CONTINUOUS CASTING LINE

Abstract: Curved continuous casting line for products (15) such as billets, blooms, slabs or similar, comprising an ingot mold (11) and a guiding and containing assembly (13) downstream of the ingot mold (11), the guiding and containing assembly (13) having a configuration such as to prevent the products (15) from having internal inclusions for a desired and pre-determined distance ("s") with respect to the outer surface of the products (15) themselves.

Description: "CONTINUOUS CASTING LINE" * * * * * FIELD OF THE INVENTION This invention concerns a continuous casting line as set forth in the main claim. To be more exact, the invention concerns a continuous casting line of the curved type suitable to produce products such as billets, blooms or slabs characterised by a high standard of inner and surface quality. The continuous casting line according to the invention is suitable to obtain products for the production of special steels. The products obtained with the casting line according to the invention are characterised in that they do not have, at the inlet to the rolling train, any inner inclusions for a desired and pre-determined distance from the outer surface of the products, and thus respect the existing norms on special steels. BACKGROUND OF THE INVENTION The state of the art includes the continuous casting of melted steel through an ingot mold inside which the steel is subjected to cooling so as to form, at the outlet of the ingot mold, at least a perimeter thickness of solidified skin. In modern continuous casting lines, the search to obtain an ever higher productivity has led Companies to increase the speed of extraction more and more, displacing the point of complete solidification of the cast product farther and farther from the outlet of the ingot mold. In other words, at the outlet of the ingot mold it is sufficient for the cast product to have a solidified skin thick enough to prevent breakages and cracks in the skin during the entire solidification step of the product. The point at which the forming half-skins unite and the solidification of the cast product becomes substantially complete is known technically as the "kissing point". This procedure also allows to achieve an optional pre- rolling with a liquid core, or soft-reduction, of the cast product, wherein the reduction of thickness is begun already during the extraction and straightening step. A further evolution has been to replace conventional substantially vertical casting lines with curved or partly curved casting lines, in order to limit the height of the sheds and to facilitate casting operations and the handling of the ladles and tundishes . Such curved casting lines normally have a constant radius and connect to the rolling line which is arranged substantially on a horizontal plane. The bending radius of the casting line is calculated in order to prevent too strong tensions and stresses on the cast product as it is solidifying, thus ensuring a regular and homogenous growth of the solidified skin. Normally, the bending radius of casting lines is in the region of 5-15 metres. In the production of high quality steels, or for other particular applications, norms on the inner quality are very strict, particularly with regard to the presence of inclusions in the final product. These norms impose strict strategies on casting procedures to prevent inclusions being formed during the progressive formation of the solid skin at an unacceptable distance from the outer surface of the billet or slab. While these norms can be easily satisfied at low casting speeds, wherein the solidification is substantially completed within a short distance from the outlet of the ingot mold, or with vertical casting, wherein possible inclusions can freely ascend vertically towards the meniscus, it has been found that in present plants and with present casting parameters, it is very difficult to obtain the required standards. In fact, it has been found in practice that in present continuous casting machines a "shadow zone" is formed between the extrados line and the meniscus, inside which possible inclusions, instead of floating towards the meniscus, are trapped in the intrados line at a position where the thickness of the solid skin which has been formed is much less than the minimum value required. This unacceptable condition, in the present state of the art, can be resolved only by reducing the speed of extraction, to allow the formation of a sufficient thickness of skin at a position near the outlet of the ingot mold, but this causes an equally unacceptable reduction in productivity, which is incompatible with the requirements of modern plants . The present Applicant has devised and tested this invention to solve this serious problem which businessmen operating in this field particularly complain about, with a simple solution which is particularly efficacious and functional . SUMMARY OF THE INVENTION The invention is set forth and characterised in the main claim, while the dependent claims describe other characteristics of the main embodiment. The purpose of the invention is to provide a curved continuous casting line for billets, blooms or slabs in which any possible inclusions, present inside the cast product in the remaining liquid part and ascending towards the meniscus, cannot be trapped in the solidified skin, on the intrados side, in a position wherein the thickness of the skin is less than a desired and pre-set minimum value. In a preferential embodiment, this minimum value is between 10 and 50 mm. Another purpose of the invention is to obtain the afore- said