IE902962A1

IE902962A1 - Process and device for the direct continuous casting of thin¹metallic products

Info

Publication number: IE902962A1
Application number: IE296290A
Authority: IE
Original assignee: Usinor Sacilor
Priority date: 1989-08-18
Filing date: 1990-08-15
Publication date: 1991-02-27
Also published as: FI904038A0; FR2650966A1; BR9004073A; PL286526A1; PT95018A; JPH0390252A; AU6107290A; CS397290A2; DD297088A5; CN1049989A; KR910004272A; HUT57102A; EP0413650A1; NO903605D0; NO903605L; CA2023504A1; HU905044D0

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.

Description

PROCESS AND DEVICE FOR THE DIRECT CONTINUOUS CASTING OF THIN METALLIC PRODUCTS The present invention relates to the field of plants for the direct continuous casting of metallic products of small thickness, in particular composed of steel, in which the solidification of the product is ensured by the contact between the liquid metal and the outer surface of a rotating roll or two rotating rolls energetically internally cooled.

The direct continuous casting of thin steel products (on the order of 10 mm. or less in thickness) is at the present time being experimented on two main types of plant: on one hand, plants in which the product is solidified in an ingot mould comprising two movable walls constituted by the outer surfaces of two rolls rotating about two parallel axes and energetically internally cooled; on the other hand, plants in which the outer surface of a single internally cooled roll licks a bath of liquid metal so as to form on the roll a film which is rapidly solidified.

The ongoing experiments have not yet resulted in products which are fully and constantly satisfactory. Among the main defects encountered is the insufficient quality of the edge portions of the product. These edge portions often present irregularities in thickness, flatness and appearance and it is then necessary to trim them. These defects are moreover quite comparable in nature and extent to those found in products manufactured by hot rolling from - 2 conventional continuously cast slabs that these directly continuously cast thin products are intended supersede.

Furthermore, when such plants will be capable of casting strips of great width (for example exceeding 1 m.), it will be of interest to have the possibility of casting in the same plant products intended to retain their normal width and products which may be easily split up so as to provide a plurality of strips of smaller width. It will be understood that the modifications to be made to a plant should not require changing excessively numerous essential parts of the plant.

An object of the invention is to facilitate the operation for trimming the edge portions of the product and even splitting this product.

For this purpose, the invention provides a process for « the continuous casting of metal products of small thickness, in particular composed of steel, by solidification of the liquid metal on the cooled surface of at least one rotating roll, characterized in that, in the course of the solidification of the metal, there is formed on at least one face of the product a line of reduced strength substantially parallel to the edges.

The invention also provides a plant for carrying out said process, characterized in that at least one of the rolls presents on its circumference at least one surface discontinuity.

In one embodiment, this surface discontinuity is-located - 3 in proximity to one of the ends of the roll and is constituted by a recessed engraving, such as a groove, which may be filled with a thermo-insulating material. In another embodiment, this surface discontinuity is constituted by a region in relief.

As will have been understood, the trimming of the edge portions or the splitting up of the product subsequent to casting is effected along lines of reduced strength formed on the product during the solidification. It is in this way IQ possible to employ cutting equipment of simpler design than in the case where the product has a shear strength which is uniform throughout its width.

A better understanding of the invention will be had from the following description with reference to the single sheet 15 of drawings in which: Fig. 1 is a diagrammatic view of the process of the solidification or freezing of the liquid metal on the surface of a cooled roll including a groove; Fig. 2 is a diagrammatic view of a plant for casting between two rolls which are fixed with respect to each other and arranged in accordance with the invention; Fig. 3 is a diagrammatic view of a similar plant in which the rolls are movable in translation with respect to each other and arranged in accordance with the invention.

