GB2444805A

GB2444805A - Nozzle

Info

Publication number: GB2444805A
Application number: GB0716905A
Authority: GB
Inventors: Shibiao Yang
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-08-30
Filing date: 2007-08-30
Publication date: 2008-06-18
Also published as: GB0617065D0; GB0716905D0

Abstract

A nozzle 500 for delivering molten metal 519,528 into a mould 526 comprises a main body defining a bore 517, wherein said bore increases in diameter from an upstream portion to a downstream portion. The increasing diameter bore improves the flow of a stream of molten metal and reduces precipitation within the bore thus reducing clogging of the nozzle. In a preferred embodiment, the shape of the middle and lower portions 512, 514 may be substantially cylindrical (fig 6a), pyramidal or elliptical (fig 6b) with the inner surface or the bore being smooth or alternatively comprising a series of steps which increase the diameter of the bore (fig 9). The bore may also comprise a series of vertical channels or grooves (fig 4).

Description

NOZZLE

FIELD OF THE INVENTION

The present invention relates to a pour nozzle suitable for use in the transfer of molten metal, for example, molten steel, in a continuous casting operation.

In particular, the present invention relates to a submerged entry nozzle or submerged-entry shroud for shrouding molten metal flowing under gravity from one vessel to another.

HACKGROUND OF THE INVENTION

In the continuous casting of metal, special refractories are used to control the flow of molten metal and protect the molten metal from oxidation as it is poured into continuous casting moulds.

In a continuous casting steelmakirig process, molten steel from a ladle is poured into a large vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows into one or more respective moulds. A refractory pour nozzle, which has the general form of an elongate conduit is located between the tundish and each mould, and is used to direct the molten steel.

The refractory pour nozzles to which this invention pertains are sometimes referred to in the art as refractory tubes, shrouds, and the like. Therefore, the use of the term "nozzle" as used in the present invention pertains to nozzles, tubes, shrouds and the like which are used in continuous casting procedures. Thus, the term "nozzle" as used herein is inclusive of such conventional refractory tubes, shrouds, and the like which are conventionally used in continuous casting of molten steel. For example, we refer to uS 4,568,007; US 4,682,718; US 5,046,647; US 5,244,130; US 5,785,880; us 5,944,261, us 5,961,874, us 5,992,711, Us 6,027,051, us 6,932,250; EP 0 737 535 Al; EP 0 950 453 Al; EP 1 036 613 Al; EP 1 036 614 Al; WO 94/19131; WO 97/48512; WO 02/043904; WO 2004/082871; WO 2005/049249; all of which are incorporated herein by reference.

Prior art refractories are usually manufactured from carbon-bonded ceramics. Such refractories are usually composed of refractOry grain such as aluminium oxide, zirconium oxide, magnesium oxide, silica, silicon carbide, silicon nitride, SiAION, clays, or other dense grain of specific mesh size. Prior art refractories may also be formed using carbon from flake graphite, amorphous graphite, carbon black, coke and the like.

Moreover, a carbonaceous binder derived from pitch or any suitable resin may be used.

It has been found that during continuous casting operations such as in the transfer of molten metal, precipitates or non-metallic build- ups may clog or restrict a bore so as to obstruct steady discharge of molten metal from a nozzle. In molten steel, precipitates and non-metallic build-ups consist primarily of alumina and other high melting point impurities.

Nozzle clogging is a serious problem in continuous casting of various types of steel such as high concentration oxygen-containing steel, high concentration Mn-containing steel, Ca-treated steel, stainless steel, and, in particular, aluminium killed steels. It has been found that nozzle clogging can effect stream dynamics, reduce pouring rate and cause large agglomerated particles to be released into a liquid steel stream. It has been found that if these particles are trapped by a solidified shell, large inclusions may be formed in the subsurface of formed slabs or billets. It has also been found that nozzle clogging affects the ability of a slide-gate valve or stopper rod to handle the flow of metal. If clogging becomes severe, a flow control device will no longer be able to compensate, and then a decrease in casting speed will result unless the nozzle is replaced Nozzle clogging can therefore give rise to both quality and productivity problems. Unfortunately, clogging is hard to predict because it is a complex function of the chemical composition of the steel and the temperature.

In the last twenty years, extensive research has been carried out to investigate the mechanism of nozzle clogging, and also great efforts have been taken to find methods for forecasting or inspecting the clogging. But until now, no appropriate technology or method has been found to solve this problem.

