Plug Rod and Long Nozzle
As shown in FIG. 15-1, the plug rod is installed on the tundish and matched with the inner immersed nozzle or tundish and the upper nozzle of the inner immersed nozzle or tundish to control the flow of molten steel from the tundish to the mold in the continuous casting process, so as to ensure the liquid level stability of the molten steel in the mold and the stability of the continuous casting process. Its size varies according to tunings, and the longest length can reach more than 1500mm. In terms of structure, it can also be designed to be vented at the rod head, that is, argon is injected into the center of the connecting rod of the plug rod and discharged at the rod head, which can reduce the inclusion in the molten steel and prevent deposition. In the material selection, to ensure safe use as the first element, once out of control, due to the non-replaceable plug rod will cause continuous casting interruption. Usually rod body, rod head, slag line with different ingredients. Rod body material fails to choose Al2O3 - C material, its main refractory raw material can be used according to the status and high-grade fused corundum raw materials or special grade bauxite clinker and slag line parts subjected to tundish covering agent and liquid steel, most often used with high-grade fused corundum as raw material of Al2O3 - C material, under the condition of strong erosion also chooses ZrO2 - C material. The rod head is the most critical part of the plug rod. The rod head and the nozzle bowl cooperate to achieve flow control. The rod head and nozzle bowl are designed in a curved form to ensure good flow control effect and closing function.
The key part to determine the flow control function and service life of the plug rod is the plug rod head. It is very important to ensure the high performance of the rod head material. There are two kinds of common materials: al2o3-c material bar head and mgo-c material bar head, depending on the reaction choice of pouring steel type and refractory material. When the vacuum degree is high, MgO will react with C, resulting in accelerated rod head erosion. When the Ca content of molten steel is high, it will react with Al2O3 to accelerate rod head erosion, which cannot be continuously cast for a long time. Vesuvius reported in plug stick head and submerged nozzle bowl used material progress, different steel grade, the different operating conditions to select different materials, such as Al2O3 - C rod head than MgO style - C rod head is more suitable for Al killed steel, steel, which is very suitable for calcium treatment JiaoGangShi rod head surface formation CaO - department of Al2O3 compounds and MA, the M2S thin membrane can effectively protect head from the molten steel slag erosion. Table 15-21 for different steel types of plug rod head and submerged nozzle bowl material performance.
The development of the plug rod also focuses on the material changes of the rod head. At present, the service life of carbon-containing material rod head is close to the limit, and it is not suitable for some highly corrosive steels. It has been reported that the life of continuous casting high-oxygen steel is reduced due to carbon oxidation, so products containing AlN are used to oxidize the surface AlN to form a dense layer of Al2O3 to improve its life, as shown in table 15-22.
Long nozzle is also known as the protection sleeve, installed in the water mouth of the steel barrel, connected to the ladle and tundish, plays the role of preventing oxidation and spatter of molten steel, preventing the inclusion of tundish slag and other multiple functions, is an important functional refractory for the protection pouring to improve the quality of steel. The basic structure is shown in figure 15-3. The size varies according to the continuous casting machine. The length is generally 1000~1600mm and the inner diameter is within 80mm. The production process of the long nozzle is basically the same as that of the plug rod and the immersed nozzle, but it has certain characteristics in terms of the ingredients in order to ensure its performance, that is, it must proceed from the two most critical requirements, one is to ensure safe use, the other is to use for as long as possible.
