regime can lead, less catastrophically, to the breakage of the root or to cracks in the root. These breaks or cracks are quite harmful because they can propagate in steady state, thus leading to the rejection of the product, otherwise and at the very least, they prevent, for example, the hot rolling of the plate without prior sawing of the root, which increases the processing. Finally, a camber that does not result in any casting rejects nevertheless constitutes a geometric singularity of the product which hinders the handling of the plates on the rollers which transport them to the following stages, and can result, during rolling, in the appearance of deformation heterogeneities in the strip which lead to an increase in metal loss.This is why specialists in the semi-continuous casting of aluminum alloys have always sought ways to minimize the camber phenomenon.

[0013] One of the avenues explored has been to modify the geometry of the false bottom by means of bosses, valleys or various edges, with the aim of modifying the distribution of hot and cold zones in the foot and thus acting on the thermomechanics of the foot to reduce the amplitude of the camber. Patent application WO2022 / 240953 thus proposes a false bottom that reduces the stresses inside the part by reducing the probability of stress crack formation. This type of solution can work but finds its limits when the alloys are sensitive to hot cracking, and therefore to the presence, by construction, of strong local thermal gradients which result in local concentrations of deformation leading to cracking.One of the solutions used was to use a curved false bottom, possibly associated with an inverted V-shaped mold, so that the center of the faces was first exposed to direct cooling, before the ends close to the edges were in turn exposed. But those skilled in the art mainly sought to adjust the heat flux extracted by the water at the first moments of the metal leaving the mold, for example by intermittent watering, called pulsation, obtained with valves feeding water into the molds at a frequency typically of the order of Hz; or by injecting CO2 into the starting water in order to get closer to the heating threshold; or by optimizing the design of the molds.

[0014] Thus, vertical semi-continuous casting devices for aluminium alloys have converged in around 80 years towards fairly complex moulds which make it possible in particular to manage the main problems of this process and attempt to minimise the phenomenon of camber whilst maintaining high productivity and satisfactory quality of the raw cast shapes.

[0015] In the oldest devices, a simple mold was cooled by a spray bar and another spray bar cooled the solidified wall of the raw form. This type of device is illustrated for example in patent US2301027 filed in 1938 or in patent US2757425 filed in 1952, in this type of device the cooling of the mold and that of the raw form are not independent because the cooling liquid sprayed on the mold trickles down onto the raw form. In an intermediate version that appeared fairly quickly, the spray bars were replaced by a spray box, supplied with water and pierced with holes allowing jets to be made around the periphery of the solidified wall of the raw form. It seems that this device was described for the first time in patent US2414269 filed in 1942.Thus in patent US2515284 filed in 1947 the spraying ramps are mentioned but the preferred device is a spray box. This type of device is also described in patent US2705353 filed in 1952. In patent US3441079 filed in 1964, the device comprising two spraying ramps as described by US2301027 in 1938 is illustrated in Figure 1 but the examples are based on a single mold device with spray box illustrated by Figure 2. The combination of the mold with the spray box so as to have only one device was then imposed, as described for example in patent US3688834 filed in 1972.

[0016] Although they are not specific to aluminum alloys and solidification conditions vary greatly between metals, patents DE813755 of 1951 may also be cited, which describes a continuous water-injection casting mold in which the spraying device is completely installed in the mold body, US3098269 of 1960, which describes a mold comprising several zones separated by thermal barriers, and FR1072748 of 1953, which describes a device in which any runoff of the coolant is avoided by projecting droplets of very small diameter. The use of spray nozzles is disclosed in the field of continuous casting for steel. Application DE 10 2009 005 679 may be cited.

[0017] According to the current state of the art, the mold, of the water box type, is pierced with multiple orifices, jets of water from the mold then allow the cooling and solidification of the metal to continue by direct contact of the water projected onto the solid metal. The design of the internal chambers and holes of the molds has evolved in particular so that the linear flow rate can be lowered typically to around 0.5 l / cm / min without loss of homogeneity, which allows the surface temperature of the metal at the exit of the mold to be uniformly higher than the so-called Leidenfrost temperature and a stable heating regime to be established.In the most recent developments, the molds have evolved in particular to include one or more rows of holes allowing jets to be projected at several angles, either by coupling such a hole design with a distribution of the rows of jets according to two optimized angles of incidence, which reduces the density of heat flux extracted at impact and makes it possible to obtain the heating regime over a wider range of linear flow rates but which can be interrupted at will by a simple increase in the flow rate (see for example application WO2013104846A1); or by ensuring that one of the two rows of holes can be activated alone before the second row of jets is also activated by a valve system internal to the mold and ends the heating regime (see for example application WO 2005 / 092540A1).We can also cite application US 2002 / 0174971 filed in 2002 which discloses the use of two jets of cooling liquid and the possibility of varying their angle of impact at the solidified surface in order to modify the extracted heat. Patent US 8365807 filed in 2012 also discloses the possibility of using a continuous jet and an intermittent jet to reduce camber. We also note patent application WO2022 / 020248, also published according to US 2023 / 0219130 which relates to a system comprising a mold and at least one cooling bar, the water box type cooling bar comprising a plurality of nozzles delivering water jets on the periphery of the metal product after the metal product has passed through the mold, in particular to increase the casting speed.

[0018] In some cases, it may also be useful to remove the cooling water from the metal surface at a distance from the mold outlet. This is typically done using baffles, as described for example in patent US4237961. An alternative to baffles is described in application WO2009 / 146527 in which the cooling water is removed from the surface by directing water jets onto the surface at an angle and with an impulse that cause the cooling water to be removed from the surface as it contacts the jets and follow a path that prevents the cooling water from re-contacting the ingot surface at a position beyond the point of removal.

