EP0430840B1 - Process and apparatus for single-roll or twin-roll continuous casting - Google Patents

Abstract

This apparatus, intended for the casting of thin metal products, comprises one or two rolls (1), which are driven in rotation and whose walls are cooled, and lateral walls (3) defining a casting space (4). <??>The surface of each roll is divided into at least three circumferential zones, at least one (5) of the said zones having a roughness greater than that of the other zones (6). <??>This apparatus permits the implementation of the process according to the invention whereby certain longitudinal zones of the skins of the cast metal, which are solidified in contact with the cooled walls of the rolls, are preferentially driven on their own. <IMAGE>

Description

The invention relates to the continuous casting of thin metallic products, in particular steel sheets, on one or between two cylinders.

This type of casting is already known, which consists in bringing molten metal into contact with a cooled moving moving surface. On contact with this cold surface, the cast metal solidifies and is driven by the scrolling movement of the surface.

One of the known problems of this type of casting is to ensure uniform solidification of the metal in contact with the cooled surface, this problem being generated by the fact that during contact of the molten metal with the cooled surface, the metal shrinks in solidifying, this constriction causing localized detachments of the skin of solidified metal relative to the casting surface. It follows that in the areas where this detachment occurs, the cooling of the metal is slowed down compared to the areas where the skin remains in good contact with the cooled surface. It appeared that on a smooth surface, these phenomena occurred in a disorderly manner, thus causing irregularities as much in the appearance of the surface of the cast product as in its structure, and being able to cause defects such as surface cracks.

To remedy this problem and ensure uniformity in the solidification of the cast metal, it has already been proposed to give the cooled surface a certain roughness which leads to a fairly regular distribution, on the roughness scale, of the zones where the cast metal is in contact with the cooled surface, and therefore at a homogeneous solidification, on the scale of the dimensions of the final product, of said metal.

Arrangements of this type are described in particular in documents EP 309247, and JP 62254953 which show casting rolls whose surface has a particular roughness defined by the dimensions and the arrangement of the reliefs, peaks and valleys of roughness.

Although the use of pouring surfaces having particular roughness characteristics makes it possible to obtain better quality products, in particular on the surface, the fact remains that during casting incidents such as breakthroughs or snagging of the Cast metal can occur and cause not only defects in the cast product but also serious disturbances during the casting operation itself. Such incidents occur for example commonly towards the edges of the casting surfaces, near the side walls, or small faces, which laterally hold the molten metal and define the width of the product. Indeed, these side walls are fixed and the edges of the cast product tend to remain attached to them while, at a distance from these side walls, the metal solidified in contact with the casting surface is entrained by the latter. This results in a difference in speed between the edges and the central zone of the product which is still imperfectly solidified, which can cause breakthroughs, or at least serious defects on the edges of the product.

The present invention aims to solve these problems, to avoid incidents of casting such as breakthroughs or snagging, and to provide a product of satisfactory quality over its entire width, and in particular on its edges.

With these objectives in view, the invention relates to a process for casting thin metallic products on one or between two cylinders driven in rotation, according to which molten metal is caused to solidify on contact with the cooled walls of said cylinder or cylinders. by forming skins of solid metal which are continuously driven by the rotation of the cylinder (s).

According to the invention, this process is characterized in that preferably only certain longitudinal zones of the solidified skins are driven.

According to a variant of the invention, said preferentially driven zones are located near the edges or at the edges of the cast product.

According to this first variant, the edges of the skins in the course of solidification are subjected to a particularly large driving force which prevents them from catching on the fixed side walls or which at least, in the event of the beginning of catching, breaks it. and thus prevents the amount of metal hanging from growing and leading to a breakthrough of liquid metal downstream of the attachment.

The improvement brought about by the preferential training only of the edges of the solidified skins could not yet be precisely explained. One hypothesis is however that the preferential entrainment of the edges, that is to say where, due to the presence of the side walls, the skins tend to be retained by friction against said walls and where they tend more to solidify more quickly, avoids the problems of snagging and breakthroughs already mentioned, while the skins solidified in the middle zone of the product are subjected to less drive forces and are not likely to be subjected to superficial stresses , caused by this effort, which could affect the quality of the product in this middle area.

According to another variant of the method according to the invention, said preferentially driven zones are located at a distance from the edges of the cast product.

According to this second variant, it is the middle zone of the skins which is preferably driven with respect to the edges. In this case, the edges of the skins in the course of solidification have a certain freedom of deformation and, in the event of catching on the side walls, only these zones are disturbed, being subjected to a longitudinal shearing effect, while the median zone of the product, being uniformly driven, does not undergo surface constraints. It seems that, in this case, the edges of the skins somehow ensure a damping effect of the stresses generated by the friction on the side walls, which are distributed over the width of the edge areas not subjected to the preferential drive, this effect being all the more important as these edge areas are wide. Obviously, these edge areas being particularly disturbed, it may be necessary to drop them downstream of the casting device.

