Multi-scale Simulation Of Solidification Collumn Crystals And Its Hot Deformation Recrystallization Of Austenitic Stainless Steels

Large forgings are formed from steel ingots.Because of the large size of the ingot,the crystallization process is slower,and the solidification structure is very coarse and uneven.Therefore,for large forgings,fine-homogenization of coarse inhomogeneous solidified grain structure is the main task and goal of thermal processing,and it is also an important way to improve the mechanical properties of large forgings.In the process of manufacturing large forgings,the fine homogenization of coarse inhomogeneous solidification structure is a process involving the whole process.First,it is necessary to understand the formation,distribution and morphology of ingot solidification structure,and then to realize grain refinement and homogenization by forging and subsequent heat treatment.The forging process plays a vital role.In particular,the fine homogenization of the columnar crystal structure(CCS)in the traditional die casting ingots and the CCS in the electroslag remelting(ESR)ingots is particularly important.The experimental method is difficult to study from different sizes.Based on the research and development of mesoscopic-scale cellular automata(CA)simulation methods and techniques for solidification columnar crystal and hot deformation recrystallization microstructure,the simulation of the formation and growth of solidified columnar crystal structure and hot deformation recrystallization grain structure were realized by macroscopic,mesoscopic(microscopic)and nanoscale simulation techniques.It is of great theoretical significance and practical value for the simulation and control of large ingot crystal structure and its hot deformation recrystallization grain structure.The main contents of the thesis are as follows.Firstly,the solidification processes of 4.2t Mn18Cr18N steel hollow ingot by bottom injection mold casting and electroslag remelting was studied by using cell automata CA microstructure simulation module of PROCAST software.With the decrease of the heat transfer coefficient of the inner and outer wall interfaces in the die casting.,the transformation(CET)of columnar grains to equiaxed grains was advanced,which inhibited the growth of columnar grains.The length of columnar grains decreased,and the primary dendrite spacing decreased.With the increase of pouring temperature,CET delayed and the proportion of columnar grains increased.The results also show that the volume nucleation density is the main parameter controlling the transformation from columnar grains to equiaxed grains and grain size.Meanwhile,the interface nucleation density controls primary dendrite spacing of columnar grains.The volume nucleation density extremes for obtaining all equiaxed grains or columnar grains in the whole wall thickness were determined as 1e+006/cm³and 2e+009/cm³respectively.The ESR results show that the ideal re-melting pool shape and solidified columnar structure can be obtained by controlling the melting rate below 0.1 mm?s^-1.The heat transfer coefficient of the bottom mold of 500 W?m²?K^-1can provide enough cooling capacity and the maximum columnar structure region at the bottom.Based on the rule of random nucleation by classical CA method,a hierarchical nucleation rule of equal undercooling surface perpendicular to the direction of heat dissipation was proposed.Based on the growth rule of periodic boundary conditions of classical CA method,the growth rule of aperiodic boundary conditions perpendicular to the direction of heat dissipation was proposed.Based on the MATLAB software platform,a simulation program for formation and growth of columnar crystal structure was developed.And the formation and growth of columnar crystals in Mn18Cr18N hollow ESR ingot macro-scale and mesoscopic scale were simulated.Experimental comparison shows the validity and accuracy of the simulation rules of columnar crystal nucleus and growing by CA during electroslag remelting and solidification.Then,using molecular dynamics simulation method,the structure model of austenitic face-centered cubic FeCrNi steels nano-columns with different loading directions was established,and the mechanical behavior,deformation mechanism and law of deformation were simulated and analyzed,and the effect of size effect on the deformation of nano-columns were studied The results show that the deformation stress of austenitic decent-centered cubic nano-column specimens is anisotropic.The maximum stress was found at the angle between the loading direction and the columnar crystal growth direction a=0°,followed by a=90?,30?,60°,45?,that is,and the minimum stress was found at the angle between the loading direction and the columnar crystal growth direction a=45°.The reason is that the internal dislocation density of columnar crystals in different loading directions was different,and the smaller the cylindrical radius,the higher the dislocation density in the compression sample model,the higher the number of dislocations and twins,and the higher the deformation stress.Hot deformation behavior and dynamic recrystallization evolution mechanism and law of Mn18Cr18N forged steel were studied by thermal modelling experiments.A constitutive model based on physical parameters of internal variables was established.The nucleation mechanism and evolution law driven by grain boundary migration at low strain rate,and the nucleation mechanism and evolution law promoted by twinning at high strain rate were revealed by establishing the hot deformation power dissipation diagram and the analysis of recrystallization microstructure.Based on this,a CA Fractal rules method was proposed to simulate the nucleation rule of hot deformation recrystallization and the grain growth.For grain boundary migration-driven recrystallization,nucleation and evolution rules based on dislocation density were proposed.The simulation of hot deformation and recrystallization behavior of Mn18Cr18N forged steel was realized by introducing weighted variables by considering?3 twin nucleation rate for dislocation-promoted recrystallization.Compared with experiments,the simulation results of recrystallized grain structure under different deformation temperature,strain rate and strain conditions verify the reliability of the simulation of the developed hot deformation recrystallization CA method.Hot deformation behavior and dynamic recrystallization evolution mechanism and law of Mn18Cr18N ESR steel were studied by thermal modelling experiments.A constitutive model based on physical parameters of internal variables was established.The mechanism and evolution law of recrystallization controlled by dislocation at low strain rate and twin controlled at high strain rate were further clarified by establishing the thermal deformation power dissipation diagram and the analysis of recrystallization structure.Based on the comparison and analysis of the microstructure of hot-deformed recrystallized wrought steel,a CA method for simulating the nucleation of primary columnar crystal boundary and secondary dendrite/deformation zone was proposed.On this basis,by comparing and analyzing the microstructure of hot deformation recrystallization of forged steel,the CA method of primary columnar crystal boundary and secondary dendrite/deformation band crystal nucleation was proposed to simulate the joint nucleation rule.And dislocation controlled recrystallization CA method simulated by dislocation controlled recrystallization CA method was also proposed.The evolution process of dynamic recrystallization controlled by columnar crystal dislocation was simulated for the first time.At last,on the basis of characteristics of column solidification structure and its hot deformation recrystallization of Mn18Cr18N ESR steel,a macro-micro multi-scale model of columnar solidification structure was established through the correspondence between finite element mesh and CA mesh.The coupling simulation of finite element macroscopic thermal parameter field-CA columnar solidification structure field-hot deformation recrystallization grain structure was realized.And coupling simulation analysis and prediction with point tracking technology were carried out.