In order to improve the performance and service life of the slit type ventilation brick, countermeasures are taken from the aspects of raw materials and production process. 1) In terms of erosion resistance and erosion resistance, raw materials with good thermal shock resistance must be selected; 2) In terms of penetration resistance, materials that are not easily wetted by molten steel at working temperature must be selected. Under high temperature and vacuum, the stability order of several refractory oxides is as follows: Al₂O₃>CaO>MgO>Cr₂O₃, and the order of molten steel wetting angle is as follows ZrO₂>Al₂O₃>MgO.
1) Corundum-spinel breathable brick
At present, the main materials of ladle breathable bricks are chrome corundum (spinel) and corundum-spinel, which are usually prepared by casting and molding at high temperature. Chrome corundum breathable bricks contain chromium, which will form Cr⁶﹢ and cause pollution to the environment. Its output is getting smaller and smaller, and it is gradually being replaced by corundum-spinel breathable bricks. The corundum-spinel castable combined with pure calcium aluminate cement has three high melting point mineral phases after high temperature firing, namely calcium hexaaluminate (CA₆, melting point 1890℃), aluminum magnesium spinel (MA, melting point) 2100°C), and corundum (melting point 2050°C).
The tabular calcium hexaaluminate in the matrix forms a good bond with corundum or spinel, which makes the material have excellent slag resistance and high temperature strength, which is called the best matrix system, but its thermal shock resistance is relatively poor , it is easy to cause thermal spalling and structural spalling during use, which limits the improvement of its service life. There have been many research reports on improving the thermal shock resistance of corundum-spinel materials, mainly including heat treatment temperature, spinel particle size and addition, fine powder content, addition of ZrO2 or zirconium corundum, and cement addition, etc. as follows:
The flexural strength of corundum-spinel castables at room temperature and the flexural strength and strength retention rate after air-cooled thermal shock at 1300℃ increased first and then decreased with the increase of heat treatment temperature. The thermal shock resistance of the samples fired at 1600℃ most. After calcination at 1600℃, the CA₆ crystal grows and develops into a plate shape obviously. There is an obvious network interweaving structure as a stress buffering mechanism in the sample. At the same time, the alumina has a proper solid solution in the spinel, which enhances the bonding strength between the substrates. . The damage caused by 1000-1600 ℃ cyclic thermal shock is the most significant.
With the increase of the mass fraction of fine powder ≤0.074 in the Al₂O₃-spinel castable (31%, 35%, 41%), the residual strength of the samples after thermal shock decreased significantly. With the increase of the amount of spinel fine powder added, the high temperature flexural strength of the corundum-spinel castable gradually increased, and the thermal shock resistance first deteriorated obviously and then gradually improved; when spinel particles and fine powder were added in combination , the thermal shock resistance is improved. With the increase of spinel particles, the fine powder of spinel decreases. Due to the solid solution of Al₂O₃ into the spinel, the lattice constant of the spinel becomes larger, and some microcracks will occur around the spinel and corundum. , which improves the thermal shock resistance, on the other hand, it also makes the sample produce a large permanent line change.
Monoclinic zirconia or zirconium corundum is introduced into the corundum-spinel material, and microcracks are generated through the phase transition of zirconia, which is beneficial to absorb and buffer thermal stress, and can be controlled by selecting the type, particle size and content of zirconium corundum. The number and size of crack formation in the castable, thereby improving the thermal shock resistance of the specimen. When the addition amount is large, the room temperature strength and the residual strength after thermal shock are obviously reduced.
By changing the amount of cement added to adjust the CaO content in the matrix, more CA₆ was formed in the matrix, which significantly improved the thermal shock resistance of the material. The reason is attributed to the plate-like CA₆ generated by the reaction at high temperature, which gradually develops and grows with the increase of temperature, and its interspersed filling plays a strengthening role in the corundum-spinel skeleton structure.
Picture legend: Effect of cement content on thermal shock resistance of corundum-spinel castables
2) Non-oxide bonded corundum breathable brick
Non-oxides (Si₃N₄, β-SiAlON, etc.) have the advantages of high high temperature strength, good thermal shock resistance, and difficulty in being wetted by metal melts and slag. Metal silicon is introduced into the corundum-silicon nitride material, and the corundum-silicon nitride breathable brick is produced by the transition plastic phase process. It has poor wettability to slag iron, no slag formation on the working surface during use, and its anti-slag iron corrosion resistance and blowing rate are better than those of corundum-spinel, chrome corundum and corundum-mullite materials.
With tabular corundum as the aggregate, Al₂O₃ micropowder, Si powder, etc. as the main matrix components, the vibrating casting method was adopted, a small amount of sintering aid Ln₂O₃ was added, and the SiAlON combined corundum breathable brick was prepared by nitriding and firing at 1450 °C. High strength, excellent thermal shock resistance and excellent slag resistance. When the mass fraction of Si powder is 9.5%-11.5%, the strength retention rate after thermal shock is high, and it has good comprehensive performance.
SiAlON-bonded corundum castables were prepared by in-situ nitridation. With the increase of SiAlON content, the amount of water required for forming increases. The room temperature strength and bulk density of SiAlON combined corundum castables decreased, the apparent porosity increased slightly, and the linear change rate changed from micro-shrinkage to micro-expansion. With the increase of SiAlON content, the thermal shock resistance of the material is obviously improved.