The invention also relates to a device for casting thin metallic products on one or between two cylinders, this device comprising one or two cylinders driven in rotation and whose walls are cooled and whose surface has a certain roughness, and walls lateral delimiting with said cylinder or cylinders a casting space.

According to the invention, this device is characterized in that the surface of said cylinder or cylinders is divided into at least three circumferential zones, at least one of said zones having a roughness greater than that of the other zones.

According to a variant of the device, the surface of the cylinders comprises two rough areas each located near a side wall, the roughness of which is greater than that of a third area located between the two said rough areas.

It will clearly appear that this first variant of the device allows in particular the implementation of the first variant of the method described above, by promoting a preferential drive of the edges of the skins solidified on the surface of the cylinder (s).

According to a second variant of the device, the surface of the cylinder comprises at least one rough zone, with roughness greater than that of the other zones, this or these rough zones being at a distance from the side walls.

It will easily be understood that this second variant of the device allows in particular the implementation of the second variant, described above, of the method, by favoring a preferential drive of a middle zone of the solidified skins, by allowing a possible sliding of the edges of said skins.

The inventors have in fact discovered that, in addition to the beneficial aspects on the regularity of solidification of the cast product, particular characteristics relating to the roughness of the casting surfaces could participate in improving the conditions of the casting operation and facilitate obtaining of a product cast on one or between two cylinders, in particular by avoiding the well-known problems caused by the contact of the metal with the fixed side walls.

Other characteristics will appear in the description which will be given, by way of example, of several embodiments of the invention.

• la figure 1 est une vue schématique d'un dispositif de coulée entre deux cylindres, conforme à l'invention,
• la figure 2 est une vue d'une variante dans le cas de cylindres axialement décalés,
• la figure 3 est une vue de dessus d'un autre dispositif dans le cas de surfaces de coulée plus rugueuses dans leur partie médiane que sur les bords,
• la figure 4 est une vue en plan d'un cylindre à rugosité élevée sur les bords dans une configuration particulière de celle-ci.

Reference is made to the appended drawings in which:

• FIG. 1 is a schematic view of a device for casting between two cylinders, according to the invention,
• FIG. 2 is a view of a variant in the case of axially offset cylinders,
• FIG. 3 is a top view of another device in the case of casting surfaces rougher in their middle part than on the edges,
• Figure 4 is a plan view of a cylinder with high roughness at the edges in a particular configuration thereof.

In Figure 1 is shown schematically a casting installation between two cylinders comprising two cylinders 1 driven in rotation according to the arrows 2 and two fixed side walls 3 disposed against the ends of the cylinders 1 to close a casting space 5 in which is poured , during casting, molten metal.

The cylindrical wall of the cylinders 1 is cooled, and as is well known, the molten metal poured into the casting space 4 solidifies on contact with these cooled walls, forming skins of solidified metal which gradually thicken and become join at the plane of the axes of the cylinders 1 to form the product cast in the form of a thin strip which is continuously pulled down.

According to the invention, the surface of the walls of the cylinders 1 comprises, near each of the side walls 3, circumferential zones 5, represented by cross hatching, the roughness of which is greater than that of the central zone 6.

By way of example, for a width of the casting surface of 800 mm, the width of the zones 5 of high roughness is approximately 10 mm, the roughness Rz of these zones 5 being 150 μm and that of the zone median 6 of 80 ”m. These values can however vary relatively considerably depending on the dimensions of the cylinders, the nature of the casting surface and that of the cast metal. Preferably, the roughness Rz of the rougher zones 5 will be substantially equal to or greater than twice that of the less rough zones.

Such roughness can be imparted to the casting surface by numerous methods such as knurling, shot blasting, EDM, hammering, etc.

The type or shape of the roughness will essentially depend on the process chosen. For example, in the case of roughness imparted by knurling, achieve a roughness of the cross knurling type, as shown by its appearance in FIG. 1, or transverse straight knurling forming stripes oriented parallel to the axis of the cylinders, as represented by its appearance in FIGS. 2.

Furthermore, the roughness of the different zones can be achieved by different methods.

In the case shown in Figure 1, where the side walls 3 are placed against the ends of the cylinders without having a projecting part between the cylinders and in contact with the cylindrical surfaces thereof, the areas 5 of high roughness are located nearby direct from the edges of the casting surfaces, i.e. at each end of the cylinders.

