It is known that in view of directly obtaining metallic products of a reduced thickness by casting of a metal, such as thin strip of a thickness of a few millimetres, more particularly of steel, a special casting technique has been developed, which is generally called continuous casting between rolls. This technique consists in pouring molten metal in a casting space defined between two cooled rolls with parallel axes and two lateral sealing walls, generally arranged against the front surfaces at the ends of the rolls. The metal solidifies upon coming into contact with the walls of the rolls, and, by rotating the latter in opposite directions, a metallic strip is pulled out in an at least partial solidified condition, the thickness thereof being substantially equal to the distance between the two rolls. This technique enables production of a thin metal strip directly from the molten metal.
The reduced thickness of this strip allows subsequently to submit it directly to a rolling treatment by a cold rolling process.
One knows also another casting technique, which is intended to obtain even thinner products. According to this technique, the liquid metal, which is cast on the surface of a single roll that rotates, completely solidifies while in contact with the roll so as to form a continuous metallic strip.
The rolls used for the implementation of these casting techniques are generally internally cooled and they comprise a core and a sleeve, coaxially arranged one with respect to the other, means for axial and rotary coupling of the sleeve on the core, as well as means for supporting and centering the sleeve on the core.
Such rolls are described for example in the document FR-A-2 654 372. This document discloses a roll comprising a core which bears a sleeve of a material having a good thermal conductivity, for example an alloy of copper. The sleeve comprises circulation channels for a cooling fluid and is maintained on the core, in an axially median zone, by a mechanical joint of the "T" or of the dovetail shape. This joint provides the axial positioning of the sleeve on the core and the transmission of the rotary movement from the core to the sleeve. This assembling method is also intended to prevent the sleeve from being deviated radially from the core as a result of the thermal expansion. To avoid the effects of this expansion upon the edges of the sleeve, these sleeves too are maintained by annular end plates which are designed in such a way that the radial displacement of the edges is prevented while their displacement in the axial direction is authorised.
Such a layout however ends up in realising the sleeve in the form of a bulky piece, in order to be able to guarantee its mounting on the core. This renders this piece expensive, all the more because the machining required for assembling this system and maintaining it on the core has to be carried out with a high precision. Moreover, the assembling system with the "T" or the dovetail joint leads to the necessity for the sleeve to be very thick in its median part and to be rather thin towards its edges; these important variations of the cross section result in irregular deformation of the sleeve during its expansion.
Other realisation methods for the rolls have been suggested. For example, the document JP-A-04.224054 shows a roll constituted by a sleeve and by two semi-cores. The bore of the sleeve is biconical, and the semi-cores show a taper which is complementary to that one of the bores of the sleeve, which is supported by the semi-cores over substantially the entity of its width. The cores are coupled one to the other by elastic connecting means, acting in the axial direction of the roll in order to maintain the contact of the sleeve with the core, even if the sleeve expands radially. The objective aimed at by this realization method is to avoid losing the contact between the sleeve and the core under the influence of the cyclic thermal deformations of the sleeve. In such an arrangement, the actuation of the sleeve by the cores is realized by friction at the level of the conical surfaces, and it is not excluded that, under the effect of the radial and axial expansions varying along the circumference, there can occur contact interruptions and consequently risks of a bad actuation, this problem being considerably worsened if the sleeve undergoes a thermal camber.
Moreover, in this accomplishment, maintaining the position of the sleeve in the axial direction of the roll is subject to a symmetrical displacement of the semicores. If these displacements are not perfectly symmetrical, there results therefrom an axial displacement of the sleeve with respect to the starting position. Such a displacement is detrimental to the required tightness between the edges of the sleeve and the lateral sealing walls of the casting space, these sealing walls having to guarantee the tightness of the casting space at the ends of the rolls as a result of the contact with the front faces at the edges of the sleeve.
Indeed, one will easily understand that in an installation for casting between two rolls, it is not possible to keep the necessary contact between the sealing wall and the edges of the sleeves if the axial displacements of the sleeves of the two rolls are not identical.
Generally speaking, it has been seen that mainly under the effect of the thermal expansions, the sleeves of the rolls have a tendency to distort, and, if the fastening of the sleeves to the rolls is not guaranteed in an optimal manner, it happens perturbations in the rotation of the sleeves, mainly under the influence of the force exerted by the metallic product during casting; the said rotation does not any longer take place in a perfectly regular and circular manner and results in a phenomenon of "false round". These perturbations can be heavily prejudicial to the quality of the cast product by generating on its surface geometrical or metallurgical defects
On the other hand, if supporting and centering of the sleeve are not realised by the same pieces, whatever the degree of expansion of the sleeve might be, the machining of the contour of the sleeve, executed while cold, in a perfectly concentric manner with regard to the rotation axis, can prove to be eccentric, when hot, due to the change of the supports.
It is the aim of the present invention to solve the aforementioned problems, and more particularly to provide an economical method of producing the casting rolls, especially their sleeves, which are pieces liable to be worn out and which must be easily exchangeable and attainable at the lowest possible costs as a result of an economy of material and of machining.
