The present invention relates to a method for heating primarily so-called nozzles of the kind through which a hot med i um i n l i qui d phase or I n a l iqui d phase l i ke cond i t i on , such as molten metal , e . g . molten steel , is intended to pass .

A hot medium of this kind which is intended to exit from a so-called nozzle or like device through an exit orifice provided therein and which medium shall be maintained at a pronounced elevated temperature may undergo radical local cooling as it passes through the nozzle, due to the lower temperature of the nozzle. This local cooling of the medium can give rise to serious disturbances in operation, such as blocking of the nozzle etc.

This problem can be solved by heating the medium to a given over-temperature. Over-heating of the medium, however, is often unsuitable, inter alia for practical reasons. Another solution is one which involves pre¬ heating the nozzle, which is effected by indirect heating methods when practicing known techniques. It is difficult to pre-heat the nozzles in this way, how¬ ever, while at the same time taking into consideration the technical requirements placed on the nozzle with regard to mechanical strength and material.

The present invention relates to a method and appara¬ tus with which the aforesaid problems are substantial¬ ly solved. According to preferred embodiments the nozzle can be heated when metal runs through a nozzle and when no metal is present in the nozzle outlet

passage.

Thus, the invention relates to a method for heating primarily a so-called nozzle through which a hot medium in liquid phase or in a liquid phase like condition, such as molten metal, e.g. molten steel, is intended to pass in con¬ junction with casting or molding operations.

The method is particularly characterized by pre-heating the nozzle with the aid of microwaves prior to bringing the medium into contact with the nozzle and/or at least periodically while medium is passing therethrough, said nozzle incorporating material which exhibits a signifi¬ cant loss factor with regard to microwaves, and said heating being effected preferably in order to prevent blockages or the like occurring in the nozzle as a re- suit of a temperature decrease in the medium passing through the nozzle.

The invention also relates to apparatus for heating primarily a so-called nozzle through which hot medium in liquid phase, or in a liquid phase like condition, such as molten metal, e.g. molten steel, is intended to pass in con¬ junction with a casting or molding operation.

The apparatus is particularly characterized in that devices are provided for pre-heating the nozzle with the aid of microwaves prior to bringing the medium into contact with the nozzle and/or for heating the nozzle at least periodically during passage of the medium therethrough, said nozzle incorporating material which exhibits a significant loss factor with regard to microwaves, and said heating being effected preferably in order to avoid blockages and the like from occurring in the nozzle as a result of a temperature decrease in the medium passing therethrough.

The invention will now be described in more detail with reference to an exemplifying embodiment thereof and with reference to the accompanying drawings, in which

Figure 1 is a schematic, central, vertical sectional view of a first embodiment of a central part of appara¬ tus according to the invention, used in conjunction with a casting operation;

Figure 2 illustrates schematically a second embodiment of a central part of an apparatus according to the invention, in conjunction with a casting operation;

Figure 3 illustrates schematically a third embodiment of an apparatus according to the invention, seen in side view;

Figure 4 illustrates the apparatus shown in Figure 3 from beneath in said Figure; and

Figure 5 is a central, vertical sectional view illus¬ trating schematically an embodiment of the apparatus by means of which a nozzle is intended to be heated from the inside of a container vessel.

The apparatus illustrated in Figure 1 includes a micro¬ wave applicator 1 intended for heating a nozzle 2, said nozzle comprising a dielectric material exhibiting a significant loss factor with regard to microwaves and thus being heated by microwaves supplied thereto. The applicator 1 forms a microwave cavity 3.

The reference 4 in Figure 1 identifies the opening of a container vessel 5, to which the nozzle 2 is connected, the nozzle being embodied in and therewith surrounded by a ceramic material 6, which preferably

exhibits an extremely small loss factor, for instance solely Al-O.,. It is preferred in many cases that parts of the cavity are formed by means of metallic parts 7, such as parts 7 of a metallic net, for example a net comprising platinum wire, embodied in, such as cast In, the ceramic material surrounding the nozzle 2. In the illustrated embodiment the cavity 3 presents a downwardly widening metallic part 8. Supplementary parts 9 of insulating brick or the like may be required in many instances, in order to obtain a suitable arrangement.

