Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-04-02
2002-03-12
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S428000
Reexamination Certificate
active
06354365
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for the continuous casting of thin metal products and to an apparatus for carrying out the same, and, more in particular to an improved continuous casting method and to an apparatus suitable for carrying out such method, of the kind constituted of two counter-rotating rolls.
Apparatus with counter-rotating rolls for performing continuous casting of metal strips are already known in the state of the art. By means of these apparatus it is possible to jump from the conventional technology—for obtaining flat products with a thickness ranging from 150 mm to 250 mm, that generally are subjected to a further hot rolling and possibly a subsequent cold rolling, to the so-called “strip casting” technology. Such technology allows to obtain flat or strip products having a thickness lower than 10 mm, that subsequently can be hot/cold rolled.
Recently, the abovesaid technologies have been utilised for the production of steel strips. In particular, according to the more widespread “Strip Casting” technology i.e. the so-called “Twin Rolls,” technology, there is provided for the casting of molten metal, such as steel, in a molten bath defined by a pair of counter-rotating rolls, which are cooled at their interior, and which have their longitudinal axes parallel and arranged horizontally, spaced each-other by a distance substantially corresponding to that of the cast strip, and a pair of containment side walls or dams made of a refractory material, that are placed in contact with the end surfaces of said rolls.
The sliding contact between the side walls or dams and the rolls must guarantee the absence of leaks of molten metal between them. This entails that said side walls or dams and the end surfaces of the rolls are subjected to particularly severe operating conditions.
Appreciable thermal deformations that modify the initial geometry of the side walls or dams are caused by a pre-heating that is carried out at two moments: i.e. before their abutting against the end faces of the rolls and after their abutting during the start operations before the actual steady-state casting.
The need of such pre-heating derives from the need of reducing to a minimum the likelihood that on the side walls or dams a undesired solidification of steel might occur, that if caught between the rolls, would cause unacceptable defects on the strip and damages to the side walls themselves.
Moreover, also the rolls are subject to thermal deformation during the casting, both in their radial and axial directions, which modify the shape of their end faces which contact the side walls,
It derives therefrom that at room temperature the two surfaces, i.e. the ones of the side walls and the ones Of the end faces of the rolls, lie in a same plane, whereas such condition is lost during the initial transient and during the casting, This entails the build-up of gaps along the coupling areas of the two surfaces with consequent leaks of metal.
A leak or spilling of metal produces consequently detects along the edges of the strip products and unevenness in carrying out the process that can lead, in the most severe cases, to a stop of the plant.
The mating surfaces of the side walls and of the end faces of the rolls are moreover subject to wear owing to their relative motion and to the abutment pressure.
Such wear reduces the useful life of the components, and increases he costs; and it is greater with the increase of the contact forces applied for maintaining the mating of said surfaces.
To the end of overcoming such inconveniences, various contrivances have been developed. As an example in the Published European Patent Applications No 546 206 and 698 433, there is disclosed that the side walls are brought in abutment with the end faces of the rolls as a rigid body, and each side wall is supported on its back by a metal plate whereon pushing forces are applied, forces that then are distributed on the whole surface of the side wall. The side wall or dam pushed in this way against the end faces of the rolls, is subjected to a wear of the parts contacting the end faces of the rolls and therefore to a forward shift of the whole side wall up to when the whole sliding surface of the side wall or dam has reached the same contour of the opposing surface of the end face of the roll, trying in this way to guarantee a seal against penetration and leaks of molten steel.
These methods are however subject to the following inconveniences:
a)the pushing force is initially localised only on the contact areas between the side wall and the surface of the end faces of the rolls, with local values of the pressure that cannot be determined in advance and that can be very high, leading possibly the fracture of the refractory material as well as a relevant wear of the same;
b)a certain time is needed so that the surface of the side wall wears and consequently matches the surface of the end face of the roll (for instance in the European Patent Application No 546 206 above referred to, a time ranging from 0.5 to 1 minutes is mentioned before starting the casting operations).
Consequently the side walls are brought in abutment with the end faces of the rolls after the pre-heating step, with a consequent decrease of the temperature of the side wall or dam because the end surfaces of the rolls are usually cooled. It derives therefrom that such step must be as short as it is possible in order to avoid an excessive cooling of the side wall, because this would lead to solidification of the molten steel on the surface of the same;
c)during the initial steps of the casting, the contact of the molten steel with the rolls and with the side walls leads to a very fast deformation of these bodies and of the abutting surfaces. Such deformation impairs the matching between the surface of the end face of the rolls and the surface of the side walls. It is therefore necessary to wait again the wear of the surfaces of the side walls up to reach a new contour that allows the matching thereof;
d)the use of refractory side walls supported in their back by a metal plate renders very difficult the heating of the side walls from a surface opposite to the one that contacts the molten steel. It derives therefrom that these side walls must be heated from the interior of the space destined to contain the molten steel by means of a device that must be retracted before the starting of the casting (see European Patent Application No 698 433 A1). During the time span between the retraction of the heating device and the start of the casting of the steel, the side walls are subject to cooling. Moreover no mention is made of the possibility of heating the side wall (s) during the casting process. This increases risks of undesired solidification of steel on the side walls and a possible damage of the same.
Consequently, notwithstanding the improvements brought forth in this field, the problems of the confinement of the molten metal in the casting cavity, and of the undue wear of the side walls, and of the quality of the edges of the cast strip, and indeed the stopping of the continuous casting process, are largely still unresolved.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is therefore to reduce to a minimum the effects caused by the abovesaid problems by providing a method and apparatus for the continuous casting of thin material, improved according to the method disclosed hereinafter, in order to reduce the possibility of leaks of molten metal between the surface of the side walls or dams and the surfaces of the end face of the rolls, and at the same time to reduce the wear both of the surface of the side walls and the surface of the end faces of the rolls.
Another object of the present invention is to reduce greatly the amount of defects in correspondence with the edges of the cast product.
A still further object of the present invention is to reduce to a minimum the likelihood of stops in the continuous casting process, caused by leaks between the side walls and the end face of the rolls, as well
Capotosti Romeo
Tolve Pietro
Tonelli Riccardo
Acciai Speciali Terni S.p.A.
Elve M. Alexandra
Pacheco Liza
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