During tapping in an electric arc furnace, the furnace body is tilted at a certain angle (generally 45°) towards the tapping direction. Molten steel flows through the tapping trough into the ladle. Besides guiding the flow of molten steel, the tapping trough also prevents molten steel from spilling out and protects the steel from secondary oxidation. Improper geometric design or use of refractory materials can lead to significant heat loss, high refractory material consumption, and even increased non-metallic inclusions in the molten steel.
The structure of a trough-type tapping furnace places the tapping opening above the molten steel surface. Previously, tapping was done with slag covering the molten steel, primarily to prevent temperature drop, improve desulfurization, and prevent oxidation. Now, the tapping opening starts close to the furnace bottom and connects to a large, upward-sloping tapping trough on the outer flat wall. The cross-section of the tapping opening and trough is shown in Figure 2-70.
The tapping trough is connected to the furnace shell, and its construction methods include ramming, casting, bricklaying, or a combination of both. The construction method for the tapping opening is relatively consistent, generally using various shapes of tapping opening bricks or a central steel pipe surrounded by high-quality magnesia castable. Some are constructed using 25mm thick ceramic pipes and 2-3 high-grade fused magnesia bricks. Typically, a new tapping opening needs to be replaced after 50-70 heats. When using a tilting tapping trough, the main damaged areas are the molten steel line, slag line, and the bell-shaped opening at the front of the trough. The refractory material must have good wear resistance, slag corrosion resistance, and thermal shock resistance, and should be designed to prevent cracking and minimize wear during use.
Foreign electric furnace tapping troughs are generally made of high-alumina, magnesia-alumina, zirconium, magnesia-carbon, or wax stone materials, or with the addition of SiC, Si3N4, and C, as well as oxidants. With maintenance and repair, their service life can reach over 100 times, and some even exceed 300 times.
Brick-built steel channel
Japan uses high-durability CFC-ZRN-15X zircon bricks. These bricks are made primarily of pure zircon, supplemented with silicon nitride to resist molten steel erosion. This gives them excellent resistance to the adhesion and erosion of molten steel and the corrosion of slag. However, they are relatively expensive. When used in a 50t electric arc furnace with a 1.95m long tapping trough, these bricks can last for 300 heats when smelting ordinary steel. In a 100t electric arc furnace with a 2.8m long tapping trough, they can last for 200 heats when smelting special steel.
In addition, CFC-ZRN-15X semi-zircon bricks are also used. These bricks are made primarily of pure zircon, with some high-silica materials used, giving them the properties of semi-zircon bricks. However, they are much cheaper than pure zircon and silicon nitride bricks. When used in a 40t electric arc furnace with a 0.9m long tapping trough, they can last for 300 heats when smelting ordinary steel.
Precast block steel outlet channel
Using precast large blocks for steel channel construction minimizes the gaps between bricks, which is beneficial for extending the service life of the steel channel, reducing construction time, allowing for mechanical hoisting, and enabling immediate use after installation. In the 1980s, Japan widely used precast block steel channels made of high-alumina, low-cement castable. This castable uses alumina as aggregate, silicon carbide and flake graphite as main raw materials, and adds special ultrafine powder and dispersants.
The 80t electric furnace uses low-cement castable and aluminum-carbon precast blocks to build the steel trough, which has a service life of 118 heats. The original resin-bonded rammed steel trough only had a service life of 20 to 40 times.
Integral steel tapping trough
Steel tapping troughs are generally made of monolithic refractory materials, which have good integrity, long service life, and low cost. The materials used include ramming mixes with phenolic resin as a binder or vibratory castables with non-cement-based binders, and can generally be prepared and constructed on-site.
Comprehensive masonry steel channel
Osaka Kyodo Refractory Brick Co., Ltd. of Japan has switched to using high-pressure pressed Al2O3-C unfired bricks to build the main part of the steel tapping channel, while the end of the steel tapping channel still uses the original Al2O3-SiO2-C-SiC rammed molding structure.
