EP0170585B1 - Temperature-homogenizing device for continuously moving metallic products - Google Patents

Abstract

Device for temperature homogenization of passing metallic products (3), housed in the bed plate of the temperature equalization zone of a reheating furnace and constituted by elongated polyphase static sliding field inductors (14) located in the extension of elements (9) supporting the metallic products in the heating zone (7) of the furnace. The device achieves efficient and rapid heating by enabling precise localization of the heating zone in a predetermined part of the metallic product to be treated, and is particularly useful for treating large dimension products, such as large slabs, in order to attenuate or eliminate skid marks (20) which are usually present upon their emergence from prior art reheating furnaces.

Description

The present invention relates to the homogenization in temperature of metal products, in particular steel slabs, intended for rolling and having previously been subjected to this effect to a not perfectly homogeneous heating.

Before being rolled, it is first necessary to bring the metal products to an adequate temperature, and secondly, desirable that this temperature be as uniform as possible throughout the mass of this product. This second objective is particularly difficult to achieve in the case of large products such as slabs.

In the usual industrial flame ovens, intended for reheating slabs before rolling, whether they are of the “pushing” type (contiguous slabs) or of “movable beams” (separate slabs), the slabs move on a hearth formed by support members arranged parallel in the direction of travel of the slabs. It will be recalled that these support members are constituted, either only by fixed slides (pushing ovens), or by a group of fixed bars and another group of mobile beams, the latter ensuring the movement of the slabs inside the oven ( beam oven). At the point of contact with the support bodies, we usually observe the presence of parts that are colder than the whole of the support and which can sometimes cross the entire thickness of the product. These colder parts are superficially manifested by a darker coloring than the rest of the product. For this reason, they are generally designated by "black traces". Such a phenomenon occurs in particular in ovens intended for the reheating of thick products where the heating takes place not only by the vault but also by the bottom, under the products, because, on the one hand, it is necessary to cool internally the support members and, on the other hand, the location of the slab-support member contact is not directly exposed to the heat, which is largely transmitted by radiation.

The presence of these black traces is the cause of local temperature gradients large enough to cause metallurgical defects in the product, and lead to super-thickeners during subsequent rolling.

We can consider mitigating them by acting, at the level of the rolling mill, on the water jets intended to rid the moving products of their scale. The resulting cooling effect can be adapted with a view to reducing black marks, for example by cooling the entire product to the exclusion of these colder zones. However, one quickly realizes that this method, of inconvenient implementation, hardly offers the desired results and, therefore, is hardly used.

Another solution consists in placing the heated products in the teminal part of the unheated oven (so-called thermal equalization zone) so as to allow a better distribution of the temperatures throughout the mass of the products. However, this process is quite slow and homogenization is only acquired at the expense of productivity or the size of the installation and at the cost of significant additional energy expenditure.

Another solution, which, to the knowledge of the inventors, has not yet received industrial application, consists in placing in the roller conveyor at the outlet of the oven, one or more inductors with a sliding magnetic field intended to locally heat the slabs at the places of the black traces (see FR-A 2372402). This inductive heating technique, certainly very elegant in itself, is however based on a discontinuous operation of the inductors, which requires the development of a system for detecting black traces and stopping phases when the slabs pass when the traces black are above the inductors.

The object of the present invention is to provide a solution which ensures at all points of the reheated product, a uniform temperature which is sufficient for the rolling operation, while avoiding the drawbacks of the previously mentioned known solutions.

To this end, the subject of the invention is a device for the temperature homogenization of moving metal products, in particular steel slabs, intended for rolling and previously raised in temperature in the heating zone of a reheating furnace having , following this heating zone provided with product support members, a thermal equalization zone comprising a hearth, device comprising a plurality of polyphase static inductors with sliding magnetic field, of elongated shape, characterized in that said inductors are placed in the thermal equalization zone of the reheating furnace, parallel to themselves at the rate of one inductor per support member and in the extension of the support members of the products in movement from the heating zone, and the active face of the inductors, facing the products, has a width at least equal to the width of said support members, preferably between 1 and 4 times this width and preferably still equal to approximately 3 times this width.

According to a variant, the conductors with which the inductors are fitted are oriented perpendicular to the direction of travel of the products.

As will be understood, the present invention recommends selective inductive heating, located in the temperature equalization zone of the reheating oven, and located under the predetermined parts of the metal product which constitute the black traces.

The positioning as well as the nature of the heating means chosen make the device according to the invention, an efficient auxiliary heating means which advantageously supplements this which is carried out in the actual heating zone.

Indeed, the location of the inductors in the extension of the metal product support rails guarantees their permanent arrangement with regard to the black traces which it is desired to eliminate.

In addition, the choice of polyphase static inductors with a sliding field of elongated shape is particularly suitable because they make it possible to locally generate large induced currents, and therefore, a concentrated calorific contribution and localized at the places of the product that one wants to heat.

