[vc_row][vc_column][vc_column_text]Functional refractories for continuous casting refer to the integral stopper, (ladle) shroud and immersion nozzle, which are called the three major pieces of continuous casting. Some people also include the ladle slide plate, the tundish slide plate, and the tundish top nozzle. Carbon refractories are commonly known as carbon-bonded refractories. In fact, the flow control structure of the tundish slide plate has become less and less common, and there is a trend of being replaced by stoppers. This article mainly discusses the raw material technology for the production of functional refractories for continuous casting.
Performance requirements for materials
Continuous casting requires its functional refractories to have good thermal shock resistance, synthetic slag corrosion resistance, molten steel corrosion resistance and suitable high-temperature strength. These characteristics and requirements are different due to the different parts of the three parts, steel grades, and smelting conditions.
- Immersion nozzle
(1) Excellent thermal shock resistance. The nozzle is generally preheated to 1100°C before use, and the temperature of continuous casting molten steel is as high as 1500°C, and the nozzle needs to withstand the thermal shock temperature difference of 400°C within 1s.
(2) Good corrosion resistance to mold powder. The zirconium-carbon material used in the slag line has enhanced slag corrosion resistance due to the introduction of ZrO2, but the zirconium-carbon material has the risk of thermal shock due to its lower carbon content and the phase change of ZrO2. Therefore, it is required that the temperature should not be too low after preheating. fast. At the same time, if the zirconium-carbon material is in direct contact with the molten steel in the inner hole, because of its low graphite content and bonding strength, it will be quickly melted under the erosion of the high-speed steel flow, so it must be used in conjunction with the main aluminum-carbon material or the lining material .
(3) Good corrosion resistance to molten steel, especially the inner hole, which must be resistant to molten steel erosion and chemical corrosion. The body part immersed in the crystallizer is rarely corroded, which is the main reason for choosing aluminum-carbon immersion nozzle.
(4) Good anti-blocking performance. The deposition and blockage of the nozzle is more serious and more common than the erosion of the inner hole, especially for the casting of aluminum killed steel. At present, it is an ideal technical route to change the material and flow pattern for the clogging problem.
(5) The high temperature strength requirement is particularly important, which is higher than that of the nozzle and stopper. The molten steel cannot fall to the bottom under impact, and cannot break when the molten steel swings. Theoretically, the high temperature flexural strength at 1200°C is required to reach 2.5MPa, and the normal temperature flexural strength after controlled firing in production should not be less than 6MPa.
- Overall stopper
(1) Better thermal shock resistance, but it is not as demanding as the immersion nozzle and shroud, because the stopper rod is only immersed in molten steel from the outside instead of the inner hole, and the heat transfer is from the outside to the inside; in addition, the stopper rod is mostly accompanied by the tundish Preheating together also reduces its thermal shock requirements.
(2) Slag erosion resistance is not a problem for stopper rods, but with the popularization and application of high-alkaline covering agents and the continuous increase in the number of continuous casting furnaces, especially the continuous casting of billets and round billets should reach continuous casting for 20h Above, the erosion of the tundish covering agent and molten slag on the slag line of the stopper is highlighted. General aluminum-carbon materials have been difficult to resist. Many manufacturers have composited zirconium-carbon materials, such as immersion nozzle slag line zirconium-carbon materials and ordinary aluminum. The carbon material is 50% (w) composite.
(3) The higher the corrosion resistance of molten steel, the better, especially the head material. This is because the rod head area is subjected to continuous erosion and erosion of high turbulent molten steel, and rapid erosion will lead to poor flow control or out of control final pouring. High-quality aluminum-carbon materials or zirconium-carbon composite materials (applicable to most carbon steels) and magnesium-carbon materials (applicable to all steel types, especially high-oxygen steel, high-manganese steel, calcium-treated steel and other aluminum-carbon materials are not suitable Applicable steel grades).
