EP0280765B1 - Method of and installation for producing castings from pressure treated melts from steel alloys - Google Patents

Abstract

A method of producing castings, such as blocks, billets or dead-mould castings from metals, in particular steel and steel-like alloys, which have higher proportions of elements with high vapour pressure or gases than at atmospheric pressure in the molten state and remain in solution during solidification is to be improved by virtue of the fact that a melt produced in an open melting unit is poured first of all under atmospheric pressure into a refractory-lined, heatable treatment vessel, a gas pressure above atmospheric pressure is then built up over the melt and is maintained during the entire heating and treatment time, the melt is continuously stirred and/or flushed by means of gas and, either by flushing with the elements or substances in the gaseous state and/or by the addition of alloys containing the element or substance, is brought to the desired content of this element or substance, the gas pressure above the melt corresponding at least to the partial pressure of the corresponding element or substance, which partial pressure is in equilibrium with the desired content of the melt, and that the melt, after the composition and the casting temperature are set, is poured directly from the treatment vessel via a closure and casting element which can be closed and opened in a suitable manner into an ingot mould, likewise under pressure, a ceramic or metallic casting mould or a continuous-casting ingot mould, or that the melt is pushed directly into a casting mould according to the die casting method by increasing the pressure above the melt, and the casting thus formed is left to solidify.

Description

The invention relates to a process for the production of castings, such as blocks, strands or molded parts made of metals, in particular of steels and steel-like alloys, which have higher contents of elements with high vapor pressure or gases than in the molten state at atmospheric pressure and remain in solution during solidification , and a device according to the preambles of claims 7, 8 and 11.

The alloying of metals and steel alloys with elements which are either gaseous at the temperature of the liquid, pourable metal or are in the vaporous state is repeatedly discussed in metallurgical process engineering. It is understood that such volatile elements or compounds have little solubility in the liquid metal. In many cases, however, efforts are made to increase the solubility of such substances in the melt, or to keep the substances in solution even when the melt solidifies.

Examples of this are the treatment of molten steel with alkaline earth metals, in particular with Ca, which usually has only a very limited solubility and has a vapor pressure of over 1 bar at the temperatures of the molten steel.

Another example is nitrogen, which is also only soluble to a limited extent, depending on the alloy composition.

In principle, this also applies to hydrogen, but this is generally not a desired accompanying element.

In order to increase the content of volatile or gaseous substances in metals, steels and alloys, melting or electroslag remelting under pressures above atmospheric pressure has been proposed, and a number of plant technology options have been described and sometimes only on a laboratory scale executed.

Treatment of the liquid melt under excess pressure with alkaline earth metals, in particular with Ca, allows, for example, the conversion of the phosphorus from the metal into the slag under reducing conditions. Such a treatment also enables the deposition of Cu, Sn etc. and their transfer into the slag.

oder
${\text{N = K . p}}^{\text{1/2}}$

On the other hand, melting under excess pressure allows the solubility of nitrogen in the metal to be increased, the solubility of the diatomic gas nitrogen in the melt being determined from the square root of the partial pressure of nitrogen over the melt according to the square root law, according to:

$\text{N = K. P (I)}$

or
${\text{N = K. p}}^{\text{1/2}}$

In equation I, N represents the nitrogen content in the melt in percent by weight, K represents a proportionality constant, the size of which is determined, among other things, by the alloy composition, and P the partial pressure of the nitrogen in bar.

The element nitrogen is an interesting alloying element in that it is able to stabilize austenite as a structural component in iron alloys and to increase its strength. Therefore, efforts are often made to set higher nitrogen contents in iron-based alloys than the solubility at atmospheric pressure.

So far, a number of process proposals have already been made for this, although many have not yet announced a large-scale implementation.

The best known processes proposed so far are pressure induction melting, pressure plasma arc melting and pressure electro-slag remelting.

In pressure induction melting, a conventional induction furnace is installed in a pressure chamber and the melt is produced under a pressure that is Setting the desired nitrogen content in the melt allows. The melt is then poured off under pressure and also allowed to solidify under pressure in the same pressure chamber. This procedure is common in laboratory systems and has been carried out several times for melt sizes up to 100 kg.

In the pressure plasma arc process, the metal is melted by a plasma arc in a melting vessel, nitrogen being added to the gas of the plasma torch. Since the nitrogen in the plasma is converted into the monatomic form, the solubility in the melt is increased at the same pressure, since this becomes directly proportional to the partial pressure of the nitrogen in the gas. In order to avoid pore formation by N₂ during the subsequent solidification of the melt - where the square root law is valid again - depending on the pressure, the plasma gas must also contain argon in addition to nitrogen in order to limit the nitrogen contents in the melt accordingly. To date, such a plant has been announced in the Soviet Union, which produces blocks up to about 1 t in weight.

