WO1995035175A1

WO1995035175A1 - Feeding of mould powder

Info

Publication number: WO1995035175A1
Application number: PCT/SE1995/000741
Authority: WO
Inventors: Bo Rogberg; Per-Olov Yhr
Original assignee: Sandvik Ab
Priority date: 1994-06-17
Filing date: 1995-06-16
Publication date: 1995-12-28
Also published as: SE9402120L; SE515012C2; AU2811995A; SE9402120D0

Abstract

A granulated casting powder is used for the continuous casting of molten metals, which powder essentially consists of substantially spherical particles. This is distributed upon the molten metal only by means of gravity from a superjacent container (1), through a valve (3) and a transport tube (5), and out upon the molten metal. The casting powder 'flows' with a constant flow independently of the filling height in the container (1), the desired flow being adjusted with the valve (3).

Description

FEEDING OF MOULD POWDER

The present invention relates to a process for feeding a casting powder on a molten metal, primarily for continuous casting. The invention also relates to a device for performing said process.

At continuous casting or strand casting of metals, the molten metal is led from a ladle over a tundish to a cooled mould via a casting tube. Usually, this mould consists of a short, double-walled, water-cooled sleeve of copper with a bottom that can be mechanically lowered with a predetermined speed, the mould being able to perform an oscillating motion. As the metal solidifies, this bottom is lowered and thereafter the solidified metal per se forms the mould bottom. The cast strand goes downwards and is usually cooled in a water spray tower, whereafter it is usually cut into suitable bloom lengths for following rolling.

Because of the quick cooling of the molten metal in the mould, surface cracks may easily arise in the solidifying strand shell. Moreover, there is a risk that the strand will stick to the inner walls of the mould, which may cause very costly operation interruptions. Therefore, a so called casting powder is always used for continuous casting, which powder melts and forms a casting slag, which functions as a release agent between the strand and the mould wall. Furthermore, this casting slag contributes to diminishing the tendency of crack formations in the strand's surface. The casting powder is spread over the open upper surface of the molten metal in the mould, whereby it melts and flows out upon the molten metal. Along the periphery of the surface of the molten metal, the molten casting powder is pulled down, with the assistance of the oscillation of the mould, by the newly formed strand and forms a continuous, covering release layer between the strand and the mould wall. After passing through the mould, this layer detaches by itself when cooling the strand with water sprays and falls down towards the bottom of the cooling tower.

In view of the above, it is easily understood how important it is that the feeding of casting powder is made at an even rate without any interruptions whatsoever. Usually, this casting powder feed is made manually in that an operator spreads the casting powder from a vat or a tank by means of a rake, the operator estimating the necessary amount visually. Besides the fact that this is a dusty, monotonous and not entirely safe work, it requires continuous surveillance. Further, it is easy to misjudge the necessary amount of casting powder, which may cause sudden interruptions in the layer between strand and mould wall. Therefore, during a long time there has been a strong wish for automatizing the feed of casting powder.

In US-A-5 067 553 an automatized arrangement is disclosed for the feeding of casting powder to the surface of molten metal. The arrangement is very costly and complicated; inter alia it comprises two robot arms and a spreading tube with a built-in, motor-driven screw-feeder. Because of the complicity of the arrangement, it is apt to disturbances and costly operation interruptions become unavoidable.

Further, in JP-A-57-195568 the principle is disclosed how an IR-detector can steer a motor-driven feeding arrangement in the feeding pipe for a casting powder for continuous casting. Besides a very scarce description of the details, also this construction is unnecessarily complicated and apt to disturbances. Also in US-A-5 158 129 a plant is disclosed for the automatic feeding of casting powder to a mould for continuous casting. However, also this requires an expensive and elaborated control equipment, inter alia an electronic control unit and a mini-computer. Further, the spreading on the surface of the molten metal is effected by the so called "chicken feeder" principle, which in practice has turned out to work unsatisfactorily.

Thus, a primary object of the present invention is to guarantee an even and continuous feed of casting powder upon a molten metal, in particular at continuous casting.

A second object of the present invention is to eliminate the safety risks which exist when casting powder is fed manually. A third object of the present invention is to as far as possible simplify the necessary equipment for the distribution of casting powder on a molten metal, in particular for continuous casting. These and further objects have been achieved in a surprisingly simple way by applying a process that comprises the measures as defined in the characterizing portion of claim 1.