Prior art solutions have also injected gas to

pressurise a pour tube and reduce alumina clogging. In conventional casting methods, nitrogen gas, argon gas or a mixture of the two gases is injected into a nozzle during casting to scrub the build-up of accretions of alumina oxide on the inside of passages and to prevent non-metallic inclusions from adhering to an inside of a nozzle. For example, we refer to GB 2,111,880 A which relates to a gas permeable immersion pouring nozzle.

However, it has been found that unfortunately, gas injection requires large volumes of gas, complicated refractory designs, and is therefore not always an effective solution. Gas may also dissolve or become entrapped within formed metal causing problems in metal quality including pinhole or porosity defects in formed steel.

A further alternative solution has been to provide a pouring nozzle with a lower melting point liner composition that prevents alumina build-up. Liner materials developed to date include the use of an MgO sleeve as disclosed in GB 2,135,918 A, use of carbon-bonded SiAION-graphite liner as described in EP 0 309 225, and use of CaO-MgO-A1203 liner as disclosed in GB 2 131. Conventionally, one of the most widely used anti-clogging liners has been a calcium zirconate graphite material. US 5,902,511 provides a typical composition of: calcium zironate from 20 to 75 weight %; graphite from 5 to 30 weight %; and calcium suicides from 0.5 to 15 weight %.

A further solution to prevent anti-clogging is to use anti-clogging liner materials such as a dense carbon free layer. According to Benson theory and as described in US 5,370,370 and US 5,681,499, a carbon bonded refractory body may be used in a casting process. In these processes, carbon monoxide (CO) gas is generated in conventional carbon containing refractories at steelmaking temperatures such that the CO reacts with aluminium dissolved in the molten steel to form alumina at the refractory surface. It is also found that a layer is formed along a selected molten metal contacting surface by first firing a pressed body in an oxidising atmosphere wherein the carbon is oxidised (i.e. decarburized), to form a porous oxidised zone on a selected steel compacting surface. A carbon free refractory slit or slurry is then infiltrated into a porous oxidised zone to create an erosion and alumina build-up resistant surface layer therein.

Previous approaches which use anti-clogging materials with nozzles, although at first hand appear to be reasonable, all these previous solutions have not been able to prevent clogging below a slag line area. When a nozzle comes in contact with a floating moulding powder such as commonly referred to as the slag line or powder line, significant clogging may occur. It is found that the clogging gets more serious near a pouring outlet. The clogging above the slag line is relatively low up to an upper portion of a nozzle, where the clogging increases.

A further solution to prevent clogging has been to use a pouring nozzle of modified geometry. For example, in GB 2 110 971 a single step submerged nozzle is disclosed. In GB 2 110 971, it was thought that when molten steel is flowing down a tube of an immersion nozzle for continuous casting, molten steel which is suddenly cooled reduces the solubility of oxygen in the steel and thus the oxygen that is present in a state of equilibrium at high temperatures is released, and this released oxygen reacts with the aluminium or calcium in the steel and creates calcium alurninate crystals or alumina. These fine crystals congeal together and build-up on the inner wall of a nozzle, finally leading to blockage of the nozzle. In GB 2 110 971 it was also found that the blockage forms especially easily from the slag line or powder line and that a closure in this area is the cause of operational problems. Thus, in GB 2 110 971 an immersion nozzle was provided having an upper portion and a lower nozzle portion wherein the inner diameter of the lower portion is greater than the top with an angle step at the border between the two portions to prevent blockage of the flow passage. The upper portion of the nozzle is comprised of alumina-graphite and the lower portion is zirconia-graphite.

Other previously used designs include pour tubes with both conical and stepped' bores such as described in US 4,566,614 which relates to an inert gas-injection nozzle having a conical bore intended to reduce pulsations in gas flow. Smoother gas flow into the bore is said to reduce clogging.

Stepped designs include pour tubes that have discontinuous changes in bore diameter. For example, in US 5,328,064 a multi-stepped submerged nozzle is provided having a main pipe with an inside diameter d' characterised in that a plurality of steps having inside diameters d1 -d are provided in the molten steel pouring hole. In the inside diameters of the molten steel pouring hole, the dimension of the inside diameters of said steps is greater from the inside diameters d' of the main pipe and preferably decreases along the direction of flow. The

B

steps are described to prevent the melt flow from staying and generating a turbulence whereby A1203 is prevented from depositing.

