Lining of molten metal handling vessels

Abstract

 The invention relates to an improved expendable lining for a molten metal handling vessel and a method of lining the vessel.
An expendable lining is sprayed inside the vessel and dried and an impregnant coating of greater refractoriness than the expendable lining is applied to the dried expendable lining whereby its permeability is reduced.

Description

[0001] This invention relates to the lining of molten metal handling vessels, e.g. a ladle or a tundish. For convenience the invention will be described with specific reference to tundishes.

[0002] In the continuous casting of metals, e.g. steel, molten metal is cast into a continuous casting mould via an intermediate vessel which acts as a constant head reservoir, this intermediate vessel being known as a tundish. The tundish has a metal floor and walls and one or more outlet nozzles set in the floor. To protect the metal floor and walls of the tundish from the effects of the molten metal it is usual to line the interior of the tundish with a relatively permanent lining, often made of bricks, or in the form of a cast monolithic lining. The tundish may additionally be provided with an inner expendable lining of refractory heat-insulating material.

[0003] Expendable inner linings have to meet various distinct requirements and certain of these conflict to a greater or lesser extent. In particular, while it is desirable for the expendable lining to be highly heat-insulating, it is also desirable for it to have substantial resistance to erosion by molten metal and slag. However, any change in the nature of the lining that improves its heat-insulating properties is usually associated with a reduction in its density, which tends also to result in reduced erosion-resistance.

[0004] Thus, a number of prior proposals have been made for the provision of an expendable lining in a tundish.

[0005] GB 1364665 describes the provision of an expendable lining in the form of refractory, heat-insulating slabs.

[0006] GB 2080505 describes an inner expendable lining of refractory, heat-insulating slabs having an inward-facing, erosion-resistant layer and a more heat-insulating backing layer.

[0007] EP 0180388 describes the use of a parting layer of heat-carbonisable sheet material between the permanent and expendable lining.

[0008] It has also been proposed to apply the expendable lining directly on to the permanent lining by trowelling, gunning or spraying rather than fitting pre-formed slabs over the permanent lining.

[0009] EP 0214882 describes a lining constituted by at least two layers, namely an inner layer which sinters throughout its mass under the action of heat from the molten metal and a further layer that either does not sinter at all or only partially sinters.

[0010] Difficulties have been encountered in the drying of sprayed linings in that it is commercially desirable that the drying period be as short as reasonably possible and drying times of more than two to two and a half hours are not really acceptable. Indeed, drying times of less than two hours are preferably required. However, it has been found that such relatively short drying times frequently result in problems in that the steam escaping through the drying lining can cause blistering and delamination of the interior surface of the lining, i.e. the surface to contact the molten metal. This is exacerbated by a tendency for fines in the spray composition to build up adjacent the inner surface and, hence, block the steam escape passages. Thus, there is a conflict of requirements in that shorter drying times are commercially needed but longer drying times are technically needed.

[0011] The present invention aims to provide an improved expendable lining formed by spraying and which ameliorates the aforementioned conflict.

[0012] Accordingly, in one aspect the invention provides a method of forming an expendable lining in a molten metal handling vessel having an outer metal casing lined with a relatively permanent lining, in which an expendable lining is sprayed over the permanent lining of the vessel, the lining is dried and an impregnant coating of greater refractoriness than the expendable lining is applied to the substantially dried expendable lining whereby the permeability of the lining is reduced.

[0013] In another aspect the invention provides a molten metal handling vessel having an outer metal casing lined with a relatively permanent lining and an inner expendable lining, the expendable lining having an impregnant coating of greater refractoriness applied to its exposed surface.

[0014] The permeability of the dried expendable lining before the impregnant coating is applied may be, for example, up to 20 AFS. This will be reduced by the impregnant coating layer so that the coated lining has a permeability of, for example, up to 4 AFS, preferably up to 3 AFS and may be as low as 2 AFS or lower. (The AFS permeability units are measured accordingly to American Foundry Society Test Procedure 226-87-S).

[0015] It will be appreciated that the amount of impregnant coating layer applied will be determined not only by the extra refractoriness desired but by the required reduction in permeability. It will be governed, therefore, by the permeability of the initial expendable layer but can be determined by the average skilled man according to the particular circumstances.

