EP0038275B1 - Means for the continuous casting of hollow blanks - Google Patents

Abstract

It incorporates a continuous casting mould (1) inclined with respect to horizontal, at the level of which asymmetrical electromagnetic means (4), powered with multiphased current, allow the rotation of the molten metal about the axis of the mould. Whenever tube blanks are being cast, the blank extracted from the mould is bent and then brought back to the mould, following an inclined path, other electromagnetic means (11) then giving the still liquid metal a rotation movement in the axial zone in order to form a well the axis of which is the same as that of the blank. Application to the making of steel tube blanks.

Description

The device which is the subject of the invention relates to the production by continuous casting of tube blanks. It relates most generally to blanks of metal tubes of all types, but more particularly applies to the manufacture of blanks of steel tubes of very different shades.

The device most often applies to the production of blanks of circular section.

French Patent No. 2,414,969 describes a process which makes it possible to obtain, by continuous casting, hollow metallic blanks having an excellent internal and external surface condition. This process consists in introducing the liquid metal into a continuous casting mold of circular section, with a substantially vertical axis, the side wall of which is cooled, then in extracting the blank in the process of solidification at the bottom of the mold according to a curved path. so that, after having passed through a low point, this blank progressively rises to a level at least equal to that of the surface of the liquid metal in the mold, and is cut above this level. In this process, the quality of the tube blank obtained results from the combined action of two sets of electromagnetic inductors. A first set is distributed annularly around the axis of the mold in the casting area of the metal and at the start of solidification of the latter in contact with the wall of this mold. This assembly is supplied with low frequency alternating current, so as to cause a rotation of the liquid metal in the mold in a manner known to those skilled in the art.

A second assembly is arranged around the blank in the zone where the latter, after passing through a low point, is reached by raising a level close to that of the liquid metal in the mold. This second set of electromagnetic inductors is distributed annularly around the blank whose periphery is solidified, but which also has a large liquid area in the vicinity of the axis. It is supplied with alternating current at a much higher frequency than the first, so as to cause a rapid rotation of the liquid metal, which forms, in the still liquid axial zone of the blank, an elongated well of substantially parabolic section. The blank being extracted continuously, we thus obtain, thanks to the rapid rotation of the still liquid metal against the internal wall, a solidification of this metal on this internal wall of the blank in an extremely regular manner which gives it an excellent surface finish and a very fine grain.

This process, although it thus makes it possible to obtain hollow blanks, having a very good surface condition in the raw casting state, has the disadvantage of requiring the curve of the blank to be bent at 180 °. out of the mold, then straighten this path, then extend it vertically.

It is understood that such successive deformations along the trajectory of a blank which has a solidified thin wall, but whose core is liquid, requires a high radius of curvature, which is a function of the section of this blank, and of the means very precise guide to avoid cracking of the wall. In addition, the cooling means must be adjusted with great precision to avoid either ruptures of too thin wall, or on the contrary the extra thicknesses which increase the stresses to which the guide means are subjected. Furthermore, this device requires a large vertical space below the mold. This space is at least equal to the vertical distance between the lower end of the mold and the low point of the trajectory of the blank, and therefore depends on the section thereof.

In the case of the example given in FR 2 414 969, for a cylindrical blank of 223 mm in diameter, the radius of curvature is already 4 m, and for products of larger cross section, radii of 10 m or more.

We therefore looked for the possibility of producing a device for the continuous casting of tube blanks which does not require a large vertical space available below the casting mold and which makes it possible to considerably limit the curvature imposed on the blank at its exit from this mold, and all along its subsequent trajectory, while preserving the product cast, both at the level of the casting mold and at the level of the extraction of the tube blank, symmetry with respect to the axes of propagation of the products in the continuous casting device.

DE 1226250 shows the beginning of a continuous casting for solid bodies in which the mold is inclined relative to the vertical.

In this document, the surface of the liquid metal is shown perpendicular to the axis of the mold but we do not find in this DE 1226250 the description of the specific means which make it possible to arrive at such an orientation of the surface of the liquid metal, means which must compensate for the asymmetry of the forces exerted around the axis of the ingot mold on the liquid metal, forces which naturally tend to create a horizontal liquid surface.

Furthermore, EP 0008376 describes a continuous casting process with a vertical mold in which, by using electromagnetic fields applied to the liquid metal being solidified, a turbulent flow is created in the core of the liquid metal, with the aim of improve the solidification structure. Such a method, applied to a continuous casting whose mold would be inclined relative to the vertical, cannot generate a surface of stable liquid metal as described in DE 1226250.

Starting from a casting device according to DE 1226250 (preamble of claim 1), the invention proposes to specify the means used to obtain, during the casting of the tube continuously, a front of liquid metal in the course of solidification which is symmetrical with respect to the axis of the tube, this so as to satisfy the requirements of vertical bulk and minimum curvature indicated above.

