System Optimization Of Alumina-spinel Castables And Its Effect On Thermal And Mechanical Properties

With the increasing demand of clean steel and the increasing refining ratio,the service environment of ladle lining refractory is becoming increasingly harsh.As the lining material of large and medium-sized ladle,alumina-spinel castables have gradually exposed a series of problems,such as poor thermal shock resistance,easy spalling and large heat loss of ladle.In recent years,although the properties of alumina-spinel castables have been optimized from different angles,systematic optimization studies of each component is still lacking.In this paper,the microstructures of alumina-spinel castables were systematically optimized and the effects of thermal and mechanical properties of alumina-spinel castables were studied from the aspects of aggregate composition and microstructure characteristics,matrix structure design and additive introduction by means of X-ray diffraction analysis,scanning electron microscopy and energy dispersive spectrum analysis.The mechanism of explosive spalling resistance of metallic aluminum powder on alumina-spinel castables was revealed by establishing the model of exhaust channel formed by Al-H₂O reaction in the castable.Based on the grey correlation theory,the correlation between the pore size distribution and the fifth stress fracture resistance factor(R'''')of alumina-spinel castables was established,and the effect of micropore structure parameters on its thermal shock resistance was also investigated.The main conclusions are as follows:From the perspective of aggregate microstructure,the effects of microporous alumina on the properties of alumina-spinel castables were systematically studied based on the fact that microporous alumina had a large number of micro pores with pore size less than 1?m.The results show that with the increase of the proportion of microporous alumina aggregate,the bulk density decreases slightly,the apparent porosity increases,and the thermal conductivity of the castable decreases greatly.The microporous structure in alumina aggregate can alleviate the tip stress of crack propagation caused by thermal shock,and effectively improve the thermal shock resistance of castables.In addition,the microporous structure of microporous alumina can reduce the penetration depth of slag in aggregate and the dissolution rate of aggregate in slag,thus improving the slag corrosion and penetration resistance of castables.From the perspective of aggregate composition,the effects of Al₂O₃-MA(Mg Al₂O₄)composite aggregate containing about 10wt%spinel phase on the properties of alumina-spinel castables were studied.The results show that the surface roughness of Al₂O₃-MA composite aggregate is higher than that of tabular alumina.By using Al₂O₃-MA composite aggregate to replace tabular alumina,the water absorption of castables increases,the density decreases,and the strength decreases after medium and low temperature heat treatment.However,due to the higher roughness of aggregate surface is beneficial to the sintering effect of aggregate and matrix at high temperature,the strengthes of castables can be improved after heat treatment at 1600?.The results of immersion corrosion test show that the cracks in the corrosion layer of Al₂O₃-MA composite aggregate are smaller and fewer than those of tabular alumina,and it is not easy to peel off after being corroded by molten slag.CA₆ phase and multiphase spinel phase are formed in the transition layer.The CA₆dense layer can prevent slag corrosion.At the same time,the spinel phase can absorb Fe O and Mn O in the slag,resulting in the increase of slag viscosity and the decrease of corrosion effect,so as to enhance the slag corrosion resistance of aggregate.By using Al₂O₃-MA composite aggregate to replace tabular alumina,the salg corrosion index and penetration index of castables decreased from 38%to 34.1%and from 80.5%to 75.6%,respectively.The effects of spinel morphology and pore forming agent on the properties of alumina-spinel castables were studied from the perspective of matrix structure design.The results show that residual strength ratio of CMOR of castables containing pre-synthesized spinel fine powder,pre-synthesized spinel micro-powder and in-situ formed spinel micro-powder are 18.5%,18.7%and 22.6%respectively.When Al(OH)₃micro-powder is added,micro pores will be formed in the castable,which reduces the volume median diameter of pores in castables from 2.05?m to 1.22?m,and enhances the ability of castables to inhibit crack propagation.Therefore,the residual strength ratio of CMOR of castables is increased from 22.6%to 24.6%.Compared with the pre-synthesized spinel fine powder,the addition of pre-synthesized spinel micro-powder can improve the slag corrosion and penetration resistance of the castable,and its slag corrosion and penetration index are reduced by 4.6%and 11.5%respectively;In-situ formed spinel micro-powder derived from addition of Mg O micro-powder is only beneficial to improve slag corrosion resistance of castables,and its slag corrosion index is reduced by 9.1%.The addition of Mg O micro-powder decreases the fluidity of castables and increases the apparent porosity,so the slag penetration index of castables is increased by 9.6%.Based on the introduction of additives,the mechanism of explosive spalling resistance of metallic aluminum powder on alumina-spinel castables was studied.The results show that the explosive spalling resistance of castables is mainly related to fracture energy and air permeability.Aluminum powder can improve the permeability of castable.The main principle is that the hydrogen generated by Al-H₂O reaction in castable overflows to produce an exhaust channel.The formation process of the exhaust channel is controlled by the process of gas generated by Al-H₂O reaction and the hardening process of castables.Through the analysis of the formation model of the exhaust passages,it can be seen that during the overlapping time of a large amount of gas generation stage and the starting and accelerating hardening stage of the castable,the gas gradually exhausts while the castable hardens,resulting in the formation of the exhaust passages.The duration of this overlapping time determines the number of exhaust channels formed in the castables.When 0.075%100-mesh aluminum powders is added,the overlapping time is the longest,reaching 230 min,and the permeability of alumina-spinel castable is the largest,reaching 22×10¹⁶/m².At the same time,the fracture energy of the castable is also larger,reaching 525 J/m².Considering the air permeability and fracture energy,there is optimum explosive spalling resistance for alumina-spinel castable when adding 0.075%100-mesh metal aluminum powders.The correlations between pore size distribution and thermal shock resistance of alumina-spinel castables were analyzed by means of grey correlation theory.The results show that the thermal shock resistance of alumina-spinel castables can be well reflected by using the fifth thermal stress fracture resistance factor R''''.After heat treatment at1100?for 3 h,the correlation degrees between pore volume and R''''are the largest when the pore size range are less than 0.2?m and 0.2?0.5?m,which are 0.8652 and0.8645 respectively,indicating that have the most significant effects on thermal shock resistance.After heat treatment at 1600?for 3 h,the relational degree between the volume content of pores with pore size range is less than 0.2?m and R''''of castables is the largest,which indicates that has the most significant effect on thermal shock resistance.