Stepped designs may also include pour tubes having a spiral bore. For example, in US 6,425,505 a pour tube with a bore comprising a plurality of fluidly connected sections that improve the flow of molten metal through the bore is described. The sections reduce asymmetric flow of a molten metal stream and the likelihood of precipitates clogging the bore. Each section comprises a converging portion and a diverging portion. The converging portion deflects the stream towards the centre of the bore, while the diverging portion diffuses the stream. The combination of converging and diverging portions produces a more symmetrical flow in the pour tube.

The process of continuous casting is subject to several different types of transient phenomena. For example, in Huang, X. and Thomas B. G., Modelling of Transient Flow Phenomena in Continuous Casting of Steel, Canadian Metallurgical Quarterly, 37 (1998) 197 -212, there is a description of this transient phenomena and the transitional transients, periodic oscillations and random fluctuations due to turbulence. Casting conditions are often upset intentionally, the casting speed may be slowed down and then ramped up to desired operational value when the ladle or tundish, or both are replaced during a grade change. The casting speed may also be lowered to change the submerged entry nozzle, or when the mould width is changed during operation. This kind of transient process can be classified as transitional transient phenomena because the system normally returns to steady state after several minutes. Other transient phenomena are of a periodic nature. These include the vertical movement of the mould during each oscillation cycle and the periodic bulging of the strand as it moves between rolls. Finally, random changes associated with turbulence are almost intrinsic in the mould region of steel casters due to the nature of fluid flow in this regime. Understanding transient phenomena is important because they can significantly affect steel quality and the mould steel level.

Most of the transient phenomena need to be controlled by a throttling device such as slide-gate valves or stopper rods. These devices can partially obstruct the entrance to the bore, and cause the stream of molten metal to enter the bore off the centre line.

The stream cart flow preferentially down one side of the bore, and exit asymmetrically or non-uniformly from a pore tube causing excess of surging and turbulence in a mould.

It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.

It is a further object of at least one aspect of the present invention to provide a nozzle suitable for use in the transfer of molten metal which substantially reduces and/or prevents clogging.

SU4.RY OF THE INVENTION According to a first aspect of the present invention there is provided a nozzle suitable for delivering molten metal, said nozzle comprising: a main body defining a bore; wherein said bore increases in diameter from an upstream portion to a downstream portion whereby clogging within the nozzle is substantially prevented or minirnised.

The nozzle according to the present invention therefore has an increasing diameter bore which improves the flow of a stream of molten metal and reduces precipitation within the bore resulting from the flowing molten metal. For example, the precipitation may be any non-metallic deposit or metal oxide deposit such as Al203.

The nozzle according to the present invention comprises a pour tube having a bore comprised of an adequate volume above a slag line which may provide moderate negative pressure during casting to continuously alter and diffuse a contained molten stream. The nozzle according to the present invention may be adapted to provide a negative pressure of about 0.05 -0.1X105 Pa (i.e. 0.05 -0.1 bar).

In the process of continuous casting, when operated at, for example, a steady state, it has been found that an increasing diameter bore when used in providing a molten stream of metal may develop higher fluid velocity near a centre line of the bore than along the sides of the bore, or lower velocity on one side of the centre line as compared to the opposite side. The sides of the stream may develop higher turbulent flow or on laminar-turbulent transition with various degrees of sweep, which can flush the inner surface of the bore in any direction.

Due to this flow, at the wall of the bore, there may be found a shearing stress retarding the flow of the molten metal as well. In laminar flow, the motion of particles of fluid is very orderly with all particles moving in straight lines parallel to the walls of the bore.

When throttling devices are in operation, especially while an opening inlet has been reduced in diameter, the larger inner bore may be in a high vacuum chamber status.

It can therefore be assumed that the drawing up of molten metal, which occurs by a mechanism of siphon, takes place substantially simultaneously with the decreasing of the opening fluid inlet. Therefore, the below molten metal may flush over the inner bore along the wall according to the siphon principle. Therefore, particles sticking on an inner surface of the bore, may be flushed away by powerful shearing forces produced on the surface of the wall. These shearing forces are provided and enhanced by the bore having a diameter which increases from the upstream (i.e. upper) portion to the downstream (i.e. lower) portion.

