Investigations On Solute Redistribution During The Solidification Of Iron-based Multicomponent Alloys And Centerline Segregation In Continuous Casting Slab

Centerline segregation encountered complex continuous casting(CC)processes is a tipical internal defect of slab.This defect is impossible to eliminate completely by subsequent hot working processes and will be inherited in the rolling plate,and finally lead to heterogeneous mechanical properties in produced steel products,delamination of plate,hydrogen-induced cracking(HIC),and so on.To further understand the formation and control measures of centerline segregation,the behaviors of solute redistribution during the solidification of Fe-based multicomponent alloy and the formation of centerline segregation during slab CC processes have been studied systematically in the thesis.The researches about the behaviors of solute redistribution during the solidification of Fe-based multicomponent alloy were carried out mainly by thermodynamic calculations.The equilibrium partition coefficient defined as the ratio of solid to liquid compositions of solute at the interface,is a critical parameter for accurately describing solute redistribution and segregation simulation during alloy solidification.In the thesis,the effects of components,temperature and phase configurations on the equilibrium partition coefficient of solutes during the solidification of Fe-based multicomponent alloy are investigated based on thermodynamic calculation model.Taking the equilibrium partition coefficient as a breakthrough point,an alloying design method to improve solute redistribution and finally reduce solute segregation during solidification is proposed.Due to the equilibrium partition coefficient of solutes for steel(especially for peritectic steel)is rare,the equilibrium partition coefficient of solutes C,Si,Mn,P and S are obtained by thermodynamic calculation.On this basis,the formation mechanism of centerline segregation and some key issues during slab CC processes are investigated.The characteristics of solute distribution in the strand,and the effects of turbulent flow and the shape of liquid pool on the solute distribution are revealed.Then,the formation mechanism of centerline segregation during slab CC processes is clarified.Finally,the effects of secondary dendrite arm spacting(SDAS)and the uniformity of secondary cooling on centerline segregation are analyzed.The main results of the thesis can be summarized as follows:(1)The effect of components,temperature and phase configurations on equilibrium partition coefficient of solutes during the solidification of Fe-based multicomponent alloy are investigated by a thermodynamic calculation model.When the content of solutes in Fe-based multicomponent alloy maintains as the same level of solutes in steel,the effect of solute self-interation on its equilibrium partition coefficient is very limited and can be ignored.The effect of solute Mn on the equilibrium partition coefficient of C(6))is relatively large,while the effects of solute of Si,P and S on6)are relatively small.During the solidification of Fe-based alloys,the phase configurations have a greater effect on the equilibrium partition coefficients of solutes C,P and S,but a smaller effect on those of Si and Mn.The redistribution tendencies of C betweenδand L is stronger than that betweenγand L,while the opposite is true for P and S.The value of k_c~γis nearly twice k_c~δ,k_p~γis almost a half of k_p~δ,k_s~γ is nearly 45%of k_s~δ,k_si~δ and k_si~γ are not much different from k_Mn~δ and k_Mn~γ.The equilibrium partition coefficient of solutes in hyper-peritectic system(e.g.,Fe-0.25%C-x%Mn)should be determined according to the different zones of phase configurations during solidification.(2)Taking the equilibrium partition coefficient as a breakthrough point,an alloying design method to improve solute redistribution and finally reduce solute segregation during solidification is proposed.These two alloy systems,Fe-0.1%C-x%Ce and Fe-0.25%C-x%Ce,are taken as examples to clarify the mechanism of Ce reducing segregation degree of C in steel and the method to determinte the optimum addition amount of Ce.The rare earth element Ce plays a role in reducing the segregation of C in steel,for the reason that Ce can enlarge the zone of austenite during the solidification of peritectic steel and increase the mean value of equilibrium partition coefficient of C.The optimum addition amount of Ce to reduce the segregation is related to the contents of C in steel.The optimum addition amount of Ce for Fe-0.1%C-x%Ce and Fe-0.25%C-x%Ce are 0.047%~0.057%and Fe-0.25%C-x%Ce,respectively.When 0.05%of Ce is added to Fe-0.1%C system,the mean value of equilibrium partition coefficient of C increases from0.157 to 0.232,increasing by 47.7%.(3)The evolution of equilibrium partition coefficient of solutes in different phase zone during Q345 solidificaiton is investigated by thermodynamic calculations and in-situ observations with high-temperature laser scanning confocal microscope(LSCM).L+δ,L+δ+γand L+γphases coexist in sequence during the solidification of steel Q345,and the tendencies of solute redistribution between solid and liquid phase in the three coexistence zones are different form each other.The calculation formula for equilibrium partition coefficient of solutes in three-phases coexistence zone(L+δ+γ)is obtained,i.e.