• un moule de coulée continue dont l'axe longitudinal est incliné par rapport à l'horizontale suivant un angle compris entre environ 30° et 60°, une ouverture d'introduction du métal liquide à l'extrémité supérieure du moule, et une ouverture d'extraction du métal partiellement solidifié à l'extrémité inférieure,

The device for continuous casting of blanks according to the invention comprises:

• a continuous casting mold, the longitudinal axis of which is inclined relative to the horizontal at an angle of between approximately 30 ° and 60 °, an opening for introducing the molten metal at the upper end of the mold, and an opening d extraction of the partially solidified metal at the lower end,

means for introducing the liquid metal into the mold,

means for cooling the mold, means for extracting and guiding the partially solidified cast metal from the extraction opening of the mold,

cooling means making it possible to obtain progressive solidification of the cast metal,

electromagnetic means for driving the liquid metal in rotation in the mold around the axis of said mold comprising inductor poles whose windings are supplied with low frequency polyphase current, and arranged around the axis of the mold, so as to creating a rotating field capable of rotating the liquid metal, the electromagnetic means comprising at least one inductor pole having an individual magnetomotor force different from the individual magnetomotive force of one or more other inductor poles in order to thus create an asymmetry with respect to the axis of the mold of the forces to which the liquid metal is subjected, and is characterized in that the asymmetry of the magnetomotor forces is obtained by increasing the individual magnetomotor force produced by at least one inductive pole in the upper zone with respect to the axis of the mold, relative to the individual magnetomotor force produced by at least one inductor pole in the area lower and in that the means for extracting and guiding the blank are arranged from the extraction opening of the mold along a curved trajectory, with concavity turned upwards, which has a low point then rises so as to reach a level corresponding substantially to that of the liquid metal in the mold, the trajectory then being made substantially rectilinear and forming an angle with respect to the horizontal of between 5 and 25 °, the speed of supply of the mold made of liquid metal, the speed of extraction of the blank and the conditions for cooling the blank along the curved path which it travels being adjusted so that when the blank reaches a level close to that of surface of the liquid metal in the mold, this blank comprises a liquid axial zone whose diameter is at least equal to the inside diameter of the blank of the tube which it is proposed to produce, a second set of electromagnetic means of e ntraining in rotation of the liquid metal being arranged around the rising trajectory of the blank, said second set of electromagnetic means for driving in rotation of the liquid metal comprising inducting poles whose windings are supplied with polyphase current, at a frequency clearly greater than that of the current which feeds the inducting poles of the assembly arranged around the axis of the mold, so as to create a rotating field capable of rotating the liquid metal around the axis of the rising path of the blank , at least one inductor pole having an individual magnetomotor force different from the individual magnetomotor force of one or more other inductor poles thus creating an asymmetry with respect to the trajectory of the blank, forces to which the liquid metal is subjected.

The frequency of the polyphase current supplying the electromagnetic means of the casting mold is advantageously between 4 and 12 Hz.

The means for introducing the liquid metal into the mold include a nozzle which is advantageously oriented so as to make an angle of about 30 to 60 ° relative to the axis of the mold and offset with respect to this axis so that the jet liquid metal coming out strikes the surface of the rotating liquid metal giving it an extra boost.

The frequency of the polyphase current supplying the second set of electromagnetic means can be between 50 and 300 Hz. At least one inductor pole of the second set of electromagnetic means may be placed in the upper zone relative to the trajectory of the blank so to produce an individual magnetomotor force greater than the individual magnetomotor force produced by at least one inductor pole in the lower zone.

The description and the figures below allow a better understanding, without limitation, of the characteristics of the device and of the process which are the subject of the invention.

• Figure 1 shows in section the device according to the invention applied to the production of tube blanks.
• FIG. 2 is a more detailed view of the casting mold shown in FIG. 1.
• Figure 3 is a view of the mold entrance of Figure 2 along the XY axis.

As shown in FIG. 1, a mold (1) of circular cross section, has an interior wall (2), for example of copper, behind which extends an annular space (3) in which, by means known per se, creating a circulation of water intended to cool the wall (2). Inside the mold (1) are arranged with a determined angular offset inductors (4) supplied by a low frequency current, for example between 4 and 12 Hz. In the example shown, the inductors are arranged at inside the mold in the space (3) and are thus themselves cooled by the water which circulates therein.

The mold is inclined so that its longitudinal axis forms with the horizontal an angle between about 30 ° and 60 °; in the case of the figure, this angle is approximately 45 °.

This mold is supplied with liquid metal from a distributor (5). This metal, for example steel, cools against the wall (2) of the mold and a circular section blank emerges at the bottom of the mold. This blank is extracted continuously by means of guide rollers (9) along a curved path. The installation also comprises, in a conventional manner, secondary cooling means by spraying with water, not shown, in the casting path, as well as means for extracting the blank and discharging it.

