JPS6144836B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nozzle for continuous casting of steel, particularly a CaO-containing graphite casting nozzle that is effective as a nozzle for use in steel types containing a large amount of Al and prone to nozzle clogging. Nozzles for continuous casting of steel include long nozzles that connect the ladle and tundish, tundishes that connect the tundish and mold, and immersion nozzles. Alumina-graphite is superior in terms of poling properties, (2) wear resistance, and (3) corrosion resistance, and is currently mainly used. This alumina-graphite nozzle satisfies properties (1), (2), and (3) above by combining alumina grains with high strength and high corrosion resistance and graphite with excellent spalling resistance. However, the disadvantage of this material is that, especially for steel types that contain a large amount of Al, the Al in the steel is oxidized to become Al ₂ O ₃ , which precipitates on the refractory wall, and this progresses and causes so-called nozzle blockage. It is easy to wake up. There have been many reports on such blockage of continuous casting nozzles, and there are many explanations for the mechanism, but the nozzle blockage itself may be caused by the type of steel being handled,
At present, it has not been completely elucidated, as there are various variations depending on the operating conditions. However, in killed steel that generally contains a large amount of aluminum, the Al in the steel is oxidized and becomes alumina, which precipitates and solidifies on the refractory wall where the flow of molten steel is slow, rapidly growing and clogging the nozzle. It is considered to be a thing. In this case, when the molten steel flow flowing through the nozzle hole is relatively calm, the flow velocity is highest near the center of the nozzle cross section, and conversely, the flow slows down near the nozzle wall due to the resistance of the wall. ing. Generally, as the viscosity of a fluid increases, this region of slow flow becomes wider. On the other hand, regarding the attached alumina particles,
This is thought to be mainly due to Al dissolved in the steel becoming alumina through oxidation.
The flow velocity of molten steel is also high near the center of the nozzle hole,
There is no such thing as merging and growing. The problem is in the area near the nozzle wall, where the speed of the molten steel flow is slow as described above, and it is thought that the alumina particles exist in a suspended state in the molten steel in this area. When fine alumina particles with a high melting point (2050°C) are suspended in the steel, the apparent viscosity becomes considerably high, and due to this effect, the flow velocity becomes even slower near the wall. It is thought that under these conditions, fine alumina particles adhere to the nozzle wall, coalesce with other particles, grow larger, and then fall off due to the resistance of the molten steel flow. When the concentration exceeds or the speed of the molten steel flow decreases below a certain speed, the alumina on the nozzle wall grows large, narrowing the holes and reducing the flow speed of the molten steel, causing more alumina grains to adhere and sinter. It is thought that this causes rapid narrowing of the pore diameter, making it impossible to cast. Various measures have been attempted to deal with the nozzle clogging phenomenon caused by alumina precipitation. For example, in terms of materials, the narrowing of the nozzle was dealt with by melting away the refractory material, and the silica content of the current Al ₂ O ₃ - graphite nozzle was increased. In addition, structural improvements include blowing inert gas through the inner wall of the nozzle to move the molten steel near the nozzle wall to prevent alumina precipitation. Although these methods have had some effect, they have not fundamentally solved the problem of abnormal erosion or nonmetallic inclusions in molten steel. The present invention focuses on using a raw material containing CaO as a refractory aggregate, and provides a nozzle material for continuous casting that solves the above problems and is also effective in cleaning steel. It is. Regarding CaO,
Features include: first,
(1) Except for the special components of CaO, the liquid phase temperature between CaO and other components is considerably lower than the solidification temperature of molten steel (≒1500℃), and non-metallic inclusions containing CaO easily float to the surface. As mentioned above, since it is in a liquid phase, it is difficult to build up on the brick surface. Also, (2)
Reacts well with slag components (Fe-Oxide, SiO ₂ , Al ₂ O ₃ ). From the above, on the inner wall of the CaO-containing continuous casting nozzle, the precipitated alumina quickly reacts with the CaO component in the nozzle material and becomes a liquid phase. That is,