It is also one of the aims of the invention to guarantee, when the rolls are cold as well as when they are hot, a continuous and constant support of the sleeve in the course of the casting, and more particularly to :
--guarantee a good axial and radial centering of the sleeve on the core,
--ensure the best possible symmetry with respect to the median plane of the roll, extending orthogonally with respect to the latter's axis,
--ensure a constant actuation of the sleeve by the core and a constant transmission of the torque notwithstanding possible deformations of the sleeve,
--limit the stresses in the sleeve and in the core, even during the deformations due to expansion,
--ensure the holding of the edges of the sleeve when the latter undergoes a thermal bulging such as due to thermal expansion,
--guarantee the concentricity of the rotation axis with the outline of the sleeve, when cold as well as when hot.
Having these aims in view, the invention is directed to a casting roll for a continuous casting installation for metals on one such roll or between two of them, each roll comprising a core and a sleeve, coaxially arranged one with respect to the other, coupling means for the sleeve and the core, including axial coupling means and rotary coupling means, as well as means for supporting and centering the sleeve radially on the core.
According to the invention, the roll is characterized in that the axial coupling means comprise axial abutment means of the sleeve against the core, which are situated in a plane substantially axially median with respect to the roll, and in that the centering means comprise means for the conical elastic centering solely of the edges of the sleeve with respect to the core.
Thanks to the invention, the axial and the radial centering of the sleeve with respect to the core are guaranteed by the same pieces, whatever the expansion condition of the sleeve might be. Indeed, the axial abutment means being located in the median plane of the roll, the expansion deformations of the sleeve in the axial direction distribute themselves equally on both sides of this plane, thus guaranteeing an accurate and fixed positioning of the sleeve and consequently an optimal symmetry of the casting surfaces with respect to said median plane.
Moreover, the conical centering means warrant the radial centering of the edges of the sleeve and consequently of the entity of the latter, independently from its axial position. One will easily understand that the invention, by disassociating the axial centering means from those for the radial centering, allows to provide the best possible positioning of the sleeve with respect to the entity of the casting installation, whatever the expansion condition or the bulging of the sleeve might be.
On the other hand, as the sleeve is not secured rigidly to the core in its center zone, an excessive thickness and a machining operation, specific for this mounting, are not necessary. Thus the section of the sleeve is more even over its entire width and this diminishes the risks of irregular deformation of its surface during its expansion.
According to a special provision of the invention, the conical centering means comprise two flanges centered and sliding on the core with the slightest possible clearance, this sliding being provided thanks to measures such as a gliding resin or a bearing of the kind with an oil film. Each flange comprises a truncated part that has a tapered, frustoconical, or inclined surface which is co-operating with a bore of a generally truncated shape provided on the corresponding edge of the sleeve and elastic means for drawing closer together the flanges, the one towards the other. Each truncated shape portion of the bore provides a tapered, frustoconical, or inclined surface that is preferably at least somewhat complementary with the tapered, frustoconical, or inclined surface of the truncated portion of the corresponding flange.
Preferably, these means for drawing together the flanges comprise elastic coupling means for the said two flanges, independently from the core and from the sleeve. Thus the forces exerted by those flanges on the edges of the sleeve are equal and this warrants a perfect symmetry of the efforts exerted on the sleeve.
According to a particular arrangement of the invention, intended to ensure the rotary coupling of the sleeve upon the core, the roll comprises pressure means in order to exert an axial load on the abutment means, in such a way that the friction of the sleeve on the core, at the level of the abutment means, is sufficient to guarantee at least in part the rotary actuation of the sleeve by the core.
According to another arrangement, which can substitute itself to or come in addition to the utilisation of the said pressure means, the flanges are rotationally linked to the core, and, thanks to this rotary coupling and to this frictional coupling of the edges of the sleeve with the said flanges, the latter transmit, at least in part, the rotary torque from the core to the sleeve.
Preferably, at least one of the conical contacting surfaces, either of the sleeve or the flanges, shows, when cold, a relieved rounded, or chamfered surface at the level of the front ends of the sleeve. This relieved surface is carried out in such a way that, at its level, the said conical surfaces are spaced apart one with respect to the other, when cold, and they are brought one closer towards the other, when hot, at the time when the sleeve is exposed to the effect of the thermal bulging.
This arrangement allows to keep steady, or even to increase, the contact area of the said conical surfaces at the time of the deformation of the sleeve by the thermal bulging. Indeed, if the conical surfaces of the sleeve and of the flange have exactly the same taper and a perfectly straight generating line, when cold, then there appears, during the thermal bulging of the sleeve, a deformation which is essentially localised on the edges of the sleeve and tends to make the said edges pivot about themselves, with the effect consisting in a reduction of the taper of the conical surfaces of the sleeve. As a result thereof the conical contact between the sleeve and the flange tends to be restricted to a circular linear contact, just at the level of the front ends of the sleeve. This reduced contact is unfavourable to a good axial centering and a good rotary actuation of the sleeve.
The relieved surface foreseen by this invention allows to avoid this reduction of the contact zone by shifting this contact zone, at the time when the bulging deformation occurs, towards the ends of the edges of the sleeve, without however entailing a separation of the conical surfaces from their contact zone lying closer to the median plane of the roll, or it allows at least to keep such a separation at a minimum. Therefore, separation of contact between the flange and sleeve is preferably minimized, even when the sleeve is hot during casting operation.
This effect will be better understood, and other characteristics and advantages of the invention will become apparent from the specification which will be given herein below, as an example only, of a casting roll for a continuous casting installation for producing thin steel products between rolls.