For illustrative but non-limiting purposes, the invention will now be further described with reference to the appended drawings. These are herewith presented:

Figure 1 shows an outline drawing of a feeding arrangement according to the invention.

Figure 2 shows a diagram over the powder flow in dependence of the valve area.

Figure 3 shows the grain size distribution for two different commercial, granulated casting powder sorts. In figure 1 a container for casting powder is designated by 1. It is for instance hung by suitable hooks on the tundish. A tube 2 with a valve 3 is connected at the container's lowermost part. The tube part 2 can be of metal or of a flexible material . To the outlet end of this valve is connected a shorter, substantially vertical tube portion 4, which in turn connects to an angled transport tube 5. This opens above the top surface of the molten metal 6, the fed casting powder melting of the heat from the molten metal and lying in the form of a liquid layer 7 on the top of the molten metal. The molten metal is fed from the tundish (not shown) through the casting tube 8 and to the copper mould 9. The cast strand is continuously led downwards, surrounded by a cover of slag of initially molten and thereafter solidified casting powder. This cracks and detaches when spraying with cooling water.

If desired, the described equipment may also comprise two separate parts in that the container 1, tube part 2 and valve 3 constitute a unit, while the tube part 4 and the transport tube 5 constitute a second unit. According to this embodiment, the outlet from the valve 3 opens into the open air, the casting powder falling freely a short distance and down into the opening of tube 4 which is placed underneath said outlet. In this case, the opening part of tube 5 is formed as a collecting trough or vessel with a widened cross-sectional area, in order to collect all the casting powder. This embodiment brings the advantage that the container with its appurtenant valve may be mounted and demounted, respectively, separately, and the transport tube 5 (with the appurtenant collecting trough 10) also separately. Further, the transport tube 5 may be shaped as a branched tube of substantially Y- shape, which makes possible that the two openings of the tube may be placed on either side of the casting tube 8 and thus guarantee a more even distribution of the casting powder upon the surface of the molten metal . The transport tube or tubes may also be constructed telescopically, which facilitates the obtainment of the desired positioning of the tube's mouth above the molten metal. Further, the tube parts 4 and 5 may be entirely or partly replaced by a flexible hose of substantially the same dimension. The essence is that also this hose then must have a minimal angle α as defined below.

An essential feature of the present invention is that the casting powder is granulated. By this expression is meant that the casting powder grains consist of substantially spherical particles with a certain grain size distribution. The chemical composition of a granulated casting powder may vary between different makes available on the market. One group of casting powders are marketed under the registered trademark "Scorialit", of which the one with the type designation SPH 107-77 II may serve as an example (given percentages refer to percent by weight) of a representative composition of a casting powder:

Basicity: 1 , 43

Si0₂ 23 , 7

CaO 33 , 0

MgO 0 , 82

A1₂0₃ 7 , 29

Na₂0 8 , 04 κ₂o 0,13 %

Ti0₂ 0,35 %

MnO 6,25 %

Fe₂0₃ 1,12 %

co₂ 9,3 %

F 6,71 %

H₂0 ( ;600 °C) 0,52 %

S 0,05 %

Softening point 1060°C

Melt:ing point 1140°C

Pouring point 1180°C

Bulk density 0,64 kg/dττv

Grading analysis:

Remainder 500 μm: 19,0 %

Remainder 125 μm: 97,2 %

Remainder 63 μm: 99,6 %

Viscosity (1300°C) : 0,7 dPa*s

A granulated casting powder for the present invention mainly consists of substantially spherical particles. These are produced in a way known per se by a spray drying process. Thanks to this geometrical configuration, an even and continuous feed of the casting powder has been obtained in an extraordinarily simple and reliable way. Thus, an even and continuous outflow from the container of casting powder is achieved by a conventional ball valve, e.g. a 9,5 mm (3/8") ball valve of brass . When the granulometry suitably is such that the average particle diameter is between 100 and 700 μm, preferably between 150 and 400 μm, and in principle no particle is larger than 1,0 mm, the powder streams or "flows" in a manner that so far has not been considered possible, provided that the transport tube 5 is not inclined towards the horizontal plane by an angle α which is smaller than the inherent angle of repose of the casting powder. For the two commercial casting powders "Scorialit" (reg. trademark) SPH A- 72 (including the additional sort SPH A-72/S) and SPH 107-77 (I and II) , this angle of repose is 31,2°. Thus, this means that the angle of inclination . of the transport tube 5 relative to the horizontal line should be equal to or larger than this value when these casting powders are used. Suitably, the inclination angle a should not be less than 30° for any casting powder. Further, the bottom surface of the container 1 should also be inclined by at least this angle relative to the horizontal plane, because otherwise the powder may stick in the container. When these conditions are fulfilled, the powder runs out from the container, through the valve and out upon the molten metal with a constant flow and independently of the filling height in the container 1. The mouth of the transport tube 5 is at a certain distance from the surface of the casting powder that has been spread upon the molten metal, wherefore the so called "chicken-feeder" principle is not used at all. This flow is linear depending upon the valve area, which is illustrated in the diagram in figure 2. In this diagram, the upper line (A) corresponds to the powder quality SPH A-72 and the lower line (B) to the powder quality SPH 107-77. The size of the valve opening is given on the x-axis in mm² and on the y-axis is given the flow of the casting powder in kg/min.

The grain size distribution of two commercial sorts of "Scorialit" is seen in the diagram in figure 3. On the x- axis is given the grain diameter in μm and on the y-axis the amount in %. As may be seen from this, the diameter distribution of the substantially spherical particles corresponds substantially to a normal distribution. These sorts may be applied directly into the process according to the invention. Since the size of the granules influences the flow, a desirable flow range may influence the choice of granulometry. Thus, a smaller average diameter gives a larger flow. If a narrower grain size distribution is desired, then some fractions can be removed by sieving.

As long as there is casting powder in the container, the preset set value of the flow is basically constant. However, if of some reason a part of the casting powder has become moist, the grains may agglomerate and a disturbance of the flow will be the result. Of this reason, an alarm sensor or a flow meter may be installed. This may for instance be fixed at the junction between the tube part 4 and the transport tube 5. Different sensors may be chosen. A suitable, tested type is a flow meter that measures the sound frequency from the casting powder that falls through the bend between the tubes 4 and 5, this frequency being transformed into an electrical signal in mA or mV. If of some reason the casting powder would stop flowing, the signal sinks below a certain minimal value in mV or mA and the alarm starts.

Claims

1. Process for feeding of casting powder upon the surface of molten metal, in particular for continuous casting, c h a r a c t e r i z e d in that the casting powder is granulated so that it essentially consists of substantially spherical particles, which are allowed to flow only by means of gravity from a superjacent container (1) , through a valve (3) and a transport tube (5) which is inclined relative to the horizontal plane, and out upon the surface of the molten metal, the desired flow being adjusted by means of the valve (3) .

2. Process according to claim 1, c h a r a c t e r i z e d in that the desired flow is adjusted only by means of said valve (3) and that the mouth of the transport tube is at a certain distance from the top surface of the casting powder layer on the molten metal.

3. Process according to claim 1 or 2, c h a r a c t e r i z e d in that the angle of the transport tube (5) relative to the horizontal plane is equal to or larger than the inherent, material-specific repose angle of the granulated casting powder.

4. Process according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the grain size distribution of the casting powder is such that the particle average diameter is between 100 and 700 μm.

5. Process according to any of the previous claims, c h a r a c t e r i z e d in that the added amount of casting powder corresponds to a flow of between 0,1 and 1,0 kg/min, preferably between 0,1 and 0,7 kg/min.

6. Arrangement for performing the process according to any of the preceding claims, c h a r a c t e r i z e d in that it comprises a container (1) for holding casting powder, a valve (3) attached at an outlet port or an outlet tube at the lowest point of the container and a transport tube (5) at the outlet end of the valve, the transport tube being inclined relative to the horizontal plane.

7. Arrangement according to claim 6, c h a r a c t e r i z e d in that the transport tube (5) is angled relative to the horizontal plane by an angle of at least 30° .

8. Arrangement according to claim 6 or 7, c h a r a c t e r i z e d in that the transport tube (5) is branched in the form of a Y-tube, in order to improve the distribution of the casting powder on the surface of molten metal.

9. Arrangement according to claims 6-8, c h a r a c t e r i z e d in that the transport tube (5) is telescopical .