To some extent, the thermal impact resistance of the long nozzle is the highest requirement of the three continuous castings. At present, most domestic steel mills use long nozzle for continuous casting directly without preheating, and the pouring begins to contact with molten steel. The inner surface temperature of nozzle instantly rises to the temperature of molten steel, while the outer surface contacts the atmosphere, and the temperature is much lower. Therefore, the long nozzle in the material design should have a good thermal shock resistance. The magnitude of the internal thermal stress of a material subjected to thermal shock can be roughly expressed as =E· hydrogen/hydrogen T (1-inverse), which is proportional to the expansion rate (hydrogen), the modulus of elasticity (E), and inversely proportional to hydrogen T. Theoretically, the ways to reduce thermal stress and improve thermal impact resistance are to reduce the expansion rate (hydrogen), the elastic modulus (E), and the temperature gradient delta T inside the material. At present, there are two ways to ensure the reliability of thermal shock at long nozzle: one is a higher graphite content in the material, and the addition of low coefficient of expansion materials -- fused quartz, feldspar, zirconium mullite, or special additives, to reduce the thermal expansion rate and elastic modulus of the material; The other is that the inner and outer layers of the nozzle are made of different materials: the inner layer of the nozzle is low carbon or no carbon in direct contact with the high-temperature liquid steel. The low thermal conductivity reduces the thermal stress in the outer material. The outer material is aluminum carbon material with low SiO2 content, which improves the corrosion resistance of the liquid steel. The purpose of all these is to improve the thermal shock resistance of products through different mechanisms, so as to meet the harsh thermal shock conditions under the use conditions.
The service life of long nozzle is related to the ability of material to resist erosion and erosion. In the use of changshuikou, the erosion rate and erosion mechanism of different parts vary with different working conditions. Several parts with severe erosion are shown in figure 15-6, as follows: slag line - eroded by tundish coating agent; The part immersed in molten steel at the outlet of molten steel flow is subject to the strong erosion, erosion and oxidation of molten steel; Neck - bias scouring and oxygen blowing cleaning of molten steel; Connection with sliding nozzle -- poor sealing results in oxidized carbon loss in inhalation, especially in the part immersed in molten steel and the neck. The measures to improve the service life of long nozzle are good for the selection of main materials for corrosion resistance of molten steel, such as white corundum, dense corundum, and other high corrosion resistance refractory materials. Highly effective anti-oxidation additives, self-repairing reinforcing additives, low silicon and low carbon, etc. In terms of composition ratio, increasing graphite content can improve the thermal shock resistance, but reduce the erosion and erosion resistance of molten steel. The amount of addition should be appropriate, generally 25%~30%. The main component is fused corundum, which plays a decisive role in resisting erosion and erosion of molten steel. Table 15-23 shows the composition ratio and performance index of long nozzle designed according to different service requirements and conditions.
Similar to many other refractories, the effectiveness of long shuikou has much to do with the optimization of operating conditions. Hideaki pointed out in his article that in the past ten years, the service life of Japan's long shuikou has been greatly developed, and many steel mills have reached a stable level of 30-40 times. The main measures include the improvement of operation technology, such as good preheating and reasonable oxygen blowing cleaning procedures. Improvement of nozzle material properties, including improvement of mechanical properties to withstand nozzle vibration stress; The lining is made of low carbon material, which can reduce the scour of molten steel. Developed advanced anti-oxidation coating on the surface of the nozzle and the use of non-sio2 materials to ensure the high stability and consistency of product quality.
In China, the service life of the long nozzle cannot be synchronized with the submerged nozzle and the integral plug rod when the continuous pouring furnace is at the second height and the continuous casting time is long in the actual production, so it is often necessary to replace the long nozzle in the continuous pouring process. The main problem is the insufficient resistance to molten steel erosion and erosion at the steel mouth and bowl. In order to meet the needs of improving the life of the long nozzle and the requirements of clean steel smelting, there is the use of silicon-free aluminum carbon long nozzle. The performance indicators are shown in table 15-24.
Another important problem in the use of long nozzle is the argon blowing seal of the bowl, especially for the continuous casting of high performance steel. The long nozzle is connected with the lower nozzle of the ladle sliding nozzle system, and the molten steel flows rapidly, which will form a certain negative pressure here. It is easy to inhale air, so that the molten steel oxidizes and cannot be completely oxidized by pouring. Therefore, most of the long nozzle in the bowl are designed to blow argon seal structure. Roughly divided, blowing argon sealing structure can be divided into three categories: fiber gland, composite or directly embedded air ring (do not contain carbon) and the nozzle end set a certain number of aeration tank, argon iron shell by nozzle bowl of outsourcing of the pipeline into, respectively by sealing structure zha shuikou bowl of the formation of circular membrane gas, isolate air suction.