So far, only the pressure-electroslag remelting process has become known on an industrial scale. Plants for the production of blocks of 2.5 and 14.5 t have already been built and operated here.

While in pressure induction melting and pressure plasma arc melting, the nitrogen absorption of the melt essentially takes place from the gas phase above the melt, this is not possible with electro-slag remelting. In this process, the melt bath is covered during the entire remelting process by a molten slag, the solubility of which is low for nitrogen.

However, this also means that nitrogen transport from the gas phase via the slag into the metal is not possible in a controllable and reproducible manner. One now makes use of the fact that either nitrogen-containing compounds are added continuously to the slag in fine-grained form or that, according to another proposal, composite electrodes are used, some of which consist of alloys which have a high solubility for nitrogen even at atmospheric pressure. This is just as possible as the addition of nitrogen-containing compounds, since when the electroslag is remelted, the bottom is mixed thoroughly and thus the nitrogen is evenly distributed.

However, both methods have disadvantages that have an unfavorable effect on operational safety and process costs. When alloys are added, this must be done continuously under pressure depending on the melting rate. The production of long, composite electrodes is complex and can only be carried out with special casting devices.

In addition, the electro-slag remelting results in a high block quality, but in itself works inexpensively, since the melting rate is limited depending on the block format.

The production of irregular, arbitrarily shaped castings is practically impossible using this method. It is also impossible to produce relatively long thin strands, as is the case with continuous casting.

A device of the type described in the introduction can be found in DE-A-32 32 551, which describes a brick ladle for casting steel, from which the steel is drawn off by a slide closure.

Knowing this prior art, the invention is based on the object, while avoiding the disadvantages described above, to bring any metal melt produced under excess pressure in a targeted manner to desired levels of elements with high vapor pressure or gases that are above solubility at atmospheric pressure and then to bring them into pour off any mold and also let it solidify under pressure.

The solution to this problem is that any pre-melt produced by any open melting unit is first poured under atmospheric pressure into a - refractory - heated treatment vessel, then a gas pressure above atmospheric pressure is built up over the melt and maintained throughout the heating and treatment time - - The melt can be covered by a slag -, the melt is continuously stirred and / or flushed with gas and either by flushing with the elements or substances in the gaseous state and / or by adding alloys containing the element or the substance to the desired content of this element or substance, the gas pressure above the melt being at least that with the desired content of the melt in Equilibrium partial pressure of the corresponding element or substance corresponds, and that the melt after adjusting the composition and the casting temperature directly from the treatment vessel via a closable and openable closure and pouring element in a likewise under pressure - conventional - mold, ceramic or metallic mold or continuous casting mold is poured. According to another solution according to the invention, the melt is instead pressed directly into a casting mold by increasing the pressure above the melt by the - known per se - method of die casting. The cast body thus formed is allowed to solidify, the pressure above the mold being maintained throughout the solidification phase; according to the invention, the pressure is kept or set as high as it corresponds to the vapor pressure of the element or substance alloyed under pressure in the melt during the transition from the liquid to the solid state.

Further features of the method can be found in claims 2 to 6.

Devices according to the features of claims 7, 8 and 11 for which independent protection is desired are particularly suitable for carrying out this method. A device according to the invention has a conventional ladle as a container, which is placed in a closable pressure tank.

In another preferred embodiment, the pressure vessel itself also forms the treatment vessel and is delivered fireproof.

Gas purging in the treatment vessel is expediently carried out using a gas purging plug, but in principle a purging plug can also be used. Stirring of the melt is generally effected by gas purging, but inductive stirring is also possible.

The melt can be heated inductively in the closed treatment vessel. Heating by means of a plasma torch or electric arc is also possible.

It is particularly important to heat the melt by one or more electrodes immersed in an electrically conductive slag according to the principle of electric slag heating.

The alloying of the melt with the substances which are only soluble to a greater extent under pressure can be carried out in the case of gases such as nitrogen by flushing the melt with this gas. However, it is also possible to add alloys in which the respective gas is bound to a greater extent, as is the case, for example, with nitrogen when using embroidered alloys or nitrides.

In the case of hydrogen, higher levels can only be achieved by flushing the melt.

