ITMI961771A1 - MOLTEN CAST BRICK FOR ADDITIONAL HEATING IN COMPENSATION HEATHS OF PUSH FURNACES AND PROCESS FOR ITS - Google Patents

Description

Description of the invention entitled:

"MOLTEN CAST BRICK FOR ADDITIONAL HEATING IN COMPENSATION HEATHS OF PUSH FURNACES AND PROCESS FOR ITS PRODUCTION"

DESCRIPTION

The present invention relates to a cast brick in the molten state for additional electric heating in compensation fires of pusher furnaces. It also relates to processes for the production of those molten cast bricks.

Pusher furnaces are used in steel rolling plants for heating the steel slabs or steel blocks before the hot rolling process. In this regard, it is known to make the sliding surfaces in the pusher furnaces, in particular in the compensation hearth of the pusher furnaces or respectively in the compensation zone, of ceramic cast in the molten state. The sliding tracks in this case are constituted by blocks in the shape of a rectangular parallelepiped which are placed one next to the other and in the direction of thrust of the bramine they form a continuous sliding track.

The molten bricks used for this purpose can have various compositions. As a rule they consist of compounds in the form of high melting oxides, such as mullite, corundum, zirconium corundum, zirconium oxide, zirconium mullite, spinel or other high melting point oxides.

In many cases the runways in the front of the pusher furnace are made up of metal guides or water-cooled metal pipes and in the back, which means in the compensation firebox, they are made of uncooled cast bricks. In this way, in the compensation firebox, it is desired to obtain a regularization of the temperature by means of the fact that the coldest points of the slabs due to the cooled sliding guides are also heated to the necessary temperature. However, full temperature compensation cannot usually be achieved in this way. Non-uniform temperatures can however, even when the temperature differences are not large, can lead to lamination defects in the laminated products in the subsequent lamination process.

Attempts have therefore been made to additionally heat at a later time, in various ways, the lower temperature zones caused by the sliding guide systems. Thus, for example, it is known that by extending the cooled sliding guides in the bottom surface of the compensation furnace it is possible to provide channel-shaped recesses and to arrange in these recesses additional heating elements by irradiation. In this known arrangement, however, it is difficult to protect the radiating elements from the oxides falling from above and from the slag falling from above, from which the heating elements can be damaged. In all the arrangements which can be implemented in practice, in order to make the temperature of the slabs uniform in the compensation hearth of the pusher furnace, relatively long residence times are necessary for the reasons already mentioned.

The present invention is based on the task of developing electrically heated runways made of molten cast bricks which have a high resistance to wear in the area of the sliding surface of the bricks and a high degree of exploitation of the electrical heating energy. for heating the slabs in the area of contact with the bricks and at the same time ensuring that the electric heating elements are safely protected from the oxides falling from above and from the slag falling from above, so that the electrically heated bricks have a long stay in operation cycle.

This task is accomplished according to the invention by means of a cast brick in the molten state which on the side that forms the sliding surface for the slabs has a homogeneous and dense microcrystalline structure and on the side opposite the sliding surface has at least one through recess in the shape of a tunnel extending in the longitudinal direction of the sliding track for receiving an electric heating element.

When the sliding tracks made according to the invention are arranged in direct extension of the cooled sliding guides, in this way the coldest points of the slab can be immediately additionally heated by means of the heated bricks, so that this additional local heating allows to obtain a particularly effective temperature compensation. In this respect, on the one hand a good protection of the electric heating element is achieved by means of the shape of the brick and on the other hand by means of the particular structure of the brick texture in the area above the heating element a conduction is achieved of particularly good heat from the heating element to the sliding surface, while at the same time a high wear resistance of the brick is achieved in the region of the sliding surface.

In a suitable configuration, the bricks according to the invention have a rectangular parallelepiped basic shape and are aligned with one another with their longitudinal sides for the formation of the sliding track. In this regard, it is advantageous that they by means of protrusions alternating on the lateral surfaces are configured in such a way that no continuous junction cracks are formed in the vertical direction, but the projections overlap each other, thus obtaining a particularly highly effective protection of the heating elements.

The cast brick according to the present invention is preferably a brick based on α-corundum, mullite, vitreous phase and baddeleyite. On the upper side, first of all, it has a dense microcrystalline structure in which the α-corundum phase, glass phase and mullite residues as well as baddeleyite are distributed approximately evenly and finely. This dense microcrystalline structure with a high α-corundum fraction (generally greater than 65% by weight) reaches up to about 5 cm deep in the brick and has very good heat conductivity. This has a value of about 6 to 7 W / (m, K) at 1300 ° C, while the usual refractory bricks of sintered mullite have thermal conductivity values of about 2 W / (m.K).