[0019] For over 50 years, there has been a prejudice among those skilled in the art that the most suitable solution is to produce a mold that allows direct cooling of the raw shape by projecting jets from a mold chamber or even from water box segments whose internal architecture and hole diameters allow relatively low flow rates to be achieved while ensuring very good uniformity of flow along the mold. The problem of camber is however not completely resolved with this type of mold, which also has certain disadvantages such as the need for complex machining that can only work for a single format.

[0020] Problem posed

[0021] The problem to be solved is to develop a vertical semi-continuous casting process and a device for implementing the vertical semi-continuous casting process, with direct cooling, which makes it possible to further limit the camber and the associated problems while resolving the disadvantages of current devices.

[0022] Subject of the invention

[0023] A first subject of the invention is a method for vertical semi-continuous casting with direct cooling of a raw form (5) made of aluminum alloy, such as a rolling plate or an extrusion billet, using a device comprising a mold with open ends (1) formed of at least one hollow element defining at least one peripheral chamber (12) having a solid internal wall (13) intended to come into contact with the aluminum alloy and allowing the circulation of a first cooling liquid capable of carrying out primary cooling, a plurality of spray nozzles (21) integrated in at least one ramp (2) independent of the mold capable of carrying out secondary cooling, independent of the primary cooling, by spraying a second cooling liquid, a false bottom (3), capable of closing the lower part of the mold at the start of casting,having an upper part (31) intended to come into contact with the aluminum alloy and a lower part (32), and a descender (4) capable of gradually lowering the false bottom during the solidification of the raw form, in which, (i) the mold (1) is cooled by primary cooling and the false bottom (3), which closes the lower end of the mold, by secondary cooling, by spraying the second cooling liquid onto the lower part (32) of said false bottom according to a first spraying mode,

[0024] (ii) the liquid aluminum alloy is fed into the mold (1) and the false bottom (3) thus cooled so as to obtain a solidified wall (52) of the aluminum alloy in contact with the walls of the mold and a base (53) in contact with the false bottom,

[0025] (iii) a transient regime is established by actuating the lowerer (4) while continuing to feed the liquid aluminum alloy, the mold being cooled by the primary cooling and the entire periphery of the solidified wall of the raw form being cooled by the secondary cooling over a portion of its height (23) according to a second spraying mode allowing cooling by direct spraying (22) and by trickling along said solidified wall (24) of this second cooling liquid, the second spraying mode being characterized by the temporal and / or spatial modulation of the spraying,

[0026] (iv) a permanent regime is established in which the conditions of the transient regime are maintained apart from the secondary cooling spraying mode for which, according to a third spraying mode, the flow rate of the second sprayed cooling liquid is greater than or equal to the maximum of that of the second spraying mode and constant.

[0027] A second subject of the invention is a device intended for implementing the method according to the invention comprising an open-ended mold (1) formed of at least one hollow element defining a peripheral chamber (12) having a solid internal wall (13) intended to come into contact with the aluminum alloy and allowing the circulation of a first cooling liquid capable of carrying out primary cooling, a plurality of spray nozzles (21) integrated in at least one ramp (2) independent of the mold, capable of carrying out secondary cooling by direct spraying (22) of a second cooling liquid over a portion of the height of the entire circumference of the solidified wall (23) and by trickling of this second cooling liquid along said solidified wall (24), a false bottom (3) capable of closing the lower part of the mold at the start of casting,and a descender (4) capable of gradually lowering the false bottom during the solidification of the raw form, the primary cooling and the secondary cooling being independent.

[0028] Description of figures

[0029] Figure 1: Sectional view of an embodiment of the method and device according to the invention. Figure 2: Top view of an embodiment of the method and device according to the invention Figure 3: Top view of an embodiment of the method and device according to the invention Figure 4: Side view of an embodiment of the method and device according to the invention Figure 5: Side view of an embodiment of the method and device according to the invention Figure 6: Sectional view of an embodiment of the method and device according to the invention during step (i)

[0030] Figure 7: Sectional view of an embodiment of the method and device according to the invention during step (ii)

[0031] Figure 8: Sectional view of an embodiment of the method and device according to the invention during step (ii)

[0032] Figure 9: Sectional view of an embodiment of the method and device according to the invention during step (iii)

[0033] Figure 10: Cross-sectional view of the principle of the reference jet cooling device used in Example 1.

[0034] Figure 11: Sectional view of the principle of the device used in example 1 to simulate the cooling of the invention.

[0035] Figure 12: Cooling curves of example 1.

[0036] Description of the invention

[0037] The present invention is part of the long quest for devices making it possible to reduce the curvature of solidified raw shapes by adjusting the heat extraction during the transient phase of starting the casting in comparison with the permanent regime, but it is a break with all the most recent solutions which were related to a type of mold which jointly ensured the confinement and cooling of the metal inside the mold and the distribution of water for the direct cooling of the metal at the outlet of the mold or even to the combination of a mold and water box segments designed on the same principle.