If elements with high vapor pressure at the temperature of the melt are to be alloyed - for example Mg or Ca or also Na - this can be done by direct addition into the pressurized melt.

In the pressurized melt treatment vessel, in combination with the simple addition of Ca compounds or pure Ca, all process steps for the decomposition of P, Cu, Sn, As, Sb and others can be advantageously carried out under reducing conditions in combination with a suitable slag guide. If, subsequently, the pressure above the melt and slag does not have to be reduced when the melt is poured, which is not necessary in the process according to the invention, a return migration of the removed steel companions into the melt can also be largely avoided. In principle, however, potting at atmospheric pressure is also possible with this treatment.

The casting of the melt treated in the treatment vessel and brought to temperature can in principle be carried out in various ways, which will be described in more detail below.

For the rest, reference is made to the content of the claims.

Further advantages, features and details of the invention result from the description of preferred exemplary embodiments and from the drawing. In its four figures, this shows sectional images through systems and devices according to the invention.

As can be seen in FIG. 1, a melt 10 is provided in a pressure vessel 12 closed by a lid 11 in a conventional ladle 13, which is opened by opening a slide closure 15 engaging under a melt outlet 14 into a one under the ladle 13 - in turn in the pressure vessel 12 - Mold or mold 16, is poured. The ladle 13 is closed by a pan lid 17 serving as radiation protection.

The melt 10 located in the ladle 13 is covered by a slag bath 20. The latter is heated in this example according to the principle of electroslag heating by means of an immersed electrode 22, which is moved via a current-carrying electrode rod 24 in the interior 25 of the ladle 13 through a bushing 23.

The current is returned from the melt 10 via a counter electrode 26 in the wall 27 of the ladle 13 to a flange 28 of the pressure vessel 12 and from there via a line 29 to a current source 30.

The ladle 13 has a gas purging plug 32 and that melt outlet 14 with slide closure 15. Below the ladle 13, the mold 16 is arranged, into which the melt 10 is also poured off under pressure after the treatment has been completed. A pressure supply and discharge line 34 is installed in the cover 11.

In the exemplary embodiment in FIG. 2, a bricked-up treatment vessel 18 closed with a vessel lid 19 receives the melt 10 covered by the slag bath 20. Heating takes place in the manner already described via an electrode 22 immersed in the slag bath 20, which is moved in the treatment vessel 18 by means of the current-carrying electrode rod 24 passing through the pressure feedthrough 23. Here too, a slide closure 15 is used as the casting device, which is installed in a slide chamber 40, which is likewise under pressure and can be closed with a closure member 36. A flange 37 adjoins the closure member 36 and can be pressure-tightly connected to a counter flange 38 on the container lid 42 of a container 44 of a casting chamber 46.

A closure member 41 is installed between the counter flange 38 and the container lid 42, with the aid of which the casting chamber 46 can be closed in a pressure-tight manner. The interior 25 of the treatment vessel 18, the slide chamber 40 and the casting chamber 46 are connected to a pressure supply and discharge line 34 - which also serves as a pressure compensation line.

Before casting, a casting mold 16 is placed in the casting chamber 46, and the flanges 37, 38 of the casting chamber 46 and slide chamber 40 are connected in a pressure-tight manner, the closure member 41 remaining open. The casting chamber 46 is then brought to the same pressure that prevails in the treatment vessel 18. The closure member 36 under the slide chamber 40 can now be opened.

The system is now ready for casting. The casting process is initiated by opening the slide closure 15. After the casting, the melt 10 is solidified under pressure. In order not to block the treatment vessel 18, the closure member 41 installed below the connecting flange 38 to the slide chamber 40 can be closed. It is then possible to release the pressure from the treatment vessel 18 and the slide chamber 40 attached to it, to release the connection of the flanges 37, 38 between the slide chamber 40 and the casting chamber 46, and either to remove the latter with the casting or to remove the treatment vessel 18 and this in to prepare for a new treatment.

Another possibility is pouring through a siphon tube 50, the melt 10 being pressed through the siphon tube 50, preferably by an overpressure in the treatment vessel 18. 3 shows the bricked-up treatment vessel 18 with the melt 10. Instead of a slide closure, here the siphon tube 50 - starting from the lowest point in the vessel bottom 48 - is provided, which can be locked by a closure member 36. For this purpose, the flange 37 for the connection to the counter flange 38 is again attached to the cover 42 of the casting chamber 46 described above. Connecting and separating the casting chamber 46 and siphon tube 50 is carried out in the manner described above.

In this arrangement, the casting is carried out by increasing the pressure in the treatment vessel 18.

