EP0891237A1

EP0891237A1 - Continuous casting method for metals and ingot mould for implementing same

Info

Publication number: EP0891237A1
Application number: EP97919470A
Authority: EP
Inventors: Jean-Marc Jolivet; Eric Perrin; Cosimo Salaris; Jacques Spiquel
Original assignee: Forges et Acieries de Dilling SA; Ugine Savoie SA; Ascometal SA; Sollac SA; Sogepass; Lorraine de Laminage Continu SA SOLLAC
Current assignee: Forges et Acieries de Dilling SA; Ascometal SA; Sollac SA; Ugitech SA; Sogepass
Priority date: 1996-04-05
Filing date: 1997-04-03
Publication date: 1999-01-20
Anticipated expiration: 2017-04-03
Also published as: DE69703793D1; AU2392997A; ES2154900T3; WO1997037794A1; ATE198285T1; JP4058561B2; DK0891237T3; CA2250786A1; CA2250786C; PT891237E; GR3035596T3; FR2747059B1; EP0891237B1; BR9708509A; KR100447466B1; JP2000508243A; KR20000005257A; FR2747059A1; DE69703793T2

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.

The continuous casting operation schematically consists, as is known, of pouring a molten metal into an ingot mold, essentially consisting of a bottomless tubular element defining a passage for the cast metal, but whose walls, of copper (more generally made of copper alloy), are vigorously cooled by circulation of water, and from which a product already solidified externally 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 products from the continuous casting of steel are often the cause of scrap, because the rolling operation does not support them well, or even amplifies them until the intolerable deterioration of the metallurgical quality of the rolled products. During casting, the molten metal, poured into the mold, forms a solid film at its free surface as soon as it comes into contact with the cooled metal walls of the mold. This film is dragged down during the extraction of the product by a jerky movement punctuated by the vertical oscillations of the mold. Simultaneously, its thickness increases due to the continued heat extraction carried out by the walls of the mold. So there is continuously creating a new film of solid metal at the free surface of the metal in the mold (called "meniscus"), this film solidifying around the entire perimeter of the internal wall of the mold and constituting a solid ring capable of contract due to the cooling experienced during its descent into the mold.

The thermal contraction of the solid skin is all the more important as the heat extraction is strong and that the cast metal has a natural tendency to contract during cooling, for example by change of solid phase at the end of solidification, as this is the case in particular for low or medium carbon steel grades or stainless steels. 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 the skin, at the start of its solidification.

Thus, it has already been attempted to reduce this heat flow 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 ingot mold a state surface likely to reduce the area of direct contact of the metal cast 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 significantly improve the surface quality of the cast product. It seems that the disturbances caused by variations in the level of the free surface of the cast metal are predominant compared to the moderating effect on the heat flow, obtained by a suitable surface condition of the walls of the mold as indicated above, so that, the solidification heterogeneities remaining, the surface defects 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 extraction of heat flow from the cast metal causing said metal to cool and gradually solidify, process by which the intensity of the heat flow extracted is reduced near the level where said metal begins to solidify on contact with said walls, characterized in what is placed above said cooled metal walls a thermally insulating riser and, during casting, the level of the free surface of the cast metal is maintained inside the said riser, and in that one injects at this height, and preferably at least at its lower end, a gas emerging in jets distributed over the inner periphery of the mold.

According to the invention, the free surface of the liquid metal bath (the meniscus) is located in the riser. As it is made of thermally insulating refractory material, the film of solidified metal begins to form regularly only from the upper edge of the metal walls, and this thanks to the blowing of the sweeping gas at the base of the riser which allows to separate this regular solidification desired on the cooled metal part from any parasitic local solidifications which may occur on the refractory part. Thus, the solidification of the cast metal begins only at a certain distance from the meniscus.

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 poured product descends.

Furthermore, since the cast metal does not initiate its solidification in the enhancement, there is no metal contraction at this level. The latter remains in contact with the wall of the riser and prevents slag infiltration between the metal and said wall. As a result, there can therefore also be no slag infiltration between the metal walls of the mold and the solidified skin in contact with them, even if the latter tends to deviate therefrom due to the withdrawal of solidification. It will also be noted that the ferrostatic pressure of the liquid metal, due to the height of this metal contained in the riser, tends to oppose this separation and therefore to keep said skin pressed against the surface of the metal walls, and this of uniformly over the entire internal periphery of the ingot mold, because the thickness and the solidification state of the skin is also uniform around 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 1 '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 flow extracted over a zone of predetermined height is reduced from the lower edge of the said riser, without however significantly modifying the amount of overall heat extracted by the ingot mold. This limited height thus ensures a reduction in the heat flux extracted in the zone where the skin of solid metal is formed, and also avoids the effect of withdrawal and detachment of the metal skin observed in the casting process according to the prior art. . 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 makes it possible to ensure a reduction in the extracted heat flux, 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 in extracted heat flow, between the enhancement where said extracted flow is as small as possible, and the cooled metal part of the mold where maximum extraction of the heat flow is sought. The invention also relates to a continuous casting ingot mold having metal walls extending substantially vertically and defining a passage for the cast metal and means for internal cooling of said walls arranged so as to ensure vigorous 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, and means for injecting a gas opening onto the inner periphery of the ingot mold, at the level of the extension and preferably at the lower end of this one just above the metal walls.