This area, and up to about 10 cm below the working side (depending on the brick size), is followed by an area with a slightly coarser crystallinity whose α-corundum fraction has been reduced in favor of the separation of the mullite. The α-corundum fraction here has a value of about 40 to 50% by weight. The thermal conductivity of this zone is only minimally reduced.

. This structural constitution of the brick, essential for the invention, can be favorably influenced by suitable additives added to the melt. These additives include, for example, fluorite, lime, magnesium oxide, carbon and molten zirconium oxide. Furthermore, the structure can be influenced in the desired direction by conducting the furnace during melting and by means of certain process parameters at the time of casting, where the most favorable conditions in individual cases can be determined by means of some preliminary tests. Among the process parameters that are able to positively influence the quality of the bricks in the sense according to the invention, there are in particular the relationship between current and voltage during electric arc melting, the duration of the melting, the casting speed, the cooling rate and gas release time.

The bricks according to the invention are basically produced as follows: during the casting of the brick, the homogeneous microcrystalline region adjacent to the sliding surface is cooled at a higher cooling rate than the remaining area of the brick. In particular, this can be obtained as follows: the casting mold forms the brick in an inverted position since the bottom of the casting mold forms what will later be the sliding surface of the brick and through the bottom of the mold in this area a higher cooling rate of the melt.

Particularly advantageously, it is possible to obtain a targeted cooling rate if graphite plates are used for forming the casting molds. Since graphite is a relatively good heat conductor and also has a relatively high heat capacity, the cooling rate of the areas near the bottom of the cast melt can be increased if plates of casting mold are used for the bottom plate of the casting mold. thicker graphite than those used for the other walls of the casting mold. For example, depending on the brick format, graphite plates with a thickness of up to 150 mm can be used for the bottom plate of the mold, while graphite plates for the side parts, for the lid plates and for the sprue , and possibly for a possible air vent well, consist of correspondingly thinner graphite plates.

As compositions for the melt, melts consisting of Al2O3-SiO2-ZrO2 in various fractions, or even melts consisting of Al2O3-MgO-Cr2O3-ZrO2-CaO alone or in mixture, proved to be valid in particular.

The melt is produced electrically and is cast with a melting temperature of 2,000 to 2,500 ° C into the graphite mold. In the graphite mold, the melt solidifies after a short time and must remain in the graphite mold for a certain time. After this time, depending on the size and shape of the brick and depending on the desired quality of the brick, after about 3 to 5 minutes the side plates, the cover plates and the graphite plates that form the sprue are removed, while the casting body remains on the bottom plate so that the necessary structure with high thermal conductivity is obtained in the lower region of the casting body. Subsequently, and to be precise, depending on the size of the brick, after 7 to 15 minutes, the casting bodies are introduced into hardening boxes. The geometry of the hardening boxes is determined in such a way that the cooling at the bottom of the casting body takes place faster than in the upper region of the casting body, so that the finely crystalline structure is maintained. The residence time in the hardening boxes is approximately 160 hours.

. While usually the ratio of current to voltage during the melting process has a value of about 150 to 180 V at about 4.0 kA, the melting time is about 2.0 hours, the gas release time is of about 30 minutes, the pouring time for casting bodies from about 170 kg is about 30-40 seconds, for the fabrication of a brick according to the invention of the format 600 mm x 300 mm x 300 mm, for example, the current / voltage ratio is about 220-240V at 5.0kA, the melting time is 2.5 hours, the gas release time is about 30 minutes, the casting time is about 60-80 seconds, and the thickness of the graphite bottom plate is 130mm.

The production of the bricks configured according to the invention is basically possible by first casting a base body in the shape of a rectangular parallelepiped whose height is approximately 50% greater than the height of the finished brick. The casting body layer adjacent to the bottom plate in the casting mold subsequently forms the top running surface of the finished brick. Approximately halfway up the brick a through hole is applied, for example with the help of a diamond core drill with a weak ligature, relatively coarse diamond grains and relatively high concentration of the diamond grains. The diameter of the hole depends on the diameter of the heating elements which are subsequently used as radiating elements. The heating elements known under the trade name KANTHAL SUPER from the company Kanthal are preferably suitable as radiant heating elements.

After the application of the through hole, whose precise position in the brick, i.e. whose distance from the sliding surface of the brick, is determined a priori by the desired life in the pusher furnace and the wear rate, the brick is sawed at the height of the hole parallel to the sliding surface and the burrs present on the hole are removed. The brick is then ready for use in the reverse position, where the finely crystalline structure above the recess provides the desired characteristics. The areas which are in proximity to the recess, with a roughly crystalline structure and possibly cavities and shrinkage voids are not disadvantageous for the use according to the invention at these points.

Bricks according to the invention with even better characteristics can be fabricated, in refinement of the present invention, if the brick is produced with the corresponding recess directly by casting in correspondingly made casting molds. In this way it is possible to produce bricks which also in the recess area have a dense structure and which above all have a more convenient tensile structure, by which the mechanical strength and resistance capacity of the bricks against mechanical loads is increased.

