PROCESS FOR MAKING A REFRACTORY ARTICLE, REFRACTORY ARTICLE AND PRESSING DEVICE

WIPO Patent Application WO/2006/029905

A2

The present invention relates to a pressing process wherein a mold is filled with one or more refractory composition(s), a first pressure is applied on the mold and further pressure is applied on a selected region of the mold. The present invention also relates to a refractory article made according to that process and a pressing device for carrying out the process.

Moriarty, Brendan Mortimer (29 Lammermuir Drive, Paisley Renfrewshire, Scotland PA2 8BP, GB)
Hanse, Eric (La couture, Orches, F-86230, FR)

PCT/EP2005/010164

March 23, 2006

September 16, 2005

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VESUVIUS CRUCIBLE COMPANY (103 Foulk Road, Wilmington, DE, 19803, US)
Moriarty, Brendan Mortimer (29 Lammermuir Drive, Paisley Renfrewshire, Scotland PA2 8BP, GB)
Hanse, Eric (La couture, Orches, F-86230, FR)

B22D41/52; B22D41/18; B22D41/50; B30B11/00

Debled, Thierry (Vesuvius Group S.A, Intellectual Property Department Rue de Douvrain 17, Ghlin, B-7011, BE)

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Claims.

1.

Process for producing a refractory article for the casting of a molten metal, comprising the steps of a) filling a mold with one or more refractory compositions; b) subjecting the refractory composition(s) in the mold to a first predetermined pressure; c) during the same pressing cycle, further subjecting the refractory composition(s) in one or more selected region(s) of the mold to a second predetermined pressure.

2.	Process according to claim 1, characterized in that the second predetermined pressure is higher than the first predetermined pressure.

3.	Process according to claim 1 or 2, characterized in that the one or more refractory compositions is (are) in powder form.

4.	Process according to any one of claims 1 to 3, characterized in that the pressing steps are carried out in an isostatic press.

5.	Process according to any one of claims 1 to 4, characterized in that it comprises one or more additional steps for subjecting the refractory composition in one or more selected region(s) of the mold to another predetermined pressure.

6.	Process according to any one of claims 1 to 5, characterized in that the region(s) of the mold subjected to further pressure is (are) filled with a different refractory composition.

7.	Process according to any one of claims 1 to 6, characterized in that the further pressing step is delayed with respect to the previous pressing step.

8.	Refractory article obtained by a process according to any one of claims 2 to 7.

9.	Refractory article according to claim 8, characterized in that at least a part of the article located in a wear zone of the said article has been subjected to a further pressing step.

10.	Refractory article according to claims 8 or 9, characterized in that said part of the article subjected to a further pressing step has a density higher than an adjacent part of the article.

11.	Refractory article according to any one of claims 8 to 10, characterized in that the density of the article continuously decreases from the said part of the article subjected to a further pressing step towards adjacent parts of the article.

12.	Refractory article according to any one of claims 8 to 11 , characterized in that the said part of the article subjected to a further pressing step has a different composition than the rest of the article.

13.	Refractory article for the casting of a molten metal made from at least one refractory composition, characterized in that a first part of the article made of a refractory composition has a density higher than an adjacent part of the article made of the same refractory composition.

14.	Refractory article for the casting of a molten metal accord in to claim 13, characterized in that the density continuously decreases from the first part towards adjacent parts of the article.

15.	Refractory article for the casting of a molten metal according to claims 13 or 14, characterized in that the first part of the article has a density higher, a porosity lower and roughly the same gravity as adjacent parts of the article.

16.	Refractory article for the casting of a molten metal according to any one claims 8 or 15, characterized in that the article is a pouring shroud for a tube changer.

17.

Pressing device for producing a refractory article for the casting of a molten metal, wherein a mold filled in with one or more refractory composition(s) is subjected to a predetermined pressure, characterized in that it comprises means for subjecting at least one region of the mold to a further pressing step.

18.	Pressing device according to claim 17, characterized in that the means comprises a processing chamber divided into at least two subchambers.

