Evolution Of Inclusions And Evaporation Of Slag During Electroslag Remelting

Electroslag remelting?ESR?is a secondly refining process,which is widely used in smelting of various metals and alloys,for its excellent quality of product and solidification mode.The requirements of the quality of ESR ingot are increasing with the advance of the industry.The inclusions in ESR ingot are mainly aluminate inclusions,which may affect the quality of steel,with the commonly use of CaF₂-Al₂O₃ and CaF₂-CaO-Al₂O₃ slags.CaF₂-CaO-SiO₂ slag can effectively improve the properties of inclusions,and the quality of ESR ingot.However,research on the properties of CaF₂-CaO-SiO₂ is not sufficiently now.This paper was carried out by industrial experiment,laboratory remelting experiment,and the constant temperature evaporation of slags.The influence of CaF₂-Al₂O₃,CaF₂-CaO-SiO₂,and CaF₂-Al₂O₃-CaO-SiO₂ slags on the inclusions evolution in steel and evaporation of each slags was studied.The conclusions are as follows:?1?The results of industrial experiments show that the deoxidization and desulphurization effect of 30%CaF₂-30%Al₂O₃ slag remelting is remarkable.The content of total oxygen?T.O?was reduced from 121×10^-6?ppm?in the electrode to 43.6ppm in the ESR ingot.The sulfur content was reduced from the 101 ppm in the electrode to 16 ppm in the ESR ingot,the desulfurization rate reached 84.16%,and the gasification desulfurization rate reached 79.2%.After ESR,the number,size and distribution of inclusions had been obviously improved.With the decrease of T.O content,the number of inclusions decreased from 855/mm² to 257/mm²,and the maximum size of inclusions decreased from 8.38?m to 4.24?m,and the ratio of the cross section of inclusions to the observed area?S_a?is significantly reduced.The inclusions in the electrode consist of Al₂O₃,MnS and?Al,Ti,Mn?O-MnS complex inclusions.After remelting,the inclusions were transformed into Al₂O₃ inclusions with the size of 1?m,and the compound inclusions with Al₂O₃ core surrounded by MnS at the outside.?2?The content of T.O is 23.3 ppm in original steel,after remelting experiment,the content of T.O in T1 to T5 steel was reduced,the T.O content is 17.1 ppm in T1,and gradually increases from 11.9 ppm to 16.8 ppm in T2 to T5.Among them,the content of T.O in T1 is the highest,but in T2 to T5,with the increase of the content of SiO₂ in the slag,the T.O content in the steel increased.The number density of inclusions in raw steel is 293 mm^-2,168 mm^-2 in T1,gradually increased from 115mm^-2 to 165 mm^-2 in T2 to T5.The maximum average size of inclusions in steel reduced from 8.02?m in the raw steel to 4.31?m in T1,3.86?m in T2,3.91?m in T3,3.98?m in T4 and 4.11?m in T5.The Sa in the raw steel is 0.05%,and after remelting,it was reduced to 0.021%,0.015%,0.017%,0.019%and 0.020%in T1 to T5.In T2 to T5,with the decrease of CaO/SiO₂ ratio,the number of inclusions in steel increased and the inclusions size became larger.?3?The types of inclusions in raw steel are mainly TiN,?Al,Mg?O,MnS and?Al,Mg,Ca?O-?Ca,Mn?,and the largest size is about 10?m.TiN and MnS inclusions had been removed after remelting.The inclusions in T1 were transformed into MgO·Al₂O₃ spinel and MgO·Al₂O₃-?Ca,Mg?S.The types of inclusions in T2 to T5were transformed into?Al,Mg,Ca,Mg?O-?Ca,Mn?S with the size of 1³?m.The oxide inclusions in raw steel and T1 are mainly MgO·Al₂O₃ spinel with high melting point,and low melting point silicate plastic inclusions in T2 to T4.Twenty-five percent of oxide inclusions are low melting point plastic inclusions and others are brittle inclusions in T5.The type of inclusions in steel can be effectively controlled by using CaF₂-CaO-SiO₂ slag.?4?In 30 min,at 1673K,the total weight loss rate of T1 slag system is 2.23%,1.30%in T2,1.86%in T3,2.28%in T4 and 2.20%in T5.With the increase of SiO₂content in the slag,the activity of SiO₂ in CaF₂-CaO-SiO₂ slag increased obviously,and the activity of CaF₂ and CaO decreased.Therefore,the selection of slag system with low SiO₂ activity can effectively reduce the weight loss of slag.