EP2100975A1

EP2100975A1 - Method and device for treating a molten metal for producing metal castings

Info

Publication number: EP2100975A1
Application number: EP08101995A
Authority: EP
Inventors: Charles Defrancq
Original assignee: Corus Technology BV
Current assignee: Corus Technology BV
Priority date: 2008-02-26
Filing date: 2008-02-26
Publication date: 2009-09-16

Abstract

The invention relates to a method for treating a molten metal suitable for producing a metal casting using an additive that is fcd into the molten metal, wherein a stream of molten metal is introduced into a treatment chamber such that the introduced molten metal rotates in the treatment chamber, the molten metal being discharged from the lower end of the treatment chamber by gravity.

According to the invention, the method is characterised in that the rotating molten metal in the treatment chamber forms an open vortex and in that the additive is fed into the molten metal by supplying granules or wire containing the additive into the centre of the open vortex.

The invention also relates to device for implementing the method.

Description

The invention relates to a method for treating a molten metal suitable for producing a metal casting using an additive that is fed into the molten metal, wherein a stream of molten metal is introduced into a treatment chamber such that the introduced molten metal rotates in the treatment chamber, the treated molten metal being discharged from the lower end of the treatment chamber by gravity. The invention also relates to a device for the implementation of the method.
Feeding an additive into a molten metal is necessary for obtaining castable molten metals or for performing specific treatments. However, it is often difficult because the additive has a lower specific mass than the molten metal and thus remains floating on the molten metal for some time, and sometimes the additive will evaporate, oxidise or bum before it is mixed with the molten metal. Thus, it is very difficult to feed such additives into the molten metal.
According to EP 0030220 A2 this problem should be solved by using a device, in which the volatile and low-temperature evaporating additive is supplied to the rotating molten metal forming a vortex using pressurised gas, or the additive is supplied through an open chute into the treatment chamber. The molten metal is supplied into the treatment chamber through a conduit ending in the lower part of the treatment chamber. It has been found however that in use the additive often remains floating on the molten metal in the treatment chamber and that only slightly more than fifty percent of the additive is recovered in the molten metal. Moreover, it has been found that in use the rotating metal does not form a vortex at all, apparently caused by the presence of one or more baffles in the treatment chamber, and that the treatment chamber is easily choked by the floating additive. As a result, the method and device according to EP 0030220 have never been used in practice.
It is an object of the invention to provide a method for producing a metal casting using an additive that is fed into a molten metal, which method is improved in comparison with the state of the art.
It is another object of the invention to provide a method for producing a metal casting using an additive, which method provides a higher recovery of the additive in the molten metal than the known methods.
It is a further object of the invention to provide a method for producing a metal casting using an additive which method provides a constant recovery of the additive in the molten metal.
According to the invention one or more of these objects are reached by using a method for treating a molten metal suitable for producing a metal casting using an additive that is fed into the molten metal, wherein a stream of molten metal is introduced into a treatment chamber such that the introduced molten metal rotates in the treatment chamber, the treated molten metal being discharged from the lower end of the treatment chamber by gravity, and wherein the rotating molten metal in the treatment chamber forms an open vortex and the additive is fed into the molten metal by supplying granules or wire containing the additive into the centre of the open vortex.
The inventors have found that, surprisingly, the recovery of the additive is very much improved by feeding the granules into the centre of the open vortex of the molten metal. By doing so, the inventors found that the granules are directly immersed in the molten metal, and as a result virtually no oxidation of the additive will take place, and that the granules will not float on the molten metal, so no clogging of the treatment chamber will occur. The inventors thus found a high recovery of the additive in the molten metal discharged from the treatment chamber. Moreover, they found that the recovery rate is fairly stable during use and that the temperature of the treated molten metal was only slightly lowered as compared to the introduced molten metal. With the term 'open vortex' is meant here the surface of a rotating fluid which in cross section presents a parabolic shape that is lower to the centre of the rotation of the fluid, due to the forced rotation.
Preferably the granules are supplied into the centre of the open vortex by gravity. Using gravity is a very simple way of supplying the granules into the molten metal, which is very reproducable as well. No carrier gas is needed, of which the pressure could change during the use of the method, which would result in varying immersion of the granules in the molten metal. The use of gravity results in a very constant supply of the granules into the centre of the open vortex of the molten metal.
