Preparation,Fracture Process Evaluation And Service Simulation Of The Corundum Refractory For Purging Plug In Steel Ladle

Corundum refractory is the preferred material for the purging plug of refined ladle and its service conditions is extremely crush.During the service,the temperature of its working surface can be up to 1650-1700°C,and both sides of the refractory need to bear the temperature difference of more than 1000°C and the repeated cold and heat cycles of casting and pouring,resulting in the thermal mechanical damage of purging plug materials.Furthermore,with the increase of the secondary refining ratio,the high frequency continuous casting accelerates the damage of materials and greatly reduces the service life of the purging plug,resulting in frequent replacement and maintenance in production,which affects the pace of refining process,and also affects the ladle online turnover and production safety.Therefore,if the life of the purging plug of corundum refractory can be further enhanced,it must be benefit to improve the efficiency and safety of the ladle refining process,which is one of the most important tasks for merallurgical workers.At present,researchers have introduced spinel,zirconia and other components to improve the thermal shock resistance of corundum refractory,but the improvement is limited.Secondly,the thermal shock resistance of materials is generally evaluated by the residual strength retention rate of materials after cold and hot cycling s,which is established without evaluation on the real fracture process scientifically.Moreover,the fracture parameters associated with the thermal shock resistance of the material can not be obtained by the traditional method,and also the effect of multi-scale cracks orignated in the materials.Finally,the damage mechanism of corundum refractory under the actual service condition is only evaluated from the materials after use.Based on the problems mentioned above,the microstructural regulation was firstly investigated in this thesis,which selected the flake calcium hexa-aluminate phase?CaO×6Al₂O₃,CA₆?and the calcium dialuminate?CaO×2Al₂O₃,CA₂?with low thermal expansion coefficent from the Al₂O₃-CaO binary system.The research content can be ascribed as the following two parts.On one hand,the microstructure?content,morphology,distribution?of ceramic phase?CaO×6Al₂O₃,CA₆ and Ca O×2Al₂O₃,CA₂?in matrix was designed by the binding phase in corundum refractory.On the other hand,calcium hexa-aluminate aggregates of different sizes were introduced into corundum refractory to improve the interface properties between aggregate and matrix,aiming to prepare corundum refractory with high thermal shock resistence.Secondly,the wedge splitting technology,digital image correlation technology and inverse optimization technology were used to systematically study the crack propagation process of materials under tensile stress,obtain the true fracture energy,tensile strength and other fracture parameters,and investigate the fracture mechanism of corundum refractory.Finally,the damage mechanism of corundum refractory under actual service condition was systematically investigated by using numerical simulation technology,which provides theoretical basis for optimizing the development of purging plug material.The details can be divided into the following parts.One was to investigate the stress intensity factor K at the crack tip of corundum refractory by the extended finite element method,revealing the law between the bearing capacity of the material and the initial crack size.The other was to take the structure of the actual purging plug as the analysis object and based on the thermo-structural coupling model,researching the the relationship between material properties and temperature and thermal stress fields in the service of the purging plug.On the basis of the work above,the main conclusions can be made as follows:?1?The phase composition and microstructure could be designed successfully by controlling the aluminate cement content of the binder during the preparation of corundum refractory,which can be benefited to improve the mechanical properties and thermal shock resistance of the refracory.After heat treatment at 1600°C,CA₆ phase was generated in situ in the material and its quantity increased,its distribution spreaded from the matrix to the aggregate,and its morphology developed from the plate to the equiaxial shape when the cement content increased from 1 wt%to 10 wt%.The interlocked network structure gradually formed,resulting in the correspondingly increase of cold and hot flexural strength.When the cement content increased from 10wt%to 15 wt%,the CA₂ phase was generated in the original position of the material,and the CA₆ phase gradually decreased,forming the CA₆/CA₂ reaction layers?inner CA₆ and outer CA₂?wrapped the corundum aggregate,which absorbed the thermal stress and improved the thermal shock resistance of the corundum refractory.?2?By introducing different sizes of calcium hexaaluminate aggregate,the interface between aggregate and matrix could be improved remarkably,and the refractory with high thermal shock resistance can be prepared successfully.CA₆ particles of 5-3,3-1and 1-0 mm were used alone or at the same time to replace the tabular corundum aggregate,which significantly reduced the thermal expansion coefficient of the material.Furthermore,the microstructure of the material was enhanced,and the strength of corundum refractory at each temperature was significantly improved.Compared with the materials without CA₆,the corundum refractory replaced by three sizes of CA₆ aggregates had the best interface combination,the cold and hot flexural strength were increased by 4.7%and 10.8%respectively after high temperature treatment.?3?By means of wedge splitting technology,digital image correlation technology and inverse optimization technology,the fracture mechanism of corundum refractory material under tensile stress could be quantitatively characterized.In the corundum refractory with high content of cement?15 wt%?,or introducing three granularity CA₆particles?5-3,1,3-1-0 mm?at the same time,the fracture tortuous path and dissipation of energy were increased,and the material fracture energy and characteristics length were also added.Correspondingly the strain on the X direction was highest and the main crack was longest,which can improve the ability of a material to resist crack propagation.?4?By using the extended finite element method and the linear elastic constitutive model,the internal law between the ultimate bearing capacity and the initial crack length of the corundun refractory was investigated.Under the tensile stress,the stress intensity factor K at the crack tip was linearly correlated with the external load at the same initial crack size.Under the same external load,the stress intensity factor K at the crack tip was linearly correlated with the square root of the initial crack size.The ultimate load of corundum refractory was inversely related with the initial crack size.?5?In service conditions,the maximum thermal stress of the plug appeared at the stages of casting and the argon blowing,and its damage occurred at the upper of the thermal surface.The corundum refractory containing three sizes of CA₆ siginificantly reduced the termperature difference an the thermal stress in the pruging plug.The main reasons for the large temperature gradient in the purging plug were ascribed as two aspects.One was the thermal shock of molten steel at the beginning of steel loading with transportation,and the other was the forced heat transfer between low-temperature argon gas and high-temperature plug during argon blowing.The thermal conductivity and thermal expansion coefficient of the material were most sensitive to the internal temperature field and thermal stress field respectively.The distribution of temperature field and thermal stress field in the structure of corumdum refractory containing three sizes of CA₆ grains were the optimal,which can effectively improve the service life of the purging plug.