FR2747059A1 - CONTINUOUS CASTING PROCESS FOR METALS AND LINGOTIERE FOR ITS IMPLEMENTATION - Google Patents

Abstract

An ingot mould has energetically cooled metal walls (1, 3) having means in their upper portion for decreasing the intensity of the heat flow extracted from the cast metal (2), such as a coating (6, 15) of a less conductive metal than that of the walls, or grooves (31) provided in said walls. On top of the walls is provided a heat insulating feeder bush (7) associated with gas injection means terminating in nozzles distributed on the inner periphery of said ingot mould. During the casting, the free surface of the liquid metal is maintained at the same level as the feeder bush, wherein the solid skin (21) only starts to solidify at the upper edge of the metal walls (3). The invention is particularly useful for the continuous casting of steel.

Description

CONTINUOUS CASTING PROCESS FOR METALS AND LINGOTIERE FOR
ITS IMPLEMENTATION.

 The present invention relates to the continuous casting of metals, in particular steel.

 As is known, the continuous casting operation consists schematically of pouring a molten metal into an ingot mold, essentially consisting of a tubular element without bottom defining a passage for the cast metal, but the walls of which, in copper or more generally made of copper alloy, are energetically cooled by water circulation, and from which a product already solidified externally over a few centimeters in thickness is also continuously extracted. Solidification then progresses towards the axis of the product and ends during the descent of the latter downstream of the mold in the so-called "secondary cooling" zone under the effect of water spray bars. The product obtained, bloom, billet or slab, is then cut to length, then laminated before shipment to the customer or processing on site, into bars, wires, profiles, plates, sheets, etc.

 Surface or subcutaneous defects of the products from the continuous casting of steel are often the cause of scrap, because the rolling operation does not support them well, even amplifies them until the intolerable deterioration of the metallurgical quality of the products laminated.

 During casting, the molten metal, brought into the mold by a nozzle, forms a solid film when it comes into contact with the cooled walls of the mold. This film is driven downwards during the extraction of the product by a jerky movement imposed by the vertical oscillations of the mold, and simultaneously its thickness increases due to the continuation of the heat extraction carried out by the walls of the mold. . There is therefore continuously creation of a new film of solid metal at the level of the free surface of the metal in the ingot mold, this film solidifying over the entire perimeter of the internal wall of the ingot mold and constituting a solid ring liable to contract. due to the cooling undergone during its descent into the mold.

 The contraction of the solid skin is all the more important as the heat extraction is strong and the cast metal has a natural tendency to contract during cooling, for example by change of solid phase at the end of solidification, as c This is the case in particular for grades of steel with 0.1 t of carbon or of stainless steel AISI 304.

 This perimeter contraction tends to cause a separation of the solidified skin relative to the wall of the mold, and therefore a decrease in the heat exchange due to the fact that the contact of said skin with the cold walls is degraded. This detachment is generally uneven depending on the perimeter of the solidified skin, which is a source of surface defects on the final product obtained.

 To avoid or limit these defects, attempts have already been made to reduce the heat flux extracted during the formation of said film, at the start of its solidification.

 Thus, it has already been attempted to reduce this heat flux by using an ingot mold provided, in its upper part where the beginning of solidification occurs, of an insert forming a thermal barrier by preventing the molten metal from coming directly into contact with the cooled walls of the mold. The holding over time of such an insert has however proved to be very uncertain, and the maintenance cost of such an ingot mold very high.

 It has also been attempted to reduce the heat flux by giving the walls of the mold a surface finish capable of reducing the area of the areas of direct contact of the cast metal with the copper of the cooled walls, for example by regular etching of the walls. forming an embossing of their surface, or by a random roughness obtained for example by sanding. However, such a method has apparently not yet made it possible to improve the surface quality of the cast product appreciably, since it appeared that the disturbances caused by variations in the level of the free surface of the cast metal were predominant by relative to the moderating effect of extracting the heat flux, obtained by using the adapted surface condition of the walls of the mold indicated above, so that, the solidification heterogeneities remaining, the surface defects do not are not eliminated.

