Refractory materials used in continuous casting have a great impact on the quality of steel. A suitable immersed nozzle structure enables the molten steel to flow in the crystallizer, and the blowing nozzle to float in the inclusions, improving the sealing performance of the ladle protective cover gasket. By improving these refractory materials for continuous casting, the quality of the steel and the durability of the refractory materials can be improved.
Keywords: continuous casting; refractory materials
Performance requirements of continuous casting technology for refractory materials (three major parts)
Continuous casting requires its functional refractory materials to have good thermal shock resistance, resistance to decomposition slag corrosion, resistance to liquid steel corrosion and suitable low-temperature strength. These characteristic requirements vary depending on the application parts, steel types, and smelting conditions of the three major parts.
(1) Excellent thermal shock resistance. The nozzle is generally preheated to 1100°C before use, but the temperature of molten steel during continuous casting is as high as 1500°C. The nozzle needs to withstand a thermal shock temperature difference of 400°C within 1 second.
(2) Good resistance to mold maintenance slag erosion. The zirconium carbon material used in the slag line has enhanced slag corrosion resistance due to the introduction of ZrO2. However, the zirconium carbon material has a risk of thermal shock explosion due to its lower carbon content and the phase change of ZrO2. Therefore, it is required not to cool down too much after preheating. quick. At the same time, if the zirconium carbon material is in direct contact with the molten steel in the inner hole, it will be rapidly melted under the erosion of the high-speed steel flow due to its low graphite content and bonding strength. Therefore, it must be combined with the main aluminum carbon material or lining material. use.
(3) It has good resistance to erosion by molten steel, especially the inner hole, which must resist both erosion by molten steel and chemical corrosion. The part of the body immersed in the crystallizer is rarely corroded, which is the main reason for choosing an aluminum carbon immersed nozzle.
(4) Good anti-infarction properties. The accumulation and blockage of the nozzle is more serious and common than the corrosion problem of the inner hole, especially for the pouring of aluminum-killed steel. At present, changing the material and flow pattern is a more ideal technical approach for the infarction effect.
(5) Low-temperature strength requirements are particularly important, and are higher than those for long nozzles and plug rods. The molten steel cannot fall to the bottom when it hits, and it cannot break when the molten steel swings. In fact, the low-temperature flexural strength at 1200°C is required to reach 2.5MPa. In practice, the normal-temperature flexural strength after burning is controlled.
(1) Good thermal shock resistance, but not as demanding as the immersed nozzle and long nozzle, because the stopper rod is only immersed in the 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 often packaged with the tundish At the same time, preheating also reduces its thermal shock resistance requirements.
(2) Resistance to slag erosion is not originally a problem for plug rods. However, 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 square billets and round billets requires continuous casting of 20 hours. As mentioned above, the erosion of the plug rod slag line by the tundish covering agent and slag is highlighted. General aluminum carbon materials are difficult to resist. Many manufacturers have compounded zirconium carbon materials, such as zirconium carbon materials for immersed nozzle slag lines and ordinary aluminum. Each carbon material is 50%(w) compounded.
(3) The higher the corrosion resistance of molten steel, the better, especially the rod head material. This is because the rod head area is subject to continuous erosion and erosion by highly turbulent molten steel. Rapid erosion will lead to poor flow control or uncontrolled final pouring. Nowadays, 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 type of steel).
(4) The high-temperature strength index is not required because the wall of the plug rod is relatively thick and has sufficient strength. Therefore, almost all manufacturers crush, screen, and dry waste products, waste materials, turning materials, and dust collection recycled materials in the production process. Roast (or burn to remove carbon) and add back into the plug body material, about 50% (w) can be introduced. Since the plug rod needs to be fixed vertically during use and performs high-frequency reciprocating impulses, there are also requirements for strength. Generally speaking, the flexural strength of the body material at room temperature after burning should not be less than 4MPa.
