EP0894556B1 - Side wall for a twin roll continuous casting machine for casting thin metal strip - Google Patents

Abstract

A lateral face (4) is claimed for closing the casting space of a continuous casting installation for the production of metal strip (7) between two rolls (1, 1') counter- rotating closely together on their horizontal axes, of the type incorporating a lower part (10) made of a first refractory material with an elevated hardness in the zone where the lower part (10) is in contact with the sides (5, 5') of the rolls (1, 1') or with the liquid metal (3) or with an edge of the solidified strip (7). The lower part (10) incorporates, on its face destined to be turned in the direction of the casting space, a recess filled with a refractory material. Preferably this refractory material enclosed by the recess is a material presenting a lower hardness than that of the first refractory material and with elevated thermal insulating properties.

Description

The invention relates to the continuous casting of metals, and more particularly installations for casting metal strips, in particular steel, of a few mm thick, between two counter-rotating cylinders with horizontal axes, internally cooled.

On these installations, whose industrial application to steel casting is currently under study, the pouring space is delimited by the lateral surfaces of the cylinders, against which the solidification of the metal takes place, and by lateral shutter plates in refractory material which are applied against the flat ends (called "edges") of the cylinders. Such an installation is described, for example, in document EP-A-0 698 433.

• du métal solidifié présent sur les surfaces des cylindres (ce qu'on appelle les "peaux solidifiées");
• du métal à l'état pâteux (partiellement solidifié) qui est présent entre les peaux solidifiées juste au-dessus du col, près des faces latérales, et qui tend à être entraíné vers le col sous l'effet des mouvements des cylindres;
• et, en-dessous du col, de la bande elle-même.

The side closing plates must be made, at least on their parts which rub against the edges of the cylinders, by a first refractory material, such as SiAlON®, having a high hardness and a great resistance to corrosion by liquid metal. . These conditions are considered necessary to obtain a satisfactory seal of the side plate / cylinder contacts, and thus prevent liquid metal from escaping from the casting space at their level. The central part of the lateral face, which is in contact only with the liquid metal and not with the cylinders, can be made of a second refractory material, for example a material based on alumina or silica. Above all, this second material is required to be a good thermal insulator, in order to prevent the liquid metal from solidifying on contact. However, in the extreme lower part of the lateral face, which, in the case of the use of cylinders with a diameter of 1500 mm, starts 5 10 centimeters above the "neck" (ie level where the cylinders are closest to each other and below which the strip is completely solidified), it is considered that the lateral face must be constituted by said first hard refractory material, or another material with equivalent properties . Indeed, this zone is subjected to significant friction, not only on the part of the cylinders, but also on the part:

• solidified metal present on the surfaces of the cylinders (so-called "solidified skins");
• metal in the pasty state (partially solidified) which is present between the solidified skins just above the neck, near the lateral faces, and which tends to be drawn towards the neck under the effect of the movements of the cylinders;
• and, below the collar, the band itself.

The safety of the side face / cylinder contact in this area during the whole casting must take precedence over the prevention of parasitic solidifications. In addition the width very reduced space separating the cylinders at the neck (equal to the thickness desired for the strip, i.e. a few mm) would make it very delicate, even impossible, the juxtaposition various materials as it exists in the remainder of the side face.

During casting, the action of pressing the side faces against the edges of the cylinders cause wear of the lateral faces by friction, but only in the areas where they are in contact with the cylinders. It follows that the parts of the lateral faces which are not subjected to this friction gradually penetrate inside of the pouring space, to a depth which can end up reaching 5 to 15 mm. Forming thus what is sometimes called a "positive insert".

When examining the worn side faces, after casting, we very often finds that the lower part of the positive insert is broken, a sign of intense stresses to which it was subjected during casting. This break has effect of abruptly decreasing the penetration depth of the positive insert in the space of pouring from a level several centimeters above the neck, or even destroying completely the positive insert below this level. It takes place according to a route random, which may depend in particular on the internal health of the refractory. So there is a great risk that this break will spread in various directions inside the refractory, causing rupture and departure of portions of the lateral face which face to areas of the casting space where the metal is in a frankly liquid state. In that case, inevitably, liquid metal is leaked out of the pouring space, which cause at least a momentary deterioration in the quality of the tape. If the break takes place at the level of a cylinder / side face contact area, the sealing of this contact can be found by pressing the side more strongly: its wear allows to restore a contact surface conforms to the geometry of the edges. But if the incident happens again often during casting, this leads to excessive consumption of the refractory hard on the side face, which may not support the entire casting if its thickness is too thin. And give this hard refractory enough thickness for him certainly support the entire casting, even when multiple incidents occur of the type which has just been described, would lead to an excessive cost price for the lateral face. Finally, if a particularly large liquid metal leak occurs, it can all simply cause the emergency stop of the pouring and deterioration of the installation.

