The performance of tundish refractory castables is directly related to the smoothness of continuous casting production and the quality of ingots. As a key process parameter before the use of the tundish, the baking temperature plays a decisive role in the physical and chemical changes, structural stability and service life of the refractory materials. Different types of refractory materials have significant differences in their response to temperature during the baking process. Reasonable control of the baking temperature is the core prerequisite for the performance of refractory material castables. The following will start with typical refractory materials such as magnesium dry materials and magnesium spray coatings to systematically analyze the key effects of baking temperature on the performance of tundish refractory materials.
1. Effect of temperature on magnesium dry refractory castables tundish refractory materials
1. Low temperature stage (<200℃): water release and structural stress control The main change of magnesium dry materials in the low temperature baking stage (usually <200℃) is the release of free water and crystal water. If the heating rate is too fast (such as more than 10℃/min), the rapid evaporation of water will form a pressure gradient inside the material, leading to microcracks or even macro cracks. Studies have shown that when the baking temperature is increased at a rate of 5-8℃/min in the range of 100-150℃, moisture can be evenly removed to avoid stress concentration. A steel plant once had a transverse crack in the working lining of magnesium dry material due to excessively fast heating (15℃/min) in the low temperature stage. The crack width reached 3mm and the length was 400-1200mm, which seriously affected the service life of the tundish. In addition, insufficient insulation time in the low temperature stage will cause residual moisture. The residual moisture will evaporate when the subsequent molten steel is poured, and may invade the molten steel to form pores, while weakening the bonding strength of the refractory material. Experimental data show that after 2 hours of insulation at 150℃, the flexural strength of the dry material can reach 7.87MPa, while the strength of the sample that was not fully insulated is only 5.2MPa, a decrease of 34%.
2. Medium temperature stage (200-800℃): binder transformation and strength fluctuation Magnesium dry refractory castables often uses resin as a binder, and will undergo the key process of resin curing and decomposition in the range of 200-600℃. 200-400℃: The resin begins to solidify and form a three-dimensional network structure, providing initial strength for the dry material. At this time, if the temperature does not stay long enough and the resin is not fully solidified, the strength of the dry material in the medium temperature zone will be significantly reduced. Experiments show that after 1 hour of insulation at 400℃, the compressive strength of the dry material can reach 7.9MPa, while the strength of the uninsulated sample is only 4.1MPa.400-800℃: The resin gradually decomposes and releases gases such as CO and CO₂, causing the internal structure of the material to be temporarily weakened and the strength to "low". When the temperature reaches 800℃, if the insulation time is insufficient (such as <2 hours), the gas produced by the decomposition of the residual resin may form pores inside the refractory material, reducing the corrosion resistance. A steel plant optimized the medium temperature stage process (600℃ insulation for 3 hours) to stabilize the medium temperature strength of the dry material at 6.5-7.2MPa, an increase of 30% compared with before optimization.
3. High temperature stage (>800℃): sintering densification and high temperature strength formation High temperature baking (800-1200℃) is the key stage for sintering densification of magnesium dry materials. In this temperature range, the magnesia particles recrystallize, and the grain boundaries fuse to form a dense structure, which significantly improves the high temperature strength and erosion resistance of the refractory castables. Studies have shown that when the baking temperature rises to 1100℃ and is kept warm for 4 hours, the compressive strength of the dry material can reach 11.33MPa, which is 57% higher than that of the medium temperature stage, and the slag erosion resistance index is increased from 1.8 to 2.5. If the temperature in the high temperature stage is insufficient (such as <1000℃) or the insulation time is short (<3 hours), the refractory material is not fully sintered, the internal porosity increases, and the erosion resistance decreases. After a steel plant increased the high temperature baking temperature from 900℃ to 1100℃, the erosion rate of the tundish working lining dropped from 5mm/furnace to 3mm/furnace, and the number of continuous casting furnaces was extended from 10 furnaces to more than 15 furnaces.
2. Effect of baking temperature on refractory castables of magnesium spray coating tundish
1. Effect of temperature on coating bonding strength: Magnesium spray coating is sprayed, and its baking temperature directly affects the bonding strength between the coating and the permanent layer. If the temperature rises too fast in the low temperature stage (<150℃), the water in the coating evaporates quickly, which will cause hollowing and peeling of the coating; the medium temperature stage (300-600℃) is the key period for dehydration of cement binder hydration products, and improper temperature control will weaken the bonding strength between the coatings. A steel plant adopts a staged heating process (150℃ insulation for 2 hours → 400℃ insulation for 3 hours → 800℃ insulation for 2 hours), so that the bonding strength between the spray coating and the permanent layer reaches 1.2MPa, which is 40% higher than the original process.
2. Effect of high temperature sintering on erosion resistance
High temperature baking (800-1000℃) of magnesium spray coating can promote the formation of magnesium-aluminum spinel phase and improve slag resistance. When the baking temperature reaches 1000℃ and is kept warm for 3 hours, the slag erosion resistance index of the spray coating increases from 1.5 to 2.2, which is 47% higher than that of the coating that is not fully sintered. If the high temperature is insufficient (such as < 900℃), the periclase crystals in the spray coating are not fully developed, and the erosion resistance is significantly reduced. A steel plant once caused the spray coating to partially peel off when casting the 5th furnace.
