Abstract:
A refractory material containing a synthetic dolomite aggregate having a calcium carbonate coating on the surface thereof and further containing magnesia, a binder, a plasticizer, an organic fiber homogenizer, a bond stabilizer, and a dispersant; and a method of forming a protective refractory surface on the lining of a tundish, steel ladle and the like by spraying, gunning or troweling the surface coated aggregate-containing refractory material thereon are disclosed. This material offers the multiple advantages of improving resistance to slaking, reducing alumina inclusions in the steel, and reducing clogging of the tundish nozzle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a refractory material, including sprayable material, gunnable material, and trowelable material, to be used principally for coating the surface of the back-up lining of a continuous casting tundish, steel ladle, and the like. 
     Conventionally, refractory material capable of being applied as a coating by spraying, gunning, or troweling, has been used for repairing the surface of a continuous casting tundish, steel ladle, and the like. Refractory materials using an aggregate of magnesia are generally used in such a coating refractory, but refractory materials using an aggregate of calcia such as lime and dolomite are known as well. 
     When a basic refractory containing a calcia aggregate is used in a tundish, CaO in the aggregate selectively adsorbs, or as it is termed in the art, &#34;catches&#34;, alumina and other non-metallic impurities which are contained in molten steel and which have a detrimental effect on final steel quality. The relative ability of a calcia-containing refractory to remove alumina and other impurities is referred to as the &#34;catch capability&#34; of the refractory. Reduction in the amount of these impurities also significantly reduces the occurrence of clogging in the tundish nozzle caused by the deposition of impurities on the inner surface wall of the nozzle. This selective catching property of CaO is metallurgically beneficial in that it results in higher purity steel. 
     However, when refractory material using a calcia aggregate is applied to the inner surface of tundishes and steel ladles, by any of the methods of spraying, gunning, or troweling, all of which require the use of water, cracks may be generated and material flaking or spalling may occur due to CaO in the aggregate reacting with the water (i.e. slaking) during subsequent curing, drying or preheating. Accordingly, the application of calcia-aggregate containing refractory material is currently limited to use in non-aqueous systems. 
     The objective of the present invention is to provide a refractory material, which can be applied by any of the methods of spraying, gunning or troweling, to a tundish or steel ladle, in either a hot or cold state, and even in a situation where water is used. This is achieved in the present invention by including in the refractory a specific synthetic dolomite aggregate having an excellent slaking resistance. 
     SUMMARY OF THE INVENTION 
     The refractory material for tundishes and steel ladles according to the present invention comprises from about 30 to about 90% by weight of an aggregate of stabilized synthetic dolomite containing at least 30% by weight of CaO. This stabilized synthetic dolomite aggregate has a coating layer of CaCO 3  on its surface and has a particle size of up to about 10 mm. Advantages of using the refractory composition of the present invention include improving the resistance to slaking, reducing the alumina content in the metal, and reducing the occurrence of clogging in the tundish nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a comparative plot of alumina catch for the present refractory material and a conventional refractory material. 
     FIG. 2 is a comparative plot of alumina inclusion in metal processed through tundishes lined with the refractory material of the present invention and a conventional refractory material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The refractory material for tundishes, steel ladles, and the like according to the present invention, is mainly used as a sprayable material, gunnable material or a trowelable material. The composition of the material is modifiable according to the method of application. The refractory material is composed of an aggregate, a binding material, a plasticizer, a homogenizer, a bond stabilizer and a dispersion/wetting agent. 
     The aggregate is a specific synthetic dolomite which has been treated with carbonate gas to form a surface coating of calcium carbonate thereon. This synthetic dolomite which has been subjected to such surface treatment, contains 30-95% by weight of CaO. When the synthetic dolomite has a CaO content less than 30% by weight, the accompanying metallurgical benefits, which occur in steelmaking due to the catching of alumina and other non-metallic impurities, is not satisfactorily exhibited. However, if the calcium carbonate coated synthetic dolomite has a CaO content greater than 95% by weight, the metallurgical benefits of a high CaO content are offset by the material having a poorer resistance to slaking. 