(4) The requirements for high-temperature strength index are not high. Because the wall of the stopper rod is relatively thick and has sufficient strength, almost all manufacturers have crushed, sieved, and dried waste products, waste materials, turning materials, and dust collection materials in the production process. Roast (or burn to remove carbon) back into the stopper body material, you can introduce about 50% (w). Since the stopper rod needs to be fixed vertically in use and undergo high-frequency reciprocating impulse, it also has a minimum requirement for strength. Generally speaking, the normal temperature flexural strength of the body material after burning shall not be less than 4MPa.
- Long nozzle
(1) Excellent thermal shock resistance. The long nozzle is generally not preheated, and the body is larger, and the requirements for thermal shock resistance are the most demanding. There are currently two major technologies to deal with this problem. The first is the oxidation method of the inner wall in Europe, represented by Vesuvius. The long nozzle firing adopts a bare firing method, with external glazing and bowl mouth shielding. The inner pore body is in an oxidizing atmosphere to form a 1 to 2 mm thick oxide layer and 1.5 mm. The left and right metamorphic layers are used to buffer thermal shock. The high-temperature molten steel above 1500℃ directly impacts the breathable insulation layer, which delays the direct thermal shock to the external aluminum-carbon body, and the thermal shock resistance is guaranteed; the second is the domestically developed insulation Inner wall method, Shengwei High Temperature Ceramics Co., Ltd. was the first to develop and own the patent. The inner wall is pre-formed with low thermal conductivity materials such as alumina or zirconia hollow balls or floating beads, which also has a good thermal shock resistance effect, but the manufacturing process Slightly complicated. In addition, some carbon-free anti-clogging technologies, such as the introduction of lining materials such as mullite, spinel, calcium zirconate, etc., are not sensitive to thermal shock due to graphite-free or greatly reduced carbon content. To a certain extent, the thermal shock resistance of the nozzle is also improved.
(2) Slag corrosion resistance, like the stopper rod, the slag line should be zirconium-carbon composite under long-term pouring.
(3) The corrosion resistance of molten steel is good, especially when there is a requirement for continuous pouring for a long time, the steel flow erosion is serious. It is required to select materials that can provide sufficient thermal shock resistance and meet corrosion resistance; in the design, it can be considered to be treated differently from the upper part of the body exposed to the atmosphere, diversified and composite , and effectively control costs .
(4) The strength requirement is also as high as possible. The minimum requirement for the strength of the shroud body is that the flexural strength at room temperature after burning reaches 6MPa.
The development of raw materials
The material system of continuous casting functional refractories has gradually developed and applied high corrosion-resistant zirconium carbon (slag line, rod head), magnesia carbon (rod head, nozzle bowl, fast End face of the water change port), spinel carbon (rod head, inner wall of the nozzle), mullite carbon or zircon carbon with high wear resistance and high silicon carbide content (end surface of the quick change nozzle), breathable material (long nozzle bowl, inner wall) ) And many anti-clogging lining materials for immersion nozzles (mullite, spinel, dolomite, calcium zirconate, etc.). The raw materials used are wide-ranging. The main raw materials use various alumina, zirconia, magnesia, silicon oxide and synthetic mullite, spinel, silicon carbide, Sialon, Aron, etc. The carbon source includes flake graphite, no Shaped carbon, asphalt, graphite powder, etc.; auxiliary materials are developed more comprehensively and finely, various antioxidants (silicon powder, silicon carbide, low melting glass powder, boron-containing materials, aluminum powder, zirconium compounds, etc.), reinforcing agents, forming aids Although the binder is still mainly based on phenolic resin, and some are supplemented by asphalt mixing, the introduction of phenolic resin (solid or liquid, thermoplastic or thermosetting), diluent blending agent (furfural resin, alcohol, ethylene Alcohol, polyethylene glycol, etc.), various modified products (boron modification, nickel modification, silicon carbide modification, white carbon black or organic silicon modification, etc.) and other companies have different developments.
