AU685999C - A flow control device for the outlet nozzle of a metallurgical vessel - Google Patents

Description

A flow control device for the outlet nozzle of a metallurgical vessel

This invention relates to a molten metal handling vessel and particularly to a means of improving the opening of the outlet nozzle of the vessel containing molten metal to allow the metal to flow out. The invention is especially concerned with the teeming of molten steel from a ladle and, although not intended to be limited thereto, it will be more specifically described below with reference to ladles.

In the steel industry, ladles are used to transfer molten metal from the melting and refining vessels to continuous casters or an ingot teeming bay. The metal flow from the ladle is controlled typically by a slide gate valve. Usually, the outlet nozzle area is filled with a refractory sand introduced from the top of the ladle to prevent steel freezing in the nozzle cavity before teeming commences. This is accomplished in a variety of ways, either manually or semi- automatically.

However, all conventional methods have been found to be inconsistent and usually result in number of failures to open. The term "free open" refers to a totally successful opening without the need to flush the nozzle with oxygen to break the formed steel/ sand skull. Free opening success rate varies from mill to mill but is usually in the range 80-90%. At the end of the cast, the ladle is closed by visually observing slag in the tundish or by detecting slag using devices sensitive to slag transfer. At the lower levels of steel in the vessel, vortex formation can occur resulting in significant slag carry-over or the need to carry sizeable quantities of steel in the ladle at the end of teeming. Slag carry-over causes a variety of problems at the caster which usually result in quality problems as well as causing difficulties with consistent repeating of the ladle and filling sequence.

It is an object of the invention to provide an improved means of opening the outlet nozzle of a molten metal handling vessel, particularly a ladle, whereby the free opening success rate may be increased.

Accordingly, in one aspect the invention provides a flow control device for the outlet nozzle of a molten metal handling vessel, the device being shaped to fit as a plug in the outlet nozzle whereby a gap is defined between the device and the closure means for the outlet, the device having apertures closed by means that melt under the influence of the molten metal in the vessel, the apertures providing passageway for the molten metal from the vessel to the outlet.

The means to close the apertures may be vented steel plates that melt under the influence of the molten metal - steel - with which the vessel is filled.

The flow control device is preferably a press-fit into the top portion of a suitably contoured ladle inner nozzle and may be a disposable item that is discarded after each teeming of the ladle. The gap between the device and the closure means of the vessel, i.e. usually a slide gate valve in a ladle, may be filled with sand which may be blown into the gap through the open slide gate valve, preferably while the empty ladle has been rotated to its horizontal position, i.e. normal to its vertical metal-containing position. The slide gate valve may then be closed and the ladle rotated to its normal vertical position to be filled with molten metal.

Accordingly in another aspect the invention provides a method of teeming a molten metal handling vessel in which a flow control device is fitted as a plug into the outlet nozzle of the empty vessel, whereby a gap is defined between the device and the closure means for the outlet, the device having apertures closed by means that melt under the influence of molten metal in the vessel, the gap is filled with sand through the closure means for the outlet, the closure means is closed and the vessel is filled with molten metal, whereby the molten metal melts the means closing the apertures of the device, and the outlet closure is opened to teem the vessel.

The flow control device preferably comprises a head portion containing the apertures and a tail portion shaped to be a close fit into the correspondingly contoured entrance of the outlet of the vessel, i.e. into the outlet nozzle. It may be mortared into a press fit in the outlet and may be placed in the desired position in the hot vessel e.g. by a mechanical arm from the top of the ladle or by an operator suitably protected by an insulating barrier. A camera may be used to ensure accurate placement. The head of the flow control device preferably sits proud of the floor of the vessel so that it is fully pre-heated with the vessel and will not cause freezing of the molten metal poured into the vessel.

As the molten metal melts the closures of the apertures of the flow control device, it contacts the sand filling the gap above the outlet closure means and the sand will sinter at the molten metal/ sand interface. When it is desired to teem the vessel, opening the outlet allows the unsintered sand to fall out and the metallostatic pressure above the sintered cap of sand breaks that cap and the metal flows out of the vessel.

The metal is then allowed to flow out until slag is detected. The transition from metal to slag is very sharp so that the invention provides improved sensitivity for slag detection, thereby eliminating or reducing premature cut-off.

At the end of teeming, the vessel may be de-slagged and the used flow control device is punched out of the nozzle to be replaced by a new device for the next sequence.

The device may be made of any suitable refractory composition, e.g. magnesia- or alumina-based refractories.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a representation of a device of the invention being positioned on an outlet in a ladle; and Figures 2 to 8 are each diagrammatic sectional views through a ladle showing successive steps from introducing a device of the invention into an empty ladle through to removing the used device from the ladle after a sequence of filling the ladle with molten steel through to emptying the steel from the ladle.

In Figure 1 a flow control device 10 of the invention has a head portion 11 integrally-formed with a tail portion 12. Tail portion 12 comprises four equi-spaced fins 13 contoured to converge inwardly away from head 11. The contoured tail portion is designed to fit into a correspondingly-contoured split upper well block plate 15 fitted in a permanent well block 16 above a nozzle 17 defining an outlet 18 in a ladle. Outlet 18 passes through an upper slide gate plate 19a and is shown closed by a lower slide gate plate 19b, these being conventional closure means.

Graphitised mortar layers 15a and 17a respectively seal the upper well block plate 15 to permanent well block 16 and the nozzle 17 to the permanent well block 16.

It will be appreciated that the flow control device 10 and upper well block plate 15 may instead be made in one-piece which is replaced after each heat. However, as shown, the upper well block plate 15 may be a semi-disposable item being replaced, for example, after 2 to 5 heats.