[0016] The permeability of the dried expendable lining is such that steam can escape during its drying cycle relatively quickly without causing harmful surface imperfections such as blistering and delamination. However, we have found that linings having such permeability are more readily susceptible to attack by slag on the surface of the molten metal. The slags in a tundish, in particular, can be very aggressive chemically and relatively non-viscous so that they can attack the linings of the above-defined permeability very rapidly. A conventional expendable lining is, therefore, expended more quickly than is acceptable, indeed, may be worn away to the extent that the permanent lining is exposed and attacked, the latter then necessitating time-consuming and hence, costly repair.

[0017] However, by protecting the expendable lining with the impregnated coating of higher refractoriness, the present invention enables thinner linings to be used than would otherwise be necessary. Thus, whereas the sprayed expendable lining when dried may be in principle for example, from 15 mm to 100 mm thick, linings without the impregnant layer would probably have to be at least 50 mm and possibly as much as 100 mm thick or more, whereas, if desired, the lining thickness of the present invention could be reduced to less than 30 mm, e.g. 15 to 30 mm thickness while still retaining satisfactory resistance to erosion so that the erosion rate is not uneconomic.

[0018] When lining thickness is reduced, any loss in insulation properties can be compensated for by reducing the density of the expendable lining material. For example, a conventional expendable lining 30 mm in thickness and of density 1.65 g/cc may have a thermal conductivity of about 0.5 watts/metre/°C. A lining of the invention of 15 mm thickness would required to be formulated to have a density of about 1.3 g/cc resulting in a thermal conductivity of about 0.45 watts/metre/°C, which is sufficiently insulating to be an adequate replacement in that respect.

[0019] The invention enables the bulk of the overall lining material used to be relatively inexpensive material. For example, the principal ingredient of the sprayable lining composition can be magnesite, particularly a magnesite/olivine mixture. Suitable mixtures are disclosed in our GB 1517749 and may contain from 1:10 to 4:1 by weight magnesia:olivine. A typical chemical composition range for the expendable dried lining may be as follows:

[0020] 18 - 22% by weight of water may be added to the solid ingredients based on the total sprayable composition to form the desired sprayable composition and typical Loss on Ignition of the solid contents on drying the lining may be from 1.5 to 4% by weight.

[0021] The bulk density of the dry powder mixture used to form the sprayed lining composition may be, for example, from 1175 to 1375 kg/m³ and the dried layer, before application of the surface impregnant coating composition may have a thermal conductivity, for example, of from 0.3 to 0.6 watts/metre/°C.

[0022] It is not necessary that the impregnant coating composition be applied over the entire internal surface of the expendable lining and it may advantageously be applied only over that portion of the lining surface that is likely to come into contact with slag. Thus, in a preferred embodiment the invention provides a molten metal handling vessel having an outer metal casing lined with a relatively permanent lining and an inner expendable lining, the expendable lining being a sprayed, dried lining having a permeability of up to 20 AFS and having an impregnant coating of greater refractoriness applied over those areas of the expendable lining that would otherwise come into contact with slag.

[0023] The impregnant coating composition may be applied by any convenient means but it is preferably applied by spraying.

[0024] The composition of the impregnant coating may be any suitable composition that gives the desired degree of refractoriness and resistance to slag attack. Suitable ingredients include zirconium silicate, magnesite in relatively pure form and silica. The composition should include a small amount of binder and it is preferred that this be a ceramic binder, as opposed to say, a clay binder.

[0025] A typical chemical composition for the impregnant could, for example, include the following:
   zirconium silicate
   inorganic binder
   aluminium oxide
   PH regulator
   water

[0026] Because such a composition has a completely ceramic gel system, it has a high solids content resulting in very rapid drying characteristics. As there are no organic compounds present the product has very low gas levels making it especially suitable for use with metals prone to gas defects.

[0027] The chemical composition can, however, vary widely and could be, for example, almost entirely of zirconium silicate or pure magnesite, with a minor proportion of binder.

