Monolithic refractory layer for metallurgical vessels and method of application

Abstract

 A seamless monolithic refractory layer for a metallurgical vessel is prepared by a method which comprises positioning within the vessel (15) a mold (16) having an outer surface (18) substantially conforming to the configuration of the inner surface (20) of the vessel so as to provide a substantially uniform space (22) between the mold outer surface and the vessel inner surface, filling the space (22) with substantially dry particulate mixture (35) comprising at least about 70 weight percent refractory aggregate and at least about 0.5 weight percent thermosetting resin, heating the mixture to cure the resin and removing the mold. The refractory aggregate may include from about 0.5 to 10 weight percent inorganic binder with from about 0.5 to 10 weight percent inorganic hydrate to improve the inorganic binder effectiveness and may also include up to about 10 weight percent fiber. The refractory layer is particularly suitable as the working layer of a tundish.

Description

[0001] This invention relates to a novel refractory layer for metallurgical vessels, particularly the protective layer for a tundish, and a method of application of the layer within such vessels.

[0002] In the continuous casting of steel, molten steel is transferred by means of a ladle from the steelmaking vessel to a tundish, from which it is constantly fed into casting molds. Such continuous feeding is accomplished by maintaining a reservoir of the molten steel within a tundish lined with such as refractory brick and having from one to five nozzles located on the tundish bottom. The tundish is customarily heated to about 1000 to 1300°C before being placed into service to minimize the tendency of the molten steel to cool and solidify during the pouring, with resultant nozzle clogging and adhesion of the solidified steel to the lining. Upon completion of a pouring, any slag or solidified steel retained in the tundish is scraped from the lining, and the eroded areas of the lining are repaired. Since the lining is readily attacked by molten steel and slag, frequent relining of the tundish is required, resulting in a considerable expenditure of time, labor and refractory material.

[0003] To extend the effective life of a tundish lining, current practice is to cover the lining with a protective layer, using either a trowelling material or lagging sheets. This protective layer should adhere well to the working lining and be strong as well as impermeable to molten steel and slag. The layer should also have a tendency to separate immediately behind any slag or solidified steel retained in the tundish at the completion of a pouring to permit the ready removal of the slag or solidified steel without damage to the lining; this tendency is commonly referred to as disintegratability.

[0004] Trowelling materials used for the protective layer are generally of magnesia, alumina or alumina-silica based refractory aggregate. The material is simply slurried in water and trowelled onto the surface of the tundish lining. Such an operation, however, requires considerable time, skill and labor. In addition, the resulting protective layer contains a considerable amount of water and has poor insulating properties. A tundish having such a protective layer must, as in the case of a tundish with only a refractory lining, be preheated for from two to five hours until its temperature is about 1000 to 1300°C prior to use. Such heating is undesirable from the standpoint of energy conservation.

[0005] The lagging sheet, which is also referred to as insulation panel or tundish board, is generally prepared from a slurry of refractory aggregate, fibrous material and thermosetting resin in water or other liquid. The slurry is drained of excess liquid and formed into a sheet, and the sheet is then oven dried to cure the resin. The lagging is installed over the refractory lining, the seams between adjacent sheets being filled with a combination of mortar plus a strip of the lagging material attached to the seams and fixed with nails. Since the lagging has good insulating properties and normally need be heated to a temperature of only about 500°C before being placed into service, considerable energy savings over the trowelling method is realized. But the installation of the lagging is difficult and time consuming, and the lagging is easily eroded, particularly at the seams.

[0006] It is therefore a primary object of the present invention to provide a refractory layer for a metallurgical vessel, particularly the protective layer of a tundish, and a method for its application which overcomes the drawbacks of the existing techniques.

[0007] Accordingly, the present invention provides a method of applying a monolithic refractory layer within a metallurgical vessel, which comprises positioning within the vessel a mold having an outer surface substantially conforming to the configuration of the inner surface of the vessel so as to provide a substantially uniform space between the mold outer surface and the vessel inner surface; filling the space with a substantially dry particulate mixture comprising at least about 70 weight percent refractory aggregate and at least about 0.5 weight percent thermosetting resin; heating the mixture to cure the resin; and removing the mold.