Therefore, by having a bore wherein the diameter increases from the upper portion to the lower portion, a continuous flushing up and down along the inside surface of the bore may prevent the deposition and adhesion of particles such as A1203 and other inorganic particulates.

The deposited particles may therefore be sheared off and flushed away thereby preventing and/or reducing clogging of the bore.

The present invention may therefore require an increased internal volume above a submerged part of the nozzle bore.

The inside surface of the bore may be substantially continuously smooth, comprise a series of steps or a series of vertical grooves or channels. The steps may increase the bore diameter gradually. However, the increasing diameter from the upstream portion to the downstream position allows a moderate negative pressure during casting to occur which may allow the shearing process to prevent or substantially reduce clogging.

Steps may be used in a cylindrical, pyramidal or elliptical bore as well as a smooth bore.

The inside surface of the bore may be coated with an anti-clogging agent to prevent clogging.

Alternatively, the bore comprising the steps which may be used to increase the diameter of the bore may be connected to one another. There may therefore be a plurality of steps ranging from about 2 to 10 increasing the diameter. The steps may occur at any point during the bore. Preferably, the steps may have a size between about -100 mm.

The bore may also comprise a plurality of substantially vertical grooves or channels extending substantially vertically down the bore. The channels or grooves may extend substantially all the way down the bore or only partially down the bore. Typically, the size of the channels or grooves may vary from 2 X 10 mm to 5 X 50 mm by deepness X width.

It has therefore been found that by increasing the diameter of the bore and providing steps or grooves may provide moderate negative pressure during casting. The smooth inner surface of the bore, steps or grooves may reduce flow asymmetry and reduce the resistance of molten melt flushing up. For example, when an inlet used to input molten metal into the nozzle is reduced in diameter, molten metal under a slag line may be swiftly drawn up along the inner bore due to the negative pressure. For example, the molten metal may be provided using a slide gate or stopper rod.

The nozzle may also comprise a converging portion below the slag line. The converging means deflects the stream toward the centre axis of the bore. The bore volume at slag line area is large enough to provide moderate negative pressure, but small enough to keep the higher mechanical strength of the nozzle body.

Therefore, unlike conventional nozzles, the increasing bore of the nozzles according to the present invention extends outwardly and upwardly above a slag line. In order to practice the invention in, for example, a multi-continuous casting, the following factors are found to be important: a) it is important to select a body material which has a porosity low enough to prevent air penetrating therethrough, but large enough to provide a high thermal shock resistance; b) it is preferred that a high strength body material is used which is high enough to prevent a nozzle breaking due to the molten steel flushing over; c) it is also preferred to make a simulation of fluid flow in a continuous casting mould if the periphery size of the nozzle is large enough relative to the breadth/thickness of a casting mould. The boundary conditions should be calculated carefully, especially taking into consideration throttling device adjustments. A redesign of the outlet pore of a nozzle may be required; d) it may also be found advantageous to select a material which prevents deposition or adherence of deposits such as A1203 and; e) it may also be found that a negative pressure inside a submerged entry nozzle may cause air suction through a joint between a collector nozzle and the submerged entry nozzle. Air suction may be the result of the ventuli effect caused by the flow of the steel.

According to a second aspect of the present invention there is provided a method for preventing or substantially minimising clogging within a nozzle, said method comprising: providing a nozzle, said nozzle having a bore which increases in diameter from an upstream portion to a downstream portion; whereby clogging within the nozzle is substantially prevented or mirilmised.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figures 1A -1C are sectional views of submerged

entry nozzles according to the prior art;

Figure 2 is a sectional view of a conventional submerged nozzle illustrating the deposition of A1203 to a nozzle; Figure 3A is a cross-sectional view of a nozzle according to the present invention with a slide gate showing the flow path of molten steel as it issues from a nozzle while operated at steady state; Figure 3B is a cross-sectional view of a further nozzle according to the present invention with a slide gate showing the flow path of molten steel as it issues from a nozzle with a reduced opening; Figures 4A -4C are sectional views of nozzles according to three embodiments of the present invention; Figures 5A 5C are cross-sectional views of nozzles according to further embodiments of the present invention; Figures 6A -6E are cross-sectional views of yet further nozzles according to embodiments of the present invention; Figure 7 is a cross-sectional view of a further nozzle according to the present invention; Figure 8 is a cross-sectional view of a further nozzle according to the present invention; Figure 8A is a top view of the nozzle shown in Figure 8; Figure 9A is a cross-sectional view of a nozzle according to the present invention suitable for slab continuous casting; and Figure 9B is a yet further nozzle according to the present invention suitable for slab continuous casting.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description as follows is directed to

preferred embodiments of the present invention for delivering a stream of molten steel from a tundish to a continuous casting mould. The present invention relates to new and improved immersion pouring nozzles for molten steel, for operation with a mould. The nozzle comprises an improved bore to provide moderate negative pressure to draw up molten metal located below and to prevent clogging.