 With the same aim of reducing the direct contact of the cast metal with the cold copper of the walls, it has also been attempted to make in them vertical grooves. However, it then appeared that these grooves quickly filled with the slag conventionally used as a cover layer for the free surface of the liquid metal, which attenuated the desired thermal effect.

 The present invention aims to solve the problems indicated above and is particularly aimed at obtaining a cast product having a very good surface quality, by ensuring an effective reduction of the heat flux extracted during the formation and the start of growth of the solidified film, and in particular avoiding the harmful effect of fluctuations in the level of the free surface of the liquid metal in the mold.

 With these objectives in view, the subject of the invention is a process for the continuous casting of molten metals, in particular steel, in an ingot mold having energetically cooled metal walls extending substantially vertically and defining a passage for the cast metal, and ensuring over their entire height an extraction of heat flow from the cast metal causing a cooling and a progressive solidification of said metal, process according to which the intensity of the heat flow extracted is reduced in the vicinity of the level where said metal begins to solidify at contact of said walls, characterized in that a thermally insulating riser is placed above said cooled metal walls and, during casting, the level of the free surface of the metal poured inside said riser is maintained.

 In accordance with the invention, the free surface of the liquid metal bath is located in said enhancement.

As the latter is made of thermally insulating refractory material, the molten metal does not solidify on contact, and the film of solidified metal only begins to form from the upper edge of the metal walls. Thus, the solidification of the cast metal begins only at a certain distance from the free surface of the liquid metal bath. The area where this solidification begins is therefore almost perfectly flat, horizontal, and not subject to fluctuations or disturbances which inevitably stir the free surface of the bath. The solid ring formed by the first solidification skin is therefore geometrically perfectly regular, and its continuous renewal is also almost perfectly regular, as is the growth of this solidified skin, as the product is lowered.

 Furthermore, since the molten metal does not solidify on contact with the riser, there is no contraction of the metal at this level; the latter remains in contact with the surface of the riser and prevents slag infiltration between the metal and said surface I1 as a result, there can therefore also be no slag infiltration between the metal walls of the ingot mold and the skin solidified on contact, even if the latter tends to deviate therefrom due to the withdrawal of solidification. It will also be noted that, at the plane where the film of solid metal begins to form and grow, the ferrostatic pressure of the liquid metal, due to the height of this metal contained by the enhancement, tends to oppose this spacing and therefore to maintain said film pressed against the surface of the metal walls, and this uniformly over the entire periphery of the internal surface of the mold, because the thickness and the solidification state of said film is also uniform on this periphery.

 Thus, due to this peripheral regularity, the heat extraction carried out in the upper part of the metal walls can also be carried out uniformly over the entire periphery of the product, avoiding localized detachments and the under-thicknesses of solid skin which would result, and the intensity of the heat flux extracted in the zone of beginning of solidification can be ensured very uniformly over the entire periphery of the passage defined by the metal walls.

 Preferably, the intensity of the heat flux extracted over a zone of predetermined height is reduced (height extending from the lower edge of said extension, this limited height ensuring a reduction in the heat flux extracted in the zone where the metal skin solid forms, and also avoiding the effect of removal and detachment of the metal skin observed in the casting process according to the prior art), without however appreciably modifying the amount of overall heat extracted by the ingot mold.

 According to a first variant, said zone has a substantially constant heat flux extraction capacity over its entire height. In this case, the reduction in the extracted heat flux can be strong but over a small height, for example of the order of 10 mm.

According to a second variant, said zone has an increasing capacity for extracting heat flow from the top to the bottom. In this case, the reduction in the extracted heat flow progressively decreases towards the bottom of the mold, over an area of greater height. This ensures a reduction in the extracted heat flow
in the zone where this reduction is carried out, even greater than in the previous case, due to the higher height of this zone, but also to ensure a kind of progressiveness in the variation of extracted heat flux, between the enhancement where the said extracted flux is as low as possible, and the cooled metal part of the mold where maximum extraction of the thermal flux is sought.

