FR2658276A3 - Device for localised reheating of metallurgical products moving past - Google Patents

Abstract

The invention relates to a device for localised reheating of metallurgical products moving past, which is intended to be implanted in the floor of a furnace for reheating said products, characterised in that it comprises at least one electric resistance heater (10) placed in a housing (14) arranged in the floor, and a protection plate (7) made of an oxidation-resistant heat-conducting material, and in that said plate closes said housing (14) in its upper part and has a free upper surface (18) which is not covered by the floor and is arranged at a small distance from or in contact with the parts of the product (1') which have to be reheated. Preferably, the resistance heater (10) is tubular and U-shaped.

Description

LOCALIZED HEATING DEVICE
OF METALLURGICAL PRODUCTS IN SCROLLING
The present invention relates to the localized heating of metallurgical products, in particular slabs, blooms and steel billets, intended for rolling and having previously been subjected to this effect to a not perfectly homogeneous heating.

 Before rolling, it is first necessary to bring the metallurgical 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. On the other hand, it is of particular importance in the case of long products made of high carbon steel intended for the manufacture of carcass wire for tires, for which a target temperature of both homogeneous and high is aimed, of the order from 11000C.

 In the usual industrial flame ovens, intended for reheating products before rolling, whether they are of the "pushing" type (joined products) or of "movable beams" (separate products), the products move either on a hearth or on support members arranged generally parallel in the direction of travel of the products, either on these two types of alternating or joint device. 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 between the product and the support members, there is usually the presence of parts that are colder than the entire product and that can sometimes pass through its entire thickness. These colder parts are superficially manifested by a darker coloration than the rest of the product. For this reason, they are generally designated by the term "black marks". Such a phenomenon occurs in particular in ovens intended for reheating thick products where the heating takes place not only by the roof but also by the bottom, under the products. On the one hand, it is necessary to internally cool the support members and, on the other hand, the location of the product-support member contact is not directly exposed to heat, which is largely transmitted by radiation. .

 In the case of long products for carcass wire, it is necessary to overheat the entire product to be laminated, so that the laminability temperature is reached even at the level of the black marks. However, this does not make it possible to avoid the presence of local temperature gradients which are large enough to cause metallurgical defects in the product, and lead to excess thicknesses during subsequent rolling.

 We can try to reduce 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 attenuating the black marks, for example by cooling the entire product to the exclusion of these cooler zones. However, one quickly realizes that this method, of inconvenient implementation, hardly offers the desired results and, therefore, is hardly used.

 One solution practiced consists in making the heated products stay in the terminal part of the weakly heated 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 acquired only at the expense of productivity or the size of the installation and at the cost of significant additional energy expenditure, or even a deterioration in the state of product surface.

 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 (French patent FR 2372402). This inductive heating technique, certainly very elegant in itself, however, is based on a discontinuous operation of the inductors, which imposes stop phases for the movement of the slabs when the black trace is above the inductors as well as the presence, in upstream of the inductor, means for detecting the passage of the black traces.

 Finally, the applicant has proposed (French patent FR 2568359) to house, in the sole of the thermal equalization zone, inductors with a sliding field of elongated shape, arranged in the extension of the members supporting the metal products in the heating zone. from the oven. This device provides localized product heating, limited to the areas affected by the black marks. However, this method proves to be relatively expensive, and an optimal coupling between the inductors and the product is difficult to obtain.

 The object of the invention is to provide a simple, reliable and efficient method of localized heating of metal products, in particular steel products.

 To this end, the subject of the invention is a device for localized heating of moving metallurgical products intended to be installed in the hearth of a furnace for reheating said products, characterized in that it comprises at least one electric heating resistance placed in a housing provided in the sole and a protective plate made of a material which conducts heat and is resistant to oxidation, and in that said plate closes said housing at its upper part and has a free upper surface not covered by the sole and placed at a short distance or in contact with the parts of the product which must be reheated.

 Preferably, this resistance is tubular and U-shaped.

 The subject of the invention is also a furnace for reheating metallurgical products, of the type comprising an area in which said products progress by resting on a continuous hearth, characterized in that it comprises at least locally devices for localized reheating of the products of previously described type implanted in said sole.

 As will be understood, in the preferred embodiment of the oven according to the invention, resistors are arranged in the hearth of the second zone of the oven, and make it possible to reheat the parts of the product which, during its movement in the first zone of the oven1 have been in contact with the support members. This reheating is obtained by means of the protective plates closing each of the resistances housings in the floor, and which are in contact with the product or in its immediate vicinity at the level of the areas affected by the black marks.

The invention will be better understood on reading the description which follows, referring to the following figures
- Figure I shows schematically, seen from above, a furnace for reheating steel products divided into two zones, the first of which has slides on which the products progress, and the second a continuous hearth on which the products rest while they are scrolled
- Figure 2 shows in section along line II-II of Figure 1 the sole of the second zone of the previous oven, showing in particular the location of the heating resistors.