[0038] In the context of the invention, the term “raw form” or “solidified raw form” refers to a product cast by direct-cooled vertical semi-continuous casting such as a rolling slab or an extrusion billet or a forging blank. The space is identified by 3 orthogonal axes X, Y and Z. The Z axis is the vertical axis which corresponds to the casting direction. The X and Y axes define a horizontal plane, perpendicular to the casting direction. A raw form of essentially rectangular section such as a rolling slab has a length along the casting direction and a width and a thickness, the width being greater than the thickness, in a plane perpendicular to the casting direction, the width extending along the Y axis and the thickness extending along the X axis.A substantially rectangular cross-sectional shape means that the solidified raw shape has two large faces and two small faces intersecting at right angles, but the angles may be rounded and the faces may be slightly curved or composed of several segments. A substantially cylindrical raw shape such as a spinning billet has a length along the casting direction and a circular cross-section in a plane perpendicular to the casting direction.

[0039] By design, the molds according to the prior art provide a fixed configuration of the watering jets, with the variation of the water flow rate or the partial activation of the holes. The present inventors have understood that this type of mold or water box segment has major drawbacks because it does not allow the cooling flow rates to be varied over a wide range of values, the pressure in the chamber cannot fall below a minimum value and is very limited as to its maximum value, and, for the casting of shapes of essentially rectangular section, it is not easy to modulate the cooling spatially by projecting more or less water depending on the position along the periphery of the shape.

[0040] The present inventors have separated mold cooling and direct spray cooling into two independent objects unlike prior art devices. The present inventors have combined a purely internally cooled mold with a plurality of spray nozzles integrated in at least one ramp allowing secondary cooling by direct spray and trickle.

[0041] The device according to the invention comprises a mold with open ends, a plurality of spray nozzles integrated in at least one ramp independent of the mold, a false bottom and a descender.

[0042] The open-ended mold is formed from at least one hollow element defining at least one peripheral chamber having a solid inner wall intended to come into contact with the aluminum alloy and allowing the circulation of a first cooling liquid capable of carrying out primary cooling. The inner wall of the peripheral chamber is solid, that is to say that it does not contain channels passing through it and allowing the cooling liquid to leave the chamber and directly spray the raw form as in molds according to the current state of the art. The mold chamber can thus be considered closed, with orifices used only for the circulation of the cooling liquid in the chamber, therefore the expression closed chamber is used in the present invention. The primary cooling obtained by circulation of the first cooling liquid in the chamber is thus purely indirect.In one embodiment, the mold has a single chamber. In another embodiment, the mold has a plurality of chambers connected to each other for the purpose of optimizing the hydraulics of the internal circulation depending on the formats considered.

[0043] In one embodiment, the mold is formed from a single monolithic hollow element. In one embodiment intended for casting rough shapes of essentially rectangular cross-section such as rolling plates, the mold is formed from four essentially rectilinear hollow elements, such as hollow profiles, connected together to form at least one peripheral chamber. In one embodiment intended for casting rough shapes of essentially rectangular cross-section, the mold comprises deformable hollow profiles, typically the hollow profiles in contact with the width of the rough shape, the shape of the hollow profiles being able to be modified from a slightly rounded shape to a rectilinear shape during casting using jacks, so as to approach a parallelepiped shape from the start of casting.In another embodiment, intended for casting a rough shape of essentially rectangular section, the mold is made up of hollow profiles and it is possible to vary the position, typically by sliding them, of the profiles parallel to the thickness of the rough shape, so as to easily vary the width of the rough shape.

[0044] The mold according to the invention is advantageous compared to certain devices of the prior art for which the mold was cooled by spraying because it makes it possible to dissociate the cooling of the mold and the direct cooling of the raw form and therefore to be able to modulate temporally and spatially the direct cooling of the raw form independently. In addition, it has an advantage in terms of safety because it limits the risk of spraying water on the liquid aluminum. Indeed, the mold according to the invention involves confining the cooling water for primary cooling. The manufacture of a mold with open ends provided on its periphery with a closed peripheral chamber is advantageous, this being much simpler to produce than a mold whose internal wall is provided with holes for cooling.To cast all the desired formats, the manufacture of new molds according to the invention is very inexpensive, due to the great simplicity of machining the mold element(s) which are reduced to simple conduits for the internal circulation of the cooling fluid. The cooling liquid is introduced into the mold by appropriate piping and valves. In an advantageous embodiment, the cooling liquid heated during casting is used for another process requiring hot water. In an advantageous embodiment, the cooling liquid circulates in a closed circuit and is cooled by a heat exchanger.

[0045] The first coolant is typically water. In one embodiment, this water is such that its alkalinity (characterized by the alkalimetric titer (AT), called P-alkalinity in English and the complete alkalimetric titer (TAC)), called M-alkalinity in English, its hardness (characterized for example by the hydrotimetric titer (TH)), its pH, its electrical conductivity, and / or its turbidity are controlled and / or optimized to improve the thermal efficiency and / or limit the fouling of the internal channel(s) of the closed chamber. In one embodiment, the first coolant is deionized water.

[0046] A plurality of spray nozzles are integrated into at least one ramp independent of the mold. Unlike cooling molds according to the prior art which allow secondary cooling using orifices (called nozzles in English), the invention proposes to carry out secondary cooling using spray nozzles. A spray nozzle dispenses the coolant in the form of droplets. In contrast, an orifice dispenses the coolant in the form of a jet. A ramp provided with spray nozzles comprises a pipe, typically of circular section, and spray nozzles independent of each other and capable of being individually adjusted in flow rate and / or pressure and / or pulsation.The spray nozzles allow secondary cooling by directly spraying a second coolant over a portion of the height of the entire periphery of the solidified wall and by trickling this second coolant along said solidified wall. The spray nozzles allow for better cooling efficiency, which makes it possible to reduce the linear flow rate of coolant. The inventors believe that this is advantageous in the transient casting regime to reduce the risks of camber. Typically, it is possible to reduce the linear flow rate of coolant below 0.5 l / min / cm using spray nozzles. This is not possible with a cooling mold according to the prior art using jets because below 0.5 l / min / cm, the flow rate is not sufficient for the jet to reach the solidified shape to be cooled.Advantageously, the portion of the height directly sprayed is 50 mm to 150 mm, preferably 80 mm to 120 mm.