The electromagnetic fields created by the inductors cause the liquid metal inside the mold to rotate around the longitudinal axis with the formation of a meniscus (10).

Experience having shown that it is not possible to obtain a stable and symmetric meniscus with respect to the axis by using a set of inductors having balanced magnetomotor forces, because of the inclination of the mold, we has introduced, according to the invention, an asymmetry in these magnetomotor forces, by increasing the individual magnetomotor force created by at least one inductive pole in the upper zone relative to the axis of the mold, compared to the individual magnetomotor force created by at least one inductor pole in the lower zone.

To compensate for the transverse component of the force of gravity with respect to the axis of the mold, the asymmetry which it is necessary to establish in the magnetomotor forces, depends on many factors: inclination of the axis of the mold, internal dimensions of the mold, physical properties of the metal or of the liquid alloy which is poured, characteristics of the electromagnetic circuits used, nature and frequency of the currents.

Figure 2 shows in more detail a casting mold similar to that of Figure 1. This copper mold (14) has a double wall inside which a hollow space (15) is traversed by cooling water introduced in (16) and discharged in (17). The longitudinal axis of this XY mold is inclined relative to the horizontal at an angle a equal to 45 °. A nozzle (25) is crossed by a stream of liquid metal (26) coming from a distributor not shown. This nozzle is substantially vertical as shown in Figure 2, and it is arranged so that the jet of liquid metal (27) strikes the surface (24) of the metal in rotation inside the mold at a point preferably offset by relative to the axis and chosen according to the direction of rotation imposed on the liquid metal by the rotating field, so as to give this metal an additional impulse. Such an impulse is all the more effective as the point of impact of the jet is close to the wall of the mold. However, care should be taken to avoid molten metal at high temperature hitting the mold, which could damage it. In practice, the point of impact of the jet preferably strikes the liquid metal at a distance from the axis of the mold of between approximately half and three quarters of the radius. As shown in Figure 3, the jet (27) of liquid metal strikes the surface of the metal in the area to the left of the axis and the rotating field is established so as to drive the liquid metal in the direction of the arrow.

It can therefore be seen that the impact of the jet effectively contributes to accelerating the rotation of the liquid.

The end (18) of the mold in the area below the axis, is extended to prevent liquid metal spills to the outside in the event of a break in the magnetic drive.

As shown in Figures 2 and 3, 4 inductor poles supplied with two-phase current are arranged in the double wall of the mold.

The upper pole (19) provides the greatest magnetomotive force and, moreover, as shown in the figures, it comprises, in the vicinity of the mold entrance, an additional winding (20) which increases the magnetomotive force in this area. The lower pole (21) provides the weakest magnetomotor force, while the two lateral poles (22) and (23) each provide an average magnetomotive force intermediate between the two extremes.

The ratio between the maximum and minimum values of the magnetomotive force is generally adjusted between 2 and 10, depending on the operating conditions. The adjustment of these magnetomotor forces is obtained in a manner known to those skilled in the art by varying in one or more poles, either the intensity of the current, or the number of turns traversed by the current, or even the direction of the current in a part of the turns.

In the case of FIGS. 2 and 3, for an internal diameter of the mold of approximately 140 mm, and a two-phase current supply at the frequency of 4 Hz, the ratio of the maximum and minimum magnetomotor forces is 8.

As shown in FIG. 1, when the device according to the invention is used for the production of hollow blanks, the extraction and guiding means which comprise series of rollers such as (9) are arranged along a curved path with concavity turned upwards, and whose radius of curvature is generally 5 to 10 m so as to cancel the slope of descent of the blank, then to make it reach a constant slope of rise, near the average level of the surface of the molten metal in the mold, having an angle of approximately 5 to 25 ° relative to the horizontal. The liquid metal feed speed, the extraction speed and the cooling conditions of the blank along its path are adjusted so that, when the blank reaches a level corresponding to that of the liquid metal in the mold, the section of the metal still liquid in the axial zone of the blank, has a diameter close to the diameter inside of the hollow blank that we want to obtain. At this level, as shown in FIG. 1, a second set of electromagnetic means (11) disposed around the trajectory of the blank, creates a rotating field which causes the liquid metal to rotate rapidly inside the blank. with the formation of a kind of well (12).

To obtain good results, it is necessary to establish at this level a rotating field much faster than that which is established around the casting mold. For this purpose, a polyphase current with a frequency between 50 and 300 Hz is preferably used.

In the case of this example, for a climb slope of 15 °, and in the case of a blank of approximately 140 mm in diameter, a set of 3 inductor poles (11) was used arranged around the blank , and supplied with three-phase current at a frequency of 200 Hz.