As beforementioned, a hot medium 10 in liquid phase or in a liquid phase like condition, such as molten metal 10, e.g. molten steel 10, is intended to pass through the nozzle 2, for instance in a casting or molding operation.

According to preferred embodiments of the invention the cavity 3 necessary for microwave heating processes is constructed so that the nozzle 2 can be heated both when metallic medium passes through the nozzle and also substantially in the absence of metallic medium in said nozzle, and so that in this regard mutually different oscillating modes can be applied, depending on the presence or absence of metallic medium.

The cavity 3 has a substantially circular cross-section, in a plane extending at right angles to the longitudi- nal axis of the nozzle, i.e. to the direction in which the through-passage 2 * of the nozzle extends therethrough when the nozzle is mounted in the cavity in the manner intended. The reference 11 identifies a waveguide ex¬ tending substantially radially into the cavity for supplying microwaves, said waveguide preferably having a rectangular cross-sectional shape. The cavity is pref¬ erably constructed to oscillate in a coaxial mode, a first mode, when metallic medium is present in the nozzle, and

in a second mode when substantially no metallic medium is present in the nozzle. In this regard, the cavity is preferably constructed so that said second mode is a cylindrical oscillating mode, preferably a mode, such as TM 010, whose resonance frequency is independent of the length of the cavity, while the diameter is determined by the resonance frequency. The cavity preferably has a length which corresponds essentially to half the wave¬ length of the intended microwaves. Thus, in some of the preferred cases the diameter and the length of the cavity are determined by the two oscillating modes per se. In some instances the mode TM 010 is written as TM _Q1Q in¬ stead.

In the embodiment illustrated in Figure 2 the nozzle 12 includes a free, outwardly projecting part 13 which ex¬ tends down from the opening 15 of a container vessel 14. An applicator 16 is arranged to form a cavity 17 having preferably a substantially cylindrical cross- section at right angles to the direction in which the nozzle part 13 extends, said nozzle part protruding into the cavity 17. The reference 18 identifies a waveguide corresponding to the waveguide 11 in Figure 1. With regard to oscillating modes, the applicator etc. illus¬ trated in Figure 2 is preferably constructed in the man- ner of the applicator illustrated in Figure 1, i.e. so that the nozzle 2 can be heated irrespective of whether molten metal 19 or the like flows therethrough or not.

In the embodiment illustrated in Figures 3 and 4 the ap¬ plicator 20 incorporated in the apparatus is not connect- ed directly to a container vessel or the like. A cavity 21 formed by means of the applicator accommodates a noz¬ zle 22 through which medium of the aforesaid kind is in¬ tended to be passed for, e.g., shaping purposes, such as

surface smoothing purposes, wherewith the medium may be in the form of an elongated object 23, which is either homogenous or hollow tubular or derplex having a central material part 23', as illustrated in Figures 1 and 2. According to preferred embodiments, the applicator 20, similar to the applicators described above with reference to Figures 1 and 2, is constructed to oscillate in modes which enable the nozzle 22 to be heated irrespective of whether metallic material is passing therethrough or not. The reference 24 identifies two end covers provided with apertures 25, while the reference 26 identifies a microwave feed waveguide that extends substantially ra¬ dially to the cavity.

Figure 5 illustrates an arrangement which is intended particularly for heating a nozzle 27 comprising a ceramic material which exhibits a significant loss factor and which is intended to be heated by microwaves. Auxiliary devices comprising a substantially cylindrical waveguide 28 for microwaves are arranged to be inserted into a container vessel 29, such as a ladle, and to be connected at one end part 30 thereof to the nozzle 27, this end part of the waveguide 19 contributing towards forming a cavity 31, in accordance with one embodiment of the in¬ vention. With regard to temperature influence, the wave- guide 28 may be cooled in some suitable manner, e.g. water cooled, and may be covered externally with an in¬ sulating material. The reference 32 identifies a micro¬ wave generator which is connected to the upper part 33 of the waveguide. The reference 34 identifies a metallic lid. The nozzle is conveniently embraced by a bush 35 made of a material which will not be heated to any ap¬ preciable extent by microwaves. Located beneath the noz¬ zle is a guard 36, preferably a metallic guard, which protects against leakage of microwaves. Embodiments are

also conceivable in which the waveguide, substantially having the construction shown in Figure 5 has an an¬ tenna affect and contributes towards transmitting micro¬ waves to the nozzle without cooperating in forming a cavity to this end.