During the heating and pressure treatment in the treatment vessel 18, the outlet of the siphon tube 50 is kept closed by that closure member 36. If a higher pressure is maintained in the siphon tube 50 than above the melt 10 in the treatment vessel 18, the melt 10 can be pushed back to the bottom and freezing in the unheated siphon tube 50 can be avoided. Alternatively, it is also possible to keep the siphon tube 50 inductively warm.

When the melt 10 is ready for casting, the flange connection 37, 38 is closed again and, with the closure member 41 open, the pressure in the casting chamber 46 is built up until it corresponds to that in the siphon tube 50. Now the closure member 36 can be opened - the system is ready to pour. The casting is now initiated by increasing the pressure above the melt 10 in the treatment vessel 18. As a result, the melt 10 is pushed up in the siphon tube 50 projecting laterally upwards until it reaches an edge at 52 at which the siphon tube 50 is guided downwards again at a distance from the bottom 48 of the vessel; the melt 10 runs over the edge 52 into the mold 16 below. During the casting, the pressure in the treatment vessel 18 is continuously increased until the mold 16 is filled.

Then the pressure above the melt 10 is reduced, whereby the casting process is interrupted. Now the shut-off device 36 is closed and - provided there is still melt 10 in the treatment vessel 18 - the pressure in the siphon tube 50 is increased in order to push the melt 10 back again.

After closing the shut-off element 41 and venting the intermediate space, the flange connection 37, 38 between the siphon tube 50 and the casting chamber 46 can be opened, the latter can be exchanged with a casting for a new casting chamber 46, and again - as described above - connected for a named casting process. If the melt 10 is consumed, only the closure member 41 is closed after the casting, and the treatment vessel 18 and the siphon tube 50 are relieved of pressure.

The flange connection 37, 38 can then be released, which makes it possible to prepare the treatment vessel 18 for the reception of a new melt 10.

In principle, instead of the siphon tube 50, a slide closure can also be attached to the side of the treatment vessel 18, which is again housed in its own slide chamber 40 _e, which is closed with a closure member and can be pressurized. An arrangement of this type for the continuous casting of pressure-treated melts is shown in FIG. 4. On the treatment vessel 18 there is a slide closure 15 on the wall 27 integrated into the lateral slide chamber 40 _e . The slide chamber 40 _e can be closed again with a closure member 36, to which a flange 37 connects, which can be closed in a pressure-tight manner with the counter flange 38 of the horizontally arranged casting chamber 46 _{e of} a horizontal container 44 _e . The horizontal continuous casting mold 16 _e can be moved up to the slide closure 15 and flanged when the closure member 36 is open.

After opening the slide closure 15, the stripping can be started. Strand pulling device and cutting device - not shown here for reasons of clarity - are also built into the casting chamber 46 _e .

Since during the solidification, ie during the transition from the liquid to the solid state of matter, the dissolving power for gases generally decreases, it may be expedient to further increase the pressure in the casting chamber 46, 46 _e immediately after the casting process until at least the corresponding one Equilibrium pressure is reached, which ensures pore- and bubble-free solidification.

Claims (15)

