CRYSTALLISER Field of the invention

The present invention relates to a crystalliser, in particular to a crystalliser for the continuous casting of blooms and billets, provided with means for distributing and lubricating its internal surface which comes into contact with the molten metal by means of oil. State of the art

Various types of crystallisers are used in continuous casting of molten metal in an ingot mould. The main tasks which must be performed by a crystalliser are to guarantee good heat transfer from the molten metal outwards, so as to facilitate its solidification, and to lubricate its internal surface which comes into contact with the molten metal to prevent problems of sticking and breakage of the skin of the billet which is forming with the consequent break-out of molten material. In the known crystallisers, the lubrication oil is distributed on the upper part along the entire perimeter of the crystalliser and by free-falling slides onto the free surface of the liquid steel, i.e. onto the meniscus. Lubrication also extends only to the upper part of the crystalliser, mainly in the meniscus zone. Another disadvantage of the known crystallisers concerns the non-compliant typology of the so-called "oscillation marks" on the surface of the cast product which, formed by effect of the turbulence of the steel casting being fed, preclude a good introduction of oil between skin and wall with consequent poor lubrication. A further disadvantage is represented by the fact that it is not possible to achieve high casting speeds, which currently do not exceed 5 m/min. An example of crystalliser which exploits porous inserts for introducing lubricant is described in patent US4579165. Such crystalliser consists of a first porous plate formed by sintering and dimensioned so as to cover the entire internal surface of the crystalliser, and by a second metallic plate which surrounds the porous plate, and by a third reinforcement plate arranged outside said first and second plates. Between the first and second plate a gap is provided, in which gas is conveyed through small holes in the first sintered plate forming a gas film between the internal surface of the crystalliser and the molten metal during casting. This crystalliser however displays some disadvantages. From a constructive point of view, it is particularly complex, providing reciprocal anchoring means of the three plates which may cause undesired vibrations in time which damage the surface

quality of the ingots. Furthermore, the porous plate is also complex to construct, being formed by several parts each comprising different types of sintered powder. Finally, the surface distribution of the lubricant thus obtained does not guarantee to allow the increase of the casting speed of blooms and billets and therefore the global system productivity.

The need for an innovative crystalliser capable of solving the aforesaid drawbacks is therefore felt.

Summary of the invention

It is the primary object of the present invention to provide a crystalliser, for the continuous casting of blooms and billets, which is simpler from a structural point of view with respect to the known types and provided with a porous insert capable of distributing the lubricant oil over the meniscus and on the internal surfaces under the meniscus. The object of the crystalliser according to the present invention is to improve lubrication making it more efficient, particularly in the area of contact between the internal surface of the crystalliser and the skin being formed.

It is a further object to provide a crystalliser which allows to considerably increase the casting speed, at least 3- to 4-fold with respect to the rates which may be obtained with crystallisers of the known art. The present invention therefore intends to reach the aforesaid objects by making a crystalliser for continuous casting of blooms and billets which, in accordance with claim 1 , comprises a tubular structure defining a longitudinal axis; a porous insert positioned along the crystalliser, with at least one surface facing the longitudinal axis and whose section on a plane orthogonal to said axis defines the shape of said blooms or billets; conduits for the flow of a lubricant in a gap between said tubular structure and porous insert; characterised in that said porous insert comprises a single element provided on its larger surfaces with a plurality of pores having a size from approximately 10 to 150 microns, and a pore/larger surface ratio from 30% to 60%. Advantageously, the porous insert of the crystalliser according to the invention allows to carry the required amount of lubricant, for example oil, by allowing the variation of the dimensions of the holes in the areas over and under the meniscus and thus the appropriate distribution of the lubricant over the meniscus and on the internal surface under the meniscus. Advantageously, by extending the lubricated surface under the meniscus in

height, the risk of sticking of the skin on the wall of the crystalliser, and the subsequent breakage of the skin itself with break-out of the liquid metal, is reduced.

Other advantages obtained with the crystalliser according to the invention are: - the possibility of eliminating oscillations thus considerably reducing the problems of the so-called "oscillation marks" on the cast product surface;

- and the consequent constructive simplification with implementation cost savings. The dependent claims describe preferred embodiments of the invention.

Brief description of the drawings Further features and advantages of the present invention will be more apparent in the light of the detailed description of a preferred, but not exclusive, embodiment, of a crystalliser illustrated by way of non-limitative example, with reference to the accompanying drawings, in which:

Fig. 1 is a longitudinal section of part of the crystalliser according to the invention; Fig. 2 shows a cross section taken along plane A-A of the crystalliser in Fig. 1 ;

Fig. 3 shows a perspective view of a component of the crystalliser in Fig. 1.

Detailed description of a preferred embodiment of the invention

With reference to the figures a crystalliser is shown comprising:

- a wall 1 , - a porous insert 2, positioned in a longitudinal housing obtained in the upper part of said wall,

- an upper flange 3.

Porous insert 2, in this preferred embodiment, comprises a monolithic tube, preferably made of copper, having either a square, or rectangular, or round cross section according to the shape to be cast.

Advantageously, the porous tube is integrally obtained by casting and the micropores are made by spark erosion or chemical evaporation. The features of porous insert 2 are advantageously the following:

- internal section equal to that of the shape to be cast; - length, measured along axis X, from approximately 100 to 250 mm;

- thickness from approximately 2 to 8 mm;

- pore dimensions from approximately 10 to 150 microns;

- total porosity, meaning the pore/larger surface ratio of the insert, from 30 to 60%. Porous insert 2 is advantageously fixed by mechanical interference in a specific

housing obtained in the thickness of the wall of the crystalliser, at one end of the crystalliser.

As shown in Fig. 1 , porous insert 2, once fitted, advantageously has its internal surface facing the inside of the crystalliser, in perfect alignment with the internal surface of the crystalliser itself so that the total internal surface is uniform and without points of discontinuity. Also the upper end of said insert 2 is aligned with the upper edge of the crystalliser.

In addition to the housing for the porous insert 2, a channel 4 is obtained on the wall of the crystalliser near the upper edge. Said channel 4 is advantageously in communication with a conduit 5 obtained in the upper flange 3 and with a longitudinal gap 6 between the wall of the crystalliser and porous insert 2. A lubrication oil is allowed to flow through conduit 5 of flange 3 reaching channel 4 which acts as introduction manifold of the oil in gap 6. For the oil to uniformly lap the entire external surface of porous insert 2, the surface of the crystalliser in gap 6 is advantageously provided with grooves 7 directly communicating with channel 4.

As shown in the cross section in Fig. 2, such grooves 7 may have different shape, for example zigzags or fretting. If one wants to increase the contact surface between the surface of the crystalliser in the gap and the porous insert, and thus privilege heat exchange, i.e. the transfer of heat from the molten metal outwards, the use of fretting type grooves 7' is recommended; if instead one wants to privilege the external surface of the porous insert wet by the oil, the use of zigzag grooves 7" is preferable. The lubrication oil passes from the external surface to the internal surface of the porous insert itself through the micro-pores in insert 2. Since the longitudinal extension of insert 2 is such that the meniscus 8 of the molten metal is approximately at half its height, the feeding of the lubrication oil is advantageously obtained both over and under the meniscus. In this way, the introduction of the lubricant is guaranteed under skin 9 being formed, for a certain length, from the first solidification point 10.

Advantageously, the crystalliser is cooled by means of water circulating in longitudinal channels 1 1 , for example through holes, obtained in the thickness of the same near its edge. The particular embodiments herein described do not limit the contents of this

application covering all the variants of the invention defined in the claims.