High speed continuous casting device and relative method

Details High speed continuous casting device and relative method High speed continuous casting device and relative method

2000-01-10

2001-11-13

Dunn, Tom (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S441000, C164S443000

Reexamination Certificate

active

06315030

ABSTRACT:

FIELD OF THE INVENTION
This invention concerns a high speed continuous casting device and the relative continuous casting method, as set forth in the respective main claims.
The invention is used in the field of steel production to cast billets or blooms of any type or section. In the following description we shall refer, for simplicity of exposition, to the application of the invention for producing billets, but the invention can be applied to other types of products too.
BACKGROUND OF THE INVENTION
In continuous casting the attainment of high casting speeds and hence an ever greater productivity, while both the surface quality and the inner quality of the product is maintained high, is correlated to the optimisation of a plurality of technological parameters concerning the characteristics of the crystalliser and the equipment connected thereto, and also the casting procedure.
These parameters concern the geometrical and dimensional characteristics of the crystalliser, the cooling system, the system to lubricate the inner walls and the processes to which the product being formed is subjected.
In conventional continuous casting plants, the problems connected with the high temperatures reached by the walls of the crystalliser condition the choice of the parameters decisively, considerably limiting the casting speeds which can be obtained because of the deformations of the crystalliser and the reduction of the mechanical properties of the copper at high temperatures.
To be more exact, the lack of uniformity in temperature along the walls of the crystalliser causes a non-uniform deformation of the walls, due to the thermal expansion of the material, and therefore problems concerned with the surface defects which this deformation causes on the product being formed.
Moreover, as it goes down the crystalliser, the skin of the product being formed tends to shrink as it solidifies.
This causes the skin to become detached from the wall of the crystalliser, which enormously reduces the heat exchange between the product and the crystalliser, to such an extent that the cooling, and thus the solidification of the skin, is practically blocked, which causes the skin to re-melt.
Cracks are created in the skin which, when the cast product is removed, may propagate and cause the product to break, and consequently the liquid metal inside the product can leak out (the break-out phenomenon).
In the case of a product with a square, rectangular or generally polygonal section, another problem is that the corners are subjected to a more intense cooling since they are subjected to simultaneous cooling on several sides of the crystalliser.
In correspondence with the corners, the skin forms more quickly and the consequent shrinkage of the material causes the product being formed to become detached from the wall of the crystalliser very quickly, thus interrupting the cooling and solidification process, and therefore making the temperature of the solidified part increase drastically.
For this reason, in correspondence with the corners, the skin of the product being formed has a lesser thickness than on its plane faces and differences of temperature are created between the edges and the plane faces of the product.
These differences generate tensions which lead to the formation of cracks and other surface defects, which lower the quality of the product and can also cause the skin to break and the liquid steel to break out.
In continuous casting plants used at present, it has been impossible to find a satisfactory solution to all these problems, and furthermore the attempt to solve some of them has led to an accentuation of others.
So, for example, the attempt to increase the casting speed has led to an unsatisfactory cooling of the product being formed, and therefore the solidification of an insufficient thickness of skin, with consequent problems in the removal and pre-rolling of the product emerging from the crystalliser.
On the other hand, any attempt to obtain an optimum cooling of the product has led to a reduction in the casting speed and therefore a reduction in productivity.
Moreover, adapting the conformation of the crystalliser to the shrinkage of the skin of the product being formed in every longitudinal zone of the crystalliser, with the purpose of guaranteeing maximum efficiency of heat exchange, has led to problems of friction between the walls of the crystalliser and the product being formed, and therefore the lowering of the surface quality of the product.
The present Applicant has devised, tested and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages, such as in particular a tangible increase in the casting speed.
SUMMARY OF THE INVENTION
The invention is set forth and characterised in the respective main claims, while the dependent claims describe other characteristics of the invention.
One of the purposes of the invention is to achieve a continuous casting device, and to perfect a relative method, which will allow to achieve high casting speeds, and hence high productivity of the plant, without compromising either the surface or inner quality of the product obtained.
Another purpose of the invention is to achieve a continuous casting device with a crystalliser subject to limited deformation, which will allow to limit the shrinkage and deformation of the skin of the billet being formed.
Another purpose of the invention is to achieve a continuous casting device wherein the crystalliser has an inner taper adapted to the casting speeds and to the type of steel which is to be cast, and wherein the position of the meniscus can be modified according to the casting parameters.
A further purpose of the invention is to achieve a continuous casting device suitable to be inserted in a casting line which can be directly associated with shearing, heating and rolling devices of a conventional type.
The continuous casting device according to the invention comprises a crystalliser characterised by dimensional and technological characteristics which allow to obtain a tangible increase in the casting speed, both with plate crystallisers and with crystallisers of the tubular type.
According to one characteristic of the invention, the crystalliser has a longitudinal development with a length of between 1050 and 1500 mm.
In the past, in the first continuous casting machines, crystallisers of 2500 mm in length were provided.
However, since such crystallisers did not have an inner taper such as to adapt to the progressive shrinkage of the product during solidification, this length was not exploited and the crystalliser, in its lower part, acted as a heat barrier between the solidified thickness of skin, which tended to become progressively detached from the wall, and the cooling system.
In a preferential embodiment, the crystalliser has an at least partly curved longitudinal development, so as to allow an efficacious in-line continuity with the removal and straightening means located downstream, at the same time allowing the bulk of the casting device to be contained in height.
The radius of curvature of the crystalliser is correlated to the radius of curvature of the guide equipment and the optional pre-rolling and straightening equipment located in the secondary cooling zone defined downstream therefrom.
In this way, already inside the crystalliser a progressive and gradual deformation of the billet is induced when it is solidifying, preventing the generation of critical tensions which create internal discontinuity (cracks) in the product being formed.
The crystalliser of the continuous casting device according to the invention is also associated with a high performance primary cooling system which achieves a high heat exchange thanks to the speed of circulation of the cooling fluid and the geometry and surface configuration of the channels wherein the cooling liquid circulates.
The cooling system also allows to maintain the walls of the crystalliser at relatively low average temperatures, considerably limiting the deformation thereof and therefore the

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