| Magnesia-carbon(MgO-C)refractory has excellent properties for thermal shock resistance and high-temperature slag corrosion resistance.MgO-C refractories are widely used in steel metallurgical equipment,such as linings of converters,electric furnaces and refining the furnace and slag lines of ladle furnaces etc.However,with the acceleration of the production rhythm,the application of new smelting technology and the improvement of energy-saving and consumption reduction,the requirements of MgO-C refractories are also getting higher.For example,carbon in MgO-C refractories will carburize into molten steel during the smelting process,which is very unfavorable for the smelting of low-carbon steel and ultra-low-carbon steel.However,the reduction of graphite content in MgO-C refractories will reduce the performances of thermal shock resistance and slag corrosion resistance and shorten its service life.Therefore,it is of practical significance to study the influence of graphite content on the comprehensive properties of MgO-C refractories.In addition,the use environments of MgO-C refractories are often complex and variable,the causes of damage can vary drastically under various conditions,and there is no unified understanding of the corrosion mechanism;further research is needed.This paper first analyzed several major factors that damage MgO-C refractories during application,and explored the relationship between damage factors and the oxidation resistance,wettability and corrosion slag resistance of MgO-C refractories,which provided a guiding direction and theoretical support for the development of higher performance MgO-C refractories.Subsequently,the effect of flaky graphite on the physical properties and oxidation resistance of such refractories was also presented to provide a reference for researching and developing MgO-C refractories with various carbon contents.Then the MgO-C refractories with different graphite contents and the dense MgO were deemed as the research materials,and the wetting and spreading processes between molten slag and such materials were observed in-situ by high-temperature wetting angle observer.The effects of graphite content and porosity on the wettability and penetration depth of the material were investigated.The different mechanisms of slag corrosion resistance between MgO-C refractory and dense MgO was discussed.Subsequently,nano Y2O3 powder was added to MgO-C refractory,and the effects of nano-Y2O3 powder on the microstructure,mechanical properties,oxidation resistance and slag corrosion resistance of MgO-C refractories were systematically studied,which provided theoretical guidance for the application of nano-powder in MgO-C refractories.Finally,based on the ion theory for slag,an electric field was applied between MgO-C refractory and molten slag,and the impact of electric field on the slag corrosion resistance of MgO-C refractory was discussed.The following main conclusions were obtained:(1)The bulk density(BD)and cold crushing strength(CCS)of the cured MgO-C refractories gradually decreased with the increasing of graphite content,and the apparent porosity(AP)changed little with the increasing of graphite content.After carbonization and oxidation experiments,the BD and CCS of MgO-C refractories decreased,and the AP increased significantly.The main reasons were the decomposition of phenolic resin at high temperatures,the oxidation of graphite,and the generation of cracks.In the oxidation resistance experiment,the oxidation rate of MgO-C refractories decreased but the mass loss rate increased with the increasing of graphite content.At 1400?,the prolonging of holding time significantly increased the oxidation rate and mass loss rate,while the holding time had little effect on the oxidation rate and mass loss rate at 1600?.The effect of indirect oxidation of carbon has a great influence on the oxidation resistance of MgO-C refractory.The oxidation rate and weight loss rate of MgO-C refractory with carbon content of 3%-12%fired at 1400? and holding 3h were higher than that of fired at 1600?.The effective diffusion coefficient and the complete oxidation time were calculated using the unreacted core model,and the relationship between them and graphite content was revealed.The effective diffusion coefficient and complete oxidation time of MgO-C refractories at 1000? were increased with the increasing of graphite content.(2)The wetting angle between slag and samples(dense MgO,MgO-C refractories,pure graphite)was directly related to the graphite content;increased with the increasing of graphite content.In addition,the wetting angle between the slag and the samples continuously decreased over time.After 60 s,the wetting angle between slag and dense MgO ceramic was about 48°,and was about 140° between slag and pure graphite substrate.When the graphite content was 8%?16%,the wetting angle between the slag and the MgO-C refractory material was little different,about 90°.Image software was used to measure the apparent height and apparent diameter of the molten slag,and the apparent volume of the molten slag on the surface of the sample was calculated using the spherical crown model.The change trend of the apparent volume was basically the same as the change trend of the wetting angle;they both decreased with time,and the higher the graphite content,the smaller the degree of change.Through the observation of the microstructure of the slag-sample interface and analysis of the phase,it is considered that the main method for penetration of slag into the dense MgO ceramic substrate was through the dissolution of MgO,while the slag has many ways to penetrate into the interior of the MgO-C refractory,including the slag dissolving MgO in the MgO-C refractory;the slag penetrating into the interior through pores existing in the MgO-C refractory;components such as FeO in the slag reacting with graphite.The reaction results in the consumption of graphite and the remaining pores penetrate into the interior.(3)An appropriate amount of nano-Y2O3 powder can improve BD and reduce AP of the MgO-C refractory and help to improve the CCS of the MgO-C refractory.When the MgO-C refractory was carbonized or oxidized at 1400?,its BD and CCS were significantly reduced,and the AP was greatly increased.However,the BD and CCS of the sample to which Y2O3 powder was added were less reduced.In addition,the oxidation resistance of the MgO-C refractory with Y2O3 powder at 1600? was significantly higher than that of the sample without Y2O3 powder.The MgO-C refractory with 2%nano-Y2O3 powder had better slag corrosion resistance.The lateral and longitudinal erosion depths were reduced by 24%and 15%,respectively,compared with the MgO-C refractories without Y2O3 powder.(4)The corrosion rate of MgO-C refractory electrode was reduced from 15.1%and 15.9%to 8.9%(anode)and 1.1%(cathode)under the action of electric field,which greatly reduced the slag corrosion of MgO-C refractory.In this study,the corrosion mechanism of MgO-C refractory was analyzed from different angles,and methods of inhibiting and reducing the damage of MgO-C refractories were proposed.It provides an important theoretical basis and reference for the application of MgO-C refractories in steel metallurgical equipment,helps to promote the development of refractory materials preparation and protection and is of great significance for the prolongation of a refractory's life. |
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