     A synthetic dolomite having a particle size greater than about 0.04 mm and less than about 10 mm is used, with from about 0.04 mm to about 5 mm being preferred. For application as a spraying or gunning material, the preferred optimum particle size of the synthetic dolomite is 3 mm or less. This is to suppress rebound loss when the material is applied. The preferred optimum particle size for application as a trowelled material is 5 mm or less. A larger particle size material can be utilized in the trowellable application because the kneading step performed when preparing a trowelable material with a mixer imparts sufficient viscosity and flexibility to the material despite the larger particle size. Therefore, material initially having a coarse particle size can be used in a trowelable material. In fact, excellent performance as to durability is obtained through the use of coarse synthetic dolomite particles in a trowelable material. The principal reason that the particle size of the synthetic dolomite is specified as 0.04 mm or coarser is because finer particles have a larger surface area which is detrimental to the slaking resistance of the material. The slaking resistance of a refractory material containing particles finer than 0.04 mm is not improved, even after treatment with gaseous CO 2  to form a carbonate coating on the particles. 
     The calcium carbonate coated synthetic dolomite is manufactured by subjecting a synthetic dolomite aggregate to an atmosphere having a CO 2  concentration of 80% or more, at a temperature of from about 400° to about 800° C., for from about 3 to about 6 hours. During this process, the surface of the synthetic dolomite contacts and reacts with CO 2  to form a coating layer of CaCO 3 . The process variables of CO 2  concentration, temperature and treatment time may be modified, depending on the aggregate to be coated. The thickness of the CaCO 3  coating layer on synthetic dolomite is from about 0.4 to about 1.0 μm, with from about 0.8 to about 1.0 μm being preferred. 
     The above surface coating process is preferably carried out after preparing the synthetic dolomite having a particle size suitable for use as an aggregate in the product formulation for the refractory material. This is to prevent the exposure of uncoated, fresh surface, which is formed upon milling, grinding or pulverizing. Uncoated fresh aggregate surface introduces hydration problems and consequently reduces the slaking resistance of the material. 
     The CaCO 3  coated synthetic dolomite may be compounded with other basic aggregates, for example, sea water magnesia, natural magnesia and olivine. In particular, the compounding of magnesia having a particle size of up to about 0.04 mm with the synthetic dolomite is preferred. This is due to synthetic dolomite having a particle size of up to about 0.04 mm exhibiting excessively large surface area, which is not suitable in this application. The compounding aggregate is added to the dolomite in an amount of from 0 to about 65% by weight in the refractory material. In addition to an aggregate, refractory material according to the present invention contains up to about 5.0 weight percent of a homogenizer in the form of an organic fiber; up to about 6.0 weight percent of a binder, which binds with the refractory material, the binder being selected from the group consisting of alkali metal silicates, alkali metal phosphates, and combinations thereof, with sodium silicate and monosodium phosphate being preferred; up to about 5.0 weight percent of a plasticizer, which enhances the ability of the refractory material to adhere to the surface to which it is applied, the plasticizer being selected from the group consisting of a clay, such as bentonite or ball clay, a silica powder, and mixtures thereof; up to about 5.0 weight percent of a bond stabilizer, the bond stabilizer being at least one of an organic acid, preferably an aliphatic di- or tri-carboxylic, more preferably, an organic acid selected from the group consisting of tartaric, citric, oxalic, and malic acids, and mixtures thereof, or calcium carbonate; and up to 1.0 weight percent of a dispersion/wetting agent, preferably a lignin calcium sulphonate. 
     According to the present invention, a refractory material contains from about 30 to about 90% by weight of the CaCO 3  coated/stabilized synthetic dolomite aggregate. The metallurgical benefits of this refractory material for a tundish or steel ladle are not fully achieved in a situation where there is less than 30% by weight of the stabilized synthetic dolomite aggregate. In contrast, a refractory material, containing more than 90% by weight of the synthetic dolomite aggregate, causes difficulty because of reduced slaking resistance of the material. 
     Refractory material of the present invention possesses remarkably improved slaking resistance compared with conventional refractory material. This is because the CaCO 3  synthetic dolomite aggregate is stable when in contact with water. The coating layer of CaCO 3  on the aggregate gives a high slaking resistance. 
     Therefore, the present refractory material can be used as a trowelable material, sprayable or gunnable material, even though water is added thereto for application by any of those means. In addition, the refractory material of the present invention can be applied to either a hot or cold surface, with no cracking or material flaking occurring. 
     Because the surface refractory material contains a CaCO 3  -based synthetic dolomite aggregate, the metallurigical benefits in the purification of steel, such as the ability to catch alumina and other non-metallic impurities from the molten steel are fully realized. Particulate impurities in the molten steel, known as &#34;inclusions,&#34; such as alumina and other non-metallic impurities, which clog a tundish nozzle are dramatically reduced by the use of the refractory material of the present invention, as compared with the use of a conventional refractory material. 
     EXAMPLES 
     The present invention can be more fully understood by the following examples, which are not intended to be in any way limiting of the scope of the invention, as set forth in the claims. 
     EXAMPLE 1 
     A. Formulation of Refractory Material 
     A sprayable refractory material of the present invention was prepared, following the compounding ratio shown below: 
     
