AU8237198A

AU8237198A - Continuous casting device with rollers and casting method with rollers

Info

Publication number: AU8237198A
Application number: AU82371/98A
Authority: AU
Inventors: Nuredin Kapaj; Anatoly Kolesnichenko; Milorad Pavlicevic; Alfredo Poloni
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 1997-09-10
Filing date: 1998-07-24
Publication date: 1999-03-29
Anticipated expiration: 2018-07-24
Also published as: EP1124659A1; ITGO970017A1; AU733520C; CA2303155A1; AU733520B2; WO1999012677A1; IT1304333B1

Description

WO 99/12677 - 1 - PCT/IB98/01128 "CONTINUOUS CASTING DEVICE WITH ROLLERS AND CASTING METHOD WITH ROLLERS" FIELD OF APPLICATION 5 This invention concerns a continuous casting device with rollers and a casting method with rollers as set forth in the respective main claims. To be more exact, the invention concerns a continuous casting device to produce strips of steel from molten metal 10 and a casting method which can be employed with this device, wherein the molten metal is fed from below and the slab or strip being formed is therefore unloaded from above. The invention is applied particularly, but not only, to produce steel strip with low and medium carbon content and 15 with thicknesses varying in the range of 2.5 to 10 mm. STATE OF THE ART The state of the art includes equipment for the continuous casting of molten metal so as to obtain products of variable shape and size. 20 It includes, for example, the use of ingot molds whose section defines the shape of the ingot or billet which is then sent for rolling to reduce the thickness. It also includes the use of equipment with large diameter, facing rollers to obtain strip to be sent to subsequent 25 processing; the rollers have at least a section of surface which is cooled so as to determine at least a partial solidification of the skin of the strip leaving the rollers. Until now continuous casting systems using rollers have been used only for stainless steels or at least with a high 30 carbon content since it is not possible to obtain steel strip with low or medium carbon content. This is because so far no solution has been found to the problem of the cracks which are formed on the skin as it forms in contact with the WO 99/12677 - 2 - PCT/IB98/01128 respective roller. In fact, in order to obtain from the rollers a strip with a solidified skin which has a certain level of consistency, such as to avoid breakages, cracks, splits, fissures, etc., 5 it requires an intense removal of heat in an extremely short space and time. This causes a very high heat shock at the moment when the liquid metal comes into contact with the cooled surface of the casting rollers; this heat shock causes micro 10 perforations, cracks and fissures on the surface of the skin, which make it impossible to achieve an acceptable quality of the product. Therefore, only stainless steels, or steels with a high carbon content, can be produced with this method, which 15 makes the system very limited and unproductive. To overcome this problem, making it possible to extend the use of continuous casting devices including rollers to steels with a low or medium carbon content, various systems have been proposed in which the surface of the rollers is 20 heated before they come into contact with the cast steel. There are examples of such systems in JP-A-05245594, JP-A 01284461, JP-A-04009251, JP-A-63093450 and JP-A-60137553. However, in all these systems the molten metal is cast from the top and extracted from the bottom, which in some 25 cases can cause operating problems, such as the bulk involved, the movement of the equipment, the management of the slab or strip extracted, or other problems. The present Applicant has designed, tested and embodied this invention to overcome this shortcoming of the state of 30 the art. DISCLOSURE OF THE INVENTION The invention is set forth and characterised in the respective main claims, while the dependent claims describe WO 99/12677 - 3 - PCT/IB98/01128 variants of the idea of the main embodiment. The purpose of the invention is to provide a simple, functional and extremely efficient solution which will make possible the continuous casting, using rollers, of any type 5 of steel, and particularly steels with low carbon content, yet ensuring high standards of quality both of the surface and internal. Another purpose of the invention is to achieve a method to obtain the strip which emphasises the main purpose. 10 According to the invention, the rollers are arranged above a container of liquid metal and the strip is extracted upwards. In this case, each roller of the casting device is immersed inside the container of liquid metal, defined on 15 two sides by the rollers themselves, and the ribbons of liquid metal gathered by each roller are coupled and joined to each other to form the strip, substantially in the position where the two rollers are nearest. According to the invention, the surface of the rollers, 20 for at least part of the outer arc of contact with the liquid metal, is kept heated by the appropriate means outside the roller itself. According to a variant, internal heating means are used. According to a further variant, both internal and external 25 heating means are used. According to the invention, before the rollers come into contact with the molten metal the surface temperature of the rollers is taken to at least 700 0 C, advantageously 750oC or more. 30 In this way, when it comes into contact with the surface of the rollers, the liquid metal at its melting temperature does not suffer any heat shock which might determine the formation of cracks or micro-fissures in the surface, since WO 99/12677 - 4 - PCT/IB98/01128 the heat exchange which creates the first solidified skin of the strip is reduced. The surface of the rollers is cooled, however, for the whole, or at least for a substantial part, of the arc placed 5 in contact with the liquid metal; this is to encourage the formation of a first, at least partly solidified skin. According to a variant, this cooling is intense along the whole surface of the roller which cooperates with the bath of molten metal. 10 According to another variant the cooling is progressive, and reaches its maximum intensity at a desired distance from the point of contact between the roller and the liquid metal. The strip is therefore obtained from the union of the two 15 ribbons of liquid metal which are formed on the cooled surface of the rollers and can be extracted through the space defined by the rollers themselves. According to a variant, the system to heat the surface of the rollers is obtained with a plurality or induction 20 resistors, which have a high heating efficiency, for example arranged outside in an arc in cooperation with the arc of the outer roller which is in contact with the liquid metal. According to a further variant, the heating system is obtained with a plurality of resistors arranged on the 25 circumference inside the roller; they can be activated in a controlled manner according to the circumferential position of the roller with respect to the liquid metal and are separated from each other by respective cooling water channels. 30 According to another variant, the roller is of the type where the cooling liquid circulates inside. According to the invention, the liquid metal is supplied to the roller device from below and the cast product, in the WO 99/12677 - 5 - PCT/IB98/01128 form of a strip with the skin at least partly solidified, comes out in an upwards direction before being extracted and possibly returned to the horizontal by the appropriate devices. 5 ILLUSTRATION OF THE DRAWING The attached Figure is given as a non-restrictive example and shows a preferred embodiment of the casting device and method according to the invention. DESCRIPTION OF THE DRAWINGS 10 The continuous casting device 10 with counter-rotating rollers lla, llb is shown in the case of casting from below in an upwards direction where the lower part of the rollers lla and llb is immersed in a container or tundish 12 to contain the liquid metal 13. 15 As they rotate, the two rollers lla and llb are immersed inside the tundish 12 and, due to the cooling of their surface in the respective ideal sector A and A' of contact with the liquid metal 13, cause the formation of two ribbons 14a and 14b, which have an at least partly solidified skin. 20 The ribbons 14a and 14b join together at the kissing point 15 to form a strip 16 which, after a possible pressing at the point of maximum proximity of the rollers lla and llb, emerges from the rollers and is extracted and possibly returned to the horizontal by an extraction and 25 straightening assembly 17. According to the invention, a substantial part of the arc of the rollers lla and llb outside the relative cooled sector A, A', is subjected to heat. In this case, there is a first heating system 18 30 comprising a plurality of induction resistors 19 arranged in an arc in cooperation with the periphery of the relative roller lla, llb and a second heating system 20 comprising resistors 21 arranged inside the relative roller lla, llb WO 99/12677 - 6 - PCT/IB98/01128 which can be activated selectively. In practice, the resistors 21 are activated as they gradually leave the exit limit M of the relative sector A, A' and are disactivated before they reach the entrance limit 5 N to the sector A, A'. The resistors 21 are separated by cooling channels where the distribution of the water is activated in the sector A and A' and disactivated in the remaining sector of the roller. 10 According to a variant which is not shown here, in place of the induction resistors 19 there are burners with a controlled flame. According to another variant which is not shown here, the surface of the rollers is heated by means of radiancy 15 systems using infra-red rays. The heating systems 18 and 20 serve to bring the surface of the rollers lla and 11b, while they are in rotation, to a temperature of at least 700 0 C, advantageously 750 0 C and more, at the moment when they come into contact with the 20 liquid metal 13, in such a way as to reduce the heat exchange between the surface of the roller and the first forming skin, and therefore the relative heat shock. As the rollers lla, llb rotate in contact with the liquid metal 13, the cooling system is activated and therefore the 25 surface temperature is returned to a much lower value, in such a way as to allow the at least partial solidification of the skin of the strip 16 in correspondence with the outlet of the rollers lla, llb. According to the invention, the device 10 makes it 30 possible to obtain strip 16 from low-alloy steels with a low or medium carbon content, at the same time obtaining a high standard of quality.