 The invention also relates to a mold for continuous casting comprising metal walls extending substantially vertically and defining a passage for the cast metal and internal cooling means of said walls arranged so as to ensure an energetic cooling of said walls on substantially their entire height, said walls being provided in their upper part with means for reducing the intensity of the heat flux caused by said cooling means and passing through their internal surface defining said passage, characterized in that it comprises an extension in thermally insulating material placed above said metal walls and extending them upwards.

 According to a first embodiment, said means for reducing the intensity of the heat flux consist of a layer of a metal having a thermal conductivity lower than that of the metal constituting the walls, said layer of metal being for example constituted of nickel, deposited by an electrolytic deposition process, on the copper or copper alloy constituting the walls of the ingot mold.

 According to a first variant, said layer is located above the walls, and therefore between them and the refractory constituting the extension, and its thickness can be for example of the order of a millimeter.

 According to another variant, said layer also extends over the internal face of the cooled metal walls, over a height which can then be of the order of a few centimeters. The layer of poorly conductive metal then forms a thermal barrier between the solidified skin of cast metal and the very good thermal conductive metal of the walls of the mold. Over the entire height over which this low-conductive layer extends, the extracted heat flux is greatly reduced (the reduction being able to reach and even exceed 50%), compared to a configuration where the cast metal would be in direct contact with said metal very conductive of the walls.

 The production of a layer of metal which is not very conductive both above the walls of copper or copper alloy and on their inner lateral face makes it possible to simultaneously modify the average value of the flow extracted in the upper part of the cooled walls and the spreading of the flux as a function of the distance from the upper edge of the metal walls, at which the solidification of the cast metal film begins, this spreading being moreover able to be preset and promoted by progressively reducing the thickness of said layer from high to low.

 According to a second embodiment, said means for reducing the intensity of the heat flux consist of grooves extending substantially vertically produced on the internal surface of said walls. One thus constitutes, in the zone of formation of the solidified film, and on the periphery of the passage formed by the walls of the ingot mold, an alternation of points of direct contact of the cast metal with the copper or copper alloy of the cooled walls, and of points, corresponding to said grooves, where the heat extraction is reduced. It will be noted that, unlike the technique mentioned at the beginning of this thesis, using such a system of grooves, and the fact that the beginning of solidification takes place, according to the invention, at a certain distance below the free surface of the metal bath and therefore in the absence of slag, there can therefore be, during casting, slag penetration in these grooves, and obstruction thereof.

 According to a variant of this arrangement, said grooves are filled at least partially with a material having a thermal conductivity lower than that of the metal constituting the walls.

 Other characteristics and advantages will appear in the description which will be given of a continuous steel casting mold according to the invention and of its implementation.

Reference is made to the appended drawings in which:
FIG. 1 is a schematic representation, in partial longitudinal section, of the upper part of the mold, in a first variant embodiment,
FIG. 2 represents the variation in the heat flux extracted during casting in such an ingot mold, as a function of the distance from the upper edge of the metal walls,
FIGS. 3 and 4 correspond respectively to FIGS. 1 and 2 in the case of a second variant embodiment of the ingot mold,
FIG. 5 schematically represents the upper part of an ingot mold wall, in a second embodiment, using grooving of the upper part of the metal wall,
FIG. 6 is a view, on an enlarged scale, of the section along a horizontal plane of the upper part of the metal wall of FIG. 5,
- Figure 7 is a view similar to Figure 6, in the case where the grooves are filled with a poorly conductive metal.

 The mold shown in FIG. 1 has metal walls 1 cooled, in a manner known per se, by an internal circulation of water, which form a tubular body and define a vertical passage for the cast steel 2. The upper part of these metal walls preferably consists of an element independent of their lower part, for example in the form of an annular part 3, also made of copper or copper alloy and provided with its own cooling circuit, shown diagrammatically by the channel 4 of water circulation.