The invention originated in the observation of the fact that, in a machine for continuously casting on a roll or between two rolls, an accidental score in the surface,.of a - 4 roll results in the formation of cracks and structural heterogeneities in the product in the course of its solidification. The mechanism of this formation is illustrated in Fig. 1. The liquid steel 1 comes into contact with the outer surface of a roll 2 provided with cooling means, not shown, such as internal ducts in which water circulates. The extraction of heat from the liquid achieved by the roll, symbolically represented by the arrows 3, 3’, has for consequence the formation of a layer 4 of metal which is solidified between the roll and the still liquid metal and whose thickness increases as the roll rotates until the completely solidified product separates from the roll. If the surface of the roll has a groove 5 which is sufficiently narrow to ensure that the liquid metal is incapable of entering the groove owing to surface tension « phenomena, the part of the solidified or frozen crust located in alignment with this groove is subjected to less extraction of heat than the neighbouring parts. Indeed, contrary to the neighbouring parts, it does not benefit from the direct contact of the crust with the cooled surface of the roll. Consequently, the solidification is abnormally slow in front of the groove and this results in a solidified thickness which is less than it should normally be. This phenomenon is aggravated by an accumulation in the less solidified part of thermomechanical stresses in the crust which causes the crust to come away from the roll in the vicinity of the edges of the groove. The result of .this - 5 process is that the structure of the product has special features where the solidification occurred in front of a groove. These special features result in greater fragility of the product and facilitate the formation of cracks or splits which are quite unacceptable when they appear on a product intended for subsequent use. They indeed constitute fracture initiators.

This explains why the surface of a casting roll must be on principle devoid of such scores capable of causing defects distributed in a random manner. On the other hand, the inventors thought that it would be of interest to create intentionally in the vicinity of the edges of the product and parallel to these edges, heterogeneities of structure resulting in increased fragility of the product and even in the immediate formation of cracks in the region in which they are located. These defects constitute regions of privileged fracture when trimming the edge portions. This trimming may therefore be carried out by machines of more rudimentary design than edge trimmers of the type employed on the output side of strip-rolling sets of rolls. In the case of the invention, a simple folding of the edge portions is indeed sufficient to separate them from the rest of the product.

In some cases, the rubbing of the edges on the refractory walls which laterally close the ingot mould is even sufficient to initiate the separation of the edge portions as soon as the product solidifies. - 6 Fig. 2 diagrammatically represents a plant for continuous casting between two rolls 2, 2' which remain fixed relative to each other during casting. The casting space is closed laterally by two refractory walls 7, 7', termed ” side dams which are applied against the ends of the rolls and against which the edges of the product solidify. The rolls include on their circumference in the vicinity of each of their ends a surface discontinuity constituted by a groove 5, 5', 51', 5'''. These grooves IQ each form on the product a line of reduced strength close to an edge along which the trimming of the edge portions will subsequently occur. The grooves acting on opposite faces of the same edge portion (5 and 511 on one hand, and 5' and 5'11 on the other) of the product are preferentially of course placed in facing relation to each other so that the lines of reduced strength they define have their effects added to each other when trimming the edge portions.

Fig. 3 diagrammatically represents a casting plant in which the roll 2 is fixed and the roll 2’' movable along its 2o axis, as indicated by the arrow. These rolls are said to be movable in translation with respect to each other. The side dams 8 and 8'1 respectively are connected to the lateral ends of the rolls 2 and 2’* respectively and slidably bear against the working surfaces of the rolls 2’1 and 2 respectively. This arrangement permits changing the width of the product, including during the casting itself. Bearing in mind the multiplicity of the positions the rdlls - 7 are capable of adopting with respect to each other, it is not possible to groove them in such manner as to cause their respective grooves to cooperate every time, as opposed to the preceding case. Consequently, on each roll, only a single groove 5, 5'' is formed in the vicinity of the end of the roll connected to the side dam 8 ,8''. In this way, the lines of reduced strength formed on the product are only one per edge. Their effect must therefore be sufficient to > nonetheless permit an easy trimming of the edge portions IQ under conditions similar to those of the preceding case.

In the case of the casting on a single roll grooved at both its ends, here again, only a single line of reduced strength can be formed on the product per edge, since the solidification of one of the faces of the product occurs in 15 a free manner. However, this drawback is minimized by the fact that the products cast in accordance with this technique are usually thinner than those cast between two rolls. The trimming of the edge portions is therefore correspondingly facilitated.

The grooves must be, on one hand, sufficiently narrow and distinctly defined so that the surface tension forces prevent the liquid metal from penetrating to the inner end of the grooves, and, on the other hand, sufficiently wide and deep to significantly upset the product cooling mode.

Dimensions of the grooves may be on the order of 0.5 mm. for the width and 0.5 mm. for the depth. The section of the groove may be of any shape, such as a triangular, or, - 8 preferably, a substantially square or rectangular shape.

