EP0891237B1 - Continuous casting method for metals and ingot mould for implementing same - 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

The present invention relates to continuous casting metals, especially steel.

The continuous casting operation consists schematically, as we know, to pour a metal into fusion in an ingot mold, essentially constituted a bottomless tubular element defining a passage for cast metal, but with copper walls (more generally made of copper alloy), are energetically cooled by circulation of water, and which we also continuously extract a product already externally solidified. Solidification progresses then towards the product axis and ends during the lowering of the latter downstream of the mold in the so-called "secondary cooling" zone under the effect of water sprinklers. The product obtained (bloom, billet or slab) is then cut to length, then laminated before shipment to customers or processing on site, in bars, wires, profiles, plates, sheets, etc.

Surface or subcutaneous defects of the products from continuous casting of steel are often the cause scrap, because the rolling operation does not support them well, even amplify them until degrading in an intolerable way the metallurgical quality of the rolled products.

During casting, the molten metal, spilled in an ingot mold, forms a solid film at its free surface as soon as it comes into contact with the walls cooled metal molds. This film is dragged down during the extraction of the product by a jerky movement punctuated by oscillations vertical of the mold. At the same time, its thickness growing due to continued heat extraction produced 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 is solidifying around the entire perimeter of the internal wall of the ingot mold and constituting a solid ring susceptible to contract due to the cooling undergone during its descent into the mold.

The thermal contraction of 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 solid phase change at the end of solidification, as is particularly the case for steel grades with low or medium carbon or stainless steel.

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

To counteract contractions, we knows by document FR 2.704.786 an ingot mold, called of continuous casting under load, comprising an extension thermal insulation intended to contain the cast metal maintained in the liquid state, and an insert located between the cooled wall of the mold and the so-called enhancer. The insert is intended to inject, at the base of the enhancer, a lubricant and a gas intended to ensure that the solidified skin of the cast product is well formed precisely from the top edge of the wall cooled, and not in contact with the insert, which is by elsewhere cooled and may have an insulating coating to prevent the cast metal from solidifying at its contact.

The document EP 0 620 062 also shows a ingot mold with insulating extension, and comprising holes for the injection of gas intended for shearing any solidification crust formed on contact of the enhancement.

To avoid or limit the faults indicated previously, we also tried to reduce the flow thermal extract during skin formation, beginning of its solidification.

Thus, there has already been an attempt to reduce this flow thermal, as indicated in FR-A-2,067,289, in using an ingot mold provided in its part upper, where the beginning of solidification occurs, an insert forming a thermal barrier preventing the molten metal to come directly into contact with the cooled walls of the mold. The outfit in the time of such an insert however proved to be very random, and the cost of maintaining such very high ingot mold.

There have also been attempts to reduce the flow by giving the walls of the mold a state likely to reduce the area of direct contact of the cast metal with the copper of the walls cooled, for example by regular etching of the walls forming an embossing of their surface, or by a random roughness obtained for example by sandblasting. A such method does not seem however allowed yet significantly improve the surface quality of the product sunk. It seems that the disturbances caused by variations in surface level free of the cast metal are predominant compared to the moderating effect on the heat flux, obtained by a adapted surface condition of the mold walls as as indicated above, so that, the heterogeneities of solidification remaining, surface defects are not not eliminated.

For the same purpose of reducing direct contact of the metal poured with cold copper from the walls, it still has been tempted to make grooves in them vertical as shown in JP "56136257. But it is then it appeared that these grooves were filling quickly slag conventionally used as a layer of covering the free surface of the liquid metal, which attenuated the thermal effect sought.

The object of the present invention is to resolve the problems indicated above and is particularly aimed at obtain a cast product having a very good quality of surface, ensuring effective reduction of flow thermal extract during the formation and the beginning of the growth of the solidified film, and avoiding in particular the harmful effect of fluctuations in the level of the free surface of the liquid metal in an ingot mold.

With these objectives in view, the invention relates to a process of the type described in FR-A-2,067,289, for continuous casting of molten metals, especially steel, in an ingot mold with walls energetically cooled metallic extending substantially vertically and defining a passage for the cast metal, and ensuring over their entire height heat flux extraction from the cast metal causing a cooling and a progressive solidification of the said metal, process by which the intensity of the flux is reduced thermal extracted near the level where the said metal begins to solidify on contact with said walls, characterized in that one places above said walls metallic elements thermally enhanced insulating and, during casting, the level of the free surface of the metal cast inside the said enhances, and in that we inject at the level of this heightening, and preferably at least at its end lower, a gas emerging in jets distributed over the inside circumference of the mold.

