Study And Optimization Of Slags For Gas Nitriding In Pressurized Electroslag Remelting Process

High nitrogen steels are widely used in many fields due to their excellent mechanical properties and good corrosion resistance.In the process of manufacturing high nitrogen steels by using pressurized electroslag remelting(PESR),high pressure nitrogen is regarded as a new source of nitrogen alloying.As an economic and advisable alloying method to increase the nitrogen content of molten steel from gas rather than nitride ferroalloys,this new technology named gas nitriding has received much attention.However,many problems,e.g.low nitrogen solubility in slags,slow rate of nitrogen absorption into molten steel from slag bath,which limit the increasing of nitrogen content in molten steel,and restrict the development and application of gas nitriding technology.Therefore,the research on mechanism and the optimization of the PESR slag using for gas nitriding are of great significance in the promotion of the manufacture technology for high quality high nitrogen steels.In this paper,a series of CaF2-CaO-Al2O3(SiO2+MgO)slags are designed based on the properties of the PESR gas nitriding process,and the physical properties of the new slags are obtained through experiment testing and calculation.Then,the thermodynamics and kinetics of nitrogen dissolution in slags under high nitrogen pressure condition are studied experimentally,which are used to investigate the dissolution behavior of nitrogen in molten slags.Meantime,the melt structure of the slags and the main existing forms of nitrogen in the nitrided slag are analyzed by means of XRD and XPS methods.Based on the study,the conclusions could be obtained as follows.The viscosity,density and alkalinity of the twelve slags designed in this paper can meet the basic requirements of slag for PESR process.Among all designed slags,S1 and S4 are not applicable to the production of Cronidur30 by PESR because their melting temperature exceed 1370?;the use of S3 and S9 will increase the power consumption of PESR process because their conductivity exceed 4.0 ?-1,The nitride capacity of slag increases with the increasing of alkaline oxides such as CaO,and dramatically decreases with the increasing of acidic oxides such as SiO2 dramatically.S3,S7?S10,S12 slags have higher nitride capacity than others,which can prove a superior ability to absorb nitrogen from gas.In the gas-slag nitriding experiments under high nitrogen pressure,the nitrogen contents in the slags increase significantly with the prolonging of melting process,and the nitrogen content dose not reach the level of saturation even after two hours melting.The results indicate that increasing nitrogen pressure and high CaO/Al2O3 ratio can significantly promote the dissolution of nitrogen,while the addition of SiO2 have a negative impact on the dissolution of nitrogen.Under the same conditions,the contents of nitrogen dissolved in S7 and S8 are the highest among all slags,which exceed 1.0 wt.%.The X-ray diffraction analyses show that the premelted slags are mainly composed of compound 11 CaO·7Al2O3·CaF2 and single phase CaF2.When the CaO/Al2O3 ratio is high,single phase CaO can be detected.When the Al2O3 content is rose to high level,the CaO phase will disappeare because of the formation of compounds;meantime,the presence of MgO can stimulate the formation of magnesite-alumina spinel MgAl2O4,which will improve the melting temperature and viscosity of slags.The X-ray photoelectron spectroscopy indicates that Ca,F and Al in the slags after gas nitriding mainly exist in the form of CaF2 and Al2O3;O exists in two forms:bridging oxygen O0 and non-bridging oxygen O-;Si exists in two forms:SiO2 and Si3N4 when 5%SiO2 is added into the S11 slag.Besides,N-C bond and Al-N bond can be detected in the fitted N spectrum due to the presence of carbon and Al2O3.These findings illustrate that the nitrogen dissolution in slags can be improved by the high carbon content,and the Si and Al in slags contribute to the the formation of incorporated nitride.