 Such an annular part 3 can be replaced more easily and cheaply than if the metal walls were formed in one piece over the entire height.

 On the upper face 5 of the annular part 3 is produced a layer 6 of electrolytic nickel, with a thickness for example of 1.5 mm.

 Above this annular part 3, the height of which is for example 40 mm, is arranged a thermally insulating riser 7 comprising an upper part 8 made of highly insulating refractory material, with a height of 200 mm for example, and a lower part 9 in a refractory material possibly less insulating but having better mechanical resistance, for example the material known under the designation SiAlON, and having for example a thickness of 20 mm.

 Between the nickel layer 6 is the SiALON 9, a space is formed forming a slot 10 of low height, for example of a few 1/10 mm, this slot opening onto the internal surface of the ingot mold around its entire periphery, and also being connected to a source (not shown) of pressurized inert gas, such as argon.

 The ingot of liquid metal is supplied to the ingot mold by a nozzle 11, of known type, comprising lateral vents 12, situated at the level of the refractory riser 7, for example approximately at the middle of its upper part 8.

 During casting, the molten steel, contained in a distributor, not shown, to which the nozzle 11 is fixed, passes into said nozzle and its gills 12 and fills the mold. The level of the free surface 13 of the liquid steel is maintained between the upper edge of the riser 7 and the louvers 12, so that these louvers are immersed in the bath of liquid steel 2. Conventionally, said surface free is covered by a layer of slag 13.

 As seen in Figure 1, the skin 21 of solid steel begins to form at the upper edge of the nickel layer 6 and, due to the cooling caused by the metal walls of the mold, gradually thickens downwards, it being understood that this skin in fact continuously moves downwards with the extraction of the cast product, and is also continuously renewed by solidification of the liquid steel coming into contact with the nickel layer 6.

 The supply of argon under pressure through the slot 10 creates gas jets, substantially perpendicular to the internal surface of the walls of the ingot mold, which serve to break any solidification primers which could occur in contact with the lower part 9 of the enhancement, so as to ensure that said skin 21 begins to solidify around the entire periphery in the same horizontal plane, situated exactly at the level of the upper edge 14 of the nickel layer 6.

 FIG. 2 represents the variation of the extracted heat flow, as a function of the vertical distance d from this edge 14. The curve in solid line 22 represents the extracted flow in the case of the use of the ingot mold of the invention shown FIG. 1, while the dotted curve 23 represents for comparison the heat flux which would be extracted in the absence of the nickel layer 6, that is to say if the solid skin 21 began to form in direct contact with the copper of the upper part 3. It will be noted that, in the vertical zone corresponding to the thickness of the nickel layer, the extracted flux is reduced, this reduction in flux being able to continue for a few millimeters below said layer of nickel, but without significantly influencing the overall flow extracted by the assembly of the annular part 3.

 FIG. 3 represents an alternative embodiment of the ingot mold, the same references as those in FIG. 1 being used to designate corresponding elements. In this variant, an additional nickel layer 15 is deposited on the inner lateral surface 16 of the annular part 3, this annular part being machined prior to the deposition of the nickel so that, after this layer 15 has been formed, the internal surface 17 of it remains substantially coplanar and in line with the internal surface of the lower part of the mold.

Preferably, the thickness of the nickel layer 15 decreases progressively from top to bottom over the height of the annular part 3.

 Figure 4, similar to Figure 2 in the case of this second variant, shows that the heat flux extracted (curve 24) overall by said annular part is very greatly reduced compared to the case where there would be no coating nickel (curve 23).

 FIG. 5 schematically represents in perspective a portion of the upper part of an ingot mold wall according to another embodiment, in which vertical grooves 31 are produced on the internal face 32 of the annular part 3, as can be seen, shown on an enlarged scale, in Figure 6. The grooves 31 may have for example a depth and a width of 0.2 mm and be spaced from each other by 1.5 mm.