[0008] In preferred embodiments of the invention, the resin is coated on the refractory aggregate, while the mixture comprises from about 70 to 99 weight percent of the refractory aggregate having a maximum particle size of about 5 millimeters, from about 0.5 to 20 weight percent of the resin and from about 0.5 to 10 weight percent inorganic binder, in which case the mixture may also comprise from about 0.5 to 10 weight percent inorganic hydrate. The mixture may additionally comprise up to about 10 weight percent fiber having a length of preferably from about 5 to 15 millimetters. Preferably, the refractory is deadburned magnes ia, while the resin is a phenol-formaldehyde resin and is cured at a temperature of from about 150 to 250 °C. The mixture is conveniently introduced to fill the space with the aid of pressurized gas, the layer preferably being the protective layer of a tundish and having a thickness of from about 10 to 50 millimeters.

[0009] The present invention further provides for a metallurgical vessel which comprises a monolithic layer of a resin-bonded particulate refractory aggregate, the layer preferably being the protective layer of a tundish and comprising from about 70 to 99 weigrht percent deadburned magnesia aggregate having a maximum particle size of about 5 millimeters, from about 0.5 to 20 weight percent thermosetting resin and from about 0.5 to 10 weight percent inorganic binder.

[0010] The novel features and advantages of the present invention will become apparent from a reading of the following description in conjunction with the accompanying drawing of a preferred embodiment.

[0011] The drawing is a pictorial representation of a preferred method of applying a monolithic refractory protective layer on the working lining of a tundish using pressurized air to aid in applying the layer.

[0012] To apply a protective layer by this preferred method to a tundish 10, which comprises a steel shell 12 with a working lining 14 of such as refractory brick or a castable, a mold 16 fabricated from mild siteel plate and having its outer surface 18 substantially conforming to the inner surface 20 of tundish 10 is used.

[0013] Mold 16 is placed within tundish 10 as shown so as to provide a substantially uniform space 22 between mold outer surface 18 and tundish inner surface 20. Space 22 may be from about 5 to 100 millimeters, but will normally be from about 10 to 50 millimeters. While substantially uniform, space 22 may be slightly greater in areas of extreme wear, such as the region of tundish 10 into which molten steel is poured from a ladle. For example, space 22 may be substantially 30 millimeters, but as much as 50 to 60 millimeters in high wear areas.

[0014] Mold 16 is equipped with air vents 24 and a heating element, either electrical or gas fired, not shown. Mold 16 is also provided with an inlet pipe 26, which is equipped with an automatic valve 28 and is connected to one end of a hose 30. The other end of hose 30 is connected through a valve 32 to the bottom of a supply tank 34 containing a dry particulate refractory mix 36. Supply tank 34 is connected through valves 38 and 40 to an air container 42, which in turn is connected to a source of compressed air, not shown.

[0015] Air container 42 is also connected through valve 44 to hose 30 just downstream of valve 32.

[0016] Before the protective layer is to be formed, tundish inner surface 20 and/or mold 16 are preferably heated to from about 150 to 250°C. Valves 38, 40 are opened to supply air pressure to supply tank 34. Then automatic valve 28 and valve 32, and at the same time valve 44, are opened to release a mixture of refractory mix 36 from supply tank 34 and compressed air from air container 42 through hose 30 into space 22. As soon as space 22 is filled with refractory mix 36, the filling normally requiring a period of less than five minutes, automatic valve 28 closes. Mold 16 is then heated to or, if previously preheated, held at 100 to 300°C, preferably 150 to 250°C, to cure the resin content of refractory mix 36 in space 22. Mold 16 is then removed to leave on working lining 14 a layer of seamless monolithic refractory which has a thickness substantially that of space 22.

[0017] Refractory mix 36 comprises at least about 70 weight percent refractory aggregate and at least about 0.5 weight percent thermosetting resin. The particulate refractory aggregate is selected from among common refractories such as deadburned magnesia, deadburned dolomite, alumina, silica, zircon and alumina-silica based refractories. Deadburned magnesia is preferred, since it generates a minimum of inclusions in molten steel. A minimum of about 70 weight percent of the refractory aggregate is required, since below this amount the resistance of the resulting protective layer to molten steel and slag is severely reduced. At least about 90 weight percent refractory aggregate is preferred. The refractory aggregate preferably has a maximum particle size of about five millimeters; aggregates of larger size are not sufficiently free flowing to make a protective layer uniform enough to prevent penetration by the molten steel and slag.

