Analysis of baking and peeling of tundish magnesia dry paint

Abstract: The reasons for the large area collapse and local peeling of the working lining of the tundish during baking and in use are analyzed for many times. The simulated baking test is introduced, and the optimization measures for the tundish baking are proposed.

1. Questions

   The long service life of the tundish working lining and the reduction of molten steel pollution have become a key part of the iron and steel industry to achieve energy saving and sustainable development strategies. Magnesium refractory has good corrosion resistance to alkaline slag. Magnesium dry paint is widely used in large and medium-sized tundish work linings because of its long service life and no pollution to molten steel. With the popularization and application of the continuous casting tundish rapid replacement technology, the magnesium tundish dry working lining shows good use effect and application prospect, and its application is increasingly widespread. In recent years, the phenomenon of large area collapse and partial peeling of the tundish in Liugang has occurred many times during baking and use. In this paper, the magnesium dry paint used in Liugang is tested and analyzed to explore the causes and solutions of the damage.

2. Test and its results

2.1 Methods and tests

   The dry paint tested includes the products of 3 suppliers including the supplier who suffered the damage and collapse accident, hereinafter referred to as sample 1 (accident material), sample 2, and sample 3 in turn. The physical and chemical properties of each supplier of magnesia dry paint are basically the same as the raw material formula. Sample 2 and Sample 3.

   Three kinds of dry coatings were rammed into a 40mm×40mm×160mm mold, and then the molds were placed in an oven to bake at 250°C for 4 hours, and then demolded after cooling. To simulate the state of dry paint after baking in the environment of small fire, medium fire and high fire, the samples were kept at a temperature of 400 °C for 8 hours, and at 900 °C, 1000 °C, and 1100 °C in a high-temperature test furnace for 3 hours. Test the compressive strength of the samples after heat treatment.

2.2 Test results

   After the dry paint was baked at 250 °C for 4 hours, the samples had higher strength, but the samples of the three groups of samples that were kept at 400 °C for 8 hours were damaged to varying degrees, and the samples were pulverized and loose. Without any strength. The three groups of samples kept at 1000℃ for 3h can keep the original shape, but the strength is only 0.1~0.2MPa; the samples kept at 1100℃ for 3h have certain strength.

3. Analysis and discussion

3.1 The combination principle of magnesia dry paint

   The main raw material of dry paint is magnesia, and magnesia needs a high temperature to sinter, so a binder is required to ensure its low temperature and medium temperature strength. At present, the most commonly used low-temperature binder for dry coatings is phenolic resin. Phenolic resin is a thermosetting binder. With the increase of temperature, it will form a network-like carbon structure and gradually decompose. Almost all decomposed at 900°C. Therefore, a sintering agent must be added. At present, the commonly used sintering agents for dry coatings are boric acid or borate. Both of these two sintering agents will form molten glassy borate at 800 ° C, which plays the role of binding magnesia particles.

3.2 Analysis of the causes of strength changes

   After the dry coating is baked at 250°C, the resin will solidify and the bulk material will form a whole, but if the baking time is too long, the resin will be completely decomposed. The dry paint sample taken from the accident bag was tested to have a w(C) of 0.1%, and the same dry paint that was not used was tested to have a w(C) of 2.9%. The time for the accident bag to be baked on a small fire is 11 hours, and the temperature is about 400 ° C. The total baking time is longer than the holding time of the simulation test. The test results show that long-time low-temperature baking will completely oxidize and decompose the resin in the dry coating, resulting in the failure of the dry coating sample to maintain its original strength and be destroyed.

   With the gradual increase of the baking temperature, the sintering agent in the dry coating begins to participate in the reaction to form a liquid phase, which promotes the combination of magnesia and forms a relatively dense whole. At 900 °C, the three groups of samples have no strength, indicating that the medium temperature binder has not started to react; at 1000 °C, the sample begins to sinter initially, but the strength is very low, only 0.1 ~ 0.2MPa; when the temperature rises to 1100 °C, the sintering reaction occurs. more sufficient, the sample achieves a more ideal strength (see Table 1). In order to ensure the use effect of tundish dry paint, the tundish baking temperature needs to be above 1100℃, at least not lower than 1000℃.

    When the intermediate bag is baked online, the permanent layer and the working lining will be separated. In severe cases, the working lining will be completely deformed and inclined to the inside, so it cannot be used normally. Detecting the accidental dry paint, it was found that the change of the permanent heating line at 1100°C for 3h was within -0.6%~-0.3%, and the change of the permanent line at 1500°C for 3h was within -2.7%~-2.5%. Excessive volumetric shrinkage of dry-in-accident coatings at moderate to high temperatures causes the working liner to deform and separate from the permanent layer. In magnesia dry paint, the main factors affecting the change of medium and high temperature line are the quality of magnesia and the type of binder. binding agent.

4. Conclusion

    (1) When the tundish is baked online, it needs to be baked at medium and high heat, and the final baking temperature cannot be lower than 1000°C, preferably 1100°C.

   (2) At present, the dry paint used by Liuzhou Iron & Steel Co., Ltd. has poor oxidation resistance, and the main binder needs to use phenolic resin with high residual carbon rate, and appropriately add metal aluminum powder and metal silicon powder as antioxidants.

   (3) The medium-temperature strength of dry paint is low. It is recommended to use a sintering agent such as boric acid or borax to reduce the sintering temperature and facilitate on-site baking.

   (4) Select higher-grade fused magnesia and suitable binder as raw materials for magnesia dry coating, which can prevent the problem of separation between permanent layer and working lining.