According to an improved embodiment the granules are supplied from a reservoir having a discharge opening which is situated perpendicularly above the centre of the open vortex. Thus, the granules will be supplied directly from the reservoir into the centre of the open vortex of the molten metal, without any additional guiding or introduction means such as a tube, which could get choked.
According to a preferred embodiment the granules have a diameter of 0,2 to 2 mm. With such a diameter, the granules can be supplied evenly into the centre of the open vortex of the molten metal. Using a smaller diameter could result in floating of the granules on the molten metal; using a larger diameter could result in clogging of the feeding means of the granules, such as the discharge opening of the granule reservoir.
It has been found that preferably the stream of molten metal is introduced into the treatment chamber in the upper half of the treatment chamber, more preferably under an angle that is slightly inclined towards the lower end of the treatment chamber. By introducing the stream of molten metal in the upper half of the treatment chamber an open vortex is formed, while it has been found that by introducing the stream of molten metal in the lower part of the treatment chamber the molten metal will rotate in the treatment chamber, but an open vortex will not easily be formed. Introducing the stream of molten metal under a slight angle has shown to form a stable open vortex.
Preferably the treated molten metal is liquid cast iron. Especially for cast iron it is essential that the additive is provided in a secure, safe way giving a constant and high recovery.
For cast iron, the additive is a nodulisation additive such as magnesium, used for producing a nodular or vermicular iron casting. On the one hand, magnesium has a boiling point that is lower than the casting temperature of cast iron, so it can easily vaporise in contact with molten iron. On the other hand, magnesium is less dense than iron and thus apt to float on molten iron. It has been found that the method according to the invention results in a high recovery of the magnesium in the molten iron.
In this case the granules or the wire preferably comprise a FeSiMg alloy. A FeSiMg alloy is the most widely used alloy for introducing magnesium into cast iron.
For liquid cast iron the additive can also be an inoculation additive such as a ferrosilicon alloy, preferably comprising 70-75 wt% Si, 0.005-3 wt% bismuth, lead or antimony and 0.005-3 wt% of at least one metal from the group of rare earths. This preferred alloy is usually called 'Sphérix'.
Other preferred embodiments of the method according to the invention are those where the treated molten metal is liquid steel and the additive is a desoxidising additive such as aluminium, for producing desoxidised steel, or where the treated molten metal is liquid steel and the additive is a grain refining additive, for producing grain refined steel.
Other additives are mentioned in WO 01/57280 , and the results are also discussed in WO 01/66813 .
According to a second aspect of the invention, a device for feeding an additive into a molten metal has been provided, wherein the device comprises a treatment chamber having an inlet opening for the introduction of molten metal and an outlet opening at its lower end, the treatment chamber being formed such that an open vortex is formed when molten metal is introduced through the inlet opening and discharged through the outlet opening during use, and wherein supply means are provided for supplying granules or wire into the treatment chamber such that the granules or wire are supplied into the centre of the open vortex during use. This apparatus can be used for implementing the method described above.
Preferably the supply means are formed by a granule reservoir having an outlet opening for the granules that is situated perpendicularly above the open vortex during use. The granule reservoir and especially the outlet opening can be formed such that they match the size of the treatment chamber and the amount of molten metal transported through the treatment chamber per second.
It can be advantageous when the outlet opening of the granule reservoir has been provided with an interchangeable valve. This is especially so when the amount of granules has to be controlled accurately and when the amount of granules has to be changed, for instance when the apparatus is to be used for different molten metal types.
According to a preferred embodiment the granule reservoir has been provided as a separate, interchangeable container in the device. In this way it is very easy to replace a container containing one type or size of granules by another container containing another type or size of granules.
Preferably a molten metal reservoir has been provided which preferably has been connected with the inlet opening of the treatment chamber by means of a conduit. Using a metal reservoir results in a constant flow of molten metal into the treatment chamber, and in that way a stable open vortex can be formed.
According to a preferred embodiment the device has been made of ceramic material. By choosing this material, the device can be used several times for treating molten metal.
Preferably one or more casting moulds are connected to the outlet opening of the treatment chamber. The molten metal treated with the additive in the treatment chamber will thus be cast into the casting mould(s) to form metal castings.
The invention will be elucidated referring to the accompanying drawing.