 Alternatively, as illustrated in Figure 7, the grooves can be filled by a deposit 33 of a weakly conductive metal, for example nickel. In this case, the grooves may for example have a width of 1 mm and a depth of 0.5 mm, and be spaced from each other by 2 mm. The deposit of nickel made in the grooves makes it possible to avoid the penetration of steel cast at the bottom of the groove. The reduction in heat flux extracted in this variant is then identical to that which would be produced by a reduction in the exchange surface proportional to the surface occupied by the grooves replenished in said poorly conductive metal.

 Although, to simplify the drawing, an inert gas injection slot has not been shown in FIG. 5, it is clear that such an injection will also be preferably used in the implementation of such an ingot mold.

 Experimental tests carried out by the inventors have given particularly satisfactory metallurgical results for the cast product, since it has been possible to observe a reduction in the heat flux extracted in the zone at the start of solidification of more than 50 k and it has been possible to obtain a product no longer having surface defects of the depression and crack type, in particular on steel grades containing 0.1% carbon, which are particularly sensitive to such defects.

 The invention is not limited to the various variants described above solely by way of example. In particular, a non-conductive coating metal other than nickel may be used. Although it is preferable, in order to reduce the maintenance costs, to carry out the reduction in heat flow extracted at the level of an upper part independent of the lower parts constituting the walls of the ingot mold, the various embodiments described above could also be implemented directly on said metal walls only over a limited height from their upper edges.

Claims (13)

 1. Process for the continuous casting of molten metals, in particular for lacing, in an ingot mold having strongly cooled metal walls (1) extending substantially vertically and defining a passage for the cast metal, and ensuring over their entire height an extraction of heat flux of the cast metal causing a cooling and a progressive solidification of said metal (2), process according to which the intensity of the heat flux extracted is reduced in the vicinity of the level where said metal begins to solidify on contact with said walls, characterized in that a thermally insulating riser (7) is placed above said cooled metal walls and, during casting, the level of the free surface of the cast metal (2) is maintained inside said riser.  2. Method according to claim 1, characterized in that the intensity of the extracted heat flux is reduced over a zone of predetermined height extending from the lower edge of said extension.  3. Method according to claim 2, characterized in that said zone has a substantially constant heat flux extraction capacity over its entire height.  4. Method according to claim 2, characterized in that said zone has an increasing heat flux extraction capacity from the top to the bottom.  5. Method according to any one of claims 1 to 4, characterized in that, at the level of said enhancement and at least at its lower edge, a gas emerging into the mold is injected in jets distributed over the entire periphery of said passage.  6. Continuous casting ingot mold comprising metal walls (1,3) extending substantially vertically and defining a passage for the cast metal (2) and means for internal cooling of said walls arranged so as to ensure an energetic cooling of said said walls over substantially their entire height, said walls being provided in their upper part with means (6,15,31) to reduce the intensity of the heat flux caused by said cooling means and passing through the internal surface of the walls, characterized in that it comprises an extension (7) of thermally insulating material placed above said metal walls and extending them upwards.  7. Ingot mold according to claim 6, characterized in that said means for reducing the intensity of the heat flux consist of a layer (6,15) of a metal having a thermal conductivity lower than that of the metal constituting the walls (1.3).  8. Ingot mold according to claim 7, characterized in that said layer (6) is located above the walls (3).  9. Ingot mold according to claim 7 or 8, characterized in that said layer (15) extends over the internal face (16) of said walls (3).  10. Ingot mold according to claim 9, characterized in that the thickness of said layer (15) decreases from top to bottom.  11. Ingot mold according to claim 6, characterized in that the said means for reducing the intensity of the heat flux consist of grooves (31) extending substantially vertically produced on the internal surface (32) of said walls (3 ).  12. Ingot mold according to claim 11, characterized in that said grooves (31) are filled at least partially with a material (33) having a thermal conductivity lower than that of the metal constituting the walls.  13. Ingot mold according to any one of claims 6 to 12, characterized in that it comprises injection means (10) for injecting at the level of said enhancement and at least at its lower edge, a gas under pressure opening into the mold in jets distributed over the entire periphery of said passage.