The material incorporated in the nozzle, such as 2, 12, 27, to be heated in accordance with the invention is selected in accordance with the use for which the nozzle is intended. In the case of molten metal, such as molten steel, the nozzle is suitably made of a ceramic material. In this regard, the nozzle comprises a ceramic material that exhibits a significant loss factor with regard to microwaves, such as ZrO-, this ceramic material prefer¬ ably forming those parts of the nozzle that come into contact with the molten medium.

It will be understood, however, that in other cases the nozzle may comprise or consist of some other dielectric material that can be heated effectively with microwaves.

The method according to the invention and the manner in which the apparatus according to the invention operates will be understood in all essentials from the aforegoing. Thus, the nozzle is heated directly with the aid of microwaves prior to hot medium being passed through the nozzle and/or while hot medium is passing through said nozzle, thereby providing a flexible and particularly useful heating arrangement. At least in those instances when a change is made between operational heating modes commensurate with the situation in which molten medium runs through the nozzle and the situation in which sub- stantially no molten medium is present in said nozzle, the conditions under which microwave heating is effected, these conditions also requiring the commensurate forma-

tion of the microwave cavity necessary for heating pur¬ poses, are suitably adapted in a manner which enables heating to be effected in both of said heating situa¬ tions, there being employed a first oscillating mode when molten medium flows through the nozzle and a sec¬ ond, different oscillating mode when substatially no molten medium is present in the nozzle. This enables the microwave heating apparatus to be used in both of the aforesaid heating situations, which is an essential step forward with regard to known applicator designs, which afford effective heating in only one of the afore¬ said situations.

It will be understood from the aforegoing that the in¬ vention affords important advantages. In the aforegoing the invention has been described with reference to a number of exemplifying embodiments thereof. It will be understood, however, that other embodiments are conceiv¬ able, and that minor modifications can be made without departing from the concept of the invention.

For example, a plurality of nozzle materials or nozzle components are embraced within the framework of the overall function, i.e.. the ability of the material to be heated with the aid of microwaves. Zirconium dioxide (ZrO-,) is one example of a suitable ceramic material for ceramic nozzles or for ceramic nozzle components.

With regard to the composition of the ceramic materials that are to be heated with the aid of microwaves, mea¬ surements have shown that the majority of ceramic ma¬ terials with solely minor modifications to the composi- tion thereof can be used when heating in accordance with the invention. Suitable materials have been found to be ceramic materials based on ZrO_ or A1-0-. supplemented with some other oxidic material or materials, such as

MgO, SiO„, Fe-,0.,. Extraordinary possibilities are to be found for controlling heating through the selection of material exhibiting mutually different loss factors, and by mixing mutually different components, e.g. ceramic components. The material from which the nozzle is made or from which parts of the nozzle are made is selected in this regard.

As will be understood from the aforegoing, the invention can be applied in many different fields. The so-called nozzle may, for example, comprise a shaping tool, e.g. a matrix or die in the extrusion of metal profiles, such as tubular profiles or other hollow profiles. The nozzle may also constitute a tool for the direct casting of wire rod, essentially in accordance with Swedish Patent Speci- fication 8003487-9, in which case heating of the nozzle, or die, is preferably effected in the form of a nozzle pre-heat prior to passing molten medium therethrough, and as back-up heat during a casting operation, thereby controlling the temperature of the nozzle, for example, in order to prevent blockages from occurring therein.

The embodiment illustrated in Figure 1 also has a stabi¬ lizing wire 10' extending through the nozzle. As will be understood from the aforegoing, additional fields in which the invention can be applied include nozzle heating processes, both pre-heating and back-up heating, in more general casting operations, such as continuous casting operations.

Applications are also conceivable in which the aforesaid medium is, for instance, a plastics material. The in- vention is therefore not limited to metallic materials, such as steel.

When the space presented between the nozzle and the ex-

ternal casing in the case of embodiments according to Figure 3 is occupied by a ceramic material, this mate¬ rial will exhibit a very low loss factor with regard to microwaves.

Consequently, the invention is not restricted to the aforedescribed embodiments and modifications can be made within the scope of the following claims.