The object of the invention is to propose a lateral face configuration which avoids that the degradation of the positive insert has harmful consequences on the quality of the tape and the flow of the casting.

To this end, the invention relates to a lateral face for closing off the space. casting of a continuous casting installation of metal strips between two cylinders close counter-rotating with horizontal axes, of the type having a lower part in a first refractory material, of high hardness, in the zone where said lower part is in contact with the edges of said cylinders, or of liquid metal likely to be in progress solidification, or an edge of the solidified strip (as described for example in document EP 0 432 073), characterized in that said part lower comprises, on its face intended to be turned in the direction of the pouring space, a recess filled with a refractory material.

Preferably, said refractory material enclosed by said recess is a material having a lower hardness than said first refractory material and high heat insulating properties.

As will be understood, the invention consists in giving the lateral face in its lowest part a bilayer structure, designed so that when the stresses exerted on the upper layer by the solidified metal become significant, it degrades without this degradation being able to extend to higher levels of the face lateral. This gives us the opportunity to control the unfolding of a phenomenon which, if occurred out of control, could lead to serious disruption of the operation of the casting machine.

• la figure 1, qui montre schématiquement, vue de face et au repos, une installation de coulée continue de bandes minces entre deux cylindres munie d'une seule de ses deux faces latérales, celles-ci étant conformes à l'art antérieur à l'invention;
• la figure 2 qui montre, vue de côté en coupe longitudinale selon II-II, la même installation au stade initial de la coulée d'une bande métallique (figure 2a), puis à un stade de la coulée où les faces latérales ont déjà subi une usure significative (figure 2b);
• la figure 3 qui montre schématiquement, vue de côté en coupe selon IIIa-IIIa (figure 3a) et vue de face (figure 3b) une face latérale selon l'invention;
• la figure 4 qui montre, vue de côté en coupe longitudinale, une installation de coulée entre cylindres de bandes métalliques équipée de faces latérales selon l'invention, à un stade de la coulée où celles-ci ont déjà subi une usure significative.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

• Figure 1, which shows schematically, front view and at rest, a continuous casting installation of thin strips between two cylinders provided with only one of its two lateral faces, these being in accordance with the prior art to the invention;
• Figure 2 which shows, side view in longitudinal section along II-II, the same installation at the initial stage of the casting of a metal strip (Figure 2a), then at a stage of casting where the side faces have already undergone significant wear (Figure 2b);
• Figure 3 which shows schematically, side view in section along IIIa-IIIa (Figure 3a) and front view (Figure 3b) a side face according to the invention;
• Figure 4 which shows, side view in longitudinal section, a casting installation between cylinders of metal strips equipped with side faces according to the invention, at a stage of casting where they have already undergone significant wear.

The dimensions which will be given as an example for the various elements of the faces lateral according to the invention are valid in the case where casting rolls are used whose the diameter is of the order of 1500 mm. If smaller cylinders are used, these dimensions should be reduced accordingly.

The prior art casting installation between cylinders shown in the Figures 1 and 2 comprises, conventionally, two cylinders 1, 1 'close to horizontal axes whose cylindrical side walls 2, 2 'are energetically cooled internally. They are rotated in opposite directions by known means not shown. The space of casting, in which the liquid steel 3 is introduced by a nozzle (not shown) connected to a container such as a pocket or a distributor, is delimited by the side walls 2, 2 ' cylinders 1, 1 'and by side faces 4, 4' made of refractory material (the side face 4 ' is not shown in Figure 1 to make visible the constitution of the side face 4). These are applied by known means (not shown) against the edges 5, 5 ', 5 "of the cylinders 1, 1 'so as to close off the pouring space laterally and thus prevent the liquid steel 3 to escape from it. The document EP-A-0 698 433 already cited describes in detail a example (non-limiting) of such means. Liquid steel 3 solidifies against the walls side 2, 2 'of the cylinders and forms two "skins" there which meet at the neck 6 (the area where the side surfaces 2, 2 'of the cylinders 1, 1' are closest together) to form a steel strip 7 which is continuously extracted from the installation by known means not represented.