The latter preferred shapes are indeed the most capable of avoiding infiltrations of liquid metal in the groove, since they result in a break in the surface of the roll which is more distinct than that possible with a groove of triangular section.

Furthermore, the groove may be filled with a thermoinsulating material. This permits the creation of a line of reduced strength which is wider than with a simple groove, since the problems of infiltration of metal to the inner end of the groove are in this way avoided.

The lines of reduced strength must be sufficiently remote from the edges to permit the good operation of the trimming tool and guarantee the complete trimming of the defective parts without however requiring the removal of an excessive amount of sound product. The cropped width may be for example 10 mm. on each side of the product.

As already mentioned, the process is also applicable to facilitate the splitting up of the product. In this case, grooves are formed on the rolls this time substantially remote from the edges so as to form on the product one or more lines of reduced strength corresponding to the lines along which the splitting will be subsequently effected. Thus, for example, the conversion of a machine which can usually cast a single product 1,500 mm. in width into a machine which permits finally obtaining two products 750 mm. in width, merely requires the replacement of the % - 9 conventional roll or rolls normally employed by one or more rolls including a groove on their median circumference and the use of a splitting tool. The latter may be disposed on the casting machine on the output side of the rolls or be independent of the machine and act on the products only subsequent to the casting and coiling of the products.

It must be understood that the scope of the invention is not intended to be limited to the described embodiments.

Other means for creating lines of reduced strength close to IQ the edges of the product may be envisaged. In particular, it is possible to imagine replacing the grooves on the rolls by any other model of a hollow engraving, such as a group of a plurality of grooves close to one another or even by regions in relief, such as ribs or pointed projections which would locally reduce the thickness of the product and thereby facilitate the trimming of the edge portions or the splitting up of the product. Furthermore, although the general orientation of the lines of reduced strength is substantially parallel to the edges, these lines may have waves.

The invention is also applicable to the case where the rolls have throughout their surface a special rugosity obtained by a series of narrow and shallow engravings (having a dimension on the order of 0.1 mm.) which have for purpose to facilitate the driving of the product by the roll or rolls and improve its surface state. They are therefore distinctly distinguished by their dimensions and their · .. - 10 action from the grooves according to the invention which form lines of reduced strength in determined regions of the product.

Claims

1. Process for the continuous casting of metallic products of small thickness, in particular composed of steel, by solidification of the liquid metal on the cooled 5 surface of at least one rotating roll, characterized in that there is formed, in the course of the solidification of the metal, on at least one face of the product at least one line of reduced strength substantially parallel to the edges of the product. 10

2. Process according to claim 1, characterized in that said line of reduced strength is located in proximity to an edge of the product.

3. Plant for the continuous casting of metallic products of small thickness, in particular composed of 15 steel, by solidification of the metal on the cooled surface of at least one rotating roll, for carrying out the process according to claim 1, characterized in that said roll has on its circumference at least one surface discontinuity.

4. Plant according to claim 3, characterized in that 20 said surface discontinuity is located in proximity to one end of the roll.

5. Plant according to claim 3 or 4, characterized in that surface discontinuity is constituted by a recessed engraving. 25

6. Plant according to claim 5, characterized in that said recessed engraving is constituted by a single groove.

7. Plant according to claim 6, characterized in'fchat said groove is filled with a thermo-insulating material.

8. Plant according to claim 3 or 4, characterized in that said surface discontinuity is constituted by a region in relief. 5

9. Plant according to any one of the claims 3 to 7, characterized in that it comprises two rolls fixed in position and said rolls include a surface discontinuity in the vicinity of each of the ends of the rolls.

10. Plant according to any one of the claims 3 to 7, 1 0 characterized in that it comprises two rolls which are movable in translation with respect to each other, said rolls each comprising a surface discontinuity in the vicinity of a single one of the ends of the rolls, and said surface discontinuities are disposed on the rolls in such -] 5 manner as to each act on a different edge portion of the product.

11. Process for the continuous casting of metallic products, substantially as hereinbefore described with reference to and as shown in Fig. 2 or Fig. 3 of the 2. Q accompanying drawing.

12. Plant for the continuous casting of metallic products, substantially as hereinbefore described with reference to and as shown in Fig. 2 or Fig. 3 of the accompanying drawing.