In the manufacture of the bricks according to the invention according to this preferred manufacturing process, the melt is poured into a graphite mold in which on one side the graphite bottom plate again has a substantially greater thickness than the remaining graphite plates. of the casting mold, and in which the graphite core is clamped to form the recess in the upper part of the mold between the side plates of the casting mold. In addition, the casting mold is provided with a relatively high pouring funnel and a relatively high vent hole. In this way it is ensured that the cavities and shrinkage voids are substantially formed in these sections, which are subsequently cut off, while the brick also in the recess area has a dense microstructure with good heat conductivity characteristics. Subsequent drilling or sawing processes for producing the recess in this way are no longer necessary.

Various embodiments of the bricks according to the invention cast in the molten state, as well as preferred processes for their production, are described again in the following in detail on the basis of the drawings.

In the drawings,

Figure 1 shows two bricks made according to the invention with overlapping side surfaces;

Figure 2 shows the microstructure of the bricks at different depths;

Figure 3 shows a graphite mold for a rectangular parallelepiped-shaped brick in longitudinal section and in cross section;

Figure 4 shows the body of the cast brick after the different processing stages, e

Figure 5 shows a graphite mold for the production of a brick according to the invention according to an alternative process. Figure 1 shows two molten bricks 1 and 2, as they are combined in alternating sequence to give a sliding track. They have a substantially rectangular parallelepiped configuration and their flat surfaces 3 and 4 form the sliding surfaces for the steel slabs. Their length L is equal for example to 600 mm, their width B is equal to 300 mm and their height H is equal to 200 mm. The lateral surfaces of the brick 1 in their lower half are provided with supporting projections 5 and the lateral surfaces of the brick 2 in their upper half are provided with protruding shoulders 6. The bricks are combined with their lateral surfaces in such a way that the shoulders 6 of the brick 2 overlap the supporting projections 5 of the brick 1. In this way it is avoided that between the bricks the formation of passages through which the oxides or the slag can penetrate.

On the lower side, the bricks 1, 2 are each time provided in the center of a recess 8 in the form of a through tunnel. The recess 8 in the case shown has a cross section of approximately semicircular shape with a radius of for example about 60 mm and in height this tunnel-shaped recess reaches approximately up to the central plane of the brick. The electric heating elements are located in this tunnel-shaped recess. To assemble the slide rails, this is done by first mounting the heating resistors in the longitudinal direction of the slide rails at the bottom of the compensation hearth. Thereafter, the bricks 1, 2 are applied in alternating sequence with their tunnel-shaped recess 8 above the heating resistances at the bottom of the compensation hearth and are combined to form the sliding track.

The bricks 1, 2, as a consequence of their special production process, seen along their height have a microstructure that changes. As already mentioned, this changing structure and related variation in composition is achieved by faster cooling of the near-bottom layers in the casting mold, where these near-bottom layers in the finished brick form the sliding surfaces. 3, 4.

How the microstructure of the brick changes is shown in the photographs of Figures 2a to 2e. The photograph of Figure 2a shows the microstructure in the layer adjacent to the sliding surfaces 3, 4. The structure here is with a particularly fine graininess. At a distance of 20 min below the sliding surface 3, 4, the internal structure is already in slightly coarse grains, but very regular, as shown in photograph 2b. Still at a depth of 40 min below the sliding surface, the microstructure, as shown by the photo in Figure 2c, is still very finely granular and regular. The wear resistance and thermal conductivity of the brick material even at this depth are very good. At a depth of 60 mm below the sliding surface, the microstructure has the appearance shown in the photo in Figure 2d. It is recognized that at this height, as a consequence of the lower cooling rate and the inevitable segregation processes, certain crystalline grains are already substantially coarser. This variation of the structure proceeds as the distance from the sliding surface increases and at a depth of 100 mm the structure is already very coarse in granulometry, as can be clearly seen from the photo in Figure 2e.

A process for producing the bricks according to the invention is described on the basis of Figures 3 and 4. In this regard, Figure 3 shows a casting mold composed of graphite plates, in a longitudinal section and in a cross section. The casting mold comprises a bottom plate 10 consisting of a graphite plate with a thickness of 130 mm, while the side walls 11, 12, the front walls 13, 14 and the lid 15 of the casting mold are made of graphite plates 40 mm thick. Along the side walls 11, 12 a graphite plate 16 with a height of 100 mm is each time arranged successively to the bottom plate 10. A further graphite body 17 is arranged on the lid 15 which, together with the casting inlet opening 18 in the lid 15, forms a casting inlet funnel. The graphite plates are joined to each other in a known manner. While the internal longitudinal dimension L and the width dimension B correspond to the corresponding dimensions of the finished brick, the mold has an internal height Hi which is approximately 50% greater than the height H of the finished brick.