[0018] The thermosetting resin of refractory mix 36 serves as a binder in the making of the monolithic protective layer. Any thermosetting resin may be used, such as an alkyd, allylic, amino (melamine and urea), epoxy, phenolic, polyester, silicone or urethane resin, as well as combinations and modified products thereof. Phenol-formaldehyde resins having a setting temperature of from about 150 to l80°C are preferred. The resin content of the mix must be at least about 0.5 weight percent, since a lower content fails to provide a protective layer of the desired strength. Preferably the maximum resin content is about 20 weight percent, since beyond this amount a protective layer having low resistance to molten steel and slag is formed. More preferably, the resin is from about 1 to 4 weight percent of the mix. The resin may be present in the mix as a separate particulate component of the mix as long as a uniform blend with the refractory aggregate is obtained. Preferably, however, the resin is coated on the refractory aggregate to insure an intimate and uniform bonding of the refractory particles.

[0019] Preferably refractory mix 36 additionally comprises a particulate inorganic binder. The purpose of the inorganic binder is to assure a strong bond between the refractory aggregate particles in the temperature range of 500 to 800°C, for at these temperatures the resin is carbonized and loses its binding ability, with resultant easy separation of the protective layer from the working lining. The inorganic binder may be selected from among the conventional binders for refractories, such as alkali metal salts of silicic acid, for example sodium or potassium silicate; borates, for example borax; phosphates, for example sodium hexametaphosphate, sodium tetrapolyphosphate, and sodium, potassium or ammonium monobasic phosphate. The inorganic binder when used is present in an amount of from about 0.5 to 10 weight percent of the mix. With inorganic binder contents less than about 0.5 weight percent, the resulting protective layer does not have the desired strength at elevated temperatures while with contents much greater than about 10 percent, the protective layer is not satisfactorily resistant to molten steel and slag and its disintegratability is low. Preferably, the inorganic binder content is from about 1 to 5 weight percent of the mix.

[0020] If refractory mix 36 has an inorganic binder, then preferably it additionally comprises from about 0.5 to 10 weight percent inorganic hydrate. By inorganic hydrate is meant a particulate inorganic compound having a water of crystallization. Examples of such hydrates include magnesium carbonate trihydrate (MgCO₃.3H₂O), magnesium nitrate hexahydrate (Mg(N03)206H20), aluminum potassium sulfate dodecahydrate (AlK(SO₄)₂.12H₂O) and aluminum sodium sulfate dodecahydrate (AlNa(SO₄)₂.12H₂O). The purpose of the hydrate is to promote the binding effect of the inorganic binder at lower temperatures as well as to improve the effectiveness of the inorganic binder. The desired effect is negligible at inorganic hydrate contents below about 0.5 weight percent, while with inorganic hydrate contents above about 10 weight percent, excess released moisture from the hydrate causes considerable energy loss during the initial heating of the protective layer. The preferred inorganic hydrate level is from about 1 to 5 weight percent of the mix. If the inorganic binder already has water of crystallization, as in the case of sodium silicate pentahydrate (Na₂SiO₃.5H₂O), aluminum sulfate octadecahydrate (A1₂S0₄.18H₂0), magnesium sulfate heptahydrate (MgSO₄.7H₂O) and sodium tetraborate decahydrate (Na₂B₄O₇.10H₂O; borax), use of the inorganic hydrate in the mix becomes unnecessary. In addition, powdered carbon having a moisture content of up to about 10 weight percent may be substituted for the inorganic hydrate; such carbons retain a dry particle characteristic during the layer preparation.

[0021] Refractory mix 36 may also contain up to about 10 weight percent fiber to improve the strength of the protective layer. Suitable fibers include ceramic fibers; metal fibers; carbon fibers; inorganic fibers such as rock wool, glass fibers and slag wool; and organic fibers such as cellulose and nylon. Since the fibers tend to reduce the flowability of the mix during formation of the protective layer, and also tend to make the formation of a seamless coating more difficult, the fiber length is limited to about 25 millimeters, preferably from about 5 to 15 millimeters, and the fiber content is limited to a maximum of about 10 weight percent.

[0022] The amount of compressed air used to inject refractory mix 36 into space 22 must be controlled.

[0023] Enough air must be used to successfully transfer the mix from supply tank 34 to space 22 but if too much is employed, it is difficult for the air to escape from the protective layer being formed in space 22 even with many vent holes 24 in mold 16. The weight ratio of refractory mix 36 to air is therefore preferably maintained at from about 50 to 200. While air has been illustrated and is preferred, any other suitable inert gas, such as nitrogen, may be used for the transfer.