Usage results show that the main body of the tapping trough (Al2O3-C material) has a lifespan of up to 6 weeks (613 heats), while the end of the trough has a lifespan of approximately 2 weeks. Overall, the melting rate of the Al2O3-C material tapping trough is 1/6 to 1/7 that of the original high-alumina material tapping trough.
Furthermore, MgO-C bricks are also used in Japanese tapping troughs. MgO-Cr2O3 bricks produced by Osaka Corporation are used in electric arc furnace tapping troughs due to their high high-temperature strength, good volume stability, corrosion resistance, and thermal shock resistance.
Lafayette of France uses a pre-cast high-alumina lining at the electric arc furnace tapping spout. The tapping spout is positioned as low as possible within the furnace, ensuring the slag level remains above the tapping spout during tapping to prevent slag overflow during tilting. The properties of RERCAST10 high-alumina castable are as follows: Chemical composition (%): Al2O3 76.3, SiO2 6.5, SiR 4.8, C 6.8; Bulk density: 3.05 g/cm3; Compressive strength at room temperature (MPa): 75 at 1100℃, 60 at 1200℃; Permanent linear shrinkage (1200℃): -0.3%.
In West Germany, magnesia-carbon bricks were used to line the tapping trough of a 100t electric arc furnace, with a service life of 100-150 cycles.
In China, tapping troughs are generally constructed using high-alumina, magnesia-alumina, or magnesia-carbon materials. When small bricks are used for the trough lining, the brick joints suffer severe melting damage, and steel easily seeps in and slag adheres, making cleaning difficult. In the 1970s, the application of bauxite cement tapping troughs in electric arc furnaces was widely promoted. These troughs, made using high-alumina clinker as aggregate and bauxite cement as binder, involved a process of mixing, tamping, vibration molding, curing, and natural drying. For electric arc furnaces under 3 tons, this type of integrally molded bauxite cement tapping trough had a lifespan of approximately 80-100 cycles, while for furnaces over 5 tons, it was approximately 20-30 cycles. Integral tapping troughs made of high-alumina rammed or vibrated material, using phosphoric acid instead of bauxite cement as a binder, had a short lifespan due to water absorption issues. Integral tapping troughs using water glass as a binder and steel fiber as reinforcement also performed poorly due to poor corrosion resistance. Later, integral tapping troughs made of low-cement castables of different materials were developed.
To improve the performance of aluminosilicate castables, the following technical measures should be taken:
(1) Matrix Composition. The matrix composition has a significant impact on the service temperature, high-temperature mechanical properties, and sintering performance of the castable. The matrix should be mainly composed of bauxite clinker with low K2O and Na2O content. The amount of cement added should be controlled, and the batching optimized to make the matrix of the castable approximate the chemical composition of mullite (A3S2). That is, the content of Al2O3 and SiO2 in the matrix should be adjusted to allow it to generate mullite at high temperatures, resulting in micro-expansion in volume, thereby improving thermal shock resistance and high-temperature strength.
(2) Binder. Pure calcium aluminate cement is selected as the binder. Its main components are CaO·Al2O3 (calcium aluminate, decomposition temperature 1600℃, very high activity, normal setting speed, fast hardening speed, high early strength, and no significant increase in later strength); and CaO•2Al2O3 (calcium dialuminate, decomposition temperature 1762℃), which has a slower hydration and hardening speed, low early strength, and high later strength. Through the hydration and setting action of this cement, the aggregates and powders are bonded together, and the addition amount should be less than 5%.
(3) Add an appropriate amount of micro-powder. The addition of micro-powder can fill the gaps between particles, improve the internal structure, and produce a cohesive bonding effect to make it denser; at the same time, it can significantly improve fluidity and greatly reduce the amount of water used in construction. At 110℃, after 24 hours, if the addition amount is greater than 8%, the shrinkage of the castable increases and there are more cracks; 4% to 5% is appropriate.