2 shows a variant of the device in which the cylinders 1 are axially offset, each side wall 3 ′ bearing on the one hand on the end face of a cylinder and on the other hand on the cylindrical wall of the other cylinder. This general configuration is already known to allow, by relative displacement of the cylinders in the axial direction, a variation of the width of the casting space and therefore a variation of width of the thin strip cast.

In this case, each cylinder will have a zone 5 ′ of high roughness on one edge of the casting surface, and another zone of high roughness at a certain distance from the other edge, this distance being defined as a function of the spacing of the side walls. The cylindrical zone 7 which is in contact with the side walls is preferably of sufficiently low roughness so as not to risk abrasion of the side wall during casting.

A cylinder such as that shown in plan in Figure 4, can be used in place of cylinders 1 in the device shown in Figure 1. The zones 8 of higher roughness of this type of cylinder have the distinction of being divided circumferentially into several segments 8 ′, 8˝ alternately having different roughnesses. For example, the segments 8 ′ have a high roughness, greater than that of the segments 8˝ which can itself be equal to or greater than that of the middle zone 6.

This particular arrangement makes it possible in particular to facilitate possible dropping of the edges of the cast product, obtained by the use of such cylinders, by creating preferential rupture zones of said edges.

It has indeed been observed that sudden variations in roughness of the surface of the cylinders, such as those caused by the alternation of the segments 8,8 ′, caused in the product, at the levels corresponding to these roughness changes, a weakening of the metal of the cast product facilitating the subsequent shedding of the banks, for example by shearing.

It will already have been understood that the use of the cylinders and devices which have just been described in relation to FIGS. 1, 2 and 3 allows the implementation of the method according to the invention taken in its first variant, by promoting, by high roughness of the casting surfaces near the side walls, entrainment of the edges of solidified metal skins.

The device shown in top view in Figure 3 is intended for the implementation of the method taken in its second variant. In this variant of the device, the cylinders 1 ′ have a median circumferential zone 6 ′ with greater roughness than the circumferential zones 9 located near the side walls.

As has already been indicated, the roughness of the zones 9 is sufficiently low to allow sliding of the edges of the solidified skins on these zones, in the event that said edges tend to be retained by hooking on the side walls. This sliding can moreover be favored by lubrication of the zones 9, prior to casting or regularly during the latter.

The roughness of the median circumferential zones 6 ′ of the cylinders 1 ′ can of course be achieved by the methods mentioned above.

The invention is not limited to the devices which have just been described by way of example. It can in particular be applied to casting installations on a single cylinder.

Claims (10)

1. Process for casting thin metal products onto one roll or between two rolls which are rotationally driven, in which molten metal to be solidified is brought into contact with the cool walls of the roll or the said rolls by forming skins of solid metal, which are continuously driven by the rotation of the roll or rolls, characterized in that only some longitudinal zones of the solidified skins are preferentially driven.
2. Process according to Claim 1, characterized in that the said preferentially driven zones are located in the vicinity of the edges or at the edges of the cast product.
3. Process according to Claim 1, characterized in that the said preferentially driven zones are located at some distance from the edges of the cast product.
4. Apparatus for casting thin metal products onto one roll or between two rolls, this apparatus including one or two rolls (1) which are rotationally driven and the walls of which are cooled and the surface of which possesses a certain rugosity, and side walls (3) delimiting, with the roll or the said rolls, a casting space (4), characterized in that the surface of the roll or the said rolls is divided into at least three circumferential zones (5, 6), at least one (5) of the said zones having a greater rugosity than that of the other zones (6).
5. Apparatus according to Claim 4, characterized in that the surface of the rolls includes two rugose zones (5) each located in the vicinity of a side wall, the rugosity of which is greater than that of a third zone (6) located between the two said rugose zones.
6. Apparatus according to Claim 4, characterized in that the surface of the roll includes at least one rugose zone (6) of greater rugosity than that of the other zones (9), this or these rugose zones being at some distance from the side walls.
7. Roll for an apparatus for continuous casting onto one roll or between two rolls whose surface possesses a certain rugosity, characterized in that its cylindrical casting surface is divided into at least three circumferential zones, at least one (5) of the said zones having a greater rugosity than that of the other zones (6).
8. Roll according to Claim 7, characterized in that it includes two zones (5) of greater rugosity which are located on the edges of the casting surface.
9. Roll according to Claim 7, characterized in that it includes a central zone (6') of greater rugosity than two other zones (9) located on the edges of the casting surface.
10. Roll according to one of Claims 7 to 8, characterized in that the rugosity Rz of the zones of greater rugosity is substantially equal to or greater than twice that of the other zones.

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