Continuous casting functional refractories are characterized by carbon content, which is determined by the application requirements of functional refractories. The main functions of flake graphite can be summarized as two: ①Enhance thermal shock resistance. Because of its higher thermal conductivity and lower coefficient of expansion, it improves the thermal diffusion ability and relieves thermal stress accumulation; in addition, it is not compatible with fire resistance. The oxide produces ceramic bonding, and a large amount of flake graphite has a blocking and isolating effect on the dense oxide ceramic matrix and bonding network, just like pores, causing thermal stress to fall into the “black hole”, which can prevent crack propagation. The more valuable point of pore insulation is that it does not wet the slag and will not absorb the liquid slag to “fill the pit”. It should be noted that, unlike common metals and their oxides, the thermal conductivity of graphite decreases with increasing temperature, and tends to become non-conducting at extremely high temperatures. This is very important for graphite as a refractory material for continuous casting. The application is also of great benefit-the material can maintain a substantially constant temperature gradient under continuous high temperature application conditions. ②Enhance corrosion resistance. Because of its non-wetting to slag, flux and molten steel, not only is it not easy to be corroded, but it can also protect the wrapped matrix particles. In addition, it is the specific high temperature resistance of graphite. Unlike general high temperature materials, the strength of graphite increases with the increase in temperature, which also gives the matrix particles and even the matrix material higher corrosion resistance.
The amount and type of graphite introduced are of decisive significance. In practice, at least 25% (w) can provide the necessary thermal shock resistance, and further increase of graphite content will reduce corrosion resistance, and graphite content below 10% (w) will not be able to form a continuous graphite space structure, so carbon composite materials To find an optimal graphite content. Large scales and high axial ratio can promote thermal shock resistance but reduce strength and are difficult to disperse. Small scales cannot provide the necessary thermal shock. Therefore, a reasonable ratio of graphite of various sizes must be matched according to the matrix particle gradation.
The purity grade of graphite has also been gradually improved from the early 85% (w) carbon content, and now it is mostly above 95% (w). European manufacturers mainly use 95% and 96% (w), and a small amount uses 98 grade. Manufacturers use a large number of grades 99 and 98, supplemented by grade 95, which is higher in quality than European ones. According to the place of origin, the ash and impurities of flake graphite are slightly different, but the main ones are the oxides and salts of the five major elements of Si, Al, Ca, Mg, and Fe. The ash composition includes SiO2, Al2O3, CaO, MgO, Fe2O3 It accounts for more than 90% (w), and there are also a small amount of Na2O, K2O, TiO2, MnO, P2O5, Li2O, etc. In fact, after natural graphite is purified by froth flotation and multi-stage grinding and separation, most of the impurity minerals have been separated and removed, and only some of the impurities are very finely impregnated in the graphite flakes, and the purity of 95% (w) can be Meet the use requirements of the three major continuous casting pieces. Just like the graphite used for magnesia carbon bricks, it is not that the higher the purity of the graphite, the better the oxidation resistance of refractory bricks. The oxidation resistance of impurities to graphite can be divided into two aspects: on the one hand, certain inclusion oxides affect graphite. Oxidation has a catalytic effect; on the other hand, the ash content of graphite has an effect on the thickness of the decarburized layer formed after the brick body is oxidized, thereby improving its oxidation resistance. It is recommended that the three main body materials of Chinese manufacturers use graphite with w(C) of 95% and 96%. Highly corrosive parts such as slag line and rod head can use graphite with w(C)=98% to save resources and reduce costs.
The particle size of graphite can be selected in a wide range according to different material systems, and particle size distribution is an important part of material design. According to the distribution of raw ore, the maximum size of graphite flakes varies within the range of 0.3-3mm, and the flake diameter is about 0.01-1.5mm, mostly 0.1-0.5mm. Graphite for refractory materials is usually divided into three categories: coarse, medium and fine according to the scale size, corresponding to 0.8～0.335, 0.335～0.074, ≤0.074mm, and 80% of the scales are in the corresponding size range. From the point of view of optimizing the mixing, the three largest pieces use flake graphite, generally the largest is 0.335mm, and 0.335, 0.2, 0.154, 0.1, 0.074mm grades are mostly used. In the formulations using amorphous carbon, a small amount of flake graphite powder is also introduced to increase the carbon content without increasing the amount of resin to prevent thermal decomposition cracks of the amorphous carbon.
