Study On Composition And Property Evolution Behavior Of Electroslag Remelting Slag System For Rack Steel

In the electroslag remelting process,the composition of the slag directly determines its physical and chemical properties and metallurgical properties,and then affects the product quality and the technical and economic indicators of the metallurgical process.Rack steel plays a very important role in the field of marine engineering.It is the best choice to research and develop marine engineering rack steel with high strength,high toughness and thick specification and adopt electroslag remelting process.In the electroslag remelting process of toothed steel,the uniformity of the composition of the electroslag ingot is a difficult problem to solve,which is closely related to the change of the slag composition in the electroslag remelting process,so it is important to study the change mechanism of the slag composition.This study is mainly aimed at the research on the composition and property evolution behavior of the traditional CaF₂-CaO-Al₂O₃-MgO slag system and the new CaF₂-CaO-Al₂O₃-MgO-B₂O₃slag system used for rack steel.Through experimental analysis of the effect of fluoride volatilization on the change of slag composition,the effect of fractional crystallization on the change of crystallization property,and the evolution law of slag property during electroslag remelting,the evolution law of slag physical and chemical properties with the change of composition is revealed from three aspects,The essential relationship between the change of slag composition and properties during electroslag remelting is clarified,which provides a basis for the control of electroslag metallurgy process and the optimization of slag system.The results show that:（1）In the 65%CaF₂-30%Al₂O₃-5%MgO slag system,the CaO content increases with the reduction of volatile Al₂O₃ and CaF₂ content of fluoride.In the CaF₂-Al₂O₃-CaO-MgO quaternary slag system,with the change of slag composition,the CaF₂crystals precipitated in the slag layer change from punctate or spherical to massive or strip,and the MgO·Al₂O₃ crystals change from spherical to dendritic.As the main precipitation phase,the calcium aluminate crystal changed from CaO·6Al₂O₃ to CaO·2Al₂O₃ and finally to 12CaO·7Al₂O₃,which was always surrounded by CaF₂crystal and MgO·Al₂O₃ crystal with irregular distribution.（2）The volatilization of slag fluoride mainly occurs in the early stage of smelting production,and the change of slag composition decreases with smelting.During production,the slag skin at the upper part of the mold has a more obvious phenomenon of layered crystallization.The precipitated phase is mainly CaF₂ crystal near the molten pool side and 12CaO·7Al₂O₃ crystal near the mold side.The distribution of crystalline phase in the lower slag skin is relatively uniform.（3）With the increase of B₂O₃ content in CaF₂-CaO-Al₂O₃-MgO-B₂O₃slag system,the morphology of 12CaO·7Al₂O₃ crystals precipitated from the slag layer changed from bulk to block-like and then to elongated strip.MgO changes from dendritic crystal to massive crystal,and MgO·Al₂O₃ crystal is spherical shape,but its volume becomes smaller.When the content of B₂O₃ in the slag is more than 2.1%,Ca₃B₂O₆ crystal begins to precipitate.（4）The melting point of the quaternary slag system and the quintic slag system continuously decreased during the change of slag composition,while the conductivity gradually increased.The viscosity of CaF₂-CaO-Al₂O₃-MgO slag system keeps decreasing,and the viscosity of CaF₂-CaO-Al₂O₃-MgO-B₂O₃ slag system keeps basically unchanged.