Numerical Simulation Of Deformation And Microstructure Evolution During Forging And Rolling Process

The steels,as typical structural materials,play very important roles in the national economy,and thus they have been widely used in energy,transportation,construction,aerospace and national defense industries.With deep applications of steels,higher mechanical properties are required,especially for special steels with thick sections,a higher requirement has been put forward to the microstructure uniformity.However,contrastive studies of different thermal deformation patterns on mechanical performance,based on the calculation method of macroscopic,mesoscopic and multi-scale coupling,and the experimental characterization,are rarely involved.In particular,for the structural parts with super thick section or large rolling ratio,the thermal deformation pattern during hot-working process has a direct influence on the evolution behavior of microstructures,which ultimately affects the comprehensive mechanical properties of steel products.Therefore,it is very important to study the influences of thermal deformation patterns on the evolution behavior of microstructures.This paper is aimed at ordinary low carbon steel,and studied the microstructure modeling of steel components in forging and rolling process.Based on the equipment of 125MN hydraulic press,the influences of thermal deformation mode on the evolution behavior of microstructure have been studied by employing the macro-finite element method,the meso-cellular automaton method and macro-mesoscopic multi-scale coupling method,combined with physical full section analysis and experimental characterizations as well as two kinds of processing technology of forging and rolling.The main research contents are listed as follows:(1)Based on the macroscopic finite element method,a finite element calculation model for forging process of steels was constructed.Taking low carbon steels as research materials,the forging process was simulated by using the general finite element software ABAQUS combined with the actual forging process.The evolution histories of physical quantities,such as the temperature field,the strain field,and the strain rate field,were obtained by simulations,which can be used as a reference to study the evolution behavior of microstructures.Employing macroscopic finite element calculations and an empirical physical metallurgy model,the microstructure evolutions during forging are acquired.Meanwhile,large forgings of 6500mm×1200mm were dissected.The low magnification microstructure of large forgings section were obtained by using multi-stage polishing technology and acid detection method of large cross section which were developed in this study in order to face the large cross-section part.The microstructure of typical location of large forgings section was also obtained by using a detailed dissection method.By comparing the experimental results(the grain size of the forgings was 18?m and the grain size of the core was 26pm)and simulation results(the grain size of the forgings was 19?m and the grain size of the core was 25?m),it was found that the physical metallurgy model of mesoscale microstructure evolution built in this thesis can be used to predict the microstructure evolution results of large forgings after multi-pass forging process.Since the plastic strain rate is small and the absolute reduction amount of single pass deformation is large in forging process,it is beneficial to the closure of micro-defects.In addition,since the cumulative plastic strain is large and the deformation temperature is high during forging process,it is beneficial to the grain recrystallization of microstructure.The grains of micrcstructure could be refined after the recrystallization process.(2)A meso-cellular automaton model of austenite dynamic recrystallization and phase transformation was built to study the microstructure evolution behavior of low carbon steel during hot-working process,which is based on a series of physical metallurgical transition processes,such as,thermal deformation,dislocation density evolution,grain deformation,nucleation,growth and coarsen of grain and so on.The proposed new model outperforms the empirical physical metallurgy model since the meso-cellular automaton model is able to describe the microstructure evolution of low carbon steel in thermal deformation starting from the transformation process of microstructure.Using this model,the dynamic recrystallization behavior under different strain rate(0.01s-1,0.1 s-1,1s-1)and different initial grain sizes(20?m,40?m,80?m)and the phase transformation behaviors under different pre-deformations(0,0.3,0.5,0.7,1)were simulated.The ability of meso-cellular automaton model to predict the typical dynamic recrystallization and phase transformation has been verified by comparing the experimental results and calculated results.The simulation result is as follows:when ?=0.01s-1,the steady grain size is 4 ?m.and when ?=1s-1,the steady grain size is 9?m.Although the initial grain size is different,the steady grain size is same.(3)A multi-scale simulation and prediction method of microstructure evolution considering the macroscopic thermal deformation was presented,which can be used to calculate the grain evolution behavior in the whole thermal deformation process by using the meso-cellular automaton model.Moreover,to describe the non-continuous thermal deformation process an integrated cellular automation of microstructure evolution,including the dynamic recrystallization process,the sub-dynamic recrystallization process and the static recrystallization process was built.This model used the dislocation density as the basic variable to simulate the microstructure evolution during complex recrystallization process by tracking the variation of intracellular dislocation density during non-continuous thermal deformation process.Also,this model has been used to simulate the microstructure evolution process during multi-pass rolling.In this paper,a 1780mm hot rolling mill is used as the research object,and the microstructure evolution of multi-