After the separation of the casting funnel from the solidified and cooled casting body, the rectangular parallelepiped-shaped brick body has the shape shown in Figure 4a in longitudinal section and in cross section, where in this representation the surface 3 which is in contact with the bottom plate already forms the top surface. Approximately in the middle of the lateral surface a through hole 20 with a diameter of about 120 mm is now obtained, as shown in Figure 4b. Subsequently, the lower part of the body of the brick is completely sawn along the A-A plane, by which the section containing the shrinkage voids of the brick is separated. The burrs 22 remaining between the intrados of the hole and the cutting surface of the brick are removed with a diamond saw. The brick then has the configuration shown in Figure 4c and the tunnel-shaped recess 8 and is used in this form.

A preferred manufacturing process for the brick according to the invention is illustrated on the basis of Figure 5. In this case, the tunnel-shaped recess in the brick is already produced at the time of the casting process. The casting mold necessary for this purpose comprises a bottom plate 24 of graphite with a thickness of about 120 mm, side walls 25, 26, front walls 27, 28 made of graphite plates with a thickness of 40 mm, and a lid 29 likewise made of <'> a graphite plate with a thickness of 40 mm. By means of a zone extension of the side walls 25, 26 and of the front wall 27 upwards as well as by means of the arrangement of a further graphite plate 30, a funnel for the pouring of the casting is formed which is covered upwards by means of a plate of graphite 31. Inside the casting mold, along the side walls 25, 26, thin graphite plates 32 are again arranged in the lower area. In the center of the longitudinal extension of the casting mold on the lid 29 between the side walls 25 26, a graphite core 35 is arranged, the cross section of which corresponds to the desired cross section of the tunnel-shaped recess of the brick. While on one side of this graphite core 35 is provided the funnel for entering the casting, on the other side of the graphite core by means of additional graphite plates 37, 38 an air vent shaft is provided so that also the gases that form in this half of the casting mold can escape.

When the brick cast in this casting mold is cooled, the upper part of the brick body is separated by sawing along the B-B plane. The brick after that is immediately ready for use.

In the figures the following references mean

1 Brick L Length 2 Brick B Width 3 Sliding surface H Height

4 Sliding surface A-A Plane

5 Projection

6 Projection

8 Recess

29 Cover 10 Bottom plate 30 Graphite plate 1 1 Side wall 3 1 Graphite plate 12 Side wall 32 Graphite plate 13 Front wall

14 Front wall

15 Cover 35 Graphite core 16 Graphite plate

17 Graphite body 37 Graphite plate 18 Funnel for introducing casting 38 Graphite plate

0 Hole

2 Deburring

4 Bottom plate

5 Side wall

6 Side wall

7 Front wall

8 Front wall

Claims (7)

CLAIMS 1. Molten cast brick for additional electric heating in compensation fires of pusher furnaces characterized in that the brick on the side forming the sliding surface (3, 4) for the slabs has a homogeneous and dense microcrystalline structure and on the side opposite the sliding surface (3, 4) it has at least one recess (8) passing through the tunnel in the longitudinal direction of the sliding track for collecting an electric heating element. 2. Cast brick in the molten state according to claim 1 characterized in that it consists of α-corundum, mullite, vitreous phase and baddeleyite and that in the layers close to the sliding surface the different crystalline phases are distributed in a uniform and fine manner and the fraction of α-corundum crystals has a value greater than 60% by weight, while in the lower layers it has a coarser crystalline zone in which the α-corundum fraction is reduced to a value of approximately 40 to 50% by weight in favor of the mullite phase. 3. Cast brick in the molten state according to claim 1 or 2 characterized by the fact that the lateral surfaces bordering one another with adjacent bricks are provided with protrusions (5, 6) which overlap in order to avoid vertical passing separating cracks . 4. Process for producing a brick according to claim 1 by pouring a molten mass into a mold characterized in that the mold is configured in such a way that the bottom of the mold forms the sliding surface of the brick and that during casting and after casting, accelerated cooling of the melt takes place in the region of the mold near the bottom through the bottom of the mold. 5. Process according to claim 4 characterized in that the molds are produced starting from graphite plates, where graphite plates of greater thickness are used for the bottom plate of the mold than those used for the remaining walls of the mold. 6. Process according to claim 4 or 5 characterized in that the brick bodies in the shape of a rectangular parallelepiped are poured with a height greater than the height of the finished bricks, the brick bodies are provided with through cylindrical holes and the containing part shrinkage voids of the brick body is separated along a plane that crosses the hole. 7. Process according to claim 4, characterized in that with the use of a casting mold with a core corresponding to the tunnel-shaped recess, the recess is already produced during the casting process, where on one side of the core is provided the The casting funnel and an air vent sleeve is provided on the other side of the core on the casting mold.