EP0100290B1 - Process for casting metals using magnetic fields - Google Patents

Abstract

A process for casting metals comprising simultaneously applying a stationary magnetic field and a variable magnetic field to the molten metal in the course of solidification. The magnetic fields act in the same direction and generate radial vibrations in the metal, causing the cast metal to have an improved homogeneous structure and surface condition.

Description

The present invention relates to a metal casting process in which magnetic fields are made to act in order to improve the structure and the surface condition of the products obtained.

It is known to cast metals such as steel, aluminum and its alloys, in the form of billets, plates or ingots, either by. molding, either by vertical continuous casting.

In the first case, the mold is filled with a known quantity of liquid metal and solidification takes place inside the mold by heat exchange with the wall of the latter.

In the second case, the molten metal is poured into a mold generally having a vertical axis of symmetry, open at its two ends and whose side walls are cooled by a fluid. On contact with the mold, the metal solidifies to form a crust which gradually thickens as the ingot progresses through the mold so that at the lower end of the mold its thickness is sufficient to retain the central part which is still liquid. . After leaving the mold, the wall of the ingot is cooled directly by a spray of water and a complete solidification of the metallic mass is thus obtained fairly quickly.

In the two processes, the ingot obtained is very heterogeneous because it has both a cortical zone which has formed by rapid solidification on contact with the mold, and a central zone resulting from slower cooling by exchange with the wall of the mold or with water through the cortical area. These zones having crystallized at different speeds, do not have at all the same structure nor the same composition. In addition, the skin of the ingot which has formed in contact with the mold is generally irregular. Also, it is most often required to subject ingots to additional scalping or peeling operations to eliminate disturbed areas and thus avoid the appearance of defects which would result during subsequent stages of processing of the ingot.

This is why those skilled in the art, with the particular aim of improving the quality of the products produced by casting, have sought solutions to solve these problems of surface and structural heterogeneity. He turned most often to the use of magnetic fields intended to agitate the liquid part of the ingot during formation, and, more particularly, to rotating fields.

Thus, in US Pat. No. 2,963,758, a process is claimed for reducing the development of basalt grains, which consists in subjecting the molten metal to a magnetic field in the direction of the thermal gradient of cooling the ingot and moving said field continuously in a direction substantially perpendicular to the direction of said gradient. Obtaining this field requires the installation of a six-pole stator, connected to a multi-phase electric current source. Hence a fairly complicated installation whose efficiency is limited to the size of the grains.

In US Pat. No. 3,153,820, the invention relates to an apparatus for improving the structure and the physical and chemical homogeneity of the cast metal by controlling the solidification phenomenon. This apparatus comprises, in combination, a plurality of external agitators operating independently, among which there are electromagnets and electromechanical vibration transducers arranged outside the mass of metal, regularly spaced apart and placed near the metal cooling zone, said stirrers producing a plurality of fields of stirring forces concentrated inside the metal.

Such a device including several types of devices of a rather complex design. although it reduces the phenomena of segregation inside the metal, does not solve the problems of surface finish.

EP-A-0 040383 teaches a method for agitating the non-solidified portion in a continuous casting, in which at least one static magnetic field and possibly an alternating magnetic field is produced which is produced by an agitator supplied with polyphase alternating current. It can be noted that these agitators are arranged on a limited portion of the wall of the mold and exert their field perpendicular to the pouring jet entering the bath.

This is why the applicant, anxious to improve the casting technique with a view to obtaining both more homogeneous structures, and surface states directly accommodating the subsequent dimensional transformation operations without going through a phase d 'peeling, sought and developed a process using a simple material but used in conditions such that the efficiency is exacerbated.

The method according to the invention consists in causing a stationary magnetic field and a variable magnetic field to act simultaneously on the metal in the course of solidification and is characterized in that a uniform stationary magnetic field is created, of direction parallel to the axis. of the mold by means of at least one coil of contour similar to that of the upper part of the mold, a variable magnetic field of the same direction as the other field by means of at least one annular inductor supplied with single-phase alternating current.

The uniform magnetic field is created by at least one coil supplied with direct current. This coil is constituted by an electric wire wound on a frame whose section by a horizontal plane has a contour similar to that of the upper section of the mold and it is placed above the mold.

Under the action of direct current, this coil creates a uniform field of general direction parallel to the axis of symmetry of the mold, that is to say substantially vertical and oriented either downwards or upwards. The lines of this field can be modified by incorporating inside the coil an iron core which follows its outline while leaving in its center sufficient space for the supply of liquid metal to the mold.

The variable magnetic field is created by an annular inductor of similar geometry, but supplied with periodic current of frequency N. This inductor is placed above the mold, either above the coil supplied with direct current, or between the latter and the mold.

It can still be leveled with the mold. In the latter case, when there are two coils supplied with direct current, it is located between them. Under the action of the periodic current, the inductor creates a collinear variable field with the stationary field which produces in the metal an induced current. The density vector is located in a generally horizontal plane and directed perpendicular to a straight line of this plane passing through the axis of the mold. All of these vectors therefore form concentric circles.

The combined action of these collinear fields generates vibrations in the metal which have a double origin. On the one hand, the stationary field and the induced current develop a force perpendicular to the plane constituted by the current density vector and the direction vector of the stationary field. This force is therefore contained in a horizontal plane and directed towards the axis of the mold. This force has an intensity which varies periodically with the same frequency N as the variable field and therefore causes vibrations in the metal.

