| In the current study,laboratory scale experiments for the modification of inclusions by calcium treatment of linepipe steels were performed with a vacuum induction furnace.The influence of different sampling methods on the amount of inclusions was investigated and validated by the measurement and the numerical simulation.In the vacuum induction furnace,the fluid flow of the molten steel under electromagnetic force favored the floating inclusions to the top so that a large number of inclusions accumulated at the top surface.More inclusions at the top surface were brought in silica pipe samplers,while most removed inclusions at the top would adhere to the inner wall of the crucible and could hardly be poured into the mold with the molten steel for the last tapped sample.The lining refractory has a great influence on the composition of inclusions since the reaction between excessive calcium and lining refractory occurred.For the experiment with a magnesia crucible,MgO content in inclusions are 10%-20%and the T.Mg content in steels increased from 13 ppm to 23 ppm.For the experiment with alumina crucibles,MgO content in inclusions was hardly measured,and more inclusions were found in steel samples when an Al2O3 crucible with a higher apparent porosity was used.Inclusions were transferred from CaO-CaS to CaO-CaS-Al2O3 and then to CaO-(CaS)-Al2O3 and finally to(CaO)-Al2o3 due to the decrease of calcium content during calcium treatment.The mass transfer coefficient of the dissolved calcium in the steel was estimated in the range of 2.35×104-3.53×10-4 m/s.Sulfur content in steel had an obvious effect on the composition of inclusions during calcium treatment.The formation and decomposition of CaS was the key point.Langmuir adsorption model was proposed for the formation of CaO and CaS just after the addition of Ca-Si powders,which was validated by the measurement.The formation of CaS highly depended on oxygen,calcium and sulfur contents in steel in a thermodynamic view.Since initially generated CaS inclusions were mostly decomposed due to the evaporation of calcium,while the extent of the modification reaction on alumina was limited by the increased sulfur content in steel.Therefore,the formation of CaS-CaO-Al2O3 inclusions with a high Al2O3 content was favored in steel with a high sulfur content in steel.Behavior of inclusions during reoxidization after caclium treatment of Al-killed steel was studied.Both Al2O3 content in inclusions and number density were increased after slight reoxidation occurred.Aluminum and calcium were oxidized firstly after the oxidation of the molten steel by the air,and then SiO2 formed due to the oxidation of silicon when the left[Al]s content was little in the molten steel.There are three stages involved during the process,i.e.,the beginning of reoxidation,aluminum oxidation and silicon oxidation,which oxidation rate gradually increased in each stage.The evolution path of inclusions was CaO-CaS-Al2O3-CaO-(CaS)-Al2O3?(CaO)-Al2O3?Al2O3-SiO2,and the number density of inclusions was higher than 200#/mm2.In addition,a kinetic model was proposed for the prediction of the evolution of inclusions during the reoxidation with air and validated with measured resutls,with consideration of the removal of inclusions.Finally,an online software was developed for the guidence of accurate control on inclusions during calcium treatment.The database was establised based on operation parameters and the steel composition in the production so that the predicted results of inclusions composition and suggestions for the addition amount of calcium are instantly available.The application of the software can siginifcantly improve the efficiency of calcium treatment,and make progress in the field of the intelligent and automatic production. |
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