As can be seen in FIG. 1, the elements made of refractory material of the faces side 4, 4 'of the example shown have two parts. Their portions 8, 9 which are in contact with the edges 5, 5 'of the cylinders 1, 1' or in the vicinity of the zones where this contact, as well as all of their lower part 10 over a height of approximately 150 mm, are made of a first material, of high hardness and having great qualities of resistance to corrosion by liquid metal. The central part 11 remaining of the faces side 4, 4 'is made of a material preferably having good properties thermal insulation and may be of a lower hardness than said first material. The Figure 2a shows the installation at the very start of casting, while the side faces 4, 4 ' are new and have facing the edges 5, 5 'of the cylinders, the liquid steel 3 and the strip 7 a uniformly flat front face. Figure 2b shows the same installation at a later stage of casting, while the side faces 4, 4 'have already undergone a significant wear. The central part 11 has been consumed at a depth "e" relatively significant by corrosion and mechanical wear in contact with liquid steel 3 in movement. The hard material portions 8, 9 and 10 were worn by friction on a depth "x" in their zones which face the edges 5, 5 'of the cylinders 1, 1', while their zones not subjected to the friction of the edges 5, 5 ′ have, in the ideal case shown, no wear. The latter therefore form during casting, with the refractory of the central part 11 which they enclose, a "positive insert" 12 penetrating into the casting space. In its lower part, this positive insert 12 is subjected on its front part to intense stresses on the part of the steel 3 which may be, in its vicinity, already in a state partially solidified. Its lateral parts are likewise subject to contact with the skins. of solidified metal against the cylinders. Consequently, under the effect of these various stresses, there is a periodic and uncontrolled rupture of the lower part of the positive insert 12, along a generally irregular line 13. In some cases, this rupture extends relatively deep into the refractory, to the point of destroying sealing the casting space and leaving the possibility for liquid steel 14 to escape out of this space by a crevice 15. This is, in particular, the case if the rupture propagates upwards from positive insert 12, therefore towards regions where the metal which borders the lateral face 4, 4 ′ is entirely in the liquid state. If this steel 14 gets immediately solidifies between the side face 4 and the edge 5, it causes the face to recede lateral 4 and contributes a little more to its deterioration, as well as to that of edge 5. If the breach 15 opens to the outside, the liquid steel 14 can flow out of the machine casting with all the risks that this entails for the installation and the personnel.

According to the invention, in order to avoid the problems caused by these uncontrolled ruptures of the lower part of the positive insert 12, we choose to deliberately create, to construct, in the lower part of the lateral face 4, 4 ′, an area where this rupture, if it occurs, will be circumscribed. For this purpose, we clean in the lower part of the lateral face 4, 4 ', on a height which extends from its lower edge up to, for example, about 70 mm above at the level of the neck 6, a recess 16 which is filled with a refractory material 17. This can be the hard, corrosion-resistant refractory that makes up the rest of the lower part 10 of the lateral face 4, 4 '. It can also be a less material resistant to abrasion by solid metal or being solidified, as well as to corrosion by liquid metal, but having good thermal insulation properties. We can choose to this effect the same refractory as that of the core 11 of the lateral face 4, 4 '. But one refractory with a lamellar structure, such as graphite or boron nitride, would also be well suited to this use, for the reasons which will be explained below. Both refractory in play are joined together according to usual methods, for example by bonding.

In the preferred embodiment of the invention which is shown, the longitudinal sections in depth of the recess 16 and of the refractory 17 which fills it have the shape of a rectangular trapezoid whose point is oriented towards the part upper part of the pouring space, and the large base of which is situated opposite this same casting space. This large base extends over a height "h ₁ " above the level of the neck 6, "h ₁ " being equal to, for example, 70 mm (this value does not include the part of the small face intended for extend under the neck 6 after its installation). The small base of the trapezoid, located at the rear of the recess, extends over a height "h ₂ " above the level of the neck 6, equal, for example, to 55 mm. The recess 16 has a maximum depth "p" equal to, for example, 15 mm. "p" must be greater than the maximum tolerated value of the wear of the parts 8, 9, 10 of hard material of the lateral face 4, 4 ′, so that the refractory 17 which fills it can play its role during the assembly of casting.

