EP0081400B1 - Magnetic induction-heating device for rectangular metallic flat products moving lengthwise - Google Patents

Description

The present invention relates to a device for heating by magnetic induction of flat rectangular metal products running in the direction of their length, of the type comprising at least one inductor capable of producing a magnetic field of constant intensity, but adjustable, oriented substantially perpendicular to a large face of the metal product to be heated, said inductor being rotatably mounted about an axis perpendicular to said large face of the metal product and comprising at least two magnetic poles having polar surfaces oriented towards said large face are parallel to these, and scanning an annular area when the inductor rotates.

It has long been known to use rotating inductors producing a magnetic field of constant, but adjustable intensity, for heating metal products intended to be shaped hot (see for example documents FR-A-916 287 and FR-A -1 387 653). The magnetic poles can be formed by permanent magnets, electromagnets or a combination of permanent magnets and electromagnets. The inductor (s) can be placed outside a tunnel made of refractory material and permeable to the magnetic field, inside which pass the metallic products to be heated.

However, the previously known magnetic induction heating devices have been relatively little used until now for heating metal products such as slabs or blanks, that is to say slabs having already undergone several rolling passes in the roughing cages of a rolling mill, but not yet passed through the finishing cages of the rolling mill. Indeed, experience has shown that with previously known heating devices, it is difficult to obtain a regular temperature profile in the transverse direction of the metal products to be heated. This problem is further complicated if we consider that the metal products to be heated can have widths varying over a wide range of values.

The present invention therefore aims to solve this problem by providing an improved device for magnetic induction heating to improve the uniformity of the heating in the transverse direction of the metal products running in the direction of their length, and this whatever the width of metal products within a given width range.

To this end, the magnetic induction heating device according to the present invention is characterized in that the pole surface of each pole has the shape of a curvilinear triangle having a vertex directed towards the axis of rotation of the inductor, two concave sides which are symmetrical with respect to a straight line passing through said vertex and perpendicular to said axis, and a convex side in an arc centered on said axis and whose radius of curvature is substantially equal to the outside radius of the annular zone swept by polar surfaces of the poles.

• La figure 1 montre schématiquement, en coupe transversale, un dispositif conventionnel de chauffage par induction magnétique.
• La figure 2 montre la zone annulaire d'action du dispositif de chauffage de la figure 1 sur le produit métallique à chauffer, ainsi que le profil de chauffage obtenu dans le sens transversal du produit métallique.
• La figure 3 montre la forme des surfaces polaires des pôles magnétiques d'un dispositif de chauffage conforme à la présente invention.
• La figure 4 est un diagramme montrant la loi idéale de variation de la puissance surfacique induite par le dispositif de chauffage dans le produit métallique à chauffer en fonction de la distance à l'axe de rotation du ou des inducteurs pour obtenir un chauffage homogène sur toute la largeur du produit métallique.
• La figure 5 est un diagramme permettant d'expliquer comment on obtient la loi de la figure 4.
• La figure 6 est un diagramme montrant comment varie au cours du temps le champ magnétique à une distance donnée de l'axe de rotation du ou des inducteurs du dispositif de chauffage.

The invention will be better understood on reading the description which follows and which is given with reference to the appended drawings in which:

• Figure 1 shows schematically, in cross section, a conventional magnetic induction heating device.
• FIG. 2 shows the annular zone of action of the heating device of FIG. 1 on the metal product to be heated, as well as the heating profile obtained in the transverse direction of the metal product.
• FIG. 3 shows the shape of the polar surfaces of the magnetic poles of a heating device according to the present invention.
• FIG. 4 is a diagram showing the ideal law of variation of the pfd induced by the heating device in the metallic product to be heated as a function of the distance to the axis of rotation of the inductor (s) to obtain homogeneous heating over all the width of the metal product.
• FIG. 5 is a diagram making it possible to explain how the law of FIG. 4 is obtained.
• FIG. 6 is a diagram showing how the magnetic field varies over time at a given distance from the axis of rotation of the inductor or inductors of the heating device.

The conventional magnetic induction heating device which is shown diagrammatically in FIG. 1 and to which the present invention can be applied comprises, for example, two inductors 1 and 2 arranged respectively above and below the metal product 3 to be heated, for example a slab, facing the large faces of the slab 3, the latter being driven by a continuous movement in a direction perpendicular to the plane of the figure, that is to say in the direction of its length. As shown in FIG. 1, each of the two inductors comprises several magnetic poles, for example two magnetic poles 4. Depending on the heating intensity which it is desired to obtain and according to the ambient temperature in the vicinity of the inductors 1 and 2, the poles 4 can be constituted by permanent magnets, by wound poles whose windings are supplied with direct current (electromagnet), or by permanent magnets surrounded by windings which can be supplied with direct current. In the case where electromagnets or permanent magnets with coils are used, the intensity of the direct current can be adjusted in a known manner in order to adjust the intensity of the magnetic field produced by the magnets and, consequently, the intensity of the heating produced by the eddy currents induced in the metallic product 3 to be heated. Usually, the poles 4 have a circular section (this shape corresponds to a maximum magnetic flux for a given length of conductor and therefore for given Joule losses in the case of wound poles).