[0047] The nature of the second coolant is identical to or different from that of the first coolant. The second coolant is typically water. In one embodiment, this water is such that its alkalinity (characterized by the alkalimetric titer (AT) and the total alkalimetric titer (TAC)), its hardness (characterized for example by the hydrotimetric titer (TH)), its pH, its conductivity, and / or its turbidity are controlled and / or optimized to improve thermal efficiency. In one embodiment, dissolved CO2 is added to the second coolant. In one embodiment, several natures of second coolant are used during casting; in particular, a different second coolant can be used between the transient phase and the steady state.In one embodiment, the second coolant is deionized water, preferably at least during the transient phase. In one embodiment, the second coolant is water whose anion content is adapted so as to control the heating threshold.

[0048] When the solidified raw form is of essentially rectangular section, the spray nozzles are preferably arranged on four straight ramps. Advantageously, the straight ramps can be connected to each other and be supplied in a single position or in two or even three positions or connected two by two at least partially so as to be supplied independently or not connected to each other and supplied by at least four separate supplies, typically for the spray ramps of the large faces, the ramps can be supplied at their two ends. In one embodiment of the invention, when the solidified raw form is of essentially rectangular section, several straight ramps are used for each large face so as to follow the shape of the raw form. In one embodiment of the invention, each spray ramp is fixed.

[0049] When the solidified raw shape is of circular section, the spray nozzles are preferably arranged on a circular ramp or two semi-circular ramps or a suitable number of ramps with an angular sector of less than 180°, typically 120° or 90° or 60° or 45°, suitable for spraying the second cooling liquid over the entire circumference of the solidified raw shape, the ramps can optionally slide to adapt to the diameter. In one embodiment, the circular ramp can be supplied in a single position or in two or even three or even four positions. In one embodiment, the semi-circular ramps can be connected to each other and be supplied in a single position or in two or even three or even four positions. In one embodiment, the ramps with an angular sector of less than 180° can be supplied in a single position or in two or even three or even four positions.

[0050] In one embodiment, two sets of ramps offset according to the casting direction and each allowing a spraying of cooling liquid around the perimeter of the raw shape are used.

[0051] Preferably, a plurality of valves are used to supply the spray nozzles with the second coolant. In one embodiment, each valve is used to supply a maximum of ten spray nozzles. In one embodiment, each valve is used to supply a maximum of nine spray nozzles. In one embodiment, each valve is used to supply a maximum of eight spray nozzles. In one embodiment, each valve is used to supply a maximum of seven spray nozzles. In one embodiment, each valve is used to supply a maximum of six spray nozzles. In one embodiment, each valve is used to supply a maximum of five spray nozzles. In one embodiment, each valve is used to supply a maximum of three spray nozzles. In one embodiment, each valve is used to supply a maximum of two spray nozzles.In one embodiment, each nozzle is supplied by an independent valve. In one embodiment, the number of spray nozzles supplied by a valve is not constant, for example, certain particularly critical spray nozzles arranged in the corners of a shape of essentially rectangular section are supplied by a single valve, while sets of three or four or five or six or seven or eight or nine or ten spray nozzles arranged towards the center of the small or large faces are supplied by the same valve. In one embodiment, each valve makes it possible to control the spray nozzles spraying a portion of the periphery of the rough shape with dimensions along the X axis or the Y axis of between 25 and 500 mm, preferably between 50 and 300 mm.

[0052] The spray nozzles used are preferably single-fluid spray nozzles in which the pressurized fluid is transformed into droplets without adding a second fluid, thanks to its geometric characteristics. A distinction is generally made between "flat jet nozzles" which allow a flat-shaped or thin-thickness brush jet to be sprayed compared to the width of the jet, and "conical jet nozzles" which allow a full jet to be sprayed in a cone of revolution. In the context of the present invention, a distinction will also be made between "elliptical jet nozzles" which allow a full elliptical jet to be sprayed and whose impact on a surface perpendicular to the central direction of the jet has an elliptical shape having a main axis and a secondary axis. In one embodiment, the spray nozzles are flat jet nozzles or conical jet nozzles or elliptical jet nozzles, these shapes being able to be optionally combined on the same ramp.In the context of the invention, it has been found that elliptical jet nozzles are advantageous. Thus, flat jet nozzles can in some cases have too much rebound, i.e. the sprayed droplets bounce off the solidified wall, and a short spray impact height. The rebound of the sprayed droplets results in a lower runoff, which can impair the cooling of the cast plate or billet.

[0053] Conical jet nozzles have the disadvantage of generating an impacting jet whose shape is a cone whose intersection with the solidified wall is circular, or elliptical if the nozzle is inclined, an ellipse whose main axis is parallel to the casting direction, which means having to size a system with a very small pitch between the spray nozzles to obtain sufficient coverage of the jets.

[0054] Elliptical jet nozzles, which are typically oriented such that the major axis of the ellipse is parallel to the horizontal plane and the minor axis is parallel to the casting direction, provide a horizontal impact length on the solidified wall sufficient to ensure overlap of the water jets, while providing a spray impact height of the same order of magnitude as the prior art waterbox-based secondary cooling solution.