(?).The temperature dependence of the equilibrium partition coefficient was quantified through regression analyses of solute C,Si,Mn,P,and S under different phase configurations.The mean values of the equilibrium partition coefficnet of solutes C,Si,Mn,P,and S in the entire mushy zone are 0.2,0.615,0.696,0.373 and 0.033.(4)A 3-D centerline segregation model of entire strand for CC processes is established,and then the characteristics of solute distribution in strand and the formation mechanism of centerline segregation during CC processes are investigated.The 3-D centerline segregation model of entire strand has relatively high accuracy and reliability,while the number of calculations is very large.The maximum deviation of segregaon degree of C on line 1/2 between predicted results and measured results reaches as low as3.22%.In the early stage of a slab CC process,the solute concentration in the turbulent zone is slightly higher than the initial concentration,and the distribution of solute elements in this zone is relatively uniform with the effect of turbulent flow.With the consecutive washing effect of jet-flow of molten steel on the strand narrow face,the rejected solute elements are taken away from the region near the narrow face and results in a negative segretion in this region.In the final stage of solidification during a slab CC process,the characteristics of solute distribution is highly depended on the shape of the liquid pool.Moreover,the solidification of molten steel in the central region of strand is not simultaneous,and generally the later complete solidification,the more serious solute segregation.In the present case,i.e.1.2m/min casting speed and 25 K superheat,the complete solidification point is located at a position roughly 595 mm from the slab center in width direction,and the length of liquid pool is 18725 mm.The combined actions of solute redistribution,solidification front moving continuously toward the centerline region,and the convection of molten steel result in centerline segregation of CC slab.When the solidification fronts are located at positions 25 mm,38.3 mm,51.9 mm,and58.8 mm from slab surface,compared with the initial concentration,the carbon composition at slab center increases 3.33%、4.67%、6.00%、8.67%、11.33%in sequence.(5)A 2-D combined 3-D hybrid model for centerline segregation during slab CC processes was established based on the flow characteristics(turbulent flow and natural convection)of molten steel in the liquid pool,and then was used to investigate the centerline segregation of CC slab.Results indicate that the number of calculations for the simulation of slab centerline segregation by the 2-D combined 3-D hybrid model is significantly reduced,and the calculation accuracy reliable.In the casting direction during slab CC processes,solute concentration in the liquid pool consecutively increases and the increasing rate of concentration becomes higher and higher.From the pure liquid zone to the mushy zone,solute concentration further increase,white the increasing rate of concentration gradually decrease.After complete solidification,solute concentration maintains a roughly constant value.The segregation tendency of solutes in steel Q345arranged in order of weakening is S,C,P,Si,and Mn.(6)Taking the permeability of mushy zone as a breakthrough point,the effect of secondary dendrite arm spacing(SDAS)on centerline segregation during CC processes is investigated.The effect of SDAS on slab centerline segregation,especially for strong segregation element,i.e.C,P and S,is significant.When the equivalent SDAS increase from 10μm to 50μm,the mean segregation degrees of C,Si,Mn,P and S on centerline increase by 13.41%,5.86%,4.51%,11.66%and 16.28%,respectively.Based on the quantitative relation between casting peed or superheat and equivalent SDAS,the effects of casting speed and superheat on centerline segregation are investigated.(7)The effect of cooling uniformity in width direction of secondary cooling zone is investigated.The cooling uniformity is loaded in the centerline segregation model as heat transfer condictions.Results indicate that the cooling uniformity directly affects the solidification profile of transverse section in the end of solidification.The solidification profile of centerline on slab transverse section presents a consecutive or discontinuous narrow band under an even or uneven secondary situation.The solute distribution on transverse section is highly depended on the solidification profile.The cooling uniformity in width direction of slab plays a great influence on the centerline segregation by affecting the solidification profile of transverse section.Even secondary cooling is beneficial to the molten steel in centerline region solidifiy simultaneously during the final state of slab CC processes,so as to promote the solute distributing uniformly on the centerline,and eventually improve centerline segregation of slab.