EP0413650A1 - Process and apparatus for continuous casting thin metallic products - Google Patents

Abstract

In a process for continuous casting of thin metal products, particularly made from steel, by solidification of the molten metal against at least one cooled rotating roll (2, 2'), a line (5) of lower mechanical strength is formed on at least one face of the product, parallel to the edges. The installation for implementing this process comprises at least one roll having, on its circumference, a surface discontinuity (interruption), such as a groove or a zone in relief. The aim of the invention is to facilitate cropping of the edges of the cast product, or its slitting. <IMAGE>

Description

The present invention relates to the field of direct continuous casting installations for metallic products, in particular steel, of small thickness, in which the solidification of the product is ensured by the contact between the liquid metal and the external surface of a single cylinder or of two cylinders in rotation and internally cooled energetically.

Direct continuous casting of thin steel products (around 10 mm thick and less) is currently being tested on two main types of installation:
- On the one hand, installations in which the product is solidified in an ingot mold comprising two movable walls constituted by the external surfaces of two cylinders rotating around the two parallel axes and internally cooled energetically;
- on the other hand, installations in which the external surface of a single cylinder internally cooled licks a bath of liquid metal, so as to form on the cylinder a film which solidifies rapidly.

The experiments in progress have not yet made it possible to obtain totally and constantly satisfactory products. Among the main defects encountered is the insufficient quality of the product edges. These often have irregularities in thickness, flatness and appearance, and it would then be necessary to drop them for later use of the product. These defects are moreover, in nature and in importance, quite comparable to those encountered on products manufactured from conventional continuous casting slabs by hot rolling, which these thin products directly cast continuously are intended to replace.

On the other hand, when such installations will be able to flow very wide strips (for example, more than 1 m), it will be interesting to have the possibility of pour on the same installation products intended to maintain their normal width and products which can be easily split to give a plurality of strips of smaller width. Of course, the modifications to be made to the installation should not require the replacement of too many essential parts of it.

The object of the invention is to facilitate the operation of falling the edges of the product or even the slitting of this same product.

To this end, the subject of the invention is a process for the continuous casting of thin metal products, in particular steel, by solidifying the liquid metal on the cooled surface of at least one rotating cylinder, characterized in that, when solidification of the metal, on at least one face of the product a line of least mechanical resistance substantially parallel to the edges.

The invention also relates to an installation for implementing this process, characterized in that at least one of the cylinders has on its circumference at least one surface discontinuity.

In one embodiment, this surface discontinuity is located near one of the ends of the cylinder, and is formed by a hollow etching, such as a groove, which can be filled with a thermal insulating material. In another embodiment, it consists of a raised area.

As will be understood, the falling of the edges of the product or its slitting, after casting, is carried out along the lines of least mechanical resistance formed on the product during solidification. It is thus possible to use a cutting apparatus of simpler design than in the case where the product has a uniform shear strength over its entire width.

• - la figure 1 schématise le processus de solidification du métal liquide sur la surface d'un cylindre refroidi comportant une rainure ;
• - la figure 2 schématise une installation de coulée entre deux cylindres fixes l'un par rapport à l'autre, et aménagés selon l'invention.
• - la figure 3 schématise une installation similaire, dont les cylindres sont translatables l'un par rapport à l'autre, et aménagés selon l'invention.

The invention will be better understood from the following description, referring to the single sheet of drawings on which,

• - Figure 1 shows schematically the solidification process of the liquid metal on the surface of a cooled cylinder having a groove;
• - Figure 2 shows schematically a casting installation between two cylinders fixed relative to each other, and arranged according to the invention.
• - Figure 3 shows schematically a similar installation, whose cylinders are translatable relative to each other, and arranged according to the invention.