[0028] The impregnant coating composition is preferably a thixotropic liquid so that it can readily be changed by stirring from a relatively viscous liquid to a thinner more mobile liquid suitable for application to the expendable lining. It may have a specific gravity of, for example, from 26 to 28.

[0029] In addition to providing a thin coating layer over the surface of the expendable lining, the impregnant coatings may penetrate into a substantial proportion of the thickness of the expendable lining, e.g. penetration of up to 25% or up to 10% or even less of the thickness of the lining may be found adequate. Moreover, the concentration of the impregnant composition in the lining will normally decrease throughout impregnated thickness, being at its greatest adjacent the surface of the lining. The thin coating layers may be up to 2 mm thick, e.g. up to 1 mm thick, i.e. in addition to the impregnated penetration, although it will be appreciated that thicker coatings could be applied, if desired.

[0030] In order to ensure that the impregnant coating composition is substantially uniformly applied over the lining surface to be coated, it may be found advantageous to colour the impregnant composition by incorporation of a suitable dye. This technique may be especially useful where it is not desired to impregnate the whole of the surface of the expendable lining but only a portion of that surface, e.g. the area containing and surrounding the 'slag line' in normal operation of the vessel.

Claims

1. A method of forming an expendable lining in a molten metal handling vessel having an outer casing lined with a relatively permanent lining, in which an expendable lining is sprayed over the relatively permanent lining of the vessel and the lining is dried, characterised in that an impregnant coating of greater refractoriness than the expendable lining is applied to the substantially dried expendable lining whereby the permeability of the lining is reduced.

2. A method according to Claim 1, characterised in that the dried expendable lining before the impregnant coating is applied has a permeability of up to 20 AFS.

3. A method according to Claim 1 or 2, characterised in that the coated lining has a permeability of up to 4 AFS.

4. A method according to Claim 1, 2 or 3, characterised in that the coated expendable lining has a thickness of not more than 30mm.

5. A method according to any preceding claim, characterised in that the expendable lining is formulated to have a density of about 1.3g/cc and a thermal conductivity of about 0.5 watts/metre/°C.

6. A method according to any preceding claim, characterised in that the sprayable composition to form the expendable lining is made by adding water to a dry powder mixture of bulk density from 1175 to 1375 kg/m³.

7. A method according to any preceding claim, characterised in that the impregnant coating composition is applied over only that portion of the lining surface that is likely to come into contact with or adjacent to slag during use of the vessel.

8. A method according to any preceding claim, characterised in that the impregnant composition is applied by spraying.

9. A molten metal handling vessel having an outer metal casing lined with a relatively permanent lining and an inner expendable lining to contact the molten metal, characterised in that the expendable lining has an impregnant coating of greater refractoriness applied to its surface which is to contact the molten metal.

10. A molten metal handling vessel according to Claim 9, characterised in that the coated expendable lining has a permeability of up to 4 AFS.

11. A molten metal handling vessel according to Claim 10, characterised in that the coated expendable lining has a permeability of up to 2 AFS.

12. A molten metal handling vessel according to Claim 9, 10 or 11, characterised in that the coated expendable lining has a thickness of not more than 30 mm.

13. A molten metal handling vessel according to any one of Claims 9 to 12, characterised in that the coated expendable lining has a density of about 1.3g/cc and a thermal conductivity of about 0.5 watts/metre/°C.

14. A molten metal handling vessel according to any one of Claims 9 to 13, characterised in that the expendable lining to which the impregnant coating is applied is of a composition comprising magnesium oxide as the principal ingredient.

15. A molten metal handling vessel according to Claim 14, characterised in that the composition contains from 58 to 85% by weight of magnesium oxide.

16. A molten metal handling vessel according to any one of Claims 9 to 15, characterised in that the composition of the impregnant coating comprises substantially zirconium silicate or pure magnesite with a minor proportion of an inorganic binder.

17. A molten metal handling vessel according to any one of Claims 9 to 16, characterised in that the impregnant coating penetrates up to 25% of the thickness of the expendable lining.

18. A molten metal handling vessel according to any one of Claims 9 to 17, characterised in that the impregnant coating is coloured by means of a dye.