Process for making a refractory article, refractory article and pressing device. Description. [0001] The present invention relates to a process for making a refractory article and the refractory article made according to the said process. It also relates to a pressing device for making such refractory article. [0002] Refractory articles are commonly used in the molten metal industry and in particular in the steel industry for the continuous casting of molten metal. [0003] It is known that continuous casting of a liquid metal is generally carried out by means of an installation comprising various refractory articles forming a channel between two successive metallurgical vessels. These refractory articles perform various functions, namely conveyance of the liquid metal, protection of the liquid metal against cooling and chemical attack from the surrounding atmosphere and, where appropriate, regulation of the pouring flow-rate of the liquid metal. These articles may be, for example, an inner nozzle, a submerged entry nozzle, a stopper, a ladle shroud, a collector nozzle, or the fixed or mobile plates of a slide gate valve. [0004] These articles are composed for example of refractory materials such as alumina, magnesia or zirconia and graphite or a mixture thereof and can be cast, pressed, extruded or others. When the article is pressed, it is commonly hydraulically or isostatically pressed. [0005] In order to perform different functions, the articles are made of different refractory compositions adapted to each function. In some applications, it is required to combine more than one function in the same article, for instance to have a good erosion resistance in one area and a good strength or thermal shock resistance in another area. [0006] Various methods of obtaining such combination of properties in a sole article are known already. One is to co-press different refractory compositions at the same time, for instance a zirconia composition at the slag line sleeve and a less expensive more thermal shock resistant alumina graphite composition in the rest of the pouring nozzle. Another method is to cement a part already pressed and fired in or around the article. These methods are known by the skilled person and widely used in the molten metal industry. For instance, WO 88/06500 relates to a refractory article made of two different refractory materials. The first material used for the body is a good thermal shock resistant material and the second material located around the aperture is a hard material capable of cutting a skin or shell of metal. WO 96/16758 relates also to a refractory article comprising different refractory materials. In this case, a layer of the article is made of a material comprising sintering aids so that the layer densifies during the preheating of the article to form a dense impermeable to gases layer, the rest of the article being made of a standard refractory material. [0007] The present invention relates to a method for improving the properties in a selected part of the article without necessarily having to use expensive refractory compositions or separated/additional process steps. [0008] The present invention relates to a process for making a refractory article for the casting of molten metal. More precisely a pressing process that permits to obtain different properties (improved properties) in different parts of the article and this in a single pressing cycle. In the frame of the present application, a pressing cycle consists of a succession of various steps wherein a composition is charged only once into a mold before subjecting the mold (wholly or partially) to the appropriate pressure(s). [0009] As stated above, most refractory articles are hydraulically or isostatically pressed. [0010] Hydraulic pressing is usually used for simple or flat articles as bricks and plates. This kind of pressing being uniaxial, the pressing is not uniform throughout the piece and a gradient of density is present. Some hydraulic pressing improvements are known already as the "double uniaxial pressing" or the compensation pressing. In the so-called double pressing, the refractory composition is compacted both from the top and from the bottom. This improves the density profile especially for thick pieces. In the so-called compensation pressing or tooling, refractory composition is added in some regions of the mold, for instance in the boss of a slide gate plate in order to compensate and to obtain a more uniform density throughout the piece. [0011] lsostatic pressing is widely used to make graphite containing generally complex pieces for the continuous casting of molten metal. The refractory composition to be pressed is filled into the mold, then placed into the processing chamber. A liquid medium, often an oil-water mixture, is pumped in and pressurised to create a "uniform" product. Advantages of isostatic pressing include the creation of product with uniform density compared to hydraulic uniaxial pressing which leads to a reduction of internal stresses, eliminating cracks, strains and lamination. An isostatic press is described for instance in documents FR2757099 and US 3,956,452. [0012] However, the uniform density is only theoretical. Toolings for isostatically pressing refractory article are comprised of rigid or fixed elements and flexible (compressible) elements (known as mold). Rigid or fixed elements, usually metallic, as for example a metallic mandrel or a bottom closure control the general dimensions and shape of the article. When the pressure is applied, it is mainly communicated to the refractory composition to be pressed through the flexible (compressible) elements. The fact that there is one or more rigid (fixed) element(s) in the device influences the pressing of the article and one can say that the pressing is no longer fully isostatic. It has been noticed that in some regions of the pressing mold and thus, on some parts of the article, the pressure does not reach the pressure of the liquid medium. For instance in some cases, the pressure reached in the proximity of the metallic mandrel or the internal bore of the article is lower than on the external diameter. [0013] It is known from GB 1 ,321 ,451 that relates to the production of refractory articles by isostatic pressing to use a flexible extensible material which projects into the mold container for pressing hollow articles. This is said to improve the pressure obtained at the proximity of the internal bore but the final shape of the article is uncertain and the properties are not improved with respect to the rest of the article. [0014] It is also known to improve the density by degasefying the refractory composition to be pressed before and/or during molding or