According to the invention, the free surface of the liquid metal bath (the meniscus) is located in the enhances. As it is made of refractory material thermally insulating, the solidified metal film does not begins to form regularly only from the edge metal walls, thanks to the blowing the purging gas at the base of the riser which helps to separate this regular solidification desired on the cooled metal part of any parasitic local solidifications which can occur on the refractory part. Thus, the solidification of the metal casting begins only a certain distance from the meniscus. The area where this solidification begins is therefore almost perfectly flat, horizontal, and not subject to the fluctuations or disturbances which agitate inevitably the free surface of the bath. The solid ring constituted by the skin of first solidification is therefore geometrically perfectly regular, and its continuous renewal is also almost perfectly regular, as well as the growth of this solidified skin, as the descent of the cast product.

Furthermore, since the cast metal does not initiate its solidification in the enhancement, there is not at this level metal contraction. The latter remains in contact with the wall of the riser and prevents infiltration of slag between the metal and the said wall. The result that there can therefore be no infiltration either slag between the metal walls of the mold and the skin solidified on contact, even if it tends to deviate from it due to withdrawal of solidification. It will also be noted that the ferrostatic pressure of the metal liquid, due to the height of this metal contained in the enhances, tends to oppose this spacing and therefore to maintain said skin pressed against the surface of metal walls, and this uniformly throughout the inner periphery of the mold, because the thickness and solidification of the skin is also uniform on this periphery.

So, because of this peripheral regularity, the heat extraction carried out in the part upper metal walls can be realized also uniformly all around the product, avoiding localized detachments and resulting sub-thicknesses of solid skin, and the intensity of the heat flux extracted in the area of beginning of solidification perhaps ensured very uniformly around the entire perimeter of the passage defined by metal walls.

Preferably, the intensity of the flow is reduced thermal extract over an area of predetermined height at from the lower edge of the said riser, without however, significantly change the amount of heat overall extracted by the ingot mold. This limited height thus ensures a reduction in the heat flux extracted in the area where the solid metal skin forms, and avoids also the effect of removal and detachment of the skin from metal observed in the art casting process prior.

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

According to a second variant, said zone has an increasing heat flux extraction capacity of the top to bottom. In this case, reducing the flow thermal extract gradually decreases down the ingot mold, over an area of greater height. This allows a reduction of the extracted heat flux, in the area where this reduction is carried out, even more stronger than in the previous case, due to the height superior of this area but also to ensure a sort progressiveness in the variation of heat flux extract, between the enhancement where the said extracted flow is the as low as possible, and the metal part cooled of the mold where extraction is sought maximum heat flux.

The invention also relates to an ingot mold for continuous casting of the type described in FR-A-2,067,289, having metal walls extending substantially vertically and defining a passage for metal sunk and internal cooling means of said walls arranged to provide cooling energetic said walls over substantially their entire height, said walls being provided in their part superior means to reduce the intensity of the flow thermal caused by said cooling means and passing through their internal surface defining the said passage, characterized in that it comprises a extension in thermally insulating material placed at the above said metal walls and extending them upwards, and means for injecting a gas leading to the inner periphery of the mold, at level of the riser and preferably at the end lower than this one just above the metal walls. According to a first embodiment, the said ways to reduce the intensity of the heat flux are formed by a layer of a metal having a thermal conductivity lower than that of metal constituting the walls, the said metal layer being by example consisting of nickel, deposited by a process of electroplating, on copper or copper alloy constituting the walls of the mold.

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

According to another variant, said layer extends in more on the inside of the metal walls cooled, to a height which can then be the order of a few centimeters. The little metal layer conductor then forms a thermal barrier between the solidified skin of cast metal and very good metal thermal conductor of the mold walls. Sure the whole height over which this layer extends conductive, the extracted heat flux is strongly reduced (the reduction can reach and even exceed 50%), compared to a configuration where the cast metal would be in direct contact with said metal very wall conductor.

The realization of a layer of low conductive metal both above the copper or alloy walls copper and on their inner side face allows simultaneously modify the average value of the extracted stream in the upper part of the cooled walls and spread of the flow as a function of the distance from from the upper edge of the metal walls, at the level from which the solidification of the metal film begins poured, this spreading can also be pre-adjusted and promoted by gradually decreasing the thickness of said layer from top to bottom.

According to a second embodiment, the so-called ways to reduce the intensity of the heat flux are formed by grooves extending substantially vertically made 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 mold, an alternation of points of direct contact of the cast metal with copper or copper alloy of the cooled walls, and of points, corresponding to said grooves, where the extraction of heat is reduced. Note that, unlike the technique mentioned at the beginning of this thesis, using a such a system of grooves, and the fact that the beginning of solidification is carried out, according to the invention, at a certain distance below the free surface of the bath of metal and therefore in the absence of slag, there can therefore have, during casting, slag penetration in these grooves, and obstruction thereof.