(1) Good thermal shock resistance. Long nozzles generally do not need to be preheated, and are large in size, so the thermal shock resistance requirements are the most stringent. There are currently two major technologies to deal with this problem. The first is the inner wall oxidation method in Europe. The long nozzle is fired in the bare firing method. The outside is glazed, the mouth of the bowl is covered, and the inner hole body is in an oxidizing atmosphere, forming an oxide layer of 1 to 2 mm and a metamorphic layer of about 1.5 mm to buffer thermal shock. When the ladle is opened and the high-temperature molten steel above 1500°C is poured, it directly impacts the breathable thermal insulation layer, which delays the direct thermal impact on the external aluminum carbon body and ensures thermal shock resistance; the second is the domestically developed thermal insulation inner wall method, which uses alumina or oxide The preformed inner wall of low thermal conductivity materials such as zirconium hollow balls or floating beads also has a good thermal shock resistance effect, but the manufacturing process is slightly complicated. In addition, the introduction of some carbon-free anti-clogging technologies, such as mullite, spinel, calcium zirconate and other lining materials, because graphite-free reduces the carbon content and is therefore insensitive to the thermal shock of opening and pouring, to a certain extent also Improved thermal shock resistance of the nozzle.
(2) Resistance to slag corrosion. Like the plug rod, the slag wire must be zirconium-carbon composite during long-term pouring.
(3) It has good resistance to the erosion of liquid steel, especially when continuous pouring is required for a long time, the erosion of the steel flow will be serious. It is required to select a material that can provide sufficient thermal shock resistance and corrosion resistance; in the design, it can be considered to treat it differently from the upper part of the body exposed to the atmosphere, diversify and compound it, and effectively control costs.
(4) The strength requirements are also as high as possible. The requirement for the strength of the long nozzle body is that the flexural strength at normal temperature after burning should reach 6MPa.
Application of refractory materials in bale lining
Large bag lined working environment
Ladle wire feeding, argon blowing and stirring, arc heating, etc. are important ladle refining technologies used in the production process of steel companies. The ladle carries molten steel or carries molten steel for a long time, and the selection of its lining material requires that it can adapt to the working environment of steel slag erosion, liquid steel erosion, and long-term high temperature.
During the selection process of the bale lining material, it is required to be able to adapt to a working environment that is 350~400°C lower than the creep temperature of the cladding material. When comprehensively using multiple refractory materials, it is required to consider the inherent properties of different refractory materials and strengthen research on the matching of different refractory materials. For example, alkaline bricks and zircon bricks can be used together. This is because zircon easily decomposes at a temperature of 1450-2430°C. Mixing bricks with higher SiO2 content with magnesia carbon bricks can promote magnesium carbon The local melting loss of bricks has a large thermal conductivity during use, so insulation materials with good thermal insulation properties can be used when selecting the permanent layer.
In order to improve the lining life and cleanliness of molten steel, it is required to use refractory varieties that can meet the corresponding operating conditions during the operation to meet the production needs of the smelting process. For example, it is not appropriate to use carbonaceous refractory materials for the smelting of IF steel, and it is not suitable to use graphite refractory materials and high alumina bricks for the smelting of boiling steel and high manganese steel.
Application of refractory materials in bale lining
Alumina-magnesia spinel carbon bricks, waxstone bricks, magnesia-carbon bricks, magnesia-calcium bricks, waxstone bricks, zirconium bricks, alumina-magnesia-carbon bricks, and low-carbon magnesia-carbon bricks are the main bale linings used in the production process. Refractory materials. It has good refractory properties, good thermal shock resistance, flexural strength and slag corrosion resistance. It is widely used in the current production process. Low carbon magnesia carbon bricks can be applied to the slag line to improve its slag resistance and corrosion resistance, which has a significant effect on improving the economic package age. Generally, alumina-magnesite, high-aluminum, and dolomite are used as castables. Magnesia carbon bricks have strong flexural strength and anti-flaking properties, which can significantly improve the slag sticking phenomenon at the bag mouth and wall, and can extend the service life of large bag linings by more than 250 times.
Application of refractory materials in slide gate plate
The position and function of slide gate plate
The sliding plate is used in both the large ladle and the tundish, which can adjust the flow rate and has the function of injecting molten steel. It is an important core component. During the pouring process, the slide gate plate tiles are subjected to various environmental influences such as mechanical erosion, chemical erosion, and sudden temperature changes. During the operation, they are subjected to wear and tear caused by hundreds of tons of high-temperature molten steel, which places high requirements on the material selection of the slide gate plate.
The slide gate plate may be damaged due to various external factors. The first is the emergence of chemical corrosion. During the calcium treatment of molten steel, melting loss is prone to occur under the erosion of high-temperature molten steel. This is mainly because Ca in the free state will corrode the sliding plate and produce Al2O3-CaO-SiO2, Al2O3-CaO, etc. Melting point compound.