(4) Dispersant. Adding an appropriate amount of dispersant (water-reducing agent) can improve the dispersibility of micro-particles in the castable, increase the distance between particles, reduce attraction and friction, further improve the fluidity of the castable, and at the same time remove free water surrounded by particle agglomerates, making the castable fluid and reducing its construction water consumption. If sodium tripolyphosphate is used as a dispersant, the appropriate addition amount is 0.05% to 0.5%.
(5) Addition of expanding agent. The high-temperature decomposition product of kyanite, quartz, reacts with (Al2O3) in the powder to generate secondary mullite (3Al2O3•2SiO2), producing volume. Micro-expansion can counteract material shrinkage to improve thermal shock resistance. 3Al₂O₃•2SiO₂ can form a solid solution with Fe₂O₃ and TiO₂ in the matrix, effectively reducing the amount of liquid phase at high temperatures to improve its high-temperature performance. The addition amount should be less than 10%.
(6) High-temperature reinforcing agent. Corundum has the characteristics of high melting point, stable chemical properties, and good high-temperature strength. The addition amount of powder with a particle size of less than 0.088 mm should be less than 15%.
(7) Construction water content. After the castable is dried, the flexural strength of the product is inversely proportional to the amount of water added during construction; the water content should be around 5%.
Composite steel tapping trough
The steel tapping trough consists of a working layer (magnesia-carbon brick, MgO ≥ 75%, C ≥ 15%, apparent porosity ≤ 6%, bulk density ≥ 2.8 g/cm³, room temperature compressive strength > 32 MPa) and a non-working layer (refractory castable). The non-working layer is assembled from precast blocks.
The composite steel tapping trough is used in a 5t electric furnace (actual steel output 18t), with a tapping temperature of 1600-1650℃. 66 troughs were used, with a maximum of 93 uses, an average lifespan of 80 uses, and an average erosion rate of 1 mm.
The composite steel tapping trough utilizes reasonable materials, is low in cost, and can reduce hot shutdown time in sync with furnace lifespan, improving operational efficiency. Its lifespan is 3-4 times longer than that of high-alumina refractory castable troughs.
It adopts an assembled, inlaid, integral structure, facilitating storage, transportation, and use.
This A1203-SiC-C castable tapping trough is used in a 5t (10t tapping capacity) electric arc furnace, with a service life of 121 cycles. The trough has an inner diameter of 0.3m and a wall thickness of 0.2m. The total castable refractory used is 1.7t. The permanent lining at the bottom is bauxite-cement refractory concrete (0.7t). The upper working layer uses Al203-SiC-C castable refractory (1.0t). This tapping trough exhibits excellent slag resistance, corrosion resistance, and erosion resistance, resulting in a smooth working surface after use, making it a highly ideal castable refractory.
The castable uses high-grade bauxite clinker as the main raw material, with appropriate amounts of SiC (particle size <0.088mm accounting for 90%), carbonaceous materials (<0.1mm) and alumina powder, and appropriate amounts of antioxidants, sintering agents and dispersants added. Pure calcium aluminate cement is used as the binder. The chemical composition (%) of the castable is: Al2O3 63.67; SiCi 2.05; SiO2 13.7; C 7.61.
At Tianjin No. 3 Steelmaking Plant, magnesia-carbon bricks were used instead of brine-magnesia ramming material in the tapping trough of the 5t electric arc furnace. The average scouring thickness was 0.3-0.66 mm per furnace, resulting in a smooth surface after use. This reduced non-metallic inclusions in the steel and extended the service life of the tapping trough.
Low-cement castable integral steel trough
The Chongqing Special Steel 10t electric arc furnace uses an integral tapping trough, with smelting temperatures reaching 1650-1750℃. During use, no cracks or structural spalling occurred, and the copper trough corrosion rate was 1.8-2.4 mm/heat. Without repair, its service life can reach over 50 heats. The tapping trough is manufactured using vibration molding with staged feeding, followed by 24 hours of static curing before demolding, and then natural curing for over 48 hours before final baking at 600℃. This tapping trough emphasizes advanced production processes.