According to a variant of this arrangement, the so-called grooves are filled at least partially with a material with thermal conductivity less than that of the metal constituting the walls.

Other characteristics and advantages will appear in the description which will be made of an ingot mold of continuous steel casting according to the invention and its Implementation.

• la figure 1 est une représentation schématique, en coupe longitudinale partielle, de la partie haute de la lingotière, dans une première variante de réalisation,
• la figure 2 représente la variation du flux thermique extrait au cours d'une coulée dans une telle lingotière, en fonction de la distance à partir du bord supérieur des parois métalliques,
• les figures 3 et 4 correspondent respectivement aux figures 1 et 2 dans le cas d'une deuxième variante de réalisation de la lingotière,
• la figure 5 représente schématiquement la partie supérieure d'une paroi de lingotière, dans un second mode de réalisation, utilisant un rainurage de la partie supérieure de paroi métallique,
• la figure 6 est une vue, à échelle agrandie, de la section selon un plan horizontal de la partie supérieure de la paroi métallique de la figure 5,
• la figure 7, est une vue similaire à la figure 6, dans le cas où les rainures sont comblées par un métal peu conducteur,
• la figure 8 illustre un mode de réalisation particulièrement avantageux de la rehausse, incorporant une résistance chauffante,
• la figure 9 est une vue schématique de la lingotière , à échelle réduite, en coupe selon la ligne IX - IX de la figure 8.

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 of 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,
• FIG. 7 is a view similar to FIG. 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 resistor,
• FIG. 9 is a schematic view of the ingot mold, on a reduced scale, in section along the line IX - IX of FIG. 8.

The mold shown in Figure 1 has metallic walls 1 cooled, in a manner known in itself, by an internal circulation of water, which form a tubular body and define a vertical passage for cast steel 2. The upper part of these walls metallic is preferably made up of a element independent of their lower part, by example in the form of an annular part 3, also in copper or copper alloy and has a circuit its own cooling, schematized by the water circulation channel 4.

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

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

Above this annular part 3, the height is for example 40 mm, is arranged a thermally insulating extension 7 comprising a part upper 8 in highly insulating refractory, with a height 200 mm for example, and a lower part 9 in one refractory material possibly less insulating but with 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 is formed a space forming a slot 10 of low height, for example by a few 1/10 mm, this slot opening out the internal surface of the mold over all its perimeter, and also being connected to a source 110 inert gas under pressure, such as argon, schematically shown in the figure.

Liquid metal ingot mold feeding is produced by a nozzle 11, of known type, comprising lateral openings 12, located at the refractory riser 7, for example about halfway up of its upper part 8.

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

As seen in Figure 1, the skin 21 solid steel begins to form at the edge upper layer of nickel 6 and, due to the cooling caused by the metal walls of the ingot mold, gradually thickens downward, being of course this skin actually moves continuously down with product extraction sunk, and is also continuously renewed by solidification of the liquid steel coming into contact with the nickel layer 6.

Feeding argon under pressure through the slot 10 creates gas jets, substantially perpendicular to the internal surface of the walls of the mold, which are used to break up any solidification primers that could occur on contact with the party lower 9 of the riser, so as to ensure that the said skin 21 begins to solidify all over perimeter in the same horizontal plane, located exactly at the level of the upper edge 14 of the layer of nickel 6.

FIG. 2 represents the variation of the extracted heat flow Φ, 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 in Figure 1, while the dotted curve 23 represents for comparison the heat flux which would be extracted in the absence of the nickel layer 6, ie if the solid skin 21 began to form in direct contact with 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 over a few millimeters below said layer of nickel, but without appreciably influencing the overall flux extracted by the assembly of the annular part 3.

Figure 3 shows an alternative embodiment of the mold, the same marks as those of the figure 1 being used to designate elements correspondents. In this variant, a layer 15 of additional nickel is deposited on the side surface 16 inside of the annular part 3, this part annular being machined before depositing nickel so that, after this layer 15 has been formed, the internal surface 17 thereof remains substantially coplanar and in the extension of the internal surface from the bottom of the mold. Preferably, the thickness of the nickel layer 15 gradually decreases from top to bottom on 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 extract (curve 24) globally by said part annular is very greatly reduced compared to the case where there would be no nickel coating (curve 23).

Figure 5 shows schematically in perspective a portion of the top of a ingot mold wall according to another embodiment, in which vertical grooves 31 are made on the internal face 32 of the annular part 3, as is see, shown on an enlarged scale, in Figure 6. The grooves 31 can have for example a depth and 0.2 mm wide and be spaced from each other 1.5 mm.

