| Magnesia castables have been widely used in tundish due to its high refractoriness and alkaline slag corrosion resistance.However,it is inevitable to meet the hydration problem of magnesia,and the formation of Mg（OH）2 in magnesia castables will lead to volume expansion,cracks and even damages of structural integrity.What’s worse,the decomposition of Mg（OH）2 and the release of constitution water during fast heating,resulting in spalling and even bursting of the castables.Therefore,the introduction of high content microsilica into magnesia castables to generate microcrystalline bonded magnesium-silicate-hydrate（MSH）not only inhibits the formation of Mg（OH）2,but also ensures the performance of materials and improves the mechanical strength.However,with the development of clean steel smelting technology,higher requirements for magnesia castables are put forward.Magnesia castables used for tundish retaining dam and weir are in direct contact with molten steel,and its redox reaction with molten steel components will affect the quality of molten steel.SiO2,in particular,is one of the most harmful oxides in refractories that brings oxygen pickup in molten steel.The content of microsilica in magnesia castables is too high（6-8wt%）,which is not conducive to the development of clean steel,at the same time,the slag corrosion resistance of magnesia castables is also adversely affected.Hence,it is significant to reduce the microsilica content in magnesia castables on the premise of ensuring their performance.However,the current research on magnesia castables shows that simply reducing the content of microsilica in magnesia castables will face the problems of poor material performance and low mechanical strength in early construction.To solve the above problems,in this work,the magnesia and microsilica in matrix of the castables first went through a co-grinding process,aiming to promote the dissolution of magnesia and microsilica in the prepared magnesia castables to form Mg2+ and HSi O-3,thus speeding up the generation of in-situ MSH,effectively reducing the content of microsilica in the castables,and the performance of the materials was guaranteed simultaneously.On this basis,the pre-synthesized MSH was used to replace part of microsilica to further reduce the total amount of microsilica in magnesia castables and optimize the properties of the materials.The effects of the addition amount of pre-synthesized MSH and its “structural memory effect” on the properties of castables were systematically studied,and the mechanism of "structural memory" effect was explained in detail.At last,the influence of magnesia castables with different content of microsilica on the cleanliness of molten steel was explored.Through the above research work,the following conclusions can be drawn:1.By means of matrix co-grinding process,the formation of in-situ MSH was accelerated by promoting the dissolution of magnesia and microsilica to form Mg2+ and HSi O-3,thus the content of microsilica in magnesia castables can be reduced effectively without sacrificing the properties of castables.The co-grinding process led to a homogeneous distribution of the matrix,effectively promoted the generation of MSH,which uniformly distributed in the castables,so that the combination of the aggregates and matrix were much more compacted,hence,the mechanical properties of the castables were guaranteed.The slow water loss of MSH and its extended dehydration temperature range greatly improved the explosion resistance of the castables.The specimens with microsilica content of 3wt% showed better explosion resistance than that of the reference samples（castables containing 6wt% microsilica using normal casting process）.After calcined at high temperature,MSH decomposed and then generated homogeneously dispersed forsterite phase,which made the aggregates firmly bonded.Therefore,the CMOR of the spacimens containing only 2wt% microsilica（19.6MPa）after 1550℃ treatment was twice that of the reference samples（8.8MPa）.Additionally,the HMOR and RUL of the castables prepared by wet milling process were both higher than that of the reference samples.2.The replacement of pre-synthesized MSH to microsilica further reduced the content of microsilica in magnesia castables and improved the properties.It was suggested that when the calcination temperature of pre-synthesized MSH was 400℃,due to its “structural memory effect”,repeated calcination and rehydration were conducive to the formation of the layered structure,therefore the content of the constitution water increased,and the hydroxyl bond became more firm.Hence,the CMOR of specimens containing 1wt% of calcined pre-synthesized MSH and only1wt% of microsilica after curing at 50℃ and drying at 110℃ reached 10.9MPa and 14.6MPa via dry-milling process,which was even higher than that of the specimens containing 6wt%microsilica.3.Magnesia castables with different content of microsilica were selected to react with IF steel,and the low microsilica bonded magnesia castables were beneficial for the molten steel to reduce the oxygen pickup and the inclusion content.The reduction of microsilica content decreased the redox reaction between refractory and molten steel,so that for the specimens containing 6wt% and 1wt% microsilica,the oxygen pickup of molten steel was reduced from197 ppm to 127 ppm.For the inclusions in steel,the maximum size of the inclusions was 8μm when more microsilica existed,while the other one was less than 5μm,besides,the amount of small size inclusions（<1μm）decreased by 75% when less microsilica existed. |