         ______________________________________CaCO.sub.3 coated synthetic dolomite aggregate product (CM-45,manufactured by Shin-Nihon Chemical Industry, Co. Ltd.)______________________________________Particle size of 1 to 3 mm                 38% by weightParticle size less than 1 mm                 27% by weightSea water magnesia    28.2% by weightParticle size less than 0.04 mmOrganic pulp fiber    1.7% by weightSodium silicate (binder)                 2.5% by weightMalic acid (bond stabilizer)                 0.6% by weightBentonite (plasticizer)                 2.0% by weight______________________________________ 
    
     To 100 parts by weight of the above material was added 22 parts by weight of water. The resulting mixture was cold sprayed onto the surface of the back-up lining of a tundish to form a layer having a thickness of about 25 mm. No cracking or flaking of the refractory material was observed upon subsequent air curing. 
     B. Reduction in Tundish Nozzle Clogging 
     A comparative test was made to demonstrate that use of the refractory material of the present invention as a tundish lining results in a reduction in tundish nozzle clogging and that there is an improvement in operability, over use of a conventional MgO clinker-based refractory material. 
     A tundish with a 20-ton capacity was cold-sprayed with the refractory material of the present invention, as in Part A, to form a removable refractory lining therein. The tundish was then preheated to a temperature of 2000° F. and placed in service in a continuous casting station. A continuous sequence of five heats of molten steel, each of 84 tons per heat, was then passed through the tundish, exiting through an exit nozzle in the tundish. 
     For comparative purposes, the above sequence was repeated using a tundish coated with a lining made of a conventional MgO clinker-based material. 
     After completion of all five heats, the tundishes were removed from service and cooled. A chemical and X-ray diffraction analysis of the impurity-containing substance deposited on the inside surface of the nozzle, which ranged in thickness of from 0 to 10 mm on the nozzle of the tundish lined with the refractory material of the present invention, but was significantly thicker, up to about 25 mm, on the surface of the nozzle of the tundish lined with a conventional refractory material, was made for both the tundishes. The chemical composition and relative presence of various mineral compounds, as disclosed by X-ray diffraction of the deposits in both tundish nozzles, is shown in Table 1. 
     
                       TABLE 1______________________________________Chemical and mineral analysis of deposits in thetundish nozzle                 Conventional MgO     Present Refractory                 Clinker     Material    Based Material______________________________________ChemicalComposition (%)MgO         2.5           7.7CaO         1.0           3.1SiO.sub.2   0.4           0.4Al.sub.2 O.sub.3       78.3          82.5Fe.sub.2 O.sub.3       15.7          4.2Minerals(Relative presence)α-Al.sub.2 O.sub.3       +++           +++MgO.Al.sub.2 O.sub.3       +             +++CaO.6Al.sub.2 O.sub.3       +             0CaO.2Al.sub.2 O.sub.3       +             03CaO.5Al.sub.2 O.sub.3       +             0______________________________________ 
    