Figure 1A is a general representation of a nozzle according to the prior art, generally designated 100. The nozzle 100 comprises a tubular body 110 containing an internal central bore 112 which defines a central flow passage from an upper vessel to a lower vessel or to a mould, during a teeming or casting process. As shown in Figure 1A, there is a circumferential portion 116 which is located towards the lower end of the tubular body 110 and which passes through a layer of molten mould powder when in use. The circumferential portion 116 is made from wear resistant material. Figure 1A also shows exit ports 118 which define the direction in which molten metal flows from the internal central bore 112 into a lower vessel or mould.

As shown in Figure 1A, the diameter along the central bore 112 at the upper area, middle and downward area are substantially the same i.e. d -dm -dd.

Although not shown in Figure 1A, the prior art nozzles also have an internal diameter for the central bore which usually follows as d > dm > dd. This was previously believed to be an effective design to discharge molten metal from a nozzle in a smooth and steady manner without interruption or disruption.

Figure 1A also shows that there is a lining 114 extending down the central bore 112.

Figure lB represents a further nozzle 200 according to the prior art. Similar to that of the nozzle shown in Figure 1A, there is a tubular body 210, a central bore 212, a lining 214, a circumferential portion 216 and exit ports 218.

Figure lC represents a further nozzle 300 according

to the prior art. The nozzle 300 comprises a

substantially tubular body 310 and a central bore 312 along with a circumferential portion 314.

Figure 2 represents a further nozzle 400 according

to the prior art. The nozzle 400 comprises a

substantially tubular body 410, a central bore 412, a lining 416 and a circumferential portion 417. As shown in Figure 2, there are two exit ports 418. Importantly, Figure 2 shows the deposition of material formed during a casting process. Although not shown in Figure 2, a tundish may be used to deliver molten steel to a caster mould. The continuous stream of molten steel may be controlled by a slide gate valve mechanism (not shown) The molten steel may be delivered to a caster mould through a submerged entry nozzle comprising a refractory tube shroud attached to a collection nozzle. The nozzle may include a collection end to couple to the slide gate mechanism or stopper rod, and a discharge end or outlet, that is immersed below the surface of molten steel contained in the caster mould.

Figure 3A represents a nozzle 500 according to the present invention. The nozzle 500 comprises a substantially tubular body with an upper section 510, a central middle portion 512 and a lower portion 514. As shown in Figure 3A, the bore increases in diameter and about half of the middle portion 512 is submerged in molten steel 528 in a mould 526. A continuous stream of molten steel is controlled by a slide gate valve mechanism wherein a gate 518 may be slid between members 516 and 520.

Figure 3A also shows a collection nozzle 524 which is used to receive the molten steel 528.

Figure 3A therefore shows a nozzle 500 according to the present invention wherein the nozzle 500 comprises an increasing-bore to receive a molten metal stream. When operated at a steady state, the molten metal stream in the nozzle 500 shown in Figure 3A may develop higher fluid velocity near the centre line of the central bore than along the sides of the bore. Alternatively, lower velocity on one side of the bore, as compared to the opposite side of the bore, may be found. The sides of the stream may develop higher turbulent flow or on laminar-turbulent transition with various degrees of sweep, which can flush the inner surface of the bore in any direction and at the wall there may be a shearing stress retarding (i.e. reducing) the flow as well. The flow of the molten steel in Figure 3A is shown by the arrows. In laminar flow, which develops in the present invention, the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the pipe walls.

Figure 3B shows the same nozzle 500 shown in Figure 3A, but where the slide gate valve mechanism has been activated to create an off centre line for the inflow of molten steel. As shown using the arrows representing the flow of the molten steel, particles sticking on the inner bore of the nozzle 500 may be flushed away by a strong shearing process.