At least one of the inductors 1 and 2 is rotated about the vertical axis z by known means not shown in Figure 1, the other inductor can be rotated synchronously either by the same means of drive, or by the magnetic field produced by the first inductor. The speed of rotation of the inductors 1 and 2 is usually significantly greater than the speed of advance of the metal product 3. During this rotation movement, the pole surfaces of the poles 4 which are located opposite the large faces of the metal product 3 scan an annular zone 5 as shown in FIG. 2. This zone 5 roughly corresponds to the zone of action of the inductors on the metal product 3 to be heated. If this product 3 were stationary, the calorific energy which is brought to it by the Joule effect of the eddy currents induced in its mass would be relatively homogeneous in the annular zone 5. However, as the metal product 3 moves, the energy heat input at any point P located at a distance d from the longitudinal median axis of the product 3 is proportional to the duration of stay of point P in the annular action zone 5 of the inductors, this duration of stay being itself proportional to the length of the segment AB shown in Figure 2. At the bottom of Figure 2, there is shown the heating profile C which is obtained with such a heater in the transverse direction of the metal product 3. As can be see it from the heating profile C shown in FIG. 2, a heating device such as that shown in FIG. 1 and having dimensions such that the outside diameter of its annular action zone 5 corresponds sensi The width of the metal product 3 to be heated does not allow homogeneous heating to be obtained over the entire width of the product 3 while it is advancing. In practice, in order to obtain almost homogeneous heating, one is led to use a heating device the dimensions of which are such that the outside diameter of its annular action zone 5 is significantly greater than the maximum width of the metallic products. 3 to be heated, so as to operate in the middle part of the heating profile C. It is therefore necessary to use heating devices which are large in relation to the width of the metal products 3 to be heated. Under these conditions, it will be noted that the magnetic flux produced by the inductors is not fully used for heating, since it does not act on the metal product 3 to be heated when, during their rotation, the magnetic poles are found beyond the longitudinal sides of product 3, hence a lower yield.

The present invention makes it possible to remedy this by providing a heating device having dimensions such that the diameter of its area of action is only very slightly greater than the maximum width of the moving metal products to be heated, and making it possible to heat the said areas. substantially homogeneously produced over their entire width with good yield. According to the present invention, this result can be obtained by using one or two inductors arranged like those of FIG. 1, but whose magnetic poles, constituted for example by electromagnets, have polar surfaces in the shape of a curvilinear triangle. FIG. 3 shows by way of example, in front view, an inductor according to the present invention comprising four magnetic poles 4 of identical shapes and of alternating polarities. Each magnetic pole 4 may comprise a magnetic core 6, for example of circular cross section, around which is arranged an excitation winding (not shown) supplied with direct current. Each core 6 is provided with a pole shoe or pole piece 7 which is an integral part of the core 6 or which is fixed to the end of the core which is adjacent to the metal product to be heated. Each pole shoe 7 has a flat pole surface parallel to one of the large faces of the metal product to be heated. As shown in Figure 3, the pole surface of each bloom 7 has the shape of a curvilinear triangle which has a vertex 8 directed towards the axis of rotation z of the inductor, two concave sides 9 and 10, which are symmetrical with respect to a straight line passing through the apex 8 and perpendicular to the z axis, and a convex side 11 in an arc of a circle centered on the z axis and whose radius of curvature is substantially equal to the outside radius R of the annular zone 6 swept by the polar surfaces.

dans laquelle k, est une constante.With the polar surfaces in the shape of a curvilinear triangle which have been described above, it is possible to obtain a more uniform transverse heating profile than with the magnetic poles with circular or square polar surfaces which were used in the previously known heating devices. This can be explained as follows. As a first approximation, if we neglect the effects of finite length and width, we can consider that the surface power induced by the inductor rotating at any point P of the metal product to be heated is only a function of the distance r of said point at the axis of rotation z of the inductor. To obtain homogeneous heating of the moving metal product, the half width of which can be between O and R (R being the maximum radius of action of the inductor, i.e. the outside radius of the zone annular swept by the magnetic poles 4), it can be demonstrated that the surface power must be an increasing function (Figure 4) of the distance r mentioned above, this function can be expressed by the following relation:

in which k, is a constant.

avec:

soit:

Pour obtenir un chauffage homogène en largeur, il faut que l'énergie moyenne E_m induite à la distance d ne dépende pas de cette distance d:

• E_m(d) = constante (6)

Indeed, starting from the above-mentioned hypothesis, the average energy E _m (d) induced at point P which moves along the segment AB at a distance d from the axis Oy (Figure 5) is proportional to:

with:

is:

To obtain a uniform heating in width, the average energy E _m induced at the distance d must not depend on this distance d:

• E _m (d) = constant (6)

d'où:

d'où:

ce qui donne bien E_m indépendant de d.The solution of equations (5) and (6) is provided by equation (1). Indeed, taking into account equations (1) and (3), equation (5) can be written:

from where:

from where:

which gives E _m independent of d.