Figure 1 schematically shows a device according to the invention for introducing granules in a treatment chamber in which introduced molten metal forms an open vortex, in cross section.
Figure 2 shows the device of figure 1 in top view.

Figure 1 shows an embodiment of a device 1 for feeding an additive into a molten metal in accordance with the invention. The device has a molten metal reservoir 2 in which molten metal that has to be treated has to be poured and a treatment chamber 3 with an inlet opening 4 and an outlet opening 5 for treating the molten metal. The molten metal reservoir has been connected to the treatment chamber by means of a conduit 6, which conduit ends in the inlet opening 4 of the treatment chamber. As shown in Figure 2, the conduit 6 ends at a side of the treatment chamber 3 in the upper half thereof, under an angle that is slightly inclined towards the outlet opening of the treatment chamber as shown in Figure 1, for reasons as elucidated below.
Above the treatment chamber a granule reservoir 7 has been supplied, in which a separate container 8 has been placed, indicated by broken lines in Figure 1 (not in Figure 2). The container has been provided with a valve 9, schematically indicated in Figure 1, for releasing granules that are present in the container during use.
The device has been engineered such that molten metal flowing through conduit 6 enters the upper part of the treatment chamber with a velocity and under an angle, resulting in a whirling movement of the molten metal. This whirling movement causes an open vortex in the molten metal. It will be clear that the outlet opening 5 of the treatment chamber should not be designed too small, as a result of which the treatment chamber would be filled with molten metal and no open vortex would be formed, and also not too large, as a result of which all molten metal would flow out of the treatment chamber and an open vortex would not be formed as well. The open vortex thus fills approximately the lower half of the treatment chamber.
The method for treating a molten metal in accordance with the invention will be elucidated below.
A container 8 with granules containing an additive is placed in the granule reservoir 7 of device 1 while the valve 9 is closed. When molten metal is poured into the molten metal reservoir 2, it streams through the conduit 6 and enters the upper part of the treatment chamber 3 through the inlet opening 4, thus engaging in a whirling motion, resulting in an open vortex in the molten metal in the treatment chamber. At the same time, the valve 9 is opened (it is known in the art what ways there are to do so) and a dosed stream of granules from the container 8 falls in the centre of the open vortex in the molten metal in the treatment chamber. Here, the granules are directly immersed in the molten metal due to the high velocity of the molten metal at the surface of the open vortex. In this way, it is not possible for the granules to remain floated on the molten metal. Due to the whirling motion of the molten metal in the treatment chamber, the granules dissolve fast in the molten metal and a high recovery rate of the additive in the molten metal is achieved.
The treated molten metal leaving the outlet opening 5 is preferably directly guided into a mould or sequentially into a number of moulds. In such a situation, it is advisable to provide a ventilation duct for the treatment chamber 3 in the device. It is however also possible to use the treated molten metal leaving the outlet opening 5 to fill a ladle, from which the metal can subsequently be cast.
The method and device according to the invention can be very well used for treating cast iron, for instance by adding granules containing a nodulisation additive, such as magnesium, or by adding an inoculation additive, such as Sphérix. The method and device can also be used for treating steel by adding a desoxidising additive such as aluminium or a grain refining additive. For such additives is referred to WO 01/57280 and WO 01/66813 , in which the results of their use is discussed.
As an example, the device 1 has been made of a ceramic material, and the molten metal is poured into the metal reservoir 2 with a pouring rate of approximately 1 kg/s. The treatment chamber 3 has a diameter of approximately 80 mm, and care has to be taken that the outlet opening 5 of treatment chamber 3 and the outlet opening of metal reservoir 2 have a diameter that is tuned to hold the treatment chamber partly filled with the molten metal. For example, the outlet opening of metal reservoir 2 has a diameter of 13 mm and the outlet opening 5 has a diameter of 24 mm. The valve 9 of the container 8 has a calibrated opening, such that the required amount of granules enters the treatment chamber.
It will be understood that the calibrated opening for the granules in the valve 9 of the container 8 has to be tuned to the kind of molten material to be treated with the additive, the kind of additive and the kind and size of the granules, and the amount of liquid molten metal supplied into the treatment chamber per second.
As an example of the way in which the device can be used, magnesium is added to liquid cast iron in the form of granules, which have the following composition: 5.8 % Mg, 46 % of Si, 0.48 % of Ca, 0.36 % of La, 1.1 % of Al, balance Fe and inevitable impurities (all elements in wt%). The amount of granules introduced in the treatment chamber is 1.1 % of the weight of the casting.
As an example, the device has been installed on top of a horizontal stack mould consisting of six moulds that are sequentially filled. The result of the magnesium analysis of the six moulds showed magnesium percentages between 0.047 % and 0.050 %. The level of regularity of the reaction thus is almost perfect. Moreover, the obtained magnesium recovery has been found to be 77 %, which is very high, and the device showed no traces of slag formation after the treatment.
It will be understood that modifications to the device and method can be made by the skilled person, as long as an open vortex is formed in the treatment chamber and granules are supplied in the centre of the open vortex.
However, it is also possible to replace the granules by a wire that is fed into the centre of the open vortex, where the wire melts and is dissolved in the molten metal. In this situation a container with a valve is not needed, and the wire has to be fed into the treatment chamber with a velocity that is tuned with the amount of molten metal supplied per second.