[0055] In another method of installing elliptical jet nozzles, the spray nozzles are oriented such that the main axis forms a non-zero angle α with the vertical axis Z, preferably between 25 and 70°, preferably between 30 and 60°, so as to ensure direct cooling impacting with the plate or billet over a greater height of the plate or billet. The angle α is obtained by rotating the spray nozzles along the X axis for the face corresponding to the width of the raw shape or along the Y axis for the face corresponding to the thickness of the raw shape.

[0056] Preferably, the conical jets of coolant have a cone angle of 20° to 100°, preferably 25° to 50°. Preferably, the elliptical jets have a cone angle along the main axis of 60° to 110°, preferably 70° to 100°, and a cone angle along the secondary axis of 10° to 50°, preferably 15° to 40°.

[0057] Preferably, the spray nozzle jets overlap in such a way as to provide a uniform linear flow rate around the entire perimeter of the solidified raw form. The closer the spray nozzles are, the easier it is to control the shape of the thermal profile along the surface of the raw form. Preferably, the spacing between two spray nozzles is 25 mm to 100 mm, and more preferably 50 to 100 mm.

[0058] The shortest distance between the end of the spray nozzles and the solidified wall is advantageously chosen in a range of 60 mm to 200 mm, preferably in a range of 80 mm to 150 mm. The average path of the coolant between the end of the nozzle and the solidified wall is advantageously in the range of 80 mm to 150 mm, preferably 100 mm to 140 mm. The method according to the invention is advantageous because it works for a wide range of distances between the spray nozzles and the metal to be cooled, which makes it possible in particular to use, for a rough shape of essentially rectangular cross-section, four straight ramps which do not precisely follow the opening profile of the mold.

[0059] Spray nozzles operate advantageously in a pressure range of 1 bar to 4 bar, and more preferably between 1 and 3 bar, because a minimum pressure is required for rapid operation of the control valves and high water pressure must be avoided in order to prevent water splashing around the mold.

[0060] In one embodiment, the spray nozzles can be oriented independently of each other. Advantageously, the spray nozzles can be oriented by rotation around the 3 axes of space. Preferably, the spray nozzles can be oriented without modification of the valves and / or the ramps, and advantageously by keeping the valves and / or the ramps in a horizontal position. Advantageously, the spray nozzles are inclined at an angle P of at least 25°, preferably 30° downwards, i.e. in the Z direction of the casting direction, relative to the horizontal plane, preferably from 30 to 60° or from 35° to 60°. The angle P is obtained by rotation of the spray nozzles along the Y axis for the face corresponding to the width of the raw form or along the X axis for the face corresponding to the thickness of the raw form.

[0061] The preferred values ​​indicated for the angles, distances and pressure allow optimal cooling to be obtained taking into account the spray nozzle technologies described, however the invention is not limited by these preferred values, in particular if equivalent cooling characteristics can be obtained with other spray nozzle technologies for which the preferred values ​​would be different.

[0062] Advantageously, the valves are fast-response valves, the opening and closing response time of which is advantageously less than 1 s, preferably less than 0.5 s, and more preferably less than 0.2 s. Advantageously, the valves are open by default, so as to ensure cooling of the plate or billet even in the event of a cut-off of the electrical or electropneumatic energy.

[0063] Feeding the spray nozzles individually with fast-response valves is advantageous because it allows each point of the height portion of the entire perimeter of the solidified wall to be cooled specifically. These response times allow, in particular, the second coolant to be sprayed with sufficient reliability and differentiated angles and faces during casting, especially at the start of casting. The device therefore allows spatial and temporal modulation of the cooling intensity of the metal during solidification.

[0064] In one embodiment, the device according to the invention further comprises means for electromagnetic stirring of the liquid metal.

[0065] The method according to the invention is a vertical semi-continuous casting method with direct cooling of a raw form such as a rolling plate or a spinning billet produced with a device according to the invention.

[0066] In a step (i) the mold is cooled by primary cooling and the false bottom, which closes the lower end of the mold, by secondary cooling, by spraying the second cooling liquid onto the lower part of said false bottom according to a first spraying mode. The first cooling liquid is introduced into the mold through appropriate piping and valves. In an advantageous embodiment, the first cooling liquid, after having circulated in the mold and thus heated, is used for another process. In another embodiment, the first cooling liquid, typically deionized water, is kept in a closed circuit and thermostatically controlled by a heat exchanger. The second cooling liquid may be of the same nature as the first cooling liquid or be different. All the spray nozzles may be supplied or only some depending on the cast format.In one embodiment, the first spraying mode consists of opening all of the valves controlling the supply of second coolant to the spray nozzles to obtain a maximum spray flow rate. In one embodiment, the first spraying mode consists of opening all of the valves corresponding to the cast format controlling the supply of second coolant to the spray nozzles.

[0067] In a step (ii) the liquid aluminum alloy is fed into the mold and the false bottom thus cooled so as to obtain a solidified wall of the aluminum alloy in contact with the walls of the mold and a base in contact with the false bottom. Thus during step (ii), the cooling conditions of step (i) are maintained, typically in an identical manner. The lower end of the mold remains closed by the false bottom during step (ii). The aluminum alloy solidifies in contact with the cooled mold to form a solidified wall. The aluminum alloy solidifies in contact with the cooled false bottom to form a solidified zone at the bottom of the mold called a base.

