EP0407323B1 - Method and apparatus for twin roll continuous casting of thin metallic products suitable for cold rolling - Google Patents

Abstract

According to this method, molten metal is cast between two rolls (1, 2) with parallel axes, driven in rotation in opposite directions and whose walls are cooled. Use is made of at least one roll whose generatrix is curved and whose diameter is slightly smaller in its axially central part than towards its ends, and the metal is kept in contact with the wall of the said roll over an arc which is longer in the said central part than towards the said ends. To this end, the casting device comprises a riser (3) whose lower edge (33) of the longitudinal walls (31) is in contact with the walls of the said rolls at the top of the nip (5) between the latter, and the line (35, 35') of contact of the lower edge of the longitudinal wall of the riser with this roll is curved so that the length of the arc of the surface (6, 7) of this roll between the said line and the said nip is greater towards the middle of the said longitudinal wall than over its edges. <IMAGE>

Description

The present invention relates to the casting between cylinders of thin metallic products, such as in particular strips or sheets of steel, according to the preambles of claims 1 and 2.

This type of casting is intended for obtaining by casting thin products which can then be directly cold rolled.

In casting installations between cylinders, the molten metal is poured into a casting space defined by the portions of the cylindrical walls of the cylinders situated above the plane passing through the parallel axes of said cylinders, and by end walls also called small faces or side walls, generally fixed, substantially perpendicular to the axes of the cylinders. During casting, the liquid metal gradually solidifies on contact with cooled cylindrical walls of the cylinders, forming solidified skins which are driven by the rotating cylinders and meet at the neck, that is to say at the said level. plane passing through the axes of the cylinders, to form the finished product which is continuously extracted downwards.

In certain cases, these installations also comprise a sort of fixed extension consisting of two longitudinal walls in sealed contact with the cylinders and two front walls extending upwards said small faces or forming an integral part thereof. This enhancement will hereinafter be designated by the term "feeder" by analogy with the part surmounting the ingot mold or the mold and intended to contain the retaining feeder in the casting installations in fixed ingot mold or molding. This "flyweight" has the particular function, in the case of casting between cylinders, of delimiting the cylinder surface on which the solidification of the cast metal takes place, and thus ensuring the regularity of this solidification, whatever the level of metal in the counterweight. In addition, the meniscus of the liquid metal is not in contact with the walls of the cylinders and the risk of entrainment of impurities floating on the meniscus is thus considerably reduced.

Document JP-A-60 54 249, on which the preambles of claims 1 and 2 are based, discloses a casting installation between cylinders having concave curvilinear generators, the concavity of which corresponds to the variation in thickness that the strip would have pouring under the effect of the thermal bulge of the cylinders.

Whatever the case, many problems remain with regard to obtaining a product which can subsequently be directly cold-rolled. Indeed, one of the desired characteristics for a flat product intended to be cold rolled is to have a slight transverse curvature. It is known that this initial convex, for example of the order of a few tens of micrometers for a sheet of a few millimeters thick, makes it possible to obtain, after rolling, a sheet of constant thickness or at least of adequate cross section while '' a flat or hollowed out draft is not acceptable by laminators.

The main problem which the present invention proposes to solve therefore comes down to obtaining a product resulting from casting which has this convex shape and which is solidified throughout its section at the level of the neck between the cylinders.

With this objective in view, the invention relates to a continuous casting process between cylinders according to which liquid metal is poured between two cylinders of parallel axes driven in rotation in opposite directions and whose walls are cooled, and minus a cylinder whose generatrix is curvilinear and whose diameter is slightly smaller in its axially middle part than towards its ends.

According to the invention, this method is characterized in that said generator is hot curvilinear and the metal is kept in contact with the wall of said cylinder on a longer arc in said middle part than towards said ends.

As will be understood, the method consists in increasing the contact time of the metal with the cooled wall of the cylinder in the axially median zone of the latter relative to the contact time in the end zones, so as to form a thicker solidified skin towards the middle than on its edges and therefore to obtain, at the level of the neck between the cylinders, a product solidified in its entire cross section and having the desired desired bulging, this bulging corresponding moreover to the "hollow" of the cylinder surface.