Alumina grows on the refractory wall and is less likely to cause nozzle narrowing. In particular, compared to the state where alumina fine particles are suspended in molten steel, the viscosity of the liquid is much lower, and therefore the region where the molten steel flow rate slows down is extremely limited, causing nozzle constriction. The possibility becomes even smaller. In addition, the generated CaO--Al ₂ O _3- based substances tend to float, and after being washed away from the nozzle, they quickly react with the slag or powder on the upper surface of the molten steel and are absorbed into the slag. In other words, non-metallic inclusions are less likely to form, and the properties of the steel are significantly improved. In this way, incorporating an appropriate amount of CaO into the material has a great effect, but
The disadvantages of CaO raw materials are (3) large thermal expansion;
While Al ₂ O ₃ expands by about 0.8% at 1000℃,
For sintered calcia raw materials, it is large at approximately 1.3%. For this reason, if the material is heated to a level that causes non-uniform temperature distribution, a large thermal stress will be generated inside the material, making it difficult to apply it to something like a continuous casting nozzle. Also, (4)
CaO reacts violently with water even at room temperature and is easily digested, so great care must be taken when storing it. The above two factors were major obstacles to making the nozzle material suitable for continuous casting. However, the drawback (3) can be solved by combining graphite, which has particularly excellent thermal shock resistance, and increasing its proportion. Regarding (4), as various patents have recently been submitted,
Clinkers with excellent digestion resistance are commercially available, and as long as they are packaged with sufficient care, there will be no practical problems. The main thrust of the present invention is to provide a continuous casting nozzle with very little nozzle clogging due to alumina precipitation by combining graphite with a refractory aggregate containing a CaO component as described above. In the present invention, first of all, graphite powder is used. Graphite can be broadly divided into natural graphite and artificial graphite, and the former can be further divided into scaly graphite and earthy graphite. Scale graphite is famous for its production in the Soviet Union, China, Madagascar, North Korea, Ceylon, Sri Lanka, etc., and its properties differ slightly depending on the production region and mine, but in general, the hexagonal layer planes are relatively stacked. It is graphite with well-regulated and well-developed crystals, and has a scale-like shape. As the name suggests, earthy graphite is earthy, has smaller crystals than scale graphite, and generally contains more impurities. On the other hand, the latter type of artificial graphite includes electrode scraps, pyrolytic graphite, and, in a unique place, hard graphite generated during the iron smelting process. Any of the above graphite can be used, but natural scale graphite is superior in terms of corrosion resistance, price, stability, and thermal shock resistance, and is widely used as a raw material for general ceramics. and is preferred in this case as well. If this graphite powder is less than 10%, it is insufficient in terms of the thermal shock resistance mentioned above, and if it exceeds 50%, the soft nature of graphite becomes dominant and wear damage becomes large, which is inappropriate. Next, as raw materials for ceramics containing CaO components,
Typical examples include sintered calcia and fused calcia, but dolomite, magdro clinker, alumina cement, and the like can also be used. The mixed powder in the present invention is composed of one or more of the above-mentioned CaO-containing raw materials, graphite powder, and the remainder is other refractory aggregate powder. In this case, the CaO component in the above-mentioned mixed powder is , if it is less than 20%, the aforementioned effect of pore blockage resistance will not be fully demonstrated, and 75
If it exceeds 60%, thermal shock resistance decreases and it is practically unusable.
If it exceeds 20% to 60%, the melting loss becomes large and there is a problem. Next, as an organic binder, any material that forms a carbon bond after firing in a non-oxidizing atmosphere can be used, but tar pitch is usually preferred because it is plastic at room temperature and has good formability. Alternatively, phenolic resins, furan resins, etc. are convenient, but in consideration of the influence of moisture, modified phenolic resins as described below are preferred. As mentioned above, CaO-containing raw materials have larger thermal expansion than other refractory aggregates such as alumina, and to compensate for this drawback, it is necessary to increase the proportion of graphite powder. In this case, graphite has a scale-like shape, so it has poor formability, and it inherently has poor reactivity with other components except for oxidation, so it can be sintered with other refractory aggregates to form a ceramic bond. Therefore, the structure deteriorates and often does not have sufficient strength. For this reason, it is effective to add 1 to 15% of metallic aluminum in place of a part of the refractory aggregate of the present invention. aluminum is
When fired in a non-oxidizing atmosphere such as CO or _N2 ,
It has the effect of producing highly corrosion-resistant reaction products such as Al ₂ O ₃ and AlN and expanding, making the material denser.