Claims

Method for treating a molten metal suitable for producing a metal casting using an additive that is fed into the molten metal, wherein a stream of molten metal is introduced into a treatment chamber such that the introduced molten metal rotates in the treatment chamber, the treated molten metal being discharged from the lower end of the treatment chamber by gravity, characterised in that the rotating molten metal in the treatment chamber forms an open vortex and in that the additive is fed into the molten metal by supplying granules or wire containing the additive into the centre of the open vortex.
Method according to claim 1, wherein the granules are supplied into the centre of the open vortex by gravity.
Method according to claim 1 or 2, wherein the granules are supplied from a reservoir having a discharge opening which is situated perpendicularly above the centre of the open vortex.
Method according to any one of the preceding claims, wherein the granules have a diameter of 0,2 to 2 mm.
Method according to any one of the preceding claims, wherein the stream of molten metal is introduced into the treatment chamber in the upper half of the treatment chamber, preferably under an angle that is slightly inclined towards the lower end of the treatment chamber.
Method according to any one of the preceding claims, wherein the treated molten metal is liquid cast iron.
Method according to claim 6, wherein the additive is a nodulisation additive such as magnesium, used for producing a nodular or vermicular graphite iron casting.
Method according to claim 7, wherein the granules or the wire comprise a FeSiMg alloy.
Method according to claim 6, wherein the additive is an inoculation additive such as a ferrosilicon alloy, preferably comprising 70-75 wt% Si, 0.005-3 wt% bismuth, lead or antimony and 0.005-3 wt% of at least one metal from the group of rare earths.
Method according to any one of the claim 1 - 5, wherein the treated molten metal is liquid steel and the additive is a desoxidising additive such as aluminium, for producing desoxidised steel.
Method according to any one of the claim 1 - 5, wherein the treated molten metal is liquid steel and the additive is a grain refining additive, for producing grain refined steel.
Device for feeding an additive into a molten metal, characterised in that the device comprises a treatment chamber having an inlet opening for the introduction of molten metal and an outlet opening at its lower end, the treatment chamber being formed such that an open vortex is formed when molten metal is introduced through the inlet opening and discharged through the outlet opening during use, and in that supply means are provided for supplying granules or wire into the treatment chamber such that the granules or wire are supplied into the centre of the open vortex during use.
Device according to claim 12, wherein the supply means are formed by a granule reservoir having an outlet opening for the granules that is situated perpendicularly above the open vortex during use.
Device according to claim 13, wherein the outlet opening of the granule reservoir has been provided with an interchangeable valve.
Device according to claim 13 or 14, wherein the granule reservoir has been provided as a separate, interchangeable container in the device.
Device according to any one of the claims 12 to 15, wherein a molten metal reservoir has been provided, preferably connected with the inlet opening of the treatment chamber by means of a conduit.
Device according to any one of the claims 12 to 16, wherein the device has been made of ceramic material.
Device according to any one of the claims 12 to 17, wherein one or more casting moulds are connected to the outlet opening of the treatment chamber.