Claims

1 - Continuous casting device with rollers for the production of steel strip, particularly but not only for steels with low to medium carbon content, comprising a pair of counter-rotating rollers (11a, lib) with an at least partly cooled surface, the strip (16) being formed by the coupling and joining of the skin (14a, 14b) of metal forming on the cooled surface of the rollers (11a, lib) and being extracted by an extraction and possibly straightening assembly (17), wherein the arc of the rollers outside the sector of contact with the liquid metal (13) is at least partly associated with a heating system (18, 20), the device being characterised in that it comprises rollers (11a, lib) partly immersed in a recipient (12) containing liquid metal arranged below the rollers (11a, lib) and an extraction and possibly straightening assembly (17) arranged above the rollers (11a, lib) and suitable to extract the strip (16) from above.

2 - Device as in Claim 1, which comprises a heating system (18) outside the rollers (11a, lib).

3 - Device as in Claim 2, in which the heating system (18) outside the rollers (11a, lib) comprises a plurality of induction resistors (19) arranged in an arc in cooperation with the periphery of the relative roller (11a, lib) . 4 - Device as in Claim 2, in which the heating system (18) outside the rollers (11a, lib) comprises a plurality of burners with a controlled flame.

5 - Device as in any claim hereinbefore, which comprises a heating system (20) inside the rollers (11a, lib) comprising a plurality of resistors (21) arranged on the circumference, separated by cooling channels and able to be activated selectively.

6 - Device as in any claim hereinbefore, in which each roller (11a, lib) comprises an ideal sector (A, A') of contact with the liquid metal (13) associated with cooling means .

7 - Device as in any claim hereinbefore, in which the heating systems (18, 20) are functionally governed to take the surface temperature of the relative roller (11a, lib) at the entrance (N) of the cooled sector (A, A' ) to at least 700°C.

8 - Device as in any claim hereinbefore, in which the cooling means are functionally governed to take the surface temperature of the relative roller (11a, lib) to a much less than 700°C.

9 - Continuous casting method for steel strip (16) adopting a device as in any claim hereinbefore, the method being characterised in that the bath of liquid metal (13) is located below the rollers (11a, lib) which are partly immersed and the strip (16) is extracted from above.