pressing. This increases the density in the whole article and requires significant additional processing time. [0015] The present invention relates to a method for making a refractory article that overcomes the above drawbacks and permits to obtain uniformly pressed articles but also permits to significantly improve the properties in a selected part of the article. [0016] The invention can be used for example to increase the density in the part of the article subject to high mechanical stress during use as for example the nose of a stopper or the sliding plate of a pouring nozzle for a tube changer. Pouring nozzle for a tube changer is described for instance in patent EP 192,019. [0017] In another aspect of the invention, the process according to the invention can be used to provide further or higher pressure in the parts of the article that are usually not well pressed. For instance, it can be interesting to provide further pressure in the proximity of a fixed (not compressible) element of the pressing mold. [0018] It can also be interesting to increase the density and the green strength (i.e., before firing the article) in the weak parts of the article, for instance where there is a sharp angle or a step. [0019] More particularly the present invention relates to a process for producing a refractory article for the casting of a molten metal, comprising the steps of a) filling a mold with one or more refractory compositions; b) subjecting the refractory composition(s) in the mold to a first predetermined pressure; c) during the same pressing cycle, further subjecting the refractory composition(s) in one or more selected region(s) of the mold to a second predetermined pressure. [0020] The refractory compositions can be in powder form but some pre-formed or pre-fired element could be placed into the mold as well. [0021] Preferably, the second predetermined pressure is higher than the first predetermined pressure, preferably significantly higher, preferably between two or three times the first predetermine pressure. This provides advantageous characteristics to the pressed article as will be seen later. [0022] Preferably, the process is carried out in an isostatic press. The isostatic press comprises a processing chamber that is divided in at least two sub-chambers so that the liquid medium can be pressurized at different pressures in said sub-chambers. The process according to the invention is not necessarily limited to three steps. One or more other regions of the mold can be subjected to further pressure as well. For instance, the head of a ladle shroud is subjected to a second predetermined pressure and the slag line sleeve of the same ladle shroud to a third predetermined pressure previously, simultaneously or afterwards. The processing chamber in this case is divided into three sub-chambers. [0023] As the refractory composition can move in the mold under the action of the pressing, it is - preferable to delay the further pressing with respect to the previous step. In case of three step's process, step c) is delayed with respect to step b). Initially, a first predetermined pressure is transmitted to the liquid medium of each sub-chamber of the processing chamber and then after a predetermined delay a second predetermined pressure is transmitted to the liquid medium of at least another sub-chamber of the processing chamber. [0024] It is still possible to use different refractory compositions for the different parts of the article. In this case the properties are improved in two ways, by the composition itself and the better pressing of this last. [0025] The process according to the invention permits to target improved properties in some parts of the refractory article only. For example, a better density is obtained only in the wear zone where it is needed so that the thermal shock resistance properties are maintained in the other parts of the article. [0026] The first and further pressures being applied to the article within the same pressing cycle, the refractory composition is still allowed to move (displace) slightly within the mold and the interface between the different parts of the article is not sharp. The change in properties is thereby continuous. The density continuously decreases from the said part of the article subjected to further pressure towards adjacent parts of the article. [0027] The continuous change in properties is advantageous. There is no risk of separation of the part of the article subjected to further pressure from adjacent parts of the article. The continuous change in properties has also a positive influence on the life of the article, the article being more homogeneous, thermal shock resistant, less sensitive to cracking, etc,... [0028] The present invention also relates to a refractory article made according to the process of the invention. The second predetermined pressure is applied on a selected part of the article, for instance a part of the wear zone of the article. When the second predetermined pressure is higher than the first predetermined pressure, the selected part of the article subjected to the further pressing has a density higher than adjacent parts. The density of the article continuously decreases from the said part of the article subjected to the further pressing step towards adjacent parts of the article. As explained above, this is not limited to one selected part of the article. A second predetermined pressure can be applied on one or more selected part(s), a third pressure on another or others selected parts, and so on. [0029] The present invention also relates to a refractory article for the casting of a molten metal made from at least one refractory composition, characterized in that a first part of the article made of a refractory composition has a density higher than an adjacent part of the article made of the same refractory composition. [0030] In the known methods, in particular method described in document WO 96/16758, to obtain improved properties especially a higher density or hardness in one selected part of the article, it is necessary to use a different refractory composition. This is also the case for the stopper rod of document WO 88/06500 in which the composition of the layer is a low carbon alumina composition comprising sintering aids so that densification occurs during preheating. [0031] In the present invention, the improved properties and in particular the higher density can be obtained without the use of another composition. The same composition is used for the entire article and by the help of the process according to claim 1 , one part of the article is subjected to a further pressing resulting in a higher density than adjacent parts. By same composition, it is