[0068] In a step (iii) a transient regime is established by actuating the descender while continuing to feed the liquid aluminum alloy, the mold being cooled by the primary cooling and the entire periphery of the solidified wall of the raw form being cooled by the secondary cooling over a portion of its height according to a second spraying mode allowing cooling by direct spraying and by trickling of this second cooling liquid along said solidified wall, the second spraying mode being characterized by the temporal and / or spatial modulation of the spraying. Step (iii) can only be started when a solidified wall and a sufficient base of the aluminum alloy in contact with the walls of the mold and the false bottom have been obtained. The mold cooling conditions are typically identical to those of steps (i) and (ii).In one embodiment, during the transient regime the temporal modulation of the second spraying mode is a pulsed spray. In one embodiment, the second spraying mode is pulsed and the third spraying mode is continuous. The pulsation is described by a reference period and a temporal percentage of supply, the reference period being the period of reproduction of the pulsation and the percentage of supply being the percentage of the reference period during which the spraying is effective. Advantageously; the reference period of the pulsation of the second spraying mode is between 0.5s and 2s, preferably from 0.8 to 1.2s and the temporal percentage of supply of the spray is between 10% and 99%. The temporal percentage of supply of 100% corresponds to the case where the spraying is no longer pulsed but continuous.Preferably, the time percentage of spray supply is between 20% and 50% at the start of step (iii). The second spray mode is advantageously not fixed and the time percentage of spray supply can typically be varied from 20% to 99% during step (iii). Thanks to the device according to the invention in which the supply of the spray nozzles can be controlled, it is indeed possible to permanently adapt the spray for each zone of the periphery of the raw shape. In one embodiment, the modulation of the spray of the second spray mode is temporal and spatial. The independence of the primary cooling and the secondary cooling is advantageous because it makes it possible to achieve low secondary cooling flow rates, which is effective in limiting camber.

[0069] In a step (iv), which corresponds to a permanent regime, the conditions of the transient regime are maintained apart from the spraying mode of the secondary cooling for which, according to a third spraying mode, the flow rate of second sprayed coolant is greater than or equal to the maximum of that of the second spraying mode and constant. Advantageously, in the third spraying mode, the valves are continuously open. In an advantageous embodiment, the third spraying mode is identical to the first spraying mode.

[0070] Advantageously, the spraying, in each of the spraying modes, can be spatially modulated by differentiated supply of the spray nozzles or groups of spray nozzles in the case where a valve supplies several spray nozzles. Advantageously, the supply of the spray nozzles can be maximum or temporally modulated, typically pulsed. In one embodiment, the supply of the spray nozzles is maximum for the first and third spraying modes and temporally modulated, typically pulsed, for the second spraying mode.

[0071] In step (iii) and step (iv) the entire perimeter of the raw shape is sprayed with the second cooling liquid, i.e. there is no unsprayed area around the perimeter, only a portion of the height, the height affected by the direct spraying, is sprayed with direct spraying. Generally, this portion increases during step (iii) until it reaches a stationary value of the portion when the descender has reached a level such that the entire jet emitted by the spray nozzles directly sprays the solidified wall. Furthermore, when the descender has reached a level such that the entire jet emitted by the spray nozzles directly sprays the solidified wall, cooling of the parts not reached by the direct spraying remains cooled by trickling of the second cooling liquid along the solidified wall.

[0072] Step (iv) begins when the maximum cooling power that will be used during casting is used. Typically, step (iv) begins for a casting length expressed in meters equal to the area of ​​the section of the rough shape perpendicular to the casting direction expressed in m 2divided by the half-perimeter of the perimeter of the rough shape expressed in meters. The permanent regime can however be interrupted in certain cases during casting and a transient regime put in place again, we then have the succession of steps (iv), (iii) and (iv). This can be the case in particular when defects appear on the rough shape. In one embodiment, an optical device such as a camera records a signal coming from the surface of the rough shape, such as an image, this signal is transmitted to a computer programmed to adjust the spraying and correct any surface defect or camber of the foot by an adapted transient regime. Thanks to the device according to the invention, it is in fact possible to modify very quickly the spatial and / or temporal characteristics of the secondary cooling.

[0073] The present invention constitutes a double break with respect to the most typical solutions of the state of the art, in that firstly, indirect cooling by the mold and direct cooling by water are obtained by two separate members instead of being combined; secondly, direct cooling is itself discretized into a set of controllable spray nozzles, which adds to the temporal modulation of the flow rate the possibility of spatial modulation of the flow rate on the periphery of the raw shapes. Thanks to the temporal modulation, the spatial modulation of the flow rate and the spatial control of the spray impact height, three anti-camber mechanisms are thus available at the start of casting. In addition, the localized spraying by each spray nozzle can advantageously be controlled and adapted throughout casting.

[0074] The device according to the invention makes it possible to obtain, in particular by adapting the spraying modes, low heat extraction at the start of the start-up phase by using a quantity of second cooling liquid that is sufficiently low and adapted to its thermal efficiency and to the choice of start-up speed and to increase the flow rate of this second cooling liquid during the casting speed increase ramp to achieve the speed and cooling parameters adapted to the permanent casting regime, thus guaranteeing the good health of the foot and minimizing its arch.Spatial modulation also allows the same spray nozzle array to be used to cast different combinations of alloys and formats, to adapt cooling on the angles and short sides of the plate for better control of cooling and solidification, which allows the structure and composition gradient associated with the cortical zone to be controlled, and to modulate the flow rate very locally at a specific time in order to eliminate hot cracking phenomena. The method according to the invention also has the advantage of not using deflectors, which are complex to use, during casting. The precise control of the spraying makes it possible to avoid excessive runoff.In one embodiment, the runoff is controlled during steady state such that the heat flux extracted by runoff is at least three times lower, preferably at least six times lower, more preferably at least ten times lower than the heat flux extracted by the impact of the direct spray.