The invention originates in the observation that, in a continuous casting machine on a cylinder or between two cylinders, an accidental scratching of the surface of a cylinder results in the formation of cracks and structural heterogeneities on the product during its solidification. The mechanism of this formation is illustrated in FIG. 1. The liquid steel 1 comes into contact with the external surface of a cylinder 2 provided with cooling means not shown, such as internal water circulation channels. The extraction of heat from the liquid operated by the cylinder, symbolized by the arrows 3, 3 ′, results in the formation of a layer 4 of solidified metal between the cylinder and the still liquid metal, a layer whose thickness grows as the cylinder rotates, until the completely solidified product separates from it. If the surface of the cylinder has a groove 5 which is sufficiently narrow so that, thanks to the phenomena of surface tension, the liquid metal cannot penetrate therein, the part of the solidified crust situated below this groove undergoes less heat extraction than the surrounding parts. In fact, unlike the surrounding parts, it does not benefit from direct contact between the crust and the cooled surface of the cylinder. It follows that, faced with groove, solidification is abnormally slow, which results in a solidified thickness less than it normally would be. This phenomenon is aggravated by the accumulation in the least solidified part of the thermomechanical stresses exerted on the crust, which causes a detachment of the crust in the vicinity of the edges of the groove. This process means that the structure of the product presents particularities where solidification took place in front of a groove. These features go in the direction of greater brittleness of the product, and are favorable to the formation of cracks, or cracks, which are unacceptable when they appear on a product intended for use. They constitute, in fact, incipient fractures.

This explains why the surface of a casting cylinder must in principle be free from such scratches which can cause defects distributed randomly. On the other hand, the inventors have thought that it is interesting to deliberately create, in the vicinity of the banks of the product and parallel to them, structural heterogeneities leading to greater fragility of the product at their location, or even to immediate formation. of coves. These defects constitute privileged places of rupture during the fall of the banks. This fall can therefore be carried out by means of machines of a more rudimentary design than the shears of banks of the type of those which are used downstream of the band trains. In the case of the invention, a simple folding of the edges is indeed sufficient to cause their separation from the rest of the product.

In some cases, the friction of the edges on the refractory walls which close the mold laterally is even sufficient to initiate the separation of the edges as soon as the product solidifies.

Figure 2 shows schematically a continuous casting installation between two cylinders 2, 2 ′ which, during casting, remain fixed relative to each other. The pouring space is closed laterally by two refractory walls 7, 7 ′ called "small faces" applied against the ends of the cylinders, and against which the edges of the product solidify. The cylinders have on their circumference in the vicinity of each of their ends, a surface discontinuity constituted by a groove 5, 5 ′, 5˝, 5˝ ′. These grooves each form on the product a line of lesser mechanical resistance close to a bank, and according to which the fall of the banks will be carried out subsequently. Of course, the grooves acting on the opposite faces of the same bank of the product (5 and 5˝ on the one hand, 5 ′ and 5˝ ′ on the other hand) must preferably be placed opposite one another. , so that the lines of least mechanical resistance that they define see their effects add up when the banks fall.

Figure 3 shows schematically a casting installation in which the cylinder 2 is fixed and the cylinder 2 ′ movable on its axis, as indicated by the arrow. These cylinders are said to be "translatable" relative to each other. The small face 8 respectively 8 ′ is integral with the lateral end of the cylinder 2 respectively 2 ′ and rests in a sliding manner on the working surface of the cylinder 2 ′ respectively 2. This device has the effect of allowing the width of the product to be changed , including during the casting itself. Given the multiplicity of positions that can take the cylinders relative to each other, it is not possible to groove them so as to always cooperate their respective grooves, unlike the previous case. Consequently, on each cylinder, only a single groove 5, 5 ′ is arranged in the vicinity of the end of the cylinder secured to the small face 8, 8 ′. In this way, the lines of least mechanical resistance formed on the product are only one in number per bank. Their effect must therefore be sufficiently marked to still allow easy shedding of the banks under conditions similar to those of the previous case.

In the case of casting on a single grooved cylinder at its two ends, here again one can form on the product only one line of least mechanical resistance per bank, since the solidification of one of the faces of the product s' performs freely. However, this drawback is minimized by the fact that the products cast according to this technique are generally thinner than those cast between two cylinders. The fall of the banks is therefore facilitated.