 The furnace for reheating steel products shown diagrammatically in FIG. 1 is divided into two zones (A) and (B), which the products 1, 1 ′ pass through successively during their progression symbolized by arrows. The products represented here are long products (blooms or billets), but could just as easily be flat products (slabs). The zone (A) constitutes a zone for heating or preheating the products, in which both their upper face and their lower face are exposed directly to heat. To do this, the products rest on slides 2, 2 ', 2 "raised relative to the hearth 3 of the oven. These slides are oriented substantially perpendicular to the axes of the products and substantially parallel to their direction of progression. Only three slides are represented, but their number can, of course, be different. Similarly, a single product 1 is represented there ~ but several products can remain simultaneously in the zone (A). If the oven is of the conventional type called "pushing oven", the products are joined and progress thanks to a thrust exerted on one of them. The furnace can also be of the "movable spar" type, in which case the products are separated from each other, and their progress is carried out step by step by means of organs acting separately on each product, in a known manner. In all cases, the progression of the product 1 on the slides 2, 21, 2 "generates on its underside" traces n oires ", according to the mechanism described above.

In zone (B), the product 1 'no longer rests on slides, but on a continuous flat hearth 4. The oven is equipped there with a radiant vault making it possible to ensure one of the following two functions, or the two successively
- provide additional reheating to the products as a whole
- only bring enough heat to the products to keep their surface or average temperature roughly constant, the function of this area of the oven (called thermal equalization area) then being to allow time for thermal stratification inside the products to fade.

Conventionally, the sole 4 is coated on its upper part with a dense refractory material 5 having good insulating power and excellent abrasion resistance, such as
This sole is at a temperature sufficient for the thermal equilibrium between it and the products to be achieved, and the products in principle do not cool on contact. In the extension of each of the slides 2, 2 ', 2 "of the zone (A) is located a device for localized heating of the products according to the invention. For this purpose grooves 6 are formed in the sole, over its entire height .Such a heating device comprises the following superimposed elements, disposed inside one of said grooves 6, from the surface to the bottom of the hearth, as shown in FIG. 2
a plate of refractory material of good thermal conductivity 7, 7 ′, 7 ", the upper face 18 of which is flush with that of the upper layer of the refractory hearths, and the lower face of which has a recess 8
= a refractory plate 9 cooperating with the recess 8 of the plate 7 so as to define a housing 14 in which is housed a heating resistor 10, 10 ′, 10 "
- a refractory mass 11 with high insulating power.

 The groove 6 is closed at its lower part by a removable metal plate 12, -fixed on U irons 13, 13 ′ secured to the plate 16 supporting the lower refractory layer 17 of the sole.

This is conventionally made of alumina bricks. It is thus possible to dismantle the above elements without it being necessary to enter the oven.

 The essential and active part of the device is the heating resistance 10, 10 ', 10 ". To be able to erase the black marks on the products, it must be able to be worn without damage and permanently a temperature of at least 11500 C. and preferably 14000 C. Among the materials making it possible to achieve this objective, we can cite the inconel 601, which is characterized by its relatively low cost for a temperature of use below 12500 C. It is also possible to use sintered metal, composed of iron, aluminum and chromium, allowing continuous operations at 14000 C. Advantageously, the resistance 10, 10 ′, 10 "has a tubular structure, so as to offer a maximum exchange surface, for minimum cross-section of the electric current. This last characteristic provides high electrical resistance. At equal dissipated power, the intensity of the current to be imposed is therefore less than in the case of a full resistor.

 The refractory plate 9, the presence of which is not compulsory, serves as a support for the resistor 10. It is, for example, made of alumina.

It forms with the refractory mass 11, which is preferably of fibrous structure, an insulating assembly which opposes as much as possible the transfer of the heat released by the resistance to the lower part of the furnace. Most of the heat flow is therefore directed to the refractory plate 7,7 ', 7 "with good thermal conductivity. Its role is to protect the resistance 10, 10, 10" and to transmit through its upper face 18 the heat given off by the resistance to metallic products, such as the billet 1 ′, which rest on the bottom 4 of the oven. The material of the protective plate 7,7 ', 7 "must therefore withstand the high temperatures involved without excessive deformation and chemical alteration, particularly in an oxidizing medium, and have excellent thermal diffusivity. Silicon carbide and UMCO 50 @ (alloy comprising 50% of cobalt, 37% of chromium and 13% of iron) are well adapted to this use. The housing 14 defined by the protective plate 7 and the refractory plate 9 must be of reduced size so that the transmission the heat given off by the resistance takes place with good efficiency.

However, it must also be large enough to prevent any contact between the resistor 10 and the protective plate 7, which could cause a short circuit. In practice, the resistor 10 and the protective plate 7 must be separated by about 5 cm.

 Preferably, each resistor has its own power supply and its own regulation system, not shown in the figures. It is thus possible to adapt as best as possible the quantity of heat communicated to the products, so as to compensate for a possible inhomogeneity of the conditions of heating of the products and of formation of black traces depending on the width of the oven.