On the other hand, due to the interaction of the variable field and the induced current both of frequency N, develops another radial force, also variable, but whose frequency is 2 N.

Thus, the metal is subjected to these two radial forces of frequency N and 2 N, from which results in a generalized vibration of the mass of liquid metal.

As a result of certain faults in the geometry of the system, it is possible to have more or less significant edge effects, from which vertical vibrations result, but the latter have relatively little importance compared to the radial vibrations.

The periodic current which supplies the variable field inductor can have a purely sinusoidal shape, but any other shape is also suitable for carrying out the invention.

As for the frequency, it can cover a whole range of values going from 5 to 100,000 hertz. However, a distinction should be made between the so-called low frequencies between 5 and 100 hertz and the higher, so-called medium frequencies.

In the first case, the so-called “variable field skin effect is reduced, that is to say that the induced current exerts its action on a thickness of metal such that there is sufficient interaction of this current with the stationary field to develop vibrations throughout the mass of metal. We then say that we are working in forced vibrations. On the other hand, as the frequency of the variable field increases, the skin effect becomes significant and the interaction of the induced current with the stationary field becomes increasingly weak. It is then necessary that the vibrations emitted, to have a suitable efficiency, can come into resonance with the proper vibrations of the liquid metal, of the dendrites being formed or of the solid mass. However, these own vibrations depend on the format of the cast product. casting speed, nature of the metal, and cooling conditions. It will therefore be necessary to choose the frequency of the current as a function of the operating conditions, which can be deduced from the calculation or from measurements made using suitable sensors.

It is certain that, in the case of low frequencies, the technology and the operating mode are simpler and that the nuisance resulting from noise due to vibration is relatively low.

In a variant of the method applied in particular to the continuous casting of steel, it is advantageous to create stationary and variable magnetic fields by means of a series of coils and inductors which alternate in succession throughout the part of the metal being solidified. Indeed, in this case, this part of the ingot can be relatively long and the effectiveness of the fields is then obtained by multiplying the number of coils and inductors.

To further increase this efficiency, it is preferable, when the variable field has a frequency lower than 100 hertz, to supply the inductors with a decreasing frequency as the solidification progresses. The skin effect is thus attenuated and the vibrations inside the ingot are developed. One can have for example at the level of the inductor supplied with 50 hertz then below, and separated each time by a DC coil, a series of inductors supplied successively with 20, 10, and 5 hertz for example.

The present invention will be better understood by referring to the appended drawing in which FIG. 1 represents a vertical section passing through the axis of a continuous casting mold to which a variable field and a uniform field according to the invention are applied. Figure 2 is a top view of the metal in the mold.

In FIG. 1, there is a mold (1) cooled by a circulation of water (2) by means of which an ingot (4) is formed from a liquid metal (3). According to the invention, the mold has been fitted on top of a coil (5) which creates a stationary field whose lines of force can be modified by the core (6). At the level of the mold is placed an inductor (7) which creates a variable field. Under the combined action of the fields, develops in the liquid of the vibra tions whose direction of propagation is represented by the arrows (8). In Figure 2, we see the drawing of the vibrating liquid metal in the directions (8).

The invention can be illustrated by means of the following non-limiting example.

An aluminum alloy of the 2024 type, previously refined, was continuously cast by adding 0.1% by weight of ATSB in the form of a plate with a section of 300 x 800 mm.

The first part was carried out in a conventional ingot mold, then the casting was continued under the same conditions of speed and cooling, but by applying near the free surface of the metal, on the one hand, a stationary magnetic field of 0 , 04 tesla, created by means of an annular coil supplied by a direct current of 17500 ampere-turns, under a voltage of 24 volts, on the other hand, a variable magnetic field of frequency 50 hertz, created by a annular coil placed below the previous one, and at mold level and supplied with an alternating current of 3800 ampere-turns at a voltage of 75 volts.

On micrographic examination of sample surfaces taken from the two parts of the plate, it was found that the number of grains was eight times greater when the method according to the invention was applied.

In addition, surface defects such as tearing, oxide skin, which appear on the first part had practically disappeared on the second.

The invention finds its application in all cases where it is sought to improve the structure and the surface condition of products molded or continuously cast, and in particular in the aluminum industry.

Claims (7)

1. A process for casting metals in an ingot mould (1) having a vertical axis of symetry in which a stationary magnetic field and a periodic magnetic field are simultaneously caused to act on the metal (3) in the course of solidification. characterized in that the stationary uniform magnetic field having a direction parallel to the mould axis is generated by a least one coil (5) the section of which being similar in contour to that of the upper section of the mould, the variable magnetic field having the same direction that the other field is generated by at least one annular inductor (7) supplied with an alternating single phase current.

2. A process according to claim 1, characterized in that the stationary magnetic field is of a value of less than 0.5 tesla.

3. A process according to claim 2, characterized in that the stationary field is modified by the presence of an iron core.

4. A process according to claim 1, characterized in that the frequency of the variable field is from 5 to 100,000 Hertz.

5. A process according to claim 4, characterized in that, when the frequency is higher than 100 Hertz, this frequency is adjusted to those which go into resonance with the natural frequencies of the liquid metal, the dendrites in the course of formation, or the solid mass.

6. A process according to claims 5 and 4, characterized in that the coils and the inductors for generating the stationary field and the variable field are disposed in alternate succession along the metal in the course of solidification.

7. A process according to claim 6, characterized in that, when the inductors generate a variable field at a frequency of less than 100 Hertz, they are supplied at a frequency which decreases in proportion to progressing solidification.

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