Under the action of thermomechanical impact, tensile stress higher than the strength of the slide gate plate will be generated, and radial micro-cracks will be formed centered on the casting hole. The micro-cracks will in turn promote chemical erosion, leading to damage to the slide gate plate. High-temperature molten steel will gradually wear away the refractory material under the constant erosion, causing the sliding plate to peel off. Therefore, it is required to select slide gate plate materials with good wear resistance and flexural strength.
Selection of slide gate plate materials
Magnesium carbon, aluminum carbon zirconium, aluminum carbon, spinel carbon refractory materials, and zirconia are the main materials currently used in slide gate plate.
Aluminum carbon has good corrosion resistance and wear resistance in use, and the slide gate plate has fewer pores, but it is prone to melting loss during use, especially when it has a high SiO2 content. Can generate 3CaO·Al2O3, 12CaO·7Al2O3, 2CaO·Al2O3·SiO2 and other substances.
Compared with aluminum carbon slide gate plate, aluminum-zirconium carbon materials have better corrosion resistance and thermal shock resistance. Currently, many steel mills use low-silicon aluminum-zirconium carbon materials.
The use of zirconia materials can increase the life of the slide plate, and its spalling resistance and corrosion resistance are outstanding. It can significantly improve the continuous casting efficiency, but at the same time it increases its use cost. The use of magnesium carbonaceous materials has significant corrosion resistance and thermal shock stability. However, graphite is easily oxidized during use. It can be used in combination with resin to improve the oxidation resistance and peeling resistance of the material. Spinel carbon has a small thermal expansion coefficient and the main crystalline phase is its main constituent form. It has strong corrosion resistance and thermal shock resistance during use. However, the spinel will erode the spinel to varying degrees during the treatment of molten steel. Crystal base material.
Application of refractory materials in tundish lining
Middle bag lining
The terminal equipment for molten steel refining is the tundish, which is a buffer between the crystallizer and the ladle. The lining material of the tundish is required to be able to adapt to chemical erosion and mechanical impact of high-temperature molten steel in the ladle. Higher requirements are put forward for the selection of materials, which should be able to meet the application conditions of gas soft stirring technology, non-metallic inclusion removal, molten steel heating and other technologies. The selected materials are required to have good low thermal expansion, low thermal conductivity, corrosion resistance and thermal shock properties, have a long service life and can reduce the pollution of molten steel.
Selection of refractory materials for lining of tundish
Magnesium insulation board has good thermal insulation performance and was the main material used in the early days. However, it is prone to deformation of steel during use and its service life is relatively limited. Magnesia-calcium coatings and magnesia coatings have better overall performance and disintegration properties, but there is a large gap between them and the current actual demand for multi-heat continuous steel casting. Magnesia and magnesia-calcium dry materials are currently the main materials used, which can achieve a service life of 24 times of the paint lining, have good corrosion resistance, and can meet the needs of multi-furnace continuous pouring operations.
At present, the most commonly used tundish working lining type is dry material lining, which is widely used in Laiwu Iron and Steel, Wuhan Iron and Steel, and Anyang Iron and Steel. In the current research and development process of the tundish lining, new binders are being actively developed to meet the current environmentally friendly production needs.
The refractory selection process requires that it be able to adapt to the current high intensity and fast pace of continuous casting production. A variety of influencing factors are comprehensively considered, and the selection is made from the overall perspective of production and based on the unit consumption of refractory materials per ton of steel. BOF-LF-(VD)RH-High-efficiency continuous casting is an important process currently used by many steel plants. It requires the selection of refractory materials for continuous casting that conform to the production process. The refractory materials are required to have better performance and longer service life. . Theoretical circles are currently actively studying this. In combination with the actual needs of current steel plant production, we are actively researching and developing. With the development of clean steel production trends, we are currently developing carbon-free and low-silicon refractory materials for continuous casting. It is required to be able to comprehensively achieve cleanliness, environmental protection and meet the needs of refractory molding strength. In order to improve the service life and production quality of refractory materials, we are actively developing new production process technologies, and actively strengthening research on the optimization of ratios between different materials to increase the service life of refractory materials and optimize their use functions.