Furthermore, when it remains in the material as Al, it significantly improves oxidation resistance. It also has good compatibility with graphite or binder carbide, filling the spaces between these grains and forming a strong bond, which greatly improves hot strength. The use of Al is also effective in the present invention. As mentioned above, it is preferable to use a phenolic resin as the organic binder, but it is also suitable to use a thermosetting resol type resin or a thermoplastic novolac type resin mixed with a curing agent such as hexamethylenetetramine. When a suitable solvent is added and used as a binder, when fired in a non-oxidizing atmosphere, a three-dimensional network is formed before entering the carbonization process, resulting in a high carbonization yield.
It has excellent strength and other properties after firing, but in this case, if the solvent is hydrophilic, it easily absorbs moisture from the air, which is natural, but it is not suitable for aggregates containing CaO. be. Furthermore, when curing by heating as described above, condensed water is released, so this is also the case. Therefore, especially for materials containing CaO, patent application No. 71053 (1983)
(Refer to Publication No. 26065) "Production method of basic refractories"
It is effective to use a method similar to the method shown in . For more information on this, please refer to the above materials, but the use of phenolic resins modified with one type of alkylene carbonate or a mixture thereof can be particularly effective in digesting CaO. be. As mentioned above, at the stage of the substrate, particles containing CaO components are almost completely covered with an organic binder, and the porosity is as low as 3 to 5%, so it is important to check the moisture content of the binder itself and moisture absorption. For example, it rarely reacts with moisture in the air and is digested, but the situation changes considerably after firing. First, after reduction firing of tar pitch, phenolic resin, etc., which are normally used in binders, the amount of residual carbon is less than 50%, and about half of it becomes _H2 , _CH4 , etc. during carbonization and volatilizes into the air.
Therefore, the porosity after firing is approximately 13% or more,
The proportion of the area of particles containing CaO components exposed to air also increases. According to the inventors' study, if clinker sintered at a sufficiently high temperature is used, there will be no problem with this level of moisture, but especially when using refractory powder containing CaO components in the fine powder region. These products use the above-mentioned crushed clinker, but if there is a more active surface on the surface due to the crushing, the digestion resistance will be significantly reduced.
Ordinary packaging methods are insufficient. In order to overcome this drawback, it is effective to impregnate the nozzle with paraffin as disclosed in Japanese Patent Application Laid-open No. 50-90608 to impart digestive resistance to the nozzle after firing. In this case, first, the impregnating agent itself is required to have no hygroscopicity and to have high water repellency. Paraffins are superior in this respect to other impregnating agents, and furthermore, when nozzles are used,
Compared to other organic impregnating agents with aromatic rings, it has the advantage of producing less smoke when heated. Regarding paraffins, it is desirable that they are solid at room temperature from the viewpoint of handling, and since it is necessary for the impregnation to easily melt them by heating, it is desirable to use paraffins with molecular weights within an appropriate range. Specifically, it is a white translucent waxy crystalline solid with a specific gravity of approximately 0.9 and a melting point of 47-68.
℃, carbon number 19 to 46, and is impregnated into a heated and molten liquid. Next, examples of the present invention will be described. Examples are shown in the table along with experimental examples and comparative examples. Experimental Example 1, Example 1, and Comparative Example 1 were fabricated with three actual immersion nozzles each. It was installed at the bottom and tested in an actual furnace. The steel types were all high-aluminum-killed steel, and the target was four continuous castings, but in Comparative Example 1, the discharge port diameter of two of the three nozzles expanded in the middle of the 2nd channel, and they were replaced with other nozzles. Also, in Example 1, two of the three have enlarged discharge ports,
Although the 2nd channel was replaced in the middle, the 4th channel of the example was completely cast without any problems. After use, the nozzle was cut vertically and the inside of the hole was observed, but no abnormal melting damage was observed, the surface was very smooth, and of course no Al ₂ O ₃ was observed. As described above, the present invention can maximize the CaO-containing characteristics as a casting nozzle by impregnating a specific binder and paraffin to impart digestion resistance to the fired product.

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Claims (1)

[Claims] 1 Mixed powder consisting of 10 to 50% graphite powder, sintered calcia, fused calcia, or other ceramic raw materials containing CaO components, and the balance being refractory aggregate powder, especially with a CaO component of 20 to 75%. The above-mentioned mixed powder is prepared, a phenolic resin modified with one type of alkylene carbonate or a mixture thereof is added and mixed, molded into an appropriate shape, fired in a non-oxidizing atmosphere, and heated and melted with paraffin. A method for manufacturing a CaO-containing graphite casting nozzle characterized by impregnation.