The inventors have indeed found experimentally that the contact time, and therefore the length of the contact arc, of the metal on the cylinder has a considerable influence on the thickness of the solidified skin. Thus for example for a cast product with a final thickness of 2 mm and a contact arc length of 600 mm, all other things being equal, a variation of 50 mm in this arc length can induce a variation in thickness of the solidified skin of approximately 50 μm. The inventors have therefore imagined using this property to obtain the desired bulge by adapting the length of the contact arc between cylinder and metal at a certain distance from the vertical median plane perpendicular to the axes of the cylinders, as a function of the thickness. of the product sought at this distance measured from the longitudinal axis of the product, or in other words, as a function of the "hollow" of the surface of the cylinders.

The invention also relates to a device for casting between rolls comprising two rolls having cooled walls, at least one of the rolls having a curvilinear generator and a slightly smaller diameter in its axially median part than towards its ends, characterized in that said generator is hot curvilinear and in that it comprises means for holding the metal in contact with the wall of at least one of the cylinders on a longer arc in said middle part than towards said ends.

Preferably, these means consist of a longitudinal wall of a counterweight, the lower edge of which is substantially in contact with the wall of a cylinder above the neck between said cylinders, and the line of contact of said lower edge with the cylinder is curved.

In other words, the length of the arc of the surface of this cylinder between said line and said neck is greater towards the middle of said longitudinal wall than on its edges.

According to a particular arrangement of the invention, said longitudinal wall of the counterweight is generally planar and has at its lower edge a chamfer more accentuated towards the middle of said wall than towards its edges.

According to another arrangement, said longitudinal wall of the counterweight has a concavity directed towards the inside of the counterweight.

As will no doubt have already been understood, these arrangements make it possible to ensure, in a simple and practical manner, the implementation of the method according to the invention described above, insofar of course as long as, during casting, it is maintained the level of liquid metal above said weight-cylinder contact line.

The invention will be better understood and other characteristics and advantages will emerge on reading the description which will be given by way of example of a device according to the invention.

• la figure 1 est une représentation schématique en perspective, partiellement en arraché, d'un dispositif de coulée entre cylindres avec masselotte ;
• les figures 2a et 2b sont des sections perpendiculaires aux axes des cylindres respectivement aux extrémités et au milieu du dispositif ;
• la figure 3 est une vue similaire à celle de la figure 1 dans le cas d'une variante ;
• les figures 4a et 4b sont des sections du produit obtenu respectivement dans le cas de l'utilisation de deux et d'une seule paroi de la masselotte conformée selon l'invention.

Reference is made to the appended drawings in which:

• Figure 1 is a schematic perspective representation, partially broken away, of a casting device between cylinders with counterweight;
• Figures 2 a and 2 b are sections perpendicular to the roll axes respectively at the ends and in the middle of the device;
• Figure 3 is a view similar to that of Figure 1 in the case of a variant;
• Figures 4 a and 4 b are sections of the product obtained respectively in the case of the use of two and a single wall of the counterweight shaped according to the invention.

It is specified that in the figures the "hollows" of the cylinders and the bulges of the product obtained have been deliberately exaggerated to facilitate understanding of the invention, it being well noted that in reality these hollows and bulges are of the order of ten micrometers.

In FIG. 1 is shown a continuous casting installation between cylinders comprising two cylinders 1 and 2 surmounted by a counterweight 3.

The cylinders 1, 2 are slightly "hollow", that is to say that their generators 11, 12 are not absolutely straight but slightly curved, with an arrow corresponding substantially to the value of the desired convex for the cast product. They are rotated in opposite directions, according to arrows 13, and their walls are cooled. It is specified that the hollow of the cold cylinders can be slightly different, in general higher, from said desired convex of the product, this in order to compensate for hot their possible variations in diameter due to expansion, during the thermal regime at the start casting, expansion whose effects are generally not perfectly identical over the entire length of the cylinders.

The counterweight made of refractory material comprises two longitudinal walls 31 parallel to the axes of the cylinders and the lower edge 33 of which is kept in contact with the cylinders with as little functional clearance as possible, and two front walls 32 which also constitute in the example shown the end walls closing off the casting space 4 delimited furthermore by the portion of the surfaces of the cylinders above the neck 5 situated at the horizontal plane P passing through the axes of said cylinders.

According to the invention, the lower edge of said longitudinal walls 31 comprises a chamfer 34 of increasing importance from the ends towards the middle of said longitudinal walls 31. Therefore the contact line 35 of each longitudinal wall with the corresponding cylinder is curve and the chamfer 34 is therefore concave. This chamfer is clearly visible in the drawings of Figure 2 wherein Figure 2a shows a section of the longitudinal wall 31 at one end thereof and Figure 2b a section of this wall towards the middle. It is clearly seen that the arc 6 on which the cast metal is in contact with the cooled wall of the cylinder towards its middle, is longer than the corresponding arc 7 towards its ends.