Figure 4 represents a further nozzle 600 according to the present invention. The nozzle 600 comprises a tubular body 610, a central bore 612, a circumferential central portion 616 and outlets 618. The nozzle 600 according to the present invention provides an improved bore volume that eliminates, or greatly reduces, non-metallic oxides deposition. Figure 4 also shows a top cross-sectional view along line A - A of the nozzle 600 which shows that the central bore 612 comprises a series of elongate channels 630. The channels 630 extend from the upper part of the nozzle 600 to the lower part. The channels 630 provide moderate negative pressure between 0.05 -0.1 X Pa during casting and act as drainage tubes while the molten steel is drawing up. The geometric shape of the central bore 612 may be substantially tapered or substantially constant in diameter, or any other appropriate design.

Although shown in Figure 4A, there are eight channels 630, any number of channels upwards from two may be used. Although the deepness and width of the channels 630 is not limited, the channels 630 preferably have a depth of 5 mm and a width of 20 mm.

The channels 630 shown in Figure 4A may be of any appropriate form such as any form of indent or recess into the central bore. For example, the channels 630 may be in any form of the following: grooves; ditches; trenches; drains; gulleys; strips; streaks; recesses; passages and the like. Typically, the elongate channels 630 extend substantially from the upper part of the nozzle 600 to the lower part.

Figure 4A also shows that the nozzle 600 comprises an upper portion, a medium portion and a lower portion, due to the geometric shape of the bore. Thus, two converging steps exist in the bore. They are the upper converging step between the upper portion and medium portion, and the lower converging step between the medium portion and the lower portion.

Figures 4B and 4C show alternative nozzles with different channels. Nozzle 700 has four channels 730 and nozzle 800 has four channels 830.

As shown in Figures 4A, 4B, 4C the geometric shape of the bore may be the general tapered, straightforward or any other suitable design. Theminimum amount of the channels is two; the depth and width of the channels is not limited. The channels are preferably substantially evenly distributed on the inner wall. The tapered bore may not increase the bore volume, but the channels may have an induction function of drawing up molten metal even at a low negative pressure.

Figure 5A shows a cross-sectional view of a further nozzle 900 according to the present invention. The nozzle 900 comprises a tubular body 910, a central bore 912, a circumferential central portion 916 and outlets 918.

Figure 5A also shows an expanded view of the central bore 912 showing the divergence of the shape of the bore 912. As shown in the expanded view, there is an angle e which is about 1 degree.

Figures SB, 5C, 6A, 6B, 6C, 6D and 6E show a variety of ranges of shapes for the central bore. Corresponding references to the previous diagrams are used to show the tubular body, the central bore, the circumferential central portion and the outlets. Any shape for the central bore is suitable which is capable of providing negative pressure by increasing the bore volume. It is preferred that the shape provides a negative pressure of 0.05 -0.1 X Pa. The upper portion of the bores can be substantially constant in diameter, tapered or converged in shape. The upper portion of the bore is simply adapted and capable of guiding and dividing incoming flow of liquid steel from a tundish into substantially equal portions so that a similar volume and velocity of liquid steel is discharged from each outlet opening into opposite sides of a caster mould under all cast steel flow rates.

The medium portion of the bore, where the slag line or powder is located has a larger diameter than the upper portion which can provide moderate negative pressure during casting. The shape of the middle portion can be reverse taper, conical/pyramidal, elliptical, spherical and in some instances can be opposite to the shape shown in Figure 6E. The lower portion of the bore, which is immersed in a casting mould, can be any of the following shapes: in the form of a column; elliptical; cylindrical; spherical; and the like. It should be noted that the nozzles shown in Figure 5C, Figure 6C and 60 which comprise a tapered lower portion and have a narrow throat, should be calculated carefully or simulated to avoid a Laval nozzle flow pattern.

It should be noted that the shape of the bore shown in Figures 4 -6 is not limited, and is simply adapted to provide moderate negative pressure to draw up molten steel from a lower portion without significantly decreasing nozzle thickness. The thickness of the body should also prevent air penetrating therethrough and should be thick enough to provide substantial strength to the body. It is found that the longer the length of the nozzle the lower the upper converging step. The upper converging step position may be between 1/4 and 1/2 of the nozzle height from the upper end, with typical values of 1/3. The desired position may depend on such factors as the length of the nozzle, the casting speed, the immersion depth of the nozzle and other features particular to a given caster design.

