Fracture Behavior Of MgO-C Refractories Used In Converter Taphole

MgO-C refractories are the preferred material for the taphole of converter,and their performance and service life directly affect the operation rate of the converter and production safety.In service,the cyclic thermal shock caused by periodic smelting and tapping of the converter is an essential reason for its damage.In order to meet the requirement of longevity of the taphole of converter,In-depth understanding of the fracture behavior of MgO-C refractories and improvement of their thermal shock resistance have become the development direction of the taphole.Usually,adjusting the carbon content and particle grading are the most straightforward and effective methods to improve the thermal shock resistance of MgO-C refractories.However,it has not been explored that the change of energy dissipation and crack propagation law during the failure process of MgO-C refractories before and after adjusting the carbon content or particle grading.Besides,flake graphite is prone to orientation distribution in the process of forming due to its flat geometric structure,which makes the anisotropy of MgO-C refractories significant.By granulating the flake graphite,its orientation distribution in the material can be suppressed,and the stress concentration can be alleviated,which is beneficial to the improvement of thermal shock resistance of MgO-C refractories.However,the influence of granulated graphite on the energy dissipation and crack propagation of MgO-C refractories during failure in different directions is still unclear.In addition,the larger thermal expansion coefficient of magnesia particles in MgO-C refractories is easy to cause thermal stress concentration in the materials and deteriorate the thermal shock resistance of the material.How to optimize the structure of magnesia to relieve stress concentration and improve the thermal shock resistance of MgO-C refractories needs to be studied.In view of the above problems,the research work of this paper includes:Firstly,the fracture behavior of MgO-C refractories with different flake graphite contents was studied based on wedge splitting test combined with digital image correlation method and acoustic emission technique to ascertain the relationship between flake graphite content and energy release,crack propagation and crack mode during the fracture process of MgO-C refractories;Secondly,the effects of particle grading on the fracture behavior of MgO-C refractories was studied to reveal the relationship between particle grading and the deformation and internal damage mechanism in the fracture process of MgO-C refractories.The strengthening mechanism of adjusting the particle grading on thermal shock resistance of MgO-C refractories was revealed by analyzing local stress magnitudes under different particle contact states using a simplified spherical particle model by means of the photoelasticity method;Thirdly,on the basis of the optimized particle grading,the flake graphite was further granulated.The fracture behavior of MgO-C refractories with flake graphite addition or granulated graphite addition in different directions was studied,and the reason for the improvement of thermal shock resistance of MgO-C refractories with granulated graphite addition was explained;Then,inspired by the above research on the fracture behavior of MgO-C refractories,a new idea to optimize the thermal shock resistance of magnesia carbon refractories was put forward.MgO@GF particles with core-shell structure were prepared by a coating process and introduced into MgO-C refractories.The correlation between microstructure evolution after thermal shock and fracture behavior was discussed,and the optimization mechanism of MgO@GF particles on thermal shock resistance of materials was revealed by fractal theory and fracture microscopy method;Finally,a new type of MgO-C refractories for the taphole was developed based on above research results,and large-scale production and industrial application were carried out.Through the above research work,the following main conclusion could be drawn:1.Wedge splitting test combined with digital image correlation method and acoustic emission technique verified that when the content of flake graphite increased from 8 wt%to 20 wt%,the nominal tensile strength of MgO-C refractories treated at 1000℃decreased from 8.4 MPa to 4.9 MPa,but the specific fracture energy and characteristic length increased from 283.1 J/m² and 112.3 mm to 374.1 J/m² and 172.7 mm,and the MgO-C refractories with lower flake graphite content had greater ability to resist crack initiation,while the MgO-C refractories with higher flake graphite content had better ability to resist further crack propagation.In addition,the mixed crack mode of tensile and shear or shear crack mode in MgO-C refractories increased with increasing flake graphite content,which was conducive to the dissipation of energy during the loading process,and thus enhanced the nonlinearity of mechanical behavior during the loading process.2.MgO-C refractories with different particle grading were designed according to Andreasen’s continuous packing theory.For the fracture process of MgO-C refractories treated at 1400℃,the transition of particle grading from tight packing（q=0.5）to non-tight packing（q=0.2）,reducing the coarse aggregate particles and increasing the proportion of fine powder,promoted more microcracks initiation or propagation earlier in the pre-peak region for consuming energy,reduced the proportion of intragranular cracks in the crack path,and promoted the crack mode transition from tensile mode to shear or mixed mode.3.MgO-C refractories exhibited a similar fracture process in the Brazilian splitting test at different loading anglesθ.The specific process was that microcracks first initiated in the stress concentration area near the loading point;With the increase of loading,the number of microcracks increased,and microcracks were begun to generate in the central region of the specimen;When the load was close to peak load value,a macrocrack was formed due to microcrack propagation and crosslinking;When the load exceeded the peak load,the macrocrack penetrated the MgO-C specimen,leading to its failure.By introducing granulated graphite,the maximum horizontal strain attenuation of MgO-C refractories in the Brazilian splitting test at different loading anglesθwas reduced from 50.7%to 11.9%.It promoted the transition of crack mode from tensile mode to shear or mixed mode.The anisotropy of the thermal expansion coefficient was reduced from 1.22 to 1.08,and the residual strength retention rate was increased from 75.37%to 81.04%when granulated graphite was introduced to MgO-C refractories.4.By introducing MgO@GF particles with a core-shell structure,the interface bond between aggregate and matrix was weakened.More abundant ceramic phases（Mg Al₂O₄,Al N）were generated in the matrix to strengthen the matrix;The crack propagation mode of MgO-C refractories changed from a single main crack to multiple secondary cracks in Brazil test,dispersing the energy concentration of main cracks and dissipating more energy,thus realizing the synchronous improvement of mechanical properties and thermal shock resistance of MgO-C refractories.5.Based on the above work,the formula with better particle grading and granulated graphite was selected for the preparation of new type of MgO-C brick for taphole,and industrial application was carried out on a converter of a steel plant.Compared with the average service life of the MgO-C bricks used initially in steel plant（125 heats）,the average service life of the new type MgO-C bricks（181 heats）was increased by 45%.