     As is seen from Table 1, the refractory material of the present invention causes a reduction in the alumina inclusion in the metal, as indicated by the lower alumina chemical content in the deposit (78.3% versus 82.5% for the conventional refractory material). This is due to the high reactivity of alumina with CaO in the refractory. The calcium aluminate phases formed in the refractory of the present invention are mineral complex species with lower melting points than alumina itself, and magnesium aluminate, which are the only species formed in the conventional MgO clinker-based refractory. These complexes more readily penetrate the refractory material of the present invention, thereby giving it a greater alumina catch capability. These additional alumina-containing species are not formed with conventional MgO clinker based refractory material. 
     This comparative test demonstrated that use of the refractory material of the present invention significantly reduces the occurrence of deposition of occluding matter in the tundish nozzle (deposited matter, namely, non-metallic &#34;inclusion&#34;, principally comprising alumina), as compared with the use of conventional sprayed refractory material. 
     C. Increased Alumina Catch Capability of Refractory Material and Reduction of Alumina Inclusion in Metal 
     A comparative test was made to demonstrate the increase in the alumina catch capability of the refractory material of the present invention over conventional MgO clinker-based refractory and to show that use of the present refractory material results in the production of a better grade metal due to a reduction of alumina inclusion in the metal. 
     A tundish with a 50-ton capacity was cold-sprayed with the refractory material of the present invention, as in Part A, to form a removable refractory lining therein. The tundish was then preheated to a temperature of 2000° F. and placed in service in a continuous casting station. A continuous sequence of four heats of molten steel, each of 250 tons per heat, was then passed through the tundish, exiting through an exit nozzle in the tundish. 
     For comparative purposes, the above sequence was repeated using a tundish coated with a lining made of conventional MgO clinker-based material. 
     After each heat, a sample of the molten metal in the tundish was removed for analysis for alumina inclusion. The comparative results for each heat, for both the present refractory material and the conventional refractory material, are shown in FIG. 1. As is seen from FIG. 1, the amount of alumina inclusion in the metal is significantly lower in each heat for the tundish which was lined with the refractory material of the present invention, than for the tundish which was lined with the conventional MgO clinker-based refractory. 
     After completion of all four heats, the tundishes were removed from service and cooled. The refractory linings were removed and analyzed for alumina catch. The comparative results are shown in Table 2 and FIG. 2. 
     
                       TABLE 2______________________________________Chemical analysis of refractory materials after use                    Conventional    Present Refractory                    MgO ClinkerChemical Material        Based MaterialComposition    Distance from surface (mm)(%)      5      10     15   20   5    10   15   20______________________________________MgO      39.9   47.3   53.3 65.3 69.1 75.3 81.0 79.2CaO      11.4   28.9   30.7 25.1 7.3  4.1  2.1  2.8SiO.sub.2     2.8   6.5    7.3  6.1  9.2  9.5  8.1  11.1Al.sub.2 O.sub.3     8.1   9.4    5.9  1.6  8.3  5.0  3.2  2.3Fe.sub.2 O.sub.3    31.4   3.0    0.8  0.3  3.2  3.0  2.7  2.0______________________________________ 
    
     As seen from Table 2 and FIG. 2, the amount of Al 2  O 3  catch in the refractory material, measured at several penetrative distances from the outer surface of the lining exposed to the molten metal was substantially higher at penetrative distances of from about 7 to about 15 mm for the refractory, thereby demonstrating the superior alumina catch capability of the refractory material of the present invention over conventional material. 
     D. Conclusion 
     The refractory material of the present invention reduces tundish nozzle clogging and reduces the amount of undesirable alumina inclusion in the metal. Without wishing to be limited to a particular explanation of the effectiveness of the refractory material of the present invention in accomplishing this, one reason is believed to be that alumina suspended in the molten metal reacts with CaO in the refractory material of the present invention and forms calcium aluminates, which have lower melting points than alumina and magnesium aluminate. These calcium aluminates are better able to penetrate into the refractory material, expressed as a higher equivalent alumina catch capability of the refractory material of the present invention. In combination with this effect, the calcium aluminates are lighter than the alumina and float to the surface of the molten metal bath where they can be skimmed-off. Together, both of these effects function to reduce the amount of alumina inclusion in the metal and the extent of alumina deposits in the tundish nozzle. 
     EXAMPLE 2 
     A trowelable refractory material of the present invention was prepared, following the compounding ratio shown below. 
     
         ______________________________________CaCO.sub.3 coated synthetic dolomite aggregate product (CM-45,manufactured by Shin-Nihon Chemical Industry, Co. Ltd.)______________________________________Particle size of 30 to 5 mm                  15% by weightParticle size of 10 to 3 mm                  15% by weightParticle size less than 1 mm                  34% by weightSea water magnesia     27% by weightParticle size less than 0.04 mmCaCO.sub.3 (bond stabilizer)                  5% by weightSilica powder (plasticizer)                  2% by weightMonobasic sodium phosphate (binder)                  0.2% by weightLignin calcium sulphonate                  0.2% by weight(dispersnat/wetting agent)(P-201, manufactured bySanyo Kokusaku Pulp, Co. Ltd)Organic pulp fiber (homogenizer)                  1.6% by weight______________________________________ 
    
     The trowelable material comprising the above ingredients was cold-trowelled onto the face of the lining base material of a tundish, to form a coating layer of a thickness of about 25 mm. When the obtained coating layer was examined, no cracking or material flaking of the coating was observed, indicating the excellent condition of the coating.