1. Method of producing castings such as ingots, billets or moulded articles from metals, in particular from steels and steel-like alloys which have higher contents of elements with high vapour pressure or gases than at atmospheric pressure in the molten state and remain in solution during solidification, characterised in that a melt (10) produced in an open melting unit is initially cast under atmospheric pressure into a heatable treatment vessel (13, 18), a gas pressure above atmospheric pressure is then built up over the melt (10) and is maintained during the entire heating and treatment period, the melt (10) is continuously stirred and/or permeated by gas and, either by permeation with the elements or substances existing in the gaseous state and/or by addition of alloys containing the element or the substance, is brought to the desired content of this element or substance, the gas pressure over the melt (10) corresponding at least to the partial pressure of the corresponding element or substance, which is in equilibrium with the desired content of the melt, and in that, after adjustment of the composition and the casting temperature, the melt is cast directly from the treatment vessel (13, 14) via a closable and openable closure and casting element (15) into a chill (16), ceramic or metallic casting mould or continuous casting mould (16e) or in that the melt (10) is pressed directly by increasing the pressure above the melt into a mould by the die casting method, whereupon the resultant casting is exposed to solidification.
2. Method according to claim 1, characterised in that the melt is covered with slag during the heating and treatment period and/or in that the pressure over the mould (16) is maintained during the entire solidification phase.
3. Method according to claim 1 or 2, characterised in that nitrogen is used as gaseous element with which the melt is alloyed and/or in that the elements existing in the gaseous state at the melting temperature and at atmospheric pressure are one or more element(s) of group 2 or 2a of the periodic system.
4. Method according to one of claims 1 to 3, characterised in that the pressure-generating gas is nitrogen or argon.
5. Method according to one of claims 1 to 4, characterised in that the gas pressure above the casting is increased during solidification sufficiently to prevent pore or gas bubble formation caused by a possible leap in solubility during solidification.
6. Method according to at least one of claims 1 to 5, characterised in that the heating of the melt (10) in the treatment vessel (13, 18) is carried out inductively or with a plasma torch or via one or more graphite electrodes by an electric arc or by means of at least one consumable or nonconsumable electrode (22) immersed into an electrically conducting slag, by the electroslag heating process.
7. Apparatus with at least one container having a refractory lining and a melt outlet (14) with closure, in particular slide closure (15) , and a gas inlet (gas flushing block 32), for a melt (10) which can be charged with heat therein, in particular for carrying out the method according to at least one of claims 1 to 6, characterised in that the container is constructed as a heatable casting ladle (13) and is arranged in a pressure vessel (12) above a mould (16) which receives the melt (10) from the casting ladle (13) and is also erected in the pressure vessel.
8. Apparatus with at least one container having a refractory lining and a melt outlet (14) with closure, in particular slide closure (15), and a gas inlet (gas flushing block 32), for a melt (10) which can be charged with heat therein, in particular for carrying out the method according to at least one of claims 1 to 7, characterised in that the container is constructed as a heatable treatment vessel (18) which can be closed in a pressure-tight manner by a vessel lid (19) and its melt outlet (14) is provided with at least one pressure-tight closure member (36) and a connecting element (flange 37) as a partner for an opposing element (opposing flange 38), the opposing element being arranged on a pressure-tightly closable container (44) for a mould (16) receiving the melt (10) from the treatment vessel (18), the opposing element (opposing flange 38) preferably resting with a closure member (41) on the container lid (42) of the container (44) limiting a casting chamber (46).
9. Apparatus according to claim 8, characterised in that a syphon tube (50) leading upwardly at an inclination substantially from the bottom of the vessel base (48) is arranged between the treatment vessel (18) and pressure-tight closure member (36) and is constructed such that it can be connected to the die casting chamber (46).
10. Apparatus according to claim 8 or 9, characterised in that the syphon tube (50) is guided downwardly again at a distance from the vessel base (48) and, substantially at the level of a pressure flange of the vessel lid (19), has an edge (52) formed by the deflection and passes from this edge (52) into a substantially vertical end.
11. Apparatus with at least one container having a refractory lining and a melt outlet (14) with closure, in particular slide closure (15), and a gas inlet (gas flushing block 32), for a melt (10) which can be charged with heat therein, in particular for carrying out the method according to one of claims 1 to 6, characterised in that the container is constructed as a heatable treatment vessel (18) which can be closed in a pressure-tight manner by a vessel lid (19) and its melt outlet (14) is provided with at least one pressure-tight closure member (36) and a connecting element (flange 37) as a partner for an opposing element (opposing flange 38), the opposing flange being arranged on a substantially horizontal casting chamber (44_e) for a chill (16_e) which receives the melt (10) from the treatment vessel (18) and is accordingly orientated.
12. Apparatus according to one of claims 8 to 11, characterised in that a pressure-tight closure member (36) as connecting element is arranged on the treatment vessel (18) downstream of its melt outlet (14) and a slider chamber (40) is optionally disposed between the pressure-tight closure member (36) and the closure (15) of the treatment vessel (18).
13. Apparatus according to at least one of claims 8 to 12, characterised in that the melt outlet (14), closed by a closure member (15), of the treatment vessel (18) with the slider chamber (40_e), the closure member (36) and the flange (37) is arranged laterally at the lowest point of the treatment vessel (18) for connection to the casting chamber (46_e).
14. Apparatus according to at least one of claims 8 to 13, characterised in that the casting chamber (46, 46_e) for receiving the chill, mould (16) or a continuous casting unit for casting under pressure has an opposing flange (38) for pressure-tight connection to the flange (37) of the pressure chamber of the treatment vessel (18), a pressure-tight closure member (41) optionally being connected to the flange (38).
15. Apparatus according to at least one of claims 7 to 14, characterised in that the optionally water-cooled casting chamber (46, 46_e) is connected to the treatment chamber (13, 18) and/or the slider chamber (40) by a pressure compensating line (34).

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