Aluminum-carbon materials are the three most basic materials. Alumina is a highly thermodynamically stable refractory, insoluble in molten steel, and reacts with carbon at extremely high temperatures. It is widely used as a matrix material in continuous casting functional refractories. Electrofusion, sintering, white corundum, brown corundum, tabular corundum, dense corundum, activated alumina and other types and forms of alumina are used. Some small and medium-sized three-piece manufacturers in Shandong also use (super-grade) bauxite as part of the alumina source, mainly supplying some domestic small and medium steel plants, especially private steel enterprises. The purity of alumina is generally required to be above 95% (w). In order to achieve a suitable particle size and required strength, alumina of various particle sizes can be used, or a large amount of activated alumina powder can be used to promote sintering. Figure II
According to the requirements of the steel grade, the number of continuous casting furnaces and the overall design of the material system, the content of alumina in the aluminum-carbon bulk material is generally within the range of 50% to 70% (w). Under the premise of ensuring thermal shock resistance, oxidation The higher the aluminum content, the higher the strength, the higher the corrosion resistance and the service life.
As the slag line material of the immersion nozzle, the zirconia-carbon material was first developed by Japan in the 1980s. It mainly solves the problem of corrosion of the aluminum carbon nozzle by the protective slag. The slag line uses a zirconium-carbon protective ring for composite reinforcement . In terms of effect, the corrosion resistance of the zirconium-carbon slag line is twice that of aluminum-carbon material, the service life can reach more than 20 hours, and the throughput of steel reaches 4000t, which is still improving. Zirconium-carbon material is currently the main material of the immersion nozzle slag line, and is transplanted to the shroud and stopper slag line area to improve corrosion resistance.
The immersion nozzle slag line zirconium carbon material, the amount of zirconia introduced is mostly 70% to 80% (w), and it can also be increased to 85% (w), but the high zirconium content brings great thermal shock resistance risk. The amount of graphite is generally 10%-20%(w), concentrated at the level of 10%-15%(w). Long nozzle zirconium carbon slag wire material, the zirconium content is low, generally under the same conditions of use, it is 50% to 70% (w) of the zirconium content of the immersion nozzle slag wire.
The application form of zirconia is mainly partially stabilized zirconia with a stability of 70% to 80%. The stabilizer is mainly composed of CaO, Y2O3, and CaO-Y2O3, and MgO is rarely used for stability. Studies have shown that Y2O3 stabilized zirconia is the most stable to mold flux, followed by CaO-Y2O3 composite stability, followed by CaO stability, and the worst is MgO stabilized zirconia. Liao Ning et al. gave a good overview of the research status of zirconia used in the immersion nozzle slag line, and also studied the stabilizing effect of carbon on zirconia, with a view to applying carburized zirconia in the nozzle. In production practice, the coarse particles are stabilized by CaO, and the fine particles are mainly stabilized by Y2O3. This combination is more economical. In addition, the original Foseco’s immersion nozzle zirconium carbon material uses a large amount of monoclinic zirconia, which is close to 50% of the total consumption, which is more economical and has good performance. It can achieve the function of cold casting.
- Magnesium oxide
Magnesium-carbon materials are mainly used as rod head materials and wear-resistant materials for built-in immersion nozzle bowls, and also as long nozzle slag line materials, especially suitable for calcium-treated steel. Vesuvius took the lead in developing and using it, and it has now become a universal material for its stopper head. Magnesia raw materials require a higher calcium to silicon ratio, preferably m(CaO)m(SiO2)>3, because high silicon content has poor corrosion resistance. Fused magnesia is more helpful than sintered magnesia to overcome the problem of tip cracking.
In the research of carbon-containing refractories, the development and application of antioxidants is a hot spot, and it is also an undisclosed secret of various manufacturers. Its essence is nothing more than the application of some elemental Si and metal (Al, Mg, Ca, Zr) powders, carbides (SiC, B4C), nitrides with large affinity for oxygen (oxygen potential) and small affinity for carbon (carbon potential) (BN, AlN, Si3N4, β-SiAlON), borides (H3BO3, ZrB2, MgB2, Na2B4O7·10H2O, CaB6), etc., before carbon and oxygen combine to form a glass body protective film (boroglass, silicate glass) or The local glass network prevents the oxidation of carbon bonding network and graphite. Currently, elemental silicon, silicon carbide and borides are mostly used in the market, and the total amount of addition is generally 3% to 5% (w). Silicon carbide has a higher protection temperature than elemental silicon, and is especially suitable for long-term continuous casting. Borides generally have a lower melting point and are easier to form glass bodies. Boron carbide contributes 4.5 times to boric acid than boric acid, but the price is higher.