During casting, the liquid metal is poured into the counterweight and fills the casting space 4. The level of liquid metal is regulated to be maintained above the contact line 35 between the counterweight and the cylinder. During the rotation of the cylinders, and because of these differences in length of contact arc, the time of contact of the metal with the wall of the cylinder is greater towards its middle, which leads to a greater thickness of solidified skin at center of the product only on its edges. The skins meet at the level of the neck 5 and form the final product having the desired bulge, as shown in FIG. 4 a . In this figure the product 8 has been shown as consisting of two curved zones 8 ', 8''each from the skins formed respectively on each cylinder. It will of course be understood that there is no real discontinuity between these two zones in the finished product.

In Figure 4b, there is shown similarly a section of the product from an installation according to another embodiment of the invention in which a single cylinder is hollow and only the longitudinal wall of the counterweight adjacent to this cylinder has the chamfer described above, the second cylinder then having rectilinear generators and the corresponding longitudinal wall of the counterweight being plane up to the line of contact with said second cylinder. In this case, the skin 9 'formed on this second cylinder is of constant thickness over its entire width, only the skin 9''having an extra thickness towards its middle. Due to the low real curvature, this asymmetry will have practically no influence on the finished product 9 which will have a curved equivalent to the product 8, since of course the "hollow" of the first cylinder and the corresponding chamfer of the longitudinal wall of the counterweight will have been increased accordingly, that is to say substantially doubled. This embodiment has the advantage of not having to perform hollow machining on a single cylinder, as well as the chamfer on a single longitudinal wall of the counterweight.

Theoretically, the lower edge 33 of the longitudinal wall 31 should also have a curvature corresponding to the "hollow" of the cylinder to maintain a constant functional clearance over its entire length between said wall and the cylinder. However this "hollow" being practically very small, even if said lower edge is rectilinear, which facilitates the manufacture of the counterweight, there is little risk of infiltration of the liquid metal by the said clearance. On the other hand, as has already explained, the variation in solidified thickness necessary to ensure the desired convexity of the cast product and the uniform solidification of the product at the level of the neck 5, requires a significant variation in the length of the contact arc on which the solidification of the skins.

By way of example, values of chamfers are given below, in an experimental installation for the casting of strip of 2 mm necessary for obtaining a convexity of the strip of the order of 50 μm.

The invention is not limited to the device described above by way of example. We can also use a counterweight as shown in Figure 3, the longitudinal walls 31 'have a concavity directed towards the regular casting space over the entire height of said walls. Other said in this variant, the longitudinal walls of the counterweight do not have a chamfer at their lower edges, but it is the general curvature of these walls which determines a curved line 35 'of counterweight-cylinder contact and therefore the variation in length of contact arc sought.

It will also be possible, in order to limit the length of the contact arc towards the edges of the cylinders, to replace the counterweight described above by any means allowing the liquid metal to be confined according to the particular geometry desired, such as for example a horizontal or oblique wall, or still containment means without contact with the cast metal.

Claims (5)

1. Method for continuous casting between rolls, in accordance with which liquid metal i cast between two rolls (1, 2) with parallel axes, driven in rotation in opposite directions, and whose walls are cooled, and at least one roll is used whose generatrix is curvilinear and whose diameter is slightly less in its axially central part than towards its ends, characterised in that the said generatrix is curvilinear when hot, and the metal is held in contact with the wail of the said roll over an arc which is longer in the said central part than towards the said ends.
2. Device for casting between rolls, comprising two rolls (1, 2) with cooled walls, at least one of the rolls has a curvilinear generatrix and a diameter slightly less in its axially central part than towards its ends, characterised in that the said generatrix is curvilinear when hot and in that it comprises means for holding the metal in contact with the wall of at least one of the rolls over an arc which is longer in the said central part than towards the said ends.
3. Device according to Claim 2, characterised in that these means consist of a longitudinal wall (31) of a feeder (3), the lower edge (33) of which is substantially in contact with the wall of a roll above the neck (5) between the said rolls, and the contact line (35, 35') of the said lower edge with the roll is incurvate.
4. Device according to Claim 3, characterised in that the said longitudinal wall (31) of the feeder is generally plane and comprises at its lower edge a chamfer (34) which is more accentuated toward the middle of the said wall than towards its edges.
5. Device according to Claim 3, characterised in that the said longitudinal wall of the feeder has a concavity directed towards the inside of the feeder.

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