In addition, the thermal map of a "black trace" is generally in the form of a family of relatively deep sinusoidal isotherms in the section of the metal. The spatial distribution of the power induced in the product by a sliding field inductor tends precisely to follow such a thermal profile.

As such, it could be determined that, under the usual operating conditions of ovens, the width of the active face of the inductors is preferably in a ratio of 3 to 1 (or approaching) with the width of the support members. Beyond a ratio of 4 to 1, a non-negligible part of the induced power is unnecessarily injected at the periphery of the black trace, while below a ratio of 1 to 2, the inductor becomes undersized for the purpose of erasing the black traces.

Furthermore, the invention presents a decisive economic interest. In fact, demand is now shifting towards metallic products having particular mechanical characteristics, in particular of resilience and weldability, which require a very precise control of the grain size in the metal. One of the means of obtaining such a characteristic consists in carrying out a so-called “low temperature” reheating, that is to say in baking the metal products at around 950 ° C. (instead of 1100-1200 ° C. as is usually does). Of course, this process, while being energetically economical, requires precise control of the heating of the product. The device according to the invention, which guarantees homogeneous heating within the metal, makes it possible to envisage the feasibility of so-called “low temperature” heating.

• La fig. 1 représente une coupe schématique longitudinale d'un four poussant pour le réchauffage de brames en acier et équipé du dispositif selon l'invention;
• la fig. 2 est un agrandissement de la partie référencée A du four exemplifiée sur la figure 1 et pourvue du dispositif selon l'invention;
• la fig. 3 représente une coupe transversale selon le plan XX_, de la figure 1 ou de la figure 2;
• la fig. 4 représente une variante de réalisation de l'invention selon une vue analogue à celle de la figure 3.

Other advantages and characteristics will become apparent in the light of the description which follows, given by way of nonlimiting example, with reference to the plates of attached drawings in which:

• Fig. 1 shows a schematic longitudinal section of a pushing furnace for reheating steel slabs and equipped with the device according to the invention;
• fig. 2 is an enlargement of the part referenced A of the oven exemplified in FIG. 1 and provided with the device according to the invention;
• fig. 3 shows a cross section along the plane XX _, of Figure 1 or of Figure 2;
• fig. 4 represents an alternative embodiment of the invention according to a view similar to that of FIG. 3.

In the pushing furnace illustrated in FIG. 1, the slabs 3 are joined by their small lateral face so as to form a continuous sheet which moves from right to left in the direction indicated by the arrow F, using a pusher symbolized in 1 at the entrance 2 of the oven. The slabs thus travel slowly in a straight line through the oven from inlet 2 to outlet 4 where they are unloaded one by one on an inclined plane 5 which deposits them on a path with transport rollers 6 for their routing to the rolling plant (not shown).

The oven comprises two thermal zones: a first zone 7 called “heating” with a length of about 18 m, in the upstream part of the oven and a second zone 8, called “equalization” with a length of 12 m approximately, in the downstream part of the oven.

The heating zone 7 is conventionally composed of a first enclosure 7 ′, called the preheating of cold products, followed by a second enclosure 7 ″ ensuring the heating of the preheated products to their nominal temperature.

In this heating zone, the slabs 3 are placed on the wear strip of metal slides 9 internally cooled by circulation of water. These slides 9, 20 cm wide, spaced from each other by a distance of the order of 1.5 m, form a parallel network oriented in the direction of movement of the slabs 3. The heating zone also includes front burners 10 and 11 respectively arranged in the upper and lower parts of the oven.

The equalization zone 8 can also be equipped with front burners 10 ′, but for the sole purpose of compensating for the heat losses which occur through the walls of the furnace.

The fumes produced in the oven circulate against the slabs 3 up to the vicinity of the oven door where they are evacuated by a chimney 12.

As mentioned above, the slab-slide contact slows down the rapid rise in temperature of the slab 3 at this point and leads to the formation of cold marks or "black marks".

In the equalization 8 (or temperature homogenization) zone, the slabs 3 are placed on a solid hearth 13 made of refractory material. This, in accordance with the invention, is fitted with inductors with a sliding magnetic field 14 of elongated shape. These inductors 14 are placed in the extension of the slides 9 at the rate of one inductor per slide, and extend, in the example under consideration, over the entire length of the equalization zone 8. In this example, the active face 15 of the inductors placed opposite the hearth 13 has a length of 6m and a width of approximately 50 cm.

Referring to Figures 2 and 3, we distinguish the arrangement made in the equalization floor 13 for the installation of an inductor 14 with a sliding field.

As can be seen, the inductor 14 is of planar structure and comprises a laminated magnetic yoke 16 having on the active face 15 regularly spaced parallel notches 17 serving as housing for conductors 18, formed of rectangular hollow copper bars to allow an internal circulation of a cooling fluid. The heads of the coils protruding laterally from the magnetic yoke are shown diagrammatically in 19 and 19 ′ and folded down to reduce their size under the floor 13.

The conductors are adequately connected to a three-phase industrial frequency power supply (not shown) to produce a mobile magnetic field sliding along the longitudinal axis of the inductor.