- Low melting point glass components
Low-melting glass clinker (generally boron glass powder) and flux (feldspar, etc.) can form a certain low-temperature ceramic bonding phase, giving the product a certain ceramic bonding strength; in addition, the low-temperature bonding phase can also close the internal pores and play a certain role. The anti-oxidation effect. Generally, about 5% (w) is added to aluminum-carbon material, which can be reduced to 1% (w) for materials with finer components and strong sinterability; zirconium-carbon materials only introduce boron-containing low-melting substances, about 1% ( w); About 2% to 3% (w) in magnesium-carbon material.
Inorganic substrates such as alumina, zirconia, magnesia and graphite will not form any form of ceramic bond at the firing temperature, and it is difficult to form a ceramic bond by itself. Therefore, the third phase, the carbon bond phase, is introduced. The carbon-containing organic binder is carbonized at high temperature.
Compared with ceramic bonding, the advantages of carbon bonding are:
① Relatively low bonding phase formation temperature. Phenolic resin can basically complete thermal pyrolysis graphitization before 900℃, forming a carbonized bonding network, while high alumina refractory materials need to form a ceramic bonding phase (high temperature phase). The firing temperature is generally higher than 1400℃, and the low temperature ceramic bonding phase, For example, although the formation temperature of the boron glass phase is low (the glass transition temperature can be as low as 250°C), the medium and high temperature is always a liquid phase (the melting point of boron oxide is 445°C) and is easy to evaporate. The dual effect of. After the phenolic resin is combined with the three major parts, it has a certain bonding strength (≥2MPa), giving the product a certain shape and room temperature strength. Different from ordinary ceramic bonding, the high-temperature strength of carbon bonding increases with the increase in temperature, and there is no brittleness of ceramic bonding after cooling, which makes it possible for continuous casting refractories to be used repeatedly for multiple times;
③Excellent heat resistance Vibration and slag erosion resistance are also due to the excellent properties of carbon.
At the beginning of the production of continuous casting functional refractories, polyols and asphalt were used as binders. Since the 1970s, phenolic resins have been used and gradually promoted. It is now the only organic binder for the three major products. Certain diluent solvents (such as furfural resin) will also form a certain carbon bonding network after thermal cracking. It can be considered that, based on the development of the bonding agent (and solvent) and its successive treatment processes (forming, curing, carbonization, etc.) over the past few decades, European technology and Asian technology have basically formed the two Dalian cast functional refractory manufacturing processes. European technology is represented by Vesuvius. When a thermoplastic solid phenolic resin containing 10% (w) curing agent is used (liquid furfural resin is the solvent, the mass ratio of the two is close to 1:1, the aluminum-carbon material contains about 15% ( w), the blank and the formed body contain about 11% (w), because about 60% (w) of furfural components decompose, evaporate, and volatilize. After curing at 200 ℃, it contains about 10% (w), 900 ~ 1000 ℃ After carbonization, it contains about 6%(w), and the comprehensive residual carbon rate is 50%-55%(w). Asian technology is represented by Shinagawa and Kurosaki of Japan, using thermoplastic solid phenolic resin, or partly adding thermosetting liquid phenolic resin, using industrial ethanol (alcohol) as solvent, aluminum carbon ingredient content about 10% (w), solvent 5% (w) ), the solid content of the blank binder is about 9% (w), about 7.5% (w) after curing, about 4% to 4.5% (w) after carbonization, and the residual carbon rate is 40% to 45%.
After more than half a century of hard work, the development and application of continuous casting functional refractory materials have achieved great success, and have made great contributions to the development of the steelmaking industry, and will continue to be for a long period of time in the future. The main protective casting material of continuous casting process. In order to better play the role of continuous casting refractories in the continuous casting industry, further work is needed in the future in terms of raw material diversification, compounding and new antioxidants.