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 top to bottom.

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 ingot mold, in a first variant embodiment, FIG. 2 represents the variation of the heat flow extracted during casting in such an ingot mold, depending on the distance from the upper edge of the metal walls,

- Figures 3 and 4 correspond respectively in 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,

- Figure 6 is a view, on an enlarged scale, of the section along a horizontal plane of the upper part of the metal wall of Figure 5, - Figure 7, is a view similar to Figure 6, in the case where the grooves are filled with a slightly conductive metal, FIG. 8 illustrates a particularly advantageous embodiment of the extension, incorporating a heating resistance, FIG. 9 is a schematic view of the ingot mold, on a reduced scale, in section along line IX - IX of Figure 8.

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 SILLON, and having for example a thickness of 20 mm. Between the nickel layer 6 is the SILLON 9, a space is formed forming a slit 10 of low height, for example of a few 1/10 mm, this slit opening onto the internal surface of the ingot mold around its entire periphery, and being moreover connected to a source 110 of inert pressurized gas, such as argon, schematically shown in the figure.

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

During a 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 gills 12, so that these gills 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 downward, it being understood that this skin is actually moving continuously downward 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 pressurized argon through the slot 10 creates jets of gases, substantially perpendicular to the internal surface of the walls of the ingot mold, which serve to break up any solidification primers which could occur in contact with the lower part 9 of the extension, so as to ensure that said skin 21 begins to solidify around the entire periphery in the same horizontal plane, located exactly at the level of the upper edge 14 of the nickel layer 6. FIG. 2 represents the variation of the extracted heat flux Φ, as a function of the vertical distance d from this stop 14. The curve in solid line 22 represents the flow extracted in the case of the use of the ingot mold of the invention represented in FIG. 1, while the curve poi ntillée 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 possibly being able to continue for a few millimeters below said nickel layer, but without appreciably influencing the overall flow extracted by the entire 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 part annular being machined prior to the deposition of nickel so that, after formation of this layer 15, the internal surface 17 thereof 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, a gas injection slot has not been shown in FIG. 5 inert, it is clear that such an injection will also preferably be used in the implementation of such an ingot mold.

The 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% and we have been able to obtain a product no longer having surface defects of the depression and crack type, in particular on steel grades with 0.1% carbon, which are particularly sensitive to such defects.

The invention is not limited to the various variants which have been 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.

Similarly, if the purging gas at the base of the riser and a "curative" means against the effects, on the solidification process of the cast product, of parasitic local solidifications on the refractory wall of the riser, this means can be supplemented by a "preventive" means consisting of heating the extension. Thus, according to the invention, it may be advantageous to incorporate into the riser 7, as is schematically represented in FIGS. 8 and 9, an electric heating resistance, for example in the form of a graphite tape 71 (type PAPYER (registered trademark) or SIGRAFLER (registered trademark)), which can be folded, without the risk of breaking it, in order to wrap it around the passage of the cast metal 2 (see Figure 9). This heating tape 71 can be molded in the refractory of the riser or preferably placed in an annular groove 72 hollowed out for this purpose in the riser, which is then produced for example in two parts 73, 74 superimposed. Consequently, if care is taken to choose an inert gas, such as argon, as the purging gas, no problem of oxidation of the graphite heating resistance is encountered.

Claims

1. Process for the continuous casting of molten metals, in particular steel, in an ingot mold having energetically cooled metal walls (1) extending substantially vertically and defining a passage for the cast metal, and ensuring over their entire height extraction of heat flow from the cast metal causing cooling and progressive solidification of said metal (2), method according to which the intensity of the heat flow 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 the said enhancer, and in that at said enhancer (7) and at least at its lower edge, a gas opening is injected into the mold in distributed jets is on the entire periphery of said passage.

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. Continuous casting ingot mold having metal walls (1,3) extending substantially vertically and defining a passage for the metal cast (2) and internal cooling means of said walls arranged so as to ensure an energetic cooling of 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 the said cooling means and passing through the internal surface of the walls, an ingot mold characterized in that it includes an extension (7) of thermally insulating material placed above the said metal walls and extending them upwards, and injection means (10) for injecting, at the level of the said riser and at least at the level of its lower edge, a pressurized gas opening into the mold in jets distributed over the entire periphery of the said passage.

6. Ingot mold according to claim 5, 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).

7. Ingot mold according to claim 6, characterized in that said layer (6) is located above the walls (3).

8. Ingot mold according to claim 6 or 7, characterized in that said layer (15) extends over the internal face (16) of said walls (3).

9. Ingot mold according to claim 8, characterized in that the thickness of said layer (15) decreases from top to bottom.

10. Ingot mold according to claim 5, characterized in that said means for reducing the intensity of the heat flux consist of grooves (31) extending substantially vertically formed on the internal surface (32) of said walls (3 ).

11. Ingot mold according to claim 10, 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.

12. Ingot mold according to claim 5, characterized in that electrical resistance heating means (71) are incorporated in the riser (7).