The dry monolithic refractory material working layer of the tundish often collapses during the baking and use process. As shown in Figure 1. The left of Figure 1 shows the large-area collapse phenomenon of the long-face working layer after the tundish is baked on the line for 100 minutes. The right of Figure 1 shows the initial collapse of the steel pouring. So, how to solve the problem of dry unshaped refractory material collapse in the tundish? Next, Rongsheng refractory manufacturer will find solutions by analyzing the problem of tundish working lining collapse.
Figure 1 The collapse of dry materials in the tundish
Tundish Working Lining Dry Monolithic Refractory Material
The working layer of the tundish is the basis for protecting the normal use of the tundish. The development of the working layer of the continuous casting tundish can be divided into 4 stages, no working layer stage, insulation board stage, coating or spraying stage, and dry monolithic refractory material stage.
Compared with the coating material, the dry working lining of the tundish has the advantages of convenient construction, high thermal efficiency, fast turnover of the tundish, long service life, good disintegration of the residual lining, and low energy consumption. It is widely used in the continuous casting tundish metallurgical industry. Magnesia materials have good corrosion resistance to high-speed iron and high-alkaline slag and have the characteristics of long service life and no pollution to molten steel. It has been gradually applied in tundish working lining refractories, with good use effect and application prospects. However, the thermal expansion coefficient of periclase is large and increases with the increase of temperature, which requires a relatively high on-line baking system. In the case of an imperfect baking system, large areas of collapse and partial peeling are prone to occur. The main reasons leading to the collapse of dry materials are low strength at medium temperature, large thermal expansion, and poor air permeability.
1.1 Reasons for construction
During the demolding of the working layer of the dry material in the tundish, if improper operation occurs, bumps or uneven application of the release agent will cause cracks in the dry material. The defects formed by these processes will form weaknesses in the baking process of the working layer, and in severe cases will lead to baking collapse.
After baking, the working layer of the tundish prepared for pouring steel usually forms an oxidative decarburization layer only on one side of the heating surface, which is very weak, and the back is a high-strength carbonized layer. The broken fragment in the collapsed packet is divided into three layers: the carbonized layer is sandwiched, and the backside also forms a decarburized layer due to oxidation, and two decarburized layers appear. The main reason is that the construction performance of dry materials is not good. Or the new permanent lining is constructed, and the bonding performance with the permanent lining is not good during the baking process. After the knot is intact, it will form a drumming phenomenon with a permanent lining. The on-site operation and construction level is not good, and the tundish turnover is tight and there is hot pack construction (permanent lining temperature exceeds 100 ℃). The uneven layout of the burner or the long baking time results in the hardening of the working lining in some positions. All of these will cause the working liner to separate from the drum, and the aforementioned sandwich layer will be formed during the in-line baking process, which will lead to the collapse of the package.
Figure 2 Schematic diagram of layering
1.2 The online baking system is not perfect
During the baking process of the tundish dry material working lining, as the temperature of the tundish increases, the phenolic resin gradually solidifies. The cured resin decomposes at 200°C to 800°C, releasing gas while leaving fixed carbon, that is, the resin is carbonized to form a carbon network structure. Although this amount of gas is not as large as the amount of water vapor produced by the paint, if the heating rate is fast and the binder decomposes too quickly, a large amount of gas will accumulate and expand. In addition, the dry material working lining is dense, and the vibration is relatively dense, which will cause the working layer to bake and collapse in severe cases.
Therefore, choose a binder with a relatively slow cracking rate and a high residual carbon content, and the amount of decomposed gas is as little as possible, which has a certain effect on reducing the risk of collapse. From the perspective of explosion-proof performance, the same principle as the castable is to introduce some fibers that can conduct air passages, or gently shake to reduce the density of the dry material, so that the gas generated by the cracking can be quickly discharged and the internal stress is relieved.
1.3 Low-temperature strength of dry materials
After the tundish dry material working layer is baked and de-moulded at low temperature, it needs to be hoisted to the pouring position to continue baking and heated to above 1100°C before it can be put into use. The sintering temperature of magnesia is high, there is no sintering at 750~1100℃. At this time, the bonding strength of the dry material working layer is at a relatively low stage. The low mid-temperature strength of the dry material working layer is an important reason for the collapse of the bag during baking.
1.4 The thermal linear expansion of dry material is relatively large
Affected by the thermal expansion and contraction factors, coupled with the thermal expansion of the magnesia itself, the dry material will expand when it is baked online to 1100°C. The release of the stress concentration will cause the bulging and the formation of cracks at the weak points, resulting in steel drilling or collapse of the ladle after pouring.
Baking collapse solution measures
2.1 Optimize construction and reduce off-drum
(1) Before construction, use a temperature measuring gun to measure the temperature of the permanent lining. Start construction when the permanent lining temperature is lower than 100°C (phenolic resin softening point 103~106°C).
(2) When baking at low temperature, turn off the heat when the thickness of the dry material changes to 1/3 of the total thickness. Avoid too long baking time to cause the working layer to deviate from the drum.
(3) When demolding, use a crane to lift the fetal membrane to the designated position. Pay attention to the uniform force when demolding, and do not damage the dry material lining to cause cracks.
2.2 Optimize the dry material plan and strengthen the medium temperature strength
When the sintering accelerator of magnesia dry ramming material is selected, the clay can form a liquid phase at medium temperature to promote sintering. But because it is composed of a variety of minerals and has no fixed melting point, it will also cause a large shrinkage of the green body. By adding boric acid and glass powder, the medium temperature strength of the dry material is obviously improved. Mainly because of its low melting point, it promotes the medium temperature sintering of the material.
The organic fiber is introduced into the brittle dry material, and the fiber is melted at a high temperature to produce micro-pores, which facilitates the discharge of the gas from the decomposition of the resin and enhances the air permeability of the lining. However, the amount of fiber added should not be too much, otherwise, the slag resistance performance is unfavorable.
2.3 Adjust the dry material formula to reduce thermal linear expansion
Phenolic resins are mainly decomposed in the range below 500°C. When the temperature exceeds 800°C, the difference in linear expansion rate caused by raw materials and sintering accelerators will begin to become obvious. At high temperatures, fused magnesia has a larger linear expansion than re-burned magnesia, and there are more pores and impurities in re-burned magnesia, which can absorb part of the expansion at high temperatures. The fused magnesia has larger crystal grains, fewer impurities, small pores, direct bonding at high temperatures, and greater linear expansion.
By optimizing the particle ratio and the selection of sintering agents, the thermal expansion of the dry material can also be reduced to a certain extent.
2.4 Permanent lining inclination and baking system
According to the decomposition characteristics of the phenolic resin and the low-temperature strength of the dry material, the low-temperature baking time during baking should be appropriately extended. Quickly pass the middle-temperature baking stage, make the dry material working layer reach the sintering state as soon as possible, and reduce the high fire to quickly bake.
For the tundish with a small inclination, especially the tundish with thinner working layer, deep, narrow, and long cladding. When designing permanent lining and working layer fetal membranes, it is necessary to consider appropriately increasing the inclination, so that the probability of collapse of the dry material will be greatly reduced. Save production costs.
In view of the smooth surface of the new permanent lining that is not easy to adhere to the dry material, it is considered to treat its surface into a hemp surface, which is also helpful to reduce the risk of collapse of the dry material.
What should I do if the refractory material cracks, falls off, and is easy to wear? To completely solve the problem, you need to dig from the root cause and analyze the real bugs. Rongsheng refractory material manufacturer ( http://www.aluminabricks.com/ ) has been engaged in refractory production, research, and development services for many years, and can solve various difficult problems of the high-temperature furnace lining.