[0075] Detailed description

[0076] In Figure 1, a sectional view of an embodiment of the method and device according to the invention is shown for casting a raw shape 5, of essentially rectangular section whose thickness extends along the X axis and whose width extends along the Y axis, having a marsh 51 and a solidified wall 52. An open-ended mold 1 provided on its periphery with a peripheral chamber 12 whose internal wall 13 is solid is capable of carrying out primary cooling of an aluminum alloy by circulation of a first cooling liquid. The peripheral chamber is closed, the orifices used for the circulation of the cooling liquid in the chamber are not shown.A plurality of spray nozzles 21 are integrated in at least one ramp 2 independent of the mold and allow secondary cooling by direct spraying 22 of a second cooling liquid over a portion of the height of the entire periphery of the solidified wall 23 and by trickling of this second cooling liquid along said solidified wall 24. The spraying 22 corresponds to the sending of fine droplets onto the solidified wall 23. The supply of the spray nozzles 21 is controlled by fast-response valves 25. The height 23 impacted by the direct spraying 22 is also called the spray impact height. The spray nozzles are inclined downwards by an angle p. The figure shows the angle P of inclination of the spray nozzles spraying the cooling liquid on the face corresponding to the width, obtained by rotation along the Y axis.The figure does not represent the angle P of inclination of the spray nozzles spraying the coolant on the face corresponding to the thickness, obtained by rotation along the X axis. A false bottom 3 is capable of closing the lower part of the mold at the start of casting, and a descender 4 is capable of gradually lowering the false bottom along the Z axis which corresponds to the casting direction during the solidification of the raw shape. The chamber illustrated in Figure 1 is unique, however in another embodiment not shown, the mold has a plurality of chambers.

[0077] In Figure 2 there is shown a top view of an embodiment of the device according to the invention for casting a rough shape of essentially rectangular section whose thickness extends along the X axis and whose width extends along the Y axis. The spray nozzles 21 are distributed over four ramps 2, i.e. one ramp per side of the periphery of the rough shape 5. In Figure 3 there is shown a top view of an embodiment of the device according to the invention in which the supply of each spray nozzle is controlled by a valve 25. The four ramps 2 are connected to each other by conduits T1 and supplied with second cooling liquid by a supply 28. In another embodiment of the device, not shown, the supply of the spray ramps is not unique but at a minimum number of one per small ramp spraying the thickness of the periphery, and two per ramp spraying the width of the periphery.

[0078] In Figure 4 there is shown a side view of an embodiment of the device according to the invention in which the impact zone of each spray nozzle is represented by an ellipse 221. The spray impact height is represented by the arrow 23. In the embodiment shown the spray nozzles are elliptical jet nozzles oriented so that the main axis of the ellipse is parallel to the horizontal plane and the minor axis parallel to the casting direction.

[0079] In Figure 5, a side view of an embodiment of the device according to the invention is shown in which the impact zone of each spray nozzle is represented by an ellipse 222. In the embodiment shown, the spray nozzles are elliptical jet nozzles having a rotation of angle a along the X axis for the face corresponding to the width, and, not shown, of angle a along the Y axis for the face corresponding to the thickness.

[0080] In Figures 6, 7, 8 and 9, four stages of an embodiment of the method according to the invention are illustrated. In Figure 6, step (i) is in progress: the mold is cooled by primary cooling and the false bottom, which closes the lower end of the mold, by secondary cooling, by spraying the second cooling liquid onto the lower part (32). In Figure 7, step (ii) has started: the liquid aluminum alloy is fed into the cooled mold (1) and the false bottom. In Figure 8, step (ii) continues and a solidified wall (52) of the aluminum alloy in contact with the walls of the mold and a sole (53) in contact with the false bottom have been obtained.

[0081] In Figure 9, the descender 4 has been actuated and step (iii) is in progress, i.e. the second coolant is sprayed over a portion of the height 23 of the entire perimeter of the solidified wall of the rough shape. The portion of the height is at this stage lower than the portion sprayed in steady state illustrated in Figure 1. The first coolant continues to circulate in the chamber 12. Example 1

[0082] Thermal tests were carried out to evaluate the cooling efficiency of a plate when it is sprayed by jets (reference case) or by spraying (according to the invention).

[0083] For this purpose, a 100 larget made of 2XXX series aluminum alloy was instrumented using thermocouples in order to record its temperature over time as a function of the cooling conditions undergone (Fig. 10 and Fig. 11). The larget is initially heated to 500°C and then immediately cooled according to 5 conditions described below.

[0084] Two reference conditions (1a and 1b) were tested. They are representative of secondary cooling conditions obtained using a conventional mold disclosed in application WO2013104846A1. In this test configuration, cooling is provided using two rows of jets oriented at impact at 32° for the first row and 22° for the second row (Fig. 10). Each row of jets is supplied with water by a water box 50. The effective linear water flow rate was set at 1 l / min / cm (condition 1a) and 0.5 l / min / cm (condition 1b).