principal purpose without slowing down the speed of extraction, and therefore without affecting the productivity of the plant, while keeping unchanged most of the structure of the conventional curved casting line. According to the invention, at least the first segment of the casting line located at the outlet of the ingot mold has a configuration defined by a first bending radius such that an inclusion ascending vertically towards the meniscus meets the intrados skin at a position wherein the thickness of skin formed is more than a pre-set value. The first bending radius of the first segment after outlet from the ingot mold is a function at least of the speed of extraction, the composition of the material and the outlet section from the ingot mold, that is to say, the parameters which most considerably influence the development of the solidified skin from the meniscus to the point where the liquid cone is closed. According to a variant, the first radius has an infinite value and the first segment of the casting line at outlet from the ingot mold is substantially vertical. According to a variant, the first segment of the casting line consists of a curvilinear segment, defined by several radii which are different from each other, which connects with a second curvilinear segment with a substantially constant radius. According to a further variant, the first segment of the casting line consists of several sectors which can be moved and appropriately oriented according to the desired bending radius to be obtained as the casting parameters vary, for example the type of steel cast or the speed of extraction. According to a further variant, these movable sectors are associated with an automatic movement and regulation system. BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment of the invention, given as a non-restrictive example wherein: Fig. 1 is a partial and diagrammatic view of a casting line according to the invention; Fig. 2 is a diagram of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The curved casting line 10 shown in the diagram in Fig. 1 substantially consists of an ingot mold 11 inside which the molten steel is cast, fed by a tundish or ladle 12. The ingot mold 11 is substantially vertical and cooperates with cooling assemblies, which are not shown in detail here, which achieve the progressive solidification of the steel until a cast product 15 is obtained, such as a billet, a bloom or a slab, which is extracted, straightened and arranged on a substantially horizontal plane to be sent directly to the rolling train or to be cooled and stored. The casting line 10 has a substantially conventional structure and we shall describe here only those parts which are necessary to understand the invention, and omit the rest, which can be considered as known. Downstream of the ingot mold 11 there is a guiding and containing assembly 13 , consisting in this case of a plurality of rollers 14 organised in sectors 114; their function is to guide the cast product 15 from the ingot mold 11 towards an extraction and straightening assembly 24 by means of which the cast product 15 is taken onto a substantially horizontal plane. In cooperation with the guiding and containing assembly 13 there are conventional assemblies of secondary cooling, which are not shown here, whose function is to complete the solidification of the cast product. The cast product 15 emerging from the ingot mold 11 has a perimeter layer 16 of solidified skin whose thickness increases as it gets farther from the outlet of the ingot mold 11. The perimeter layer 16 defines inside a cone-shaped liquid core 17, whose vertex 18 constitutes the point wherein the perimeter parts of solidified skin join together and the solidification of the cast product 15 is substantially completed. The vertex 18 of the liquid cone is technically known as the "kissing point" . The liquid core 17, in curved or partly curved casting lines, obviously follows the curvilinear development of the line itself, and is defined by an extrados line 19 and an intrados line 20 which progressively converge towards the said vertex 18. According to the preferential embodiment of the invention, the casting line 10 has a curved development defined by at least two segments, a first segment 10a arranged immediately downstream of the ingot mold 11 and a second segment 10b, consecutive to the first segment 10a, which connects and links up to the substantially horizontal plane. According to the invention, the first segment 10a, which can include one, two or more sectors 114 of the guiding and containing assembly 13, is defined by at least a first radius Rl, such that the vertical tangent 21 to the intrados line 20 of the liquid core 17 defines a point of tangency P wherein the thickness of the solidified skin 16 is at least equal to a pre-determined value "s". To be more exact, the value "s" is the minimum distance from the outer surface of the cast product 15 wherein there must not be any inclusions inside the cast product 15 itself, according to the norms on special steels, or according to the specific requirements for the final material . In other words, having defined at least the speed of extraction, the type of steel during the casting step and a defined outlet section from the ingot mold 11, the curvilinear configuration at least of the first segment 10a of the casting line 10 is such that an inclusion which rises vertically towards the steel which is still in the ingot mold, and therefore towards the meniscus, meets the solidified skin of the intrados 20 at a position wherein the thickness of the skin 16 formed is more than