Effect Of Pressure On Solidification Structure And Macrosegregation In Nitrogen Containing Steel Ingot

The pressurization has been great progress in metallurgy and solidification of nonferrous.And some are at the mature industrialization stage.However,it is quilt difficult and dangerous to increase pressure at the higher liquid temperature of ferrous metals.Thus,there is little research and application of pressurization on iron and steel.There exists a typical case that pressure is used to improve the nitrogen solubility and beneficial influence of nitrogen on the performance of steel.Up to now,the research of pressurization in ferrous metallurgy mainly focuses on the manufacture of nitrogen containing steel,especially high nitrogen steel.And very little attention and investigation has been given to the effect of pressure on solidification structure and phase transformation in nitrogen containing steel.In this research,the mechanism for pressure to solidification phenomenon has been investigated with the Fe-Cr-Mn-Mo-N nitrogen containing steel ingot.The solidification phenomenon includes thermodynamic and kinetic parameters,heat transfer from ingot to mould,solidification defects(shrinkage,porosity,pore and segregation),growth of dendritic arm and columnar-to-equiaxed-transition(CET)etc.The change in thermodynamic and kinetic parameters of nitrogen containing steel with pressure has been investigated using Thermo-Calc and DICTRA software.Pressure can suppress the formation of ferrite phase and ferrite trap,enlarge the uniphase austenite area,improve the liquidus/solidus temperature and nitrogen solubility,and promote the solidification mode transforming from FA to A mode.With increasing pressure,the partition coefficients decrease,leading to the deterioration of both Mn and Mo microsegregation.And the increment of C,N and Cr partition coefficients results in eliminating their microsegregation.However,both partition coefficient and microsegregation of Si increase with pressure,because the partition coefficient of Si is bigger than 1.Increasing pressure can decrease the diffusion coefficient of C in ferrite phase and N in both ferrite and austenite phase,and increase the diffusion coefficient of C in austenite phase.Additionally,increasing pressure is conducive to promote driving force for phase transformation and decrease the critical nucleation radius of grain,which results in the exponential increment of nucleation rate and refinement of grain during solidification of nitrogen containing steel.The mathematical model inversing interfacial heat transfer coefficient has been developed.Combing with the experimental result of embedded thermocouple measurement and empirical equation,this mathematical model,as a quantitative way,has been used to investigate the change in interfacial heat transfer coefficient with pressure.Increasing solidification pressure reduces the size of air-gap and accelerates the formation of perfect contact between ingot and mould surfaces.Meanwhile,both interfacial heat transfer coefficient and cooling rate of ingot increase with pressure.Thus,increasing pressure can obviously enhanced heat transfer from ingot to mould.Based on the shrinkage porosity criterion,the effect of pressure on shrinkage porosity has been clarified in 19Cr14Mn0.9N nitrogen contianing steel.With increasing pressure,solidification style gradually transforms from pasty state to layer-by-layer,and the pressure gradient existing in the residual liquid surrounding dendrites is improved.Increasing solidification pressure enhances interdendritic feeding by decreasing interdendritic feeding resistance and distance,and then reduces the whole area of dispersing porosity and shrinkage.The columnar to equiaxed transition(CET)with pressure has been mesured.Combined with the simulated results of temperature field and solidification structure by ProCast software,the CET,dendrite spacing and grain number were clarified.Due to cooling rate accelerated by increasing solidification pressure,the temperature gradient becomes higher,which has a negative effect on the nucleation and growth of equiaxed grains.Thus,CET is postponed,and CET position gradually moves to the central line of ingot.With increasing solidification pressure,dendritic arm spacing exhibits a decreasing tendency,and grain number increases,which indicate that the solidification structure is refined under higher solidification pressure.With the calculation and experimental results,the mathematical model calculating pressure in nitrogen pore has been developed,and the formation mechanism of nitrogen gas pores was clarified during the solidification process of nitrogen duplex steel(21.5Cr5Mn1.5Ni0.25N).The segregation behavior of nitrogen was revealed with respect to the phase transformation.The process associated with the formation of nitrogen gas pores was illustrated clearly.The effects of shrinkage,solidification pressure and alloying elements(N,Mn and Cr)on the formation of gas pores in nitrogen containing steel were investigated in detail.The nitrogen-depleted phase(ferrite phase)is beneficial to suppress nitrogen segregation in the residual liquid surrounding dendrites.Nitrogen-rich phases(austenite phase,AlN and HCP phase)are helpful to eliminate nitrogen segregation and increase the solubility of nitrogen in liquids,which is responsible for the decrease of pressure in nitrogen pore and elimination of gas pore defects.The shape of gas pores is approximately elliptical,and regular gas pores initially form in locations consisting of the austenite phase.Meanwhile,solidification shrinkage promotes the formation of gas pores by decreasing hydrostatic pressure.An increased Mn content is beneficial to eliminate gas pore defects,and Cr exhibits a dual effect on gas pore formation.As the nitrogen content is increased,the enrichment of nitrogen in the residual liquid becomes more significant,which is favorable to the formation of gas pores.With the pressure increasing,there is an obvious reduction in the number and an increment in the formation height of gas pores in the ingot.This indicates that increasing pressure is an effective meathod to suppress the formation of gas pore defects during solidification.According to the nucleation of the equiaxed grains,mass and momentum conservation etc,the 2D mathematical model has been developed to predicte the effect of pressure on solidification structure and macrosegregation in nitrogen containing steel ingot.The present model has very good accuracy and credibility,which is verified by the comparison with references.The effect of pressure on macrosegregation has been revealed by the 2D mathematical model.The number density of equiaxed grains and cooling rate increased by pressure has completely different impacts on macrosegregation.And more research will be needed in the combined influence of these two factors.In the aspect of increasing number density of equiaxed grains,increasing pressure can enlarge the zone of equiaxed grains and negative segregation,improve the formation height of maximum segregation,and suppress macrosegregation.From the aspect of accelerating cooling rate,increasing pressure is beneficial to postpone the CET,decrease the zone of equiaxed grains,and eliminate nitrogen macrosegregation.Additionality,there are other thermodynamic and kinetic parameters of solidification related with pressure,including distribution coefficient and diffusion rate etc.Thus,more comprehensive and systematic investigation should be given to the effect of pressure on macrosegregation in nitrogen containing steel.