[0024] The protective layer and method of preparation of the instant invention as herein described have numerous advantages which those in the art can readily appreciate. The high disintegratability of the layer facilitates the subsequent servicing of the tundish. Since it is formed essentially dry and has superior insulating properties, requiring less energy consumption and a- preheat to only about 500 to 800°C before being placed in service, the instant layer is superior to that prepared by the trowelling method. The instant layer is also superior to the lagging sheet since it is seamless and firmly secured to the working lining; in contrast to the lagging sheet, it is highly resistant to slag and molten steel and does not tend to separate from the working lining during the casting operation. Further, the instant method allows preparation of the protective layer in a fraction of the time required by either the trowelling or the lagging sheet method.

[0025] The following example is merely illustrative and is not to be construed as limiting the invention, the scope of which is defined by the claims.

EXAMPLE

[0026] A dry particulate refractory mix for the protective layer of a tundish is prepared by intimately blending in a mixer the following ingredients:

[0027] A tundish comprised of a steel shell lined with chamotte brick and with inner dimensions measuring 230 centimeters long by 60 centimeters wide by 70 centimeters deep and having two nozzles in the bottom is selected for applying a protective layer as described herein.

[0028] A mold having vent holes at various locations in its side and bottom portions is placed within the tundish with the aid of a crane such as to provide a substantially uniform space of about 30 millimeters between the outer surface of the mold and the inner surface of the tundish. The portions of the mold in contact with the nozzles and with the peripheral edge of the tundish are covered with heat resistant sealant. The refractory mix is transferred to a supply tank and the mold connected to the supply system as shown in the Figure.

[0029] Following the procedure described herein, the space between the mold and tundish is filled with about 350 kilograms of the refractory mix in about one minute, the surface of the tundish being maintained at a temperature of about 160°C during the transfer. The mold is then disconnected from the supply system, and the tundish together with the mold is transferred to a preheating zone where the mold is heated for about 5 minutes with a low flame from gas burners to heat the mold outer surface to about 180°C.

[0030] Following an additional 5-minute holding period at 160-180°C to cure the resin, the mold is removed from the tundish.

[0031] The tundish is then enclosed with a tundish cover, preheated to a temperature of 500 to 800°C for about 30 minutes, and immediately transferred to a casting site where continuous castings are performed. The protective layer is found to have a high resistance against the molten steel and slag, which can be easily removed from the tundish after service.

[0032] While the present invention has been described in connection with the protective layer of the tundish, those in the art can readily appreciate that it is not so limited. The present layer with its method of preparation is readily adaptable,.for example, to the tundish working lining itself. In such case, the layer would be heated sufficiently to form high temperature bonding in order to give the layer the high strengrth required of the working lining. The method is also applicable to the linings of other vessels such as ladles, crucibles and even steelmaking vessels themselves.

Claims

1. A method of applying a monolithic refractory layer within a metallurgical vessel, which comprises:

positioning within said vessel a mold having an outer surface substantially conforming to the configuration of the inner surface of said vessel so as to provide a substantially uniform space between said mold outer surface and said vessel inner surface;

filling said space with a substantially dry particulate mixture comprising at least about 70 weight percent refractory aggregate and at least about 0.5 weight percent thermosetting resin;

heating said mixture to cure said resin; and removing said mold.

2. The method of claim 1 wherein said resin is coated on said refractory aggregate.

3. The method of claim 1 wherein said mixture comprises from about 70 to 99 weight percent of said refractory aggregate having a maximum particle size of about 5 millimeters, from about 0.5 to 20 weight percent of said resin and from about 0.5 to 10 weight percent inorganic binder.

4. The method of claim 3 wherein said mixture additionally comprises from about 0.5 to 10 weight percent inorganic hydrate.

5. The method of claim 1 wherein said mixture additionally comprises up to about 10 weight percent fiber having a length of from about 5 to 15 millimeters.

6. The method of claim 1 wherein said refractory is deadburned magnesia.

7. The method of claim 1 wherein said resin is a phenol-formaldehyde resin and is cured at a temperature of from about 150 to 250°C.

8. The method of claim 1 wherein said mixture is introduced to fill said space with the aid of pressurized gas.

9. The method of claim 1 wherein said layer is the protective layer of a tundish.

10. The method of claim 9 wherein the thickness of said layer is from about 10 to 50 millimeters.

11. A metallurgical vessel, which comprises a monolithic layer of a resin-bonded particulate refractory aggregate.

12. The vessel of claim 11 wherein said layer is the protective layer of a tundish and comprises from about 70 to 99 weight percent deadburned magnesia aggregate having a maximum particle size of about 5 millimeters, from about 0.5 to 20 weight percent thermosetting resin and from about 0.5 to 10 weight percent inorganic binder.

Drawing