Fundamental Study Of Inclusions Behavior In Heat Resistant Steel Containing Rare Earth Elements

A certain amount of rare earth elements is added to heat resistant steel to develop the high-temperature oxidation resistance and creep resistance properties.For example,0.03-0.08%of cerium is specified in the 253MA steel grade.During the manufacturing process,plenty of rare earth oxides and oxysulfides with high melting point are generated from the reaction between cerium and oxygen,sulfur,and the severe nozzle clogging issues arises consequently.One heat of steel cannot even be accomplished before the nozzle being blocked during the productive process of 253MA steel in one stainless steel plant of China.It will be of great significance to control the nonmetallic inclusions with high melting point for the purpose of solving the nozzle clogging and assuring continuous casting process smooth.Firstly,the liquid regions in ternary phase diagrams containing rare earth oxides were predicted and validated.Then a thermodynamic model of inclusions precipitation and solutes microsegregation during solidification of heat resistant steel was worked out.At last the dynamic experiments of inclusions modification were taken out base on the thermodynamic calculations.Under the experimental conditions in this paper,the crucial conclusions are obtained as follows.(1)The thermodynamic properties of RE2SiO5 and RE4.67Si3O13(RE=La,Ce)were calculated.The interaction parameters of CaO-Al2O3,CaO-SiO2,CaO-La2O3,La2O3-Al2O3,La2O3-SiO2,Al2O3-SiO2,Ce2O3-SiO2 and Ce2O3-Al2O3 binary systems were optimized using Redlich-Kister expressions.(2)The Kohler method was adopted to extrapolate the phase diagrams of Al2O3-SiO2-La2O3,CaO-Al2O3-La2O3,CaO-SiO2-La2O3 and Al2O3-SiO2-Ce2O3 systems.The calculated Al2O3-SiO2-La2O3 diagrams are reasonably reliable comparing with the literature and equilibrium experimental results.(3)Based on the interaction parameters obtained from phase diagram calculations,a thermodynamic model of inclusions precipitation which contains comprehensive data of rare earth compounds,especially the liquid inclusions was developed.Then the model was validated comparing with the commercial tests,lab scale experiments and results from literature.It is confirmed that the thermodynamic model is accurate to predict the precipitation behavior of inclusions in heat resistant steel containing rare earth elements.(4)The precipitation behavior of inclusions during the solidification of 253MA steel,the stability region of inclusions in solidification structure and liquid region of liquid inclusions at 1600? were worked out.The inclusions are modified to liquid when 0.01%aluminum is added and the liquid region of inclusions is expanded as the initial oxygen content increases.(5)After the simulation experiments at high temperature,it is declared that the inclusions are Ce2O3 and Ce2O2S after Ce addition no matter what deoxidant is chosen at low oxygen content.When Si is chosen as the deoxidant at high oxygen content,the inclusions composition change in the order of Ce2O3(Ce2O2S)?Ce-Si-(Al)-O?Ce-Si-Al-O after Ce addition,of which the Ce-Si-Al-O inclusions are layered or uniform solid phase.When Si-Al alloy is chosen as the deoxidant at high initial oxygen content,the inclusions composition change as Ce2O3(Ce2O2S)-Ce-Si-Al-O?Ce4Si5Al5O23.5 after Ce addition,of which the Ce-Si-Al-O inclusions are layered and the Ce4Si5Al5O23,5 inclusions are homogeneous liquid phase.Through the phase diagrams computation,the thermodynamic calculation and simulation experiments at high temperature,the rare earth inclusions with high melting point were successfully modified to liquid phase in this paper.The abundant thermodynamic properties of rare earth compounds and phase diagram information supply the theoretical and experimental foundation to solve the nozzle clogging of heat resistant steel containing rare earth elements.