Metal closure sheets 14 are fitted to head 11 to close apertures formed by the castellated outline of the lower edge 1 la of the head. The sheets, usually of steel, have vent holes 14b to allow upward venting through the outlet nozzle when the device is fitted in a ladle.

Use of the flow control device of Figure 1 is now described with reference to Figures 2 to 8.

In Figure 2 an empty ladle 20 is shown rotated to its side or horizontal position. The ladle has walls 21 and a bottom 22. An outlet 23 in the bottom contains a well block 24, an inner nozzle 25 fitted in the well block to define an outlet passageway 26 and an upper well block plate 24a similar to plate 15 of Figure 1.

A conventional slide gate valve 27 is attached to the outside of the bottom 21 of the ladle surrounding the outlet.

The upper surface of the upper well block plate 24a is contoured to receive the tail portion of flow control device 10 so that its head 11 stands proud of the surface of the bottom of the ladle. Device 10 is fitted into the ladle using a removable positioning arm 28 and is set in place in the top of the inner nozzle with mortar.

As can be seen in Figure 2, the head 11 of the device and the closure plates 14 completely close off access to the outlet from inside the ladle but leave a gap or empty volume 29 in the outlet between the device and the sliding gate valve.

In Figure 3, ladle sand 30 is shown being introduced into gap 29 through the open slide gate valve 27 using a hose 31 and blowing means (not shown). In Figure 4 gap 29 has been filled with sand and the slide gate valve 27 has been closed.

In Figure 5 ladle 20 has been turned to its vertical position, pre-heated and filled with molten steel 32 covered by a slag layer 33. The slide gate valve 27 is still closed and the molten metal dissolves the closure plates 14 to expose apertures 14A through which it flows to contact the sand at the top of gap 29 to form a sintered cap of sand (not shown) at the sand /metal surface. The steel is refined and the ladle taken to the caster (not shown).

In Figure 6, teeming of the steel from the ladle is commenced. Slide gate valve 27 is opened and the free sand 30 runs out. The pressure of the steel 32 above any sintered cap of sand breaks the cap and the steel can flow out through the outlet.

In Figure 7 is shown the end of teeming. Slide gate valve 27 has been closed leaving the slag 33 and a very small amount of steel 32 in the ladle.

Finally, in Figure 8, the nearly empty ladle 20 has been rotated into its side position. Slag 33 is removed and the flow control device 10 is punched out using punch 34 through the outlet.

In a typical conventional slab caster using ladles of 250 to 300 ton capacity, a free-opening rate of up to about 95% is usual. At the end of teeming about 1 to 3% of the steel is left in the ladle if high quality grade products are being made. The use of the invention as described above enables the steel left in the ladle to be reduced to about 0.5% and can give a virtually unheard of >99% free-opening rate.

Claims (12)

1. A flow control device for the outlet nozzle of a molten metal handling vessel (20), the device being shaped to fit as a plug in the outlet nozzle (15, 17, 18) whereby a gap (29) is defined between the device (10) and the closure means (27) for the outlet (18), characterised in that the device (10) has apertures (14a) closed by means (14) that melt under the influence of the molten metal (32) in the vessel (20), the apertures (14a) providing passageway for the molten metal (32) from the vessel (20) to the outlet (18).

2. A flow control device according to Claim 1, characterised in that the means (14) to close the apertures (14a) are steel plates that melt under the influence of the molten metal (32) in the vessel (20).

3. A flow control device according to Claim 2, characterised in that the steel plates (14) have vent holes (14b) to allow upward venting when the device (10) is fitted in the vessel outlet nozzle (15, 17, 18).

4. A flow control device according to Claim 1, 2 or 3, characterised in that the device (10) is a press-fit into the top portion of a suitably contoured ladle inner nozzle (15).

5. A flow control device according to any preceding claim, characterised in that the device (10) is a disposable device to be discarded after one teeming of the vessel.

6. A flow control device according to any preceding claim, characterised in that the device (10) comprises a head portion (11) containing the apertures (14a) and a tail portion (12) shaped to be a close fit into the correspondingly contoured entrance ( 15) of the outlet (18).

7. A flow control device according to Claim 6, characterised in that the head portion in use sits proud of the floor of the vessel (20).

8. A flow control device according to Claim 6 or 7, characterised in that the apertures (14a) are defined by a castellated outline of the lower edge (11a), in use, of the head portion (11) of the device (10).

9. A method of teeming a molten metal handling vessel in which a flow control device is fitted as a plug in the outlet nozzle (15, 17, 18) of the empty vessel (20) whereby a gap (29) is defined between the device (10) and the closure (27) for the outlet (18), characterised in that the device (10) is formed with apertures (14a) closed by means (14) that melt under the influence of the molten metal (32) in the vessel (20), the gap (29) is filled with sand through the closure (27) for the outlet (18), the closure (27) for the outlet (18) is closed and the vessel (20) is filled with molten metal (32), whereby the molten metal melts the means (14) closing the apertures (14a) of the device (10) and the outlet closure (27) is opened to teem the vessel (20).

10. A method according to claim 9, characterised in that the device is placed in the outlet nozzle (15, 17, 18) of the vessel (20) while the vessel is hot using a mechanical arm (28).

11. A method according to Claim 9 or 10, characterised in that the device (10) is a disposable device and is punched out of the nozzle after teeming to be replaced by a new device for the next teeming sequence.

12. A method according to Claim 9, 10 or 11, characterised in that the device (10) is press-fitted into the top portion of a suitably contoured ladle inner nozzle (15).