intended that a zone of the article is made of one single composition, one part of this zone being subjected to further pressing. Of course, in another zone of the article a different composition can be used as for instance to form the slagline sleeve of a nozzle. [0032] When the same refractory composition is used for the part of the article subjected to further pressing and the adjacent parts, the said part of the article subjected to further pressing has a higher density, a lower porosity and roughly the same gravity than adjacent parts of the article. The gravity is the apparent specific gravity (i.e. the ratio of the weight in air of a unit volume of the impermeable portion of the article to the weight in air of equal volume of water). [0033] The present invention is particularly advantageous for the manufacture of a pouring shroud for a tube changer. A higher density and thus a better mechanical resistance is obtained in the sliding plate while the rest of the shroud remains at a lower density for thermal shock resistance; the transition between the two parts of the article being gradual, separation is avoided. [0034] The invention also relates to a pressing device for producing a refractory article for the casting of a molten metal wherein a mold filled in with one or more refractory composition(s) is subjected to a predetermined pressure, characterized in that it comprises means for subjecting at least one region of the mold to a further pressing step. [0035] The means may be a processing chamber divided in at least two sub-chambers as explained above but any other means giving the same effect and leading to the same refractory article could be used. The manufacturing of a pressing device according to the invention can be carried out by a skilled person in the art. [0036] In one embodiment, the pressing device is a dry-bag isostatic press. In the dry-bag type of press, the mold is not in contact with the fluid. The mold is placed itself within a deformable sheath (known as a master bag) which is fixed permanently in the pressure vessel. The pressure of the fluid within the vessel is increased to a determined pressure. The master bag transmits the pressure to the removable mold which deforms and transmits the pressure to the powder. A dry-bag press is described in document FR 2 757 099. [0037] Advantageously, the pressing device according to the invention may also comprise a delayer for delaying the further pressing step with respect to the previous one. [0038] In order to facilitate the understanding of the invention, embodiments will now be described, only as examples which do not limit the scope of the invention, with the help of the enclosed figures wherein: Figure 1 illustrates step b) of pressing a ladle shroud according to the invention; Figure 2 illustrates a further step of pressing a ladle shroud according to the invention; Figure 3 illustrates the pressing of a pouring shroud for a tube changer according to the invention; Figure 4 illustrates another embodiment for pressing a pouring shroud for a tube changer according to the invention; Figure 5 illustrates another embodiment for pressing a pouring shroud for a tube changer according to the invention; Figure 6 illustrates the pressing of a stopper according to the invention; Figure 7 illustrates the pressing of a slag line sleeve of a shroud according to the invention; Figures 1 to 7 are schematic figures aimed at illustrating the principle of the present invention. Notably, in some figures, the regions of the mold and parts of the article are discrete and represented by straight line, this is in fact not the case as explained above. Figure 8 illustrates the sampling of a pouring shroud for a tube changer made according to the process of the figure 5. Figure 9 illustrates a pressing device according to the prior art comprising the removable tooling. Figure 10 illustrates a pressing device according to one embodiment of the present invention. Figure 11 illustrates a pressing device according to one embodiment of the present invention comprising the removable tooling. Figure 12 illustrates a pressing device according to one embodiment of the present invention comprising the removable tooling. Figure 13 illustrates the pressure transmission elements of figure 12. [0039] Figure 1 relates to the pressing of a ladle shroud (1). The processing chamber is divided into two sub-chambers acting on the two regions (4,5) of the mold. A pressure of 150 MPa is applied to the entire shroud (1). Afterwards as shown in figure 2 another pressure of 300 MPa is applied only in the region corresponding to the head (2) of the ladle shroud. The properties and in particular the mechanical strength is increased in the head (2) of the shroud with respect to the tubular part (3) of the shroud. The shroud is more resistant during handling and connection to the collector nozzle. [0040] Figure 3 relates to pressing a pouring shroud (6) for a tube changer. As for the ladle shroud a higher pressure is applied in the sliding plate (7) to improve the density and wearing resistance. In this figure the fixed elements of the tooling, the bottom closure (9) and the pressing mandrel (8) are shown. [0041] Figure 4 relates to pressing of a pouring shroud (6) for a tube changer. This time the higher pressure is applied on the internal bore near the sliding surface of the plate (7) to improve the density and strength in the corner area (10). [0042] Figure 5 relates to the pressing of a pouring shroud (6) for a tube changer. The higher pressure is applied on the sliding surface of the plate (7). [0043] Figure 6 relates to the pressing of a stopper (11 ). The higher pressure is applied on the stopper nose (12). The refractory composition filled in the mold in the nose can be different and more erosion resistant than the refractory composition of the rest of the stopper (11 ). [0044] Figure 7 relates to another embodiment for pressing a shroud. A pressure is applied first to the entire shroud (1 ), a second pressure is applied through region (4) only to increase the density of the slag line sleeve (13) and a third pressure is applied through region (14) to improve the • strength of the head of the shroud. [0045] Figure 8 relates to the pressing of a pouring shroud (6) for a tube changer. A refractory composition of alumina graphite is used for filing the mold. Pressing process is similar to the one related to figure 5. First pressing pressure is 150 MPa and second pressing pressure is 300 MPa. The shroud is cured and then fired at 10000C. Three samples (S1 ,S2,S3) are taken at different depth of the sliding surface and one (S4) in the tubular part of the pouring shroud. The results are shown in the table 1 below.