Alternatively, as illustrated in Figure 7, the grooves can be filled with a deposit 33 of a metal weakly conductive, for example nickel. In this case, the grooves can have for example a width 1 mm and a depth of 0.5 mm, and be distant from each other by 2 mm. The nickel deposit produced in grooves prevent penetration of cast steel at the bottom of the groove. Reduction of heat flux extracted in this variant is then identical to that which would be produced by a reduction in the area exchange proportional to the area occupied by grooves replenished in said non-conductive metal.

Although, to simplify the drawing, we do not have shown in Figure 5 of gas injection slot inert, it is clear that such an injection will also preferentially used in the implementation of a like an ingot mold.

The experimental tests carried out by the inventors have given particularly successful results metallurgically satisfactory for the product sunk, since a reduction in the flow could be observed thermal extract in the zone of beginning of solidification more than 50% and we were able to obtain a product with more surface defects such as depressions and cracks, in particular on steel grades with 0.1% carbon, particularly sensitive to such defaults.

The invention is not limited to the different variants that have been described above only at as an example. In particular, a non-conductive coating metal other than nickel. Although it is preferable, to reduce the costs of maintenance, achieve reduction of heat flow extracted at the level of a superior room independent of lower parts constituting the walls of the ingot mold, the different embodiments described above could also be implemented directly on said metal walls only over a limited height from their edges superiors.

Similarly, if the sweeping gas at the base of the enhances and a "curative" means against the effects, on the solidification process of the cast product, local parasitic solidifications on the wall refractory of the extension, this means can be completed by a "preventive" means consisting in heating the enhances. Thus, according to the invention, we can have advantage to incorporate into the extension 7, as is schematically shown in Figures 8 and 9, a resistance electric heater, for example in the form of a graphite tape 71 (PAPYER type (registered trademark) or SIGRAFLER (registered trademark)), which can be folded, without risk of breaking it, in order to wrap it around the passage cast metal 2 (see Figure 9). This heating tape 71 can be molded into the refractory of the riser or preferably placed in an annular groove 72 dug at this effect in the enhancement, which is then achieved by example in two parts 73, 74 superimposed. Therefore, if care is taken to choose a gas for sweeping gas inert, like argon, no oxidation problem the graphite heating resistance is not encountered.

Claims (11)

1. Process for the continuous casting of molten metals, especially steel, in a mould comprising vigorously cooled metal walls (1) extending substantially vertically and defining a passage for the cast metal, and extracting, over their entire height, a heat flux from the cast metal causing cooling and gradual solidification of the said metal (2), according to which process the intensity of the heat flux extracted in the vicinity of the level where the said metal starts to solidify on contact with the said walls is reduced,
      characterized in that a thermally insulating feed head (7) is placed on top of the said cooled metal walls and, during the casting run, the level of the free surface of the cast metal (2) is maintained within the said feed head and in that a gas is injected level with the said feed head (7) and at least level with its lower edge, the said gas emerging in the mould as jets distributed over the entire periphery of the said passage.
2. Process according to Claim 1, characterized in that the intensity of the extracted heat flux is reduced over a region of predetermined height, the said region having a heat-flux extraction capacity which is approximately constant over its entire height.
3. Process according to Claim 1, characterized in that the intensity of the extracted heat flux is reduced over a region of predetermined height, the said region having a heat-flux extraction capacity which increases from the top downwards.
4. Continuous casting mould comprising metal walls (1, 3) extending substantially vertically and defining a passage for the cast metal (2) and means for the internal cooling of the said walls, the said means being placed so as to ensure that the said walls are vigorously cooled over approximately their entire height, the said walls being provided in their upper part with means (6, 15, 31) for reducing the intensity of the heat flux caused by the said cooling means and passing through the internal surface of the walls, which mould is characterized in that it includes a feed head (7) made of thermally insulating material placed on top of the said metal walls and extending them upwards, and injection means (10) for injecting a gas under pressure level with the said feed head and at least level with its lower edge, the said gas emerging in the mould as jets distributed over the entire periphery of the said passage.
5. Mould according to Claim 4, characterized in that the said means for reducing the intensity of the heat flux consist of a layer (6, 15) of a metal having a thermal conductivity less than that of the metal of which the walls (1, 3) are composed.
6. Mould according to Claim 5, characterized in that the said layer (6) is located above the walls (3).
7. Mould according to Claim 5 or 6, characterized in that the said layer (15) extends over the internal face (16) of the said walls (3).
8. Mould according to Claim 7, characterized in that the thickness of the said layer (15) decreases from the top downwards.
9. Mould according to Claim 4, characterized in that the said means for reducing the intensity of the heat flux consist of grooves (31) extending approximately vertically and produced at the internal surface (32) of the said walls (3).
10. Mould according to Claim 9, characterized in that the said grooves (31) are at least partially filled with a material (33) having a thermal conductivity less than that of the metal of which the walls are composed.
11. Mould according to Claim 4, characterized in that electrical resistance heating means (71) are incorporated into the feed head (7).

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