It is also necessary at this level, to allow the obtaining of a liquid metal well stable and symmetrical with respect to the axis of the blank, to introduce a significant asymmetry between the magnetomotor forces produced in the lower zone located below the axis of the blank, and in the upper zone located above this axis. To achieve this, the 3 inductor poles were arranged so that one of them was in the lower zone, its axis being substantially vertical to the axis of the blank, and the other 2 poles being in the upper area. In addition, the individual magnetomotor force of the lower pole was calculated so as to be 3 times weaker than that of each of the other 2 poles. Under these conditions, the formation of a liquid metal well has indeed been observed, the depth of which, measured parallel to the axis of the blank, is clearly greater than the diameter. The liquid walls of this well have a section along a plane passing through the axis of the roughly parabolic blank, the axis of the parabola being practically coincident with that of the blank.

The rapid rotational movement of the liquid metal at this level, and the asymmetry of the magnetomotor forces which makes it possible to compensate for the effects of gravity, make it possible to equalize the internal surface of the solid wall of the blank and thus obtain a blank hollow whose internal walls have a great regularity of thickness and an excellent surface condition. Beyond the electromagnetic assembly (11), and after complete solidification, known cutting-off means (13) make it possible to cut the blank into sections of the desired length. Numerous embodiments of the device according to the invention and of the method for implementing it can be envisaged.

Claims (5)

1. A device for the continuous casting of tube blanks comprising :

a continuous casting mould (1), the longitudinal axis of which is inclined to the horizontal by an angle of from 30 to 60°, an opening for the introduction of the molten metal at the upper end of the mould and an opening for the extraction of the partially solidified metal at the lower end ;

means (5) for the introduction of the molten metal into the mould ;

mould cooling means (3) ;

means for the extraction and guidance of the partially solidified cast metal from the opening for extraction from the mould ;

cooling means permitting the gradual solidification of the cast metal ;

electromagnetic means (4) for setting the molten metal in the mould into rotation about the axis of the said mould, comprising inductor poles, the windings of which are supplied with low frequency polyphase current and arranged round the axis of the mould so as to create a revolving field capable of setting the molten metal into rotation, electromagnetic means (4) comprising at least one inductor pole (21) having an individual magnetomotive force which is different from the individual magnetomotive force of one or more other inductor poles (22), thus creating dissymmetry, relative to the axis of the mould, of the forces to which the molten metal is subjected, characterised in that the dissymmetry of the magnetomotive forces is produced by increasing the individual magnetomotive force produced by at least one inductor pole (19, 20) in the upper zone relative to the axis of the mould, relative to the individual magnetomotive force produced by at least one inductor pole (21) in the lower zone, and in that the means (9) for the extraction and guidance of the blank are arranged from the mould extraction opening along a curved path, with its concavity turned upwards, which has a low point and then rises so as to meet a level corresponding approximately to that of the molten metal in the mould, the path thus being substantially rectilinear and forming an angle with the horizontal of from 5 to 25°, the speed at which the mould is supplied with molten metal, the speed of extraction of the blank and the conditions for cooling the blank along the curved path along which it travels being adjusted in such a manner that when the blank reaches a level close to that of the surface of the molten metal in the mould, this blank has an axial liquid zone, the diameter of which is at least equal to the inner diameter of the blank of the tube to be made, a second group of electromagnetic means (11) for setting the molten metal into rotation being arranged around the rising path of the blank, the said second group of electromagnetic means for setting the molten metal into rotation comprising inductor poles, the windings of which are supplied with polyphase current at a frequency which is substantially higher to that of the current supplying the inductor poles of the group arranged round the axis of the mould so as to create a revolving field capable of setting the molten metal into rotation around the axis of the rising path of the blank, at least one inductor pole having an individual magnetomotive force which is different from the individual magnetomotive force of one or more other inductor poles, thus creating dissymmetry, relative to the path of the blank, of the forces to which the molten metal is subjected.

2. A device according to claim 1, characterised in that the frequency of the polyphase current supplying the electromagnetic means of the casting mould is from 4 to 12 Hz.

3. A device according to claim 1 or claim 2, characterised in that the means (5) for the introduction of the molten metal into the mould comprise a nozzle (25), oriented so that it forms an angle of from about 30 to 60° relative to the axis of the mould and offset relative to this axis so that the jet of molten metal (27), issuing therefrom, strikes the surface of the revolving molten metal, thereby giving it a supplementary thrust.

4. A device according to one of claims 1 to 3, characterised in that the frequency of the polyphase current supplying the second group of electromagnetic means is from 50 to 300 Hz.

5. A device according to one of claims 1 to 4, characterised in that at least one inductor pole of the second group of electromagnetic means, arranged in the upper zone relative to the path of the blank, produces an individual magnetomotive force produced by at least one inductor pole in the lower zone.

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