The same sampling on a pouring shroud made according to the prior art process has shown the results of table 2.

The sliding surface of the pouring shroud according to the invention has a much better visible aspect after usage. Less crack and scratch are visible and air infiltration into the molten metal is reduced. [0046] Figure 9 is a dry-bag type pressing device (15) for pressing a pouring shroud (6) for a tube changer according to the prior art. The device (15) comprises a pressure vessel (24), a top closure (not shown), a bottom closure (9), a master bag (19) and retaining ring (22) with seals (21). The tooling comprising the pattern mold bag (23) and the pressing mandrel (8) is raised into the press supported on the bottom closure (9). A cavity is formed between the outer peripheral wall of the mold, the inner peripheral wall of the vessel (24) and the retaining ring (22). This cavity forms the chamber (17) in which pressurising fluid is introduced. A seal (21 ) is positioned on the ring (22) to prevent leakage of the pressurising fluid, the fluid is generally oil. The fluid is fed (16) under pressure in the chamber (17). The master bag (19) transmits the pressure to the pattern mold (23) and the refractory powders (20) are molded and compressed. After the said powders (20) have been molded to form the molded shroud (6), the pressure in the chamber (17) is reduced. ' [0047] Figure 10 is a dry-bag type pressing device (15) according to the invention. The removable tooling is not shown. The chamber comprises two sub-chambers . A first chamber (17) and a second chamber (18). The pressing device comprises a secondary fluid inlet (26) for pressing at different pressure in the chamber (18). The pressing process in a device as illustrated in figure 10 is for instance the following: - filling the mold with refractory composition; - injecting the fluid under a determined pressure in both sub-chambers; - after a few seconds, further increasing the pressure of the fluid in the second chamber only so as to further compress the powders in the region located beyond the second chamber. [0048] Figure 11 shows the dry-bag type pressing device (15) of figure 10 comprising the removable tooling for pressing a pouring shroud (6) for a tube changer according to the invention. The chamber comprises two sub-chambers . A first chamber (17) and a second chamber (18). The pressing device comprises a secondary fluid inlet (26) for pressing at different pressure in the chamber (18). The fluid is injected first at the same pressure in the two chambers (17,18). Afterwards, the fluid is brought at higher pressure in chamber (26) and secondary master bag (27) pushes on the region of the mold corresponding to the sliding plate of the pouring shroud. This way, the sliding plate is pressed at higher pressure than the rest of the refractory article. [0049] Figure 12 is a dry-bag type pressing device (15) for pressing a pouring shroud (6) for a tube changer according to the invention. The tooling is designed this time for further pressing in the part of the plate located around the casting bore (30). Secondary master bag (27) pushes on pressure transmission element (29) which passes through the support pillars (28). The pressure transmission element (29) deforms transmitting the pressure to the powder of the part of the article located around the casting bore (30). In the embodiment of figure 12, the gradiant of properties within the refractory article will be present in both, vertical and horizontal direction [0050] Figure 13 illustrates the pressure transmission element of figure 12. The pressure transmission element (29) is preferably an elastomer which hardness is typically 20 to 30 A shores. The pressure transmission element comprises holes (31) for support pillars (28). The surface (32) of the transmission channel adjacent to the refractory powder is made of an harder material with respect to the transmission element (29) to avoid sticking. Hardness of surface (32) is generally about 50 A shores. Reference list

1 ladle shroud 2 head of ladle shroud 3 tubular part of the shroud 4 region 5 region 6 pouring shroud 7 sliding plate 8 pressing mandrel 9 bottom closure 10 corner area 11 stopper 12 stopper nose 13 slag line sleeve 14 region 15 pressing device of the prior art 16 fluid inlet 17 chamber (for pressurising oil) 18 second chamberi 9 master bag 20 refractory powder 21 seal 22 ring 23 pattern mold 24 pressure vessel 25 pressing device according to the invention 26 secondary fluid inlet 27 secondary master bag 28 support pillars 29 pressure transmission element 30 part of the shroud pressed at higher pressure 31 holes for support pillars 32 surface of pressure transmission element adjacent to the refractory powder

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