In Figure 4 we have drawn the curve representative of the function f (r) defined by equation (1) for r between -R and + R. From this curve we can see that to obtain a homogeneous heating on the entire width of a metallic product having a width equal to 2 R, that is to say equal to the diameter of the annular zone of action of the inductor, the induced pfd should theoretically have an infinite value at the periphery of said annular zone, which is of course impossible to achieve in practice. In practice, for a given maximum width of the metal products to be heated, it will suffice to size the inductor so that its radius of action R is slightly greater than half of said given maximum width and that the curve representative of the variations in the induced pfd as a function of the distance r has a shape similar to that of the curve in FIG. 4, but with finite values of the power for the values of r close to R.

If it is assumed that the magnetic field is uniform under each pole 4 of the inductor, that the metal product to be heated is limited to the zone of action of the inductor and that the armature reaction is negligible, the variation in the time of the magnetic field B seen by the point P, of polar coordinates r, a (figures 3 and 5) during the rotation of the inductor can be represented by a succession of slots alternately positive and negative as shown in the figure 6. Each slot corresponds to the passage of a pole 4 in front of the point P and has a width which corresponds to the length of the polar arc 8 (figure 3) of each pole 4 at the distance r at which the point P is located This waveform of the magnetic field B seen by the point P can be decompensated in Fournier series and expressed by the relation:

où k₂ est une constante de proportionnalité et A_o est l'amplitude de la composante fondamentale du champ. A_o peut être tiré de l'équation (10) en faisant p = 0, soit:

L'équation (11) devient alors:

ce qui peut encore s'écrire:

avec:

If we consider for simplicity that the pfd induced at point P is proportional to the square of the amplitude of the fundamental component of the magnetic field, this can be expressed by the following equation:

where k ₂ is a proportionality constant and A _o is the amplitude of the fundamental component of the field. A _o can be taken from equation (10) by making p = 0, that is:

Equation (11) then becomes:

which can still be written:

with:

From equation (14), we therefore see that the length of the polar arc 0 of each magnetic pole 4 at the distance r from the center O of the inductor is an increasing function of the distance r, d ' where the concave shape of the sides 9 and 10 of each of the pole surfaces 7 (Figure 3).

Using the above equations, it is possible to determine, for a given width of the metallic products to be heated and neglecting the edge effects, a theoretical shape of the polar surface making it possible to obtain a homogeneity of heating over the entire width of the metal products for heating. Taking edge effects into account, which depend on the speed of rotation of the inductor, the number and shape of the poles, the physical characteristics of the metal product to be heated and the value of the air gap is complex. The edge effects can be taken into account by iteratively modifying the theoretical shape determined by the calculation for a given width of the metallic products. For reasons of simplicity of manufacture, it is possible to adopt for the pole surface of each of the pole shoes 7 the shape of a curvilinear triangle whose concave sides 9 and 10 are arcs of a circle having a profile which approaches the determined ideal profile. as described above, and the convex side 11 of which is an arc of a circle having a radius of curvature substantially equal to the outside radius of the annular zone swept by the poles 4, this outside radius itself being slightly larger than the half the maximum width of metal products to be heated. In addition, each polar surface in the shape of a curvilinear triangle is preferably symmetrical with respect to the straight line passing through its apex 8 and through the center O of the rotating inductor, in order to obtain better balancing of the rotating masses. In addition, as shown in FIG. 3, the apex 8 of each curvilinear triangle is preferably truncated to avoid the leakage of magnetic flux between the poles 4 of opposite polarities.

Claims (4)

1. A device for the magnetic induction heating of rectangular, flat metal products (3) travelling in the direction of their length, comprising at least one inductor (1, 2) capable of producing a magnetic field of constant but adjustable intensity, oriented essentially perpendicularly to one main surface of the metal product (3) to be heated, said inductor (1, 2) being mounted so as to rotate around an axis (z) which is perpendicular to said main surface of the metal product and which comprises at least two magnetic poles (4) having polar surfaces oriented towards said main surface and parallel to the latter, and scanning over an annular zone (5) when the inductor rotates, characterized in that the polar surface of each pole (4) has the form of a curvilinear triangle with an apex (8) directet towards the rotation axis (z) of the inductor (1, 2), with two concave sides (9, 10) which are symmetrical with respect to a straight line passing through said apex (8) and perpendicular to said axis (z), and with a convex side (11) shaped according to a circular arc which is centered on said axis (z) and the radius of curvature (R) of which is essentially equal to the outside radius of the annular zone (5) scanned by the polar surfaces of the poles (4).

2. A heating device according to claim 1, characterized in that said concave sides (9, 10) have the form of a circular arc.

3. A heating device according to claim 1 or 2, characterized in that said apex (8) is truncated.

4. A heating device according to one of claims 1 to 3, in which each pole (4) comprises a core (6) surrounded by a coil and supplied with a pole shoe (7), characterized in that said polar surface with the form of a curvilinear triangle is the polar surface of the pole shoe (7) of the respective pole (4).

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