 The resistors 10, 10 ', 10 "can be in the form of simple rectilinear bars. However, it is better to bend them in a U shape, so as to bring their two electrical connection terminals together. If one does not wish or cannot bend the tube, one can quite simply replace the bending by a deep welding of two parallel tubes according to the desired spacing on a thick plate of inconel 601. This makes it possible to reduce to a very small value the length conductors connecting these terminals to the power supply, if the latter is placed a short distance from the ends of the branches of the U. Under these conditions, it is advisable to provide a protuberance 15 on the protective plate 7,7 ', 7 " , 15 ', 15 "forming a partition dividing over most of its length the housing 14 into two compartments, each containing one of the two branches of the U formed by the resistor. This protuberance 15.15', 15" captures the part of the shelf nt emitted by the branches towards the inside of the U. Without this, this part of the radiation would be essentially captured by the resistance itself and not by the protective plate 7,7 ', 7 ". Its transmission to the product 1 ′ would therefore not be optimally ensured.

 The width of the protective plates 7,7 ', 7 "is of the order of magnitude that of the black traces to be eliminated, which is itself substantially equal to that of the slides 2,2', 2". The ~ length of the plates 7,7 ', 7 "and of each of the branches of the resistors 10,10', 10" must be calculated so that, taking into account the value of the thermal deficit to be compensated for on the products, energy dissipated by the resistors, the energy efficiency of its transmission to the product and the speed of progression of the product in the oven, the black traces are already eliminated when the products reach beyond the ends of the protective plates 7,7 ', 7 " .

 For example, an industrial furnace of the pushing type intended for the heating of steel billets of 120 mm side and 12 m long comprising in its zone (A) 6 slides was equipped with a set of 6 devices localized heating of the type which has just been described and shown, placed in zone (B) in the extension of the slides. The U-shaped inconel 601 resistors have an outside diameter of 60 mm, a tube thickness of 3 mm, and each branch of U has a length of 2 m. This length is approximately equal to that of the protective refractory plates placed end to end, themselves wide by 0.50 m and long by 0.35 m. The resistors are supplied with an intensity varying from 1500 to 3000 A and an adjustable potential difference of the order of 10 to 20 V (corresponding to a maximum of 5.5 V per meter of length). Each of these devices allows '' provide the products with a heat flux of 7.5 kW / m2, sufficient to remove the black marks on the product.

 Of course, the invention is not limited to the example which has just been described and shown. In particular, the configurations of the resistor and of the protective refractory plate may be different. One can also plan to install several resistors in the same unitary device.

 Another possible configuration is not to place the upper surface of the protective plates at the same level as the surface of the sole, but to arrange it in a slight hollow relative to the surface of the sole. The drop can reach a few centimeters. The products then rest only on the floor, and the heat given off by the resistors is transmitted to them by the radiation emitted by the protective plates, and no longer by conduction. The efficiency of this transmission is less good than in the case of plates flush with the floor. However, this prevents the product from slipping on the plates, which it is feared will cause localized wear of the products, if their surface hardness is insufficient.

It is also thus possible to compensate in advance for the differential wear between the plates and the sole. It was found that filling the space between the plate and the product with scale did not affect the performance of the installation. Conversely, if the problem of product wear does not arise or can be neglected, the upper surface of the protective plates may be raised relative to the surface of the sole, the wear of which by the friction of the products is thus deleted.

 The arrangement of the refractory portion of the hearth of the furnace is not an essential characteristic of the device and can also be modified. On the other hand, it is entirely conceivable to build an oven or a portion of an oven, the entire bottom of which would be constituted by protective plates which conduct heat well overhanging resistances, and which make it possible to heat the product. by the floor in addition to conventional heating by the roof. The heating provided by each of the resistors would preferably be independently modulable, which would make it possible to compensate for any heterogeneities of the previous reheating, such as black traces due to slides.

 The invention is not limited to the heating of steel products: it can be applied to the heating of any metallurgical product in an oven of the type described.

Claims (6)

 1) Device for localized heating of moving metallurgical products intended to be installed in the bottom of a furnace for reheating said products, characterized in that it comprises at least one electric heating resistance (10, 10 ′, 10 ″) placed in a housing (14) formed in the sole, and a protective plate (7,7 ', 7 ") made of a heat conductive material and resistant to oxidation, and in that said plate closes said housing (14 ) at its upper part, and has a free upper surface (18) not covered by the sole and disposed at a short distance or in contact with the parts of the product (1 ') which must be reheated.  2) Device according to claim 1, characterized in that the electric heating resistor (10,10 ', 10 ") is tubular.  3) Device according to claim 1 or 2, characterized in that the electric heating resistance (10,10 ', 10 ") is in the form of U.  4) Device according to claim 3, characterized in that the lower wall (8) of the protective plate (7,7 ', 7 ") has a protuberance (15) dividing the housing (14) for receiving the resistance in two parts, each of which contains one of the resistance branches (10,10 ', 10 ").  5) Device according to one of claims 1 to 5, characterized in that said protective plate (7,7 ', 7 ") is made of silicon carbide.  7) Oven for reheating metallurgical products, of the type comprising a zone (B) in which said products (1 ') progress while resting on a continuous hearth (4), characterized in that it comprises at least locally reheating devices localized products of the type according to claim 1 implanted in said sole.