Figure 4 shows in operation a casting installation between cylinders equipped with side faces 4, 4 'according to the invention, under the same conditions as the installation shown in Figure 2b. In both cases, we observe the formation of a positive insert 12, of maximum thickness "x" at the levels where the side faces 4, 4 'are formed by the hard refractory 10. But the parts of the side faces 4, 4 'According to the invention which face the strip 7 or areas where the metal 3 is in the partially solidified state undergo wear the quantitative importance of which is a function of the stresses to which they are subjected. These stresses tend to increase with the solid fraction of the metal 3, and consequently, the penetration depth of the positive insert 12 decreases as one approaches the neck 6. Below the neck 6, or even already a little above, the positive insert 12 can possibly, as shown in Figure 4, be fully consumed. As the refractory 17 lining the recess 16 of each lateral face 4, 4 ′ according to the invention does not have a very high hardness, its wear is regular and we do not normally observe the presence of rupture lines of tormented shapes and random as line 13 in Figure 2b. On the other hand, if nevertheless a momentary too strong stress resulted in a rupture of the refractory 17 lining the recess 16, this rupture would necessarily stop at the limits of the recess 16. It would therefore not propagate in the higher parts of the lateral face 4, 4 ′, that is to say those which are in contact with an entirely liquid metal 3 which would therefore risk escaping out of the casting space by crevices that would create the propagation of this rupture. The height "h ₁ " over which extend above the neck 6 the recess 16 and the refractory 17 which garnishes it must be determined accordingly.

We said above that materials with a lamellar structure such as graphite or boron nitride were particularly indicated to constitute the refractory 17 garnishing the recess 16. This is because this lamellar structure makes it easier to wear them on a regular and progressive mode, and the evolution of the shape of the positive insert 12 can thus be better mastered. However, it may also be entirely acceptable to use a material of the type usually used to form the central part 11 of the faces side 4, 4 'and which have a relatively low hardness and high insulating power. The use of these materials also makes it possible to avoid too early solidification of the metal liquid 3 above the neck 6. Finally, it would remain in the spirit of the invention to use, for constitute the refractory 17 lining the recess 16, a hard refractory of the type which forms the lower part 10 and the edges 8, 9 of the side faces 4, 4 '. Certainly it would be this refractory 17 could undergo irregular ruptures, which could go as far as cause its complete destruction. But at least we would be sure that these ruptures would stop at the limits of the recess 16 and would not reach areas of the faces side 4, 4 'where they could cause dangerous liquid metal leaks 3 out of casting space.

In the example which has just been described and shown, the recess 16 has a section longitudinal trapezoidal shape. We therefore aim to ensure that, once the consumption of refractory 17 primed, the hard refractory 10 exposed to the liquid metal 3 or partially solidified a beveled surface offering only a relatively limited grip mechanical wear. A rectangular section for this recess 16 would leave after wear of the refractory 17 a hard refractory 10 having a sharp angle making it more sensitive to wear and more prone to sudden and irregular ruptures. That said, such a section rectangular of the recess 16 would essentially remain within the scope of the invention such that defined by the appended claims.

Alternatively, it is also possible to provide that the recess 16 has a section longitudinal in the shape of a right triangle.

The invention can easily be adapted to lateral faces which would have shapes different from the one shown in the previous example.

The following description has been given with reference to the casting of steel strips (at carbon to stainless), but it goes without saying that the invention can be applied to casting between cylinders of other types of ferrous or non-ferrous alloys, where the problems mentioned higher would be likely to occur.

Claims (5)

1. Side wall (4, 4') for closing off the casting space of a plant for the continuous casting of metal strip (7) between two counter-rotating rolls (1, 1') which are close together and have horizontal axes, of the type including a lower part (10) made of a high-hardness first refractory in the region where the said lower part (10) is in contact with the end faces (5, 5') of the said rolls (1, 1'), or liquid metal (3) likely to be in the course of solidifying, or an edge of the solidified strip (7), characterized in that the said lower part (10) includes, on its face intended to be turned towards the casting space, a recess (16) filled with a refractory (17).
2. Side wall (4, 4') according to Claim 1, characterized in that the said refractory (17) enclosed by the said recess (16) is a material having a lower hardness than the said first refractory and high thermal insulation properties.
3. Side wall (4, 4') according to Claim 2, characterized in that the said refractory (17) enclosed by the said recess (16) is a material having a lamellar structure, such as graphite or boron nitride.
4. Side wall (4, 4') according to one of Claims 1 to 3, characterized in that the said recess (16) has, depthwise, a longitudinal section in the form of a right-angled trapezium.
5. Side wall (4, 4') according to one of Claims 1 to 3, characterized in that the said recess (16) has, depthwise, a longitudinal section in the form of a right-angled triangle.

Images