a desired value, functional to the final product to be obtained. In the case where special steels or high quality steels are to be obtained, the value "s" imposed by the norms, below which no inclusions must be present, is generally between 10 and 50 mm. According to the invention, when we have precise knowledge of the development of the forming skin 16, as determined principally by the type of cooling and the data on velocity, on material, on the section, and on the type of cooling, lubrication and other parameters, it is possible to determine the curvilinear configuration of the first segment 10a so as to obtain the desired result. In one embodiment of the innovation, the radius Rl is infinite, the first segment 10a assuming a substantially vertical rectilinear configuration. According to a variant, the first segment 10a is defined by a multi-radial segment wherein each of the sectors 114 of the guiding and containing assembly 13 is oriented according to a pre-determined arc and a different radius (Rl, Rl ' , Rl"...) from the previous sector 114 and the subsequent one, so as to obtain the result mentioned above. In the solution shown in Fig. 2, the first segment 10a has three different radii Rl , of which the first radius Rl , located at the outlet of the ingot mold 11, assumes an infinite value and the subsequent radii Rl ' and Rl" assume decreasing values with Rl" less than Rl ' . According to a further variant, the sectors 114 can be configured on each occasion, according to the specific parameters of the casting cycle, such as in particular the speed of extraction, the material cast, the dimensions, etc. According to a further variant, the sectors 114 at least of the first segment 10a are associated with an automatic regulation mechanism which determines its position according to the pre-set casting parameters . The second segment 10b of the casting line 10 is the segment which connects to the horizontal plane and has a substantially constant radius, with a conventional value, normally of between 5 and 15 metres. The second segment 10b can be connected directly to the rolling train or to a shearing assembly 22 cooperating with means 23 to discharge and store the starter bar; these means are of a known type and shall not therefore be described in any detail here. From this description it is clear how the invention allows to obtain significant advantages in operational and technological terms with minimum modifications to conventional casting lines. It is obvious however that modifications and additions may be made to this invention, but these shall remain within the field and scope thereof.

Claims: CLAIMS 1 - Curved continuous casting line for products (15) such as billets, blooms, slabs or similar, comprising an ingot mold (11) and a guiding and containing assembly (13) downstream of the ingot mold (11) , the line being characterised in that the guiding and containing assembly (13) has a configuration such as to prevent the products (15) from having internal inclusions for a desired and pre-determined distance ("s") with respect to the outer surface of the products (15) themselves . 2 - Curved continuous casting line for products (15) such as billets, blooms, slabs or similar, comprising an ingot mold (11) and a guiding and containing assembly (13) downstream of the ingot mold (11), wherein the guiding and containing assembly (13) defines an outlet path for the products (15) from the ingot mold (11) at least partly curved and defined by at least one radius, the cast product (15) including, at outlet from the ingot mold (11) , a cone-shaped liquid core (17) defined inside a perimeter of solidified skin (16) of a progressively increasing thickness, the liquid core (17) having a curved development and defining a vertex (18) wherein the cast product (15) is completely solidified, the liquid core (17) having an intrados line (20) and an extrados line (19) , the line being characterised in that the outlet path from the ingot mold (11) has a curved development such that the vertical tangent (21) to the intrados line (20) of the liquid core (17) of the cast product (15) defines a point of tangency wherein the thickness of the solidified skin (16) is at least equal to a value ("s"), the value ("s") being the minimum distance from the outer surface of the cast product (15) wherein there must be no inclusions inside the cast product (15) . 3 - Curved continuous casting line as in Claim 2, characterised in that the outlet path from the ingot mold (11) has a first segment (10a) defined by at least a radius (Rl) and a second segment (10b) with a radius (R2) to connect with a substantially horizontal plane. 4 - Curved continuous casting line as in Claim 3, characterised in that the radius (Rl) of the first segment (10a) is infinite. 5 - Curved continuous casting line as in Claim 3, characterised in that the first segment (10a) consists of a multi-radial curvilinear segment which progressively connects with the second segment (10b) . 6 - Curved continuous casting line as in any Claim hereinbefore, characterised in that at least the first segment (10a) consists of a plurality of sectors (114) each comprising a plurality of successive guiding and containing rollers (14) , each of the sectors (114) being able to be positioned so as to define a curvilinear development functional to the casting parameters, at least in terms of speed of extraction and type of cast product.

Inventors: LESTANI DARIO, RUZZA DOMENICO WOGLER, LESTANI, DARIO, RUZZA, DOMENICO, WOGLER
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 1999
Main classification: B22D11
IPC codes: B22D11