As shown in the expanded representation in Figure 5A, the upper converging step face comprises an incline surface having an inclination angle e between the inclined surface and the central axis. The angle e may range from 0.5 to 90 . The step face to which the stress is concentrated is related to the degree of inclination angle e. It is found that an optimisation of the magnitude of e, the height and volume of the medium portion can shift, minimise or eliminate the stress concentrating at the step face, so that the damage of the nozzle can be remarkably reduced.

Furthermore, the lower converging step can be left out as shown in Figures 4, 5A and 6A. This means that the medium portion transition to lower portion occurs smoothly. It is found that if a lower converging step is required, it is better to have this at or below a slag line area to facilitate molten steel being drawn up.

Figure 7 represents a further nozzle generally designated 1700 having a different bore structure. The nozzle 1700 comprises a tubular body 1710, a central bore 1712, a circumferential central portion 1714 and outlets 1718. As shown in Figure 7, the essential bore has a number of horizontal steps 1750 altering the diameter of the central bore 1712. Although the height and thickness of the steps is not limited, it is preferred that each step has a horizontal depth of 1 mm and increasing the bore diameter by about 1 -2% with each step and in total by about 10%.

Figure 8 represents a nozzle generally designated 1800 having a yet further different bore structure. The nozzle 1800 has a body similar to that of the nozzle 1700 shown in Figure 7, the only difference being that a series of vertical channels 1830 are used to construct the bore.

It should be noted that the structure of the bore shown in Figures 7 and 8 corresponds to the structure shown in Figures 5A -6D. The minimum amount of steps may be 1. The height and thickness of the steps is not limited. The minimum amount of channels is 2, the depth and width of the channels is not limited. The steps and channels can exist in the whole bore, the medium portion and/or the lower portion.

Figures 9A and 9B show further nozzles 1800,1900, respectively, according to the present invention. The nozzles 1800,1900 have different outer geometric shapes, except the general straightforward/tapered column/cylinder as illustrated in Figures 1 -8 and other special-shaped nozzles used for thin slab continuus casting and the like. These embodiments may be used in slab continuous casting as shown in Figures 9A and 9B.

The medium and/or the lower portions of the nozzle body can be shaped to allow slab continuous casting to occur.

Whilst specific embodiments of the invention have been described above, it will be appreciated that departure from the described embodiments may still fall within the scope of the invention. For example, any appropriate shape which increases the volume of the inner bore may be used to prevent clogging of a nozzle during a casting process.

Claims

1. A nozzle suitable for delivering molten metal, said nozzle comprising: a main body defining a bore; wherein said bore increases in diameter from an upstream portion to a downstream portion and whereby clogging within the nozzle is substantially prevented or minirnised.

2. A nozzle according to claim 1, wherein the bore comprises an upper portion, a middle portion and a lower portion.

3. A nozzle according to any of claims 1 or 2, wherein the bore comprises a series of grooves or channels.

4. A nozzle according to claim 3, wherein the grooves or channel are substantially vertical.

5. A nozzle according to claim 2, wherein the shape of the middle and lower portion of the bore is any of the following: substantially cylindrical; substantially pyramidal; substantially elliptical.

6. A nozzle according to any preceding claim, wherein the bore is substantially tapered in direction along at least part of the bore.

7. A nozzle according to any preceding claim, wherein an inner surface of the bore is substantially smooth.

8. A nozzle according to any of claims 1 to 6, wherein the nozzle comprises a series of steps which increase the diameter of the bore.

9. A nozzle according to claim 8, wherein the steps exist in substantially all of the length of the bore, the middle portion and/or the lower portion.

10. A nozzle according to any preceding claim, wherein a converging step located between an upper and lower portion of the bore, is between about 1/4 to about 1/2 of the nozzle height from an upper end of the nozzle.

11. A nozzle according to claim 10, wherein the upper converging step comprises an inclined surface having an inclination angle between the inclined surface and a central axis, where the inclination angle ranges from any of about 0.5 to 90 .

12. A nozzle according to any preceding claim, wherein the nozzle comprises a converging step adapted to be provided below a slag line.

13. A method for reducing or substantially minimising clogging within a nozzle, said method comprising: providing a nozzle comprising a main body defining a bore; wherein the bore increases in diameter from an upper portion to a lower portion and whereby clogging within the nozzle is substantially prevented or minimised.

14. A method according to claim 13, wherein the nozzle is capable of creating moderate negative pressure during casting to substantially continuously alter and diffuse a molten stream of metal.