Development Of Electroslag Remelting Slag For COST-FB2 Steel

The most promising turbine rotor material for application temperature up to 620? in the USC power generation plant is FB2,which was developed by the Cooperation in Science&Technology(COST)programs.FB2 contains 0.01%B and can significantly improve the thermal efficiency of power generation plants.Now some companies are trying to produce FB2 via electroslag remelting for the good uniformity of its chemistry distribution and solidification structure.During the melting process,the slag system is a crucial factor for the purity,homogeneity,burning loss of B and solidification structure of ingots.This paper is mainly focused on the physical and chemical parameters of slag systems and the results of controlling the burnig loss of boron during the remelting to identify a suitable slag system for electroslag remelting FB2.For that,the paper conducted three experiments.Firstly,based on the primary requirement of slag system for remelting FB2,we developed six slag systems containing 0,0.5%,1%,1.5%,2%,3%amount of B2O3 to the basic slag system of 55%CaF2-20%CaO-22%Al2O3-3%MgO.Secondly,we measured and calculated the parameters of the slag systems such as the density,optical basicity,electrical conductivity,viscosity and melting temperature of the slag systems.Then we smelted FB2 by the vaccum induction melting(VIM).Finally we conducted the experiments of equilibrium between slag and meltal in the MoSi2 furnance and detected the contents of composition of the steel samples and slag samples.By analysizing the data,these conclusions can be achieved:(1)Using the VIM to produce FB2 containing nitrogen,the vacuum degree should be 2073?8290 Pa;(2)The melting temperature is rising rapiadly while the viscosity,density,optical basicity and electrical conductivity is decreasing with the increasing additional content of B2O3 for the educt of different kinds and amount of solid phases,but all the parameters are suitable for electroslag remelting FB2;(3)The viscosity and the temperature of turning temperature is decreasing with the increasing additional content of B2O3;(4)Content of O in the steel is dominated by the content of Si,so the only way to limite the content of O to the standard extent is to control the content of Si in the steel and the content of SiO2 in slag.To control the w[Al]/w[Si]in the metal we should adjust the w(Al2O3)/W(SiO2)in slag;(5)Addition of 0.5%B2O3 to the basic slag system can effectively control the boron content within the target range,while with the increasing amount of B2O3,the content of B in the melt is increasing;(6)To control the content of B in the steel,we should base on the ratio of w[B]/w[Si]in the steel to vary additional amount of B2O3 according to the corresponding w(B2O3)/w(SiO2)in the slag;(7)The ratio between equibrium content of B in the metal and B2O3 in the slag is approximately 0.02;(8)From all the experiments,0.5%B2O3 additional to the basic slag system is the most appropriate system for the FB2 remelting,but the specific remelting effect using the electroslag remelting still needs the verification of the following experiments.