[0085] Three other cooling conditions (2a, 2b, 2c) representative of preferred embodiments of the invention were tested. Cooling is here obtained using a spray nozzle 21 (Fig. 11). The supply to the spray nozzle 21 is controlled by a fast response valve 25. The spray nozzle is inclined downwards by an angle of 30° corresponding to an angle of 60° at impact. The linear flow rate was varied by drawing or not drawing the water supply using the fast valves 25.Thus, the cooling obtained was evaluated if the spray nozzle is supplied continuously (100% - the nozzle is open for 100% of the time - condition 2a), or at 50% of the time (the pulsation is made on the basis of a one-second cycle: the nozzle is open for 500 milliseconds, the nozzle is closed for 500 milliseconds - condition 2b) or at 25% (in the same way, the nozzle is open for 250 milliseconds, the nozzle is closed for 250 milliseconds - condition 2c). In all three cases, there is a water supply of 0.5 l / min, whose effective linear flow rate corresponds to 0.5 l / min / cm for condition 2a, 0.25 l / min / cm for condition 2b and 0.125 l / min / cm for condition 2c (see Table 1). The evolution of the temperature of the larget as a function of time is presented in Fig. 12. [Table 1].

[0086] Better cooling efficiency is observed under the conditions of the invention. Indeed, at the same effective linear flow rate, cooling is faster with a spray nozzle (condition 2a) rather than a jet (condition lb). The system according to the invention thus has the advantage of reducing coolant consumption. It is also observed that a spray system with spray makes it possible to achieve significantly lower cooling speeds than the reference cooling system with jets.

Claims

Claims 1. Method for vertical semi-continuous casting with direct cooling of a raw form (5) made of aluminum alloy, such as a rolling plate or an extrusion billet, using a device comprising an open-ended mold (1) formed of at least one hollow element defining at least one peripheral chamber (12) having a solid internal wall (13) intended to come into contact with the aluminum alloy and allowing the circulation of a first cooling liquid capable of carrying out primary cooling, a plurality of spray nozzles (21) integrated in at least one ramp (2) independent of the mold capable of carrying out secondary cooling, independent of the primary cooling, by spraying a second cooling liquid, a false bottom (3), capable of closing the lower part of the mold at the start of casting,having an upper part (31) intended to come into contact with the aluminum alloy and a lower part (32), and a descender (4) capable of gradually lowering the false bottom during the solidification of the raw form, in which, (i) the mold (1) is cooled by primary cooling and the false bottom (3), which closes the lower end of the mold, by secondary cooling, by spraying the second cooling liquid onto the lower part (32) of said false bottom according to a first spraying mode, (ii) the liquid aluminum alloy is fed into the mold (1) and the false bottom (3) thus cooled so as to obtain a solidified wall (52) of the aluminum alloy in contact with the walls of the mold and a base (53) in contact with the false bottom, (iii) a transient regime is established by actuating the lowerer (4) while continuing to feed the liquid aluminum alloy, the mold being cooled by the primary cooling and the entire periphery of the solidified wall of the raw form being cooled by the secondary cooling over a portion of its height (23) according to a second spraying mode allowing cooling by direct spraying (22) and by trickling along said solidified wall (24) of this second cooling liquid, the second spraying mode being characterized by the temporal and / or spatial modulation of the spraying, (iv) a permanent regime is established in which the conditions of the transient regime are maintained apart from the spraying mode of the secondary cooling for which, according to a third spraying mode, the flow rate of the second cooling liquid sprayed is greater than or equal to the maximum of that of the second spraying mode and constant.

2. The method of claim 1 wherein the second spray mode is pulsed and the third spray mode is continuous.

3. Method according to claim 2 wherein the reference period of the pulsation of the second spray mode is between 0.5s and 2s and the spray supply percentage is between 10% and 99%.

4. Device for implementing the method according to any one of claims 1 to 3 comprising an open-ended mold (1) formed of at least one hollow element defining a peripheral chamber (12) having a solid internal wall (13) intended to come into contact with the aluminum alloy and allowing the circulation of a first cooling liquid capable of carrying out primary cooling, a plurality of spray nozzles (21) integrated in at least one ramp (2) independent of the mold, capable of carrying out secondary cooling by direct spraying (22) of a second cooling liquid over a portion of the height of the entire circumference of the solidified wall (23) and by trickling of this second cooling liquid along said solidified wall (24), a false bottom (3) capable of closing the lower part of the mold at the start of casting,and a descender (4) capable of gradually lowering the false bottom during the solidification of the raw form, the primary cooling and the secondary cooling being independent.

5. Device according to claim 4 in which the raw shape is of essentially rectangular section and in which the spray nozzles are arranged on four straight ramps.

6. Device according to any one of claims 4 or 5 in which a plurality of valves makes it possible to supply the spray nozzles with the second cooling liquid.

7. Device according to claim 6 in which each valve makes it possible to supply a maximum of ten spray nozzles.

8. Device according to any one of claims 4 to 7 in which the spray nozzles are capable of producing jets of coolant in flat form, “flat jet nozzles” or conical “conical jet nozzles” or elliptical “elliptical jet nozzles”, these shapes can optionally be combined on the same ramp.

9. Device according to any one of claims 4 to 8 in which the spray nozzles can be oriented independently of each other.

10. Device according to any one of claims 4 to 9 in which the valves are fast response valves.

11. Device according to any one of claims 4 to 10 in which the spray nozzles are elliptical jets having a cone angle along the main axis of 60° to 110°, preferably 70° to 100°, and a cone angle along the secondary axis of 10° to 50°, preferably 15° to 40°, and in which the spray nozzles are oriented such that the main axis forms a non-zero angle α with the vertical axis, preferably between 30 and 60° or in which the spray nozzles are conical jets and have a cone angle of 20° to 100°, preferably 25° to 50°.

12. Device according to any one of claims 5 to 11 wherein the spray nozzles are inclined at an angle P of at least 30°, preferably 35° to 60°, downwards relative to the horizontal plane.

Images