According to the invention, these inductors 14 are placed under the floor 13 in the extension of the slides 9. In this way, the black traces 20 generated locally in the slabs 3 in contact with these slides, naturally come to pass above inductors during the progression of the slabs in the zone 8 of thermal equalization.

In the example described, an insert 21 made of a refractory material with high thermal insulating properties (for example of fibrous refractory based on alumina) was housed in a recess 22 made for this purpose on the underside of the sole. 13 above the inductor 14. The role of this insert is to reinforce the thermal protection of the inductor 14 with respect to the heat given off by the slab 3.

Furthermore, to increase the heating efficiency of the inductor 14, it is advantageous to bring it closer to the slab 3 by a distance less than the thickness of the hearth 13 in order to reduce the air gap (distance separating the active face 15 of the slab inductor 3). FIG. 3 shows that this result can be easily obtained using an insert 21 with a concave base, serving as an imprint on the inductor 14.

In addition, the action of the inductors 14 can be regulated in the usual way, for example by acting on the intensity of the excitation current, or on its frequency.

Experience has shown that, in the case of the oven described above, in order to "completely erase" the black marks in slabs 250 mm thick and 3.5 m long by 2 m wide, reheated to At 950 ° C, a power of 58 kW / inductor must be used at a frequency of 50 Hz of current when the oven production is 85 t / h.

On the other hand, it is necessary to induce a power of 220 kW / inductor in the case where the production of this oven is 160 t / h.

It goes without saying that the invention is not limited to the example described above.

An advantageous variant consists in providing an active curved face of the inductor.

Such an arrangement makes it possible to increase the power induced in the center and to reduce it at the two edges due to the variation thus obtained of the air gap depending on the width of the active face 15.

This arrangement can have an advantage insofar as the spatial distribution of the heating power induced in the slab 3 thus tends to approach even more the sinusoidal thermal profile of the "black trace" 20.

Furthermore, it is clear that other variants of the structure of the inductor can be envisaged to provide a curved active face, by giving, for example, a semi-cylinder structure, or in the form of a circular sector to the inductor. .

Likewise, the sliding direction of the magnetic field can be modified according to the orientation of the busbars. In the case of the examples described above, a transverse arrangement of the conductive bars 18 (that is to say perpendicular to the direction of travel of the slabs 3) was used.

One can very well opt for a longitudinal arrangement of the busbars (that is to say parallel to the direction of travel of the slabs 3) so as to provide a field which slides in a direction perpendicular to that of the movement of the slab 3 Such a variant has been represented in FIG. 4. The essential difference between this variant and the embodiment described above resides in the fact that the conductive bars 18 ′ are arranged in longitudinal notches 17 _′ parallel this time to the direction of scrolling of the slabs 3 and the magnetic yoke 16 ′ of the inductor 14 ′ is laminated according to cutting planes parallel to the plane of the figure.

In addition, with regard to the arrangement of the inductors in the role of equalization of the furnace, it goes without saying that other variants than that described above can be envisaged. In particular, one can think of distributing the inductors at different locations in the equalization hearth, each inductor being of course arranged in the extension of the slides.

In addition, it is easily understood that the longer the length of the inductors increases, the less the surface heating power of each inductor must be significant. At the limit, it is possible to provide inductors which cover the entire length of the equalization sole.

Claims (7)

1. An apparatus for homogenizing the temperature and removing "skid marks" in continuously moving metal products, in particular steel slabs (3), intended for rolling and preliminarily increased in temperature in the heating zone (7) of an ingot reheating furnace having, after said heating zone provided with support members (9) for the products (3), constituted by girders or slide rails, a soaking zone (8) comprising a bed (13), said apparatus comprising polyphase static inductors with a sliding magnetic field and being characterized in that said inductors (14) of elongate shape are placed in the soaking zone (8) of the reheating furnace, in the extension of the support members (9), and extend parallel to each other, at the rate of one inductor per support member (9), in the advancing direction of the product, the active surfaces (15) of the inductors (14) being turned opposite said skid marks on the products (3) and having a width at least equal to the width of said support members (9).

2. An apparatus according to claim 1, characterized in that said active surfaces (15) have a width between one and four times the width of said support members (9).

3. An apparatus according to claim 2, characterized in that the width of said active surfaces is equal to approximately three times the width of said support members (9).

4. An apparatus according to claim 1, characterized in that the inductors (14) comprise conductors (18) oriented perpendicular to the direction of movement of the products (3).

5. An apparatus according to any of claims 1 to 4, characterized in that the active surfaces (15) of the inductors (14) are flat.

6. An apparatus according to any of claims 1 to 4, characterized in that the active surfaces (15) of the inductors (14) are bulged.

7. An apparatus according to any of claims 1 to 6, characterized in that the active surfaces (15) of the inductors (14) are covered by a thermally insulating refractory material, said material forming an insert (21) in the lower surface of the bed (13) of the soaking zone (8) of the furnace.

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