The grooves must on the one hand be sufficiently narrow and clearly drawn so that the surface tension forces prevent the liquid metal from entering the bottom of the grooves, and, on the other hand, sufficiently wide and deep to create a significant break in the mode. product cooling. Possible dimensions for the grooves are of the order of 0.5 mm for the width, and 0.5 mm for the depth. The section of the groove can be of any shape, such as triangular, or, preferably, substantially square or rectangular. These latter forms are in fact the most suitable for avoiding the infiltration of liquid metal into the groove, since they cause a rupture of the surface of the cylinder more straightforward than a groove of triangular section would do.

On the other hand, the groove can be filled with a heat insulating material. This makes it possible to create a line of lesser mechanical strength of greater width than with a simple groove, since this eliminates the problems of metal infiltration at the bottom of the groove.

The lines of least mechanical resistance must be far enough from the edges to allow the cutting tool to function properly and guarantee complete dropping of the defective parts, without however requiring the removal of an excessive amount of healthy product. The dropped width can be, for example, 10 mm on either side of the product.

As mentioned, the process can also be applied to facilitate the slitting of the product. In this case, grooves are formed on the cylinders, this time substantially away from the edges, so as to form on the product one or more lines of lesser mechanical resistance corresponding to the lines along which the slitting will later be carried out. Thus, for example, the transformation of a machine usually making it possible to pour a single product of 1500 mm wide into a machine making it possible ultimately to obtain two products of 750 mm wide requires only the replacement of the cylinder (s) conventional normally used by one or more cylinders having a groove on their median circumference, and the implementation of a slitting tool. This can be placed on the casting machine downstream of the cylinders, or be independent of the machine and only act on the products after their casting and winding.

Of course, the invention is not limited to the examples described. Other means of creating lines of less mechanical resistance near the edges of the product are conceivable. One can imagine, in particular, replacing the grooves on the cylinders by any other hollow engraving model, such as a set of several grooves close to each other, or even by raised areas, such as ribs or pins, which locally reduce the thickness of the product and thus facilitate the cutting of its edges or its slitting. On the other hand, if the general orientation of the lines of least mechanical resistance is substantially parallel to the edges, these lines may have undulations.

Finally, the invention is also applicable to the case where the cylinders have over their entire surface a particular roughness obtained by a series of engravings of small width and shallow depth (of the order of 0.1 mm), which aim favor the entrainment of the product by the cylinder or cylinders and improve its surface condition. By their dimensions and their action, they are thus clearly distinguished from the grooves according to the invention which form lines of less mechanical resistance in specific areas of the product.

Claims (10)

1) Process for the continuous casting of thin metal products, in particular steel, by solidification of the liquid metal on the cooled surface of at least one rotating cylinder, characterized in that, when the metal solidifies, it is formed on at least one face of the product at least one line of least mechanical resistance substantially parallel to the edges. 2) Method according to claim 1, characterized in that said line of least mechanical resistance is located near a bank of the product. 3) Installation for continuous casting of thin metal products, in particular steel, by solidifying the metal on the cooled surface of at least one rotating cylinder, for implementing the method according to claim 1, characterized in that said cylinder has on its circumference at least one surface discontinuity. 4) Installation according to claim 3, characterized in that said surface discontinuity is located near one end of the cylinder. 5) Installation according to claim 3 or 4, characterized in that said discontinuity is constituted by a hollow engraving. 6) Installation according to claim 5, characterized in that said hollow etching is constituted by a single groove. 7) Installation according to claim 6, characterized in that said groove is filled with a heat insulating material. 8) Installation according to claim 3 or 4, characterized in that said surface discontinuity is constituted by a raised area. 9) Installation according to one of claims 3 to 7, characterized in that it comprises two fixed cylinders and in that these cylinders have a surface discontinuity in the vicinity of each of their ends. 10) Installation according to one of claims 3 to 7, characterized in that it comprises two cylinders translatable relative to each other, in that these cylinders each comprise a surface discontinuity in the vicinity of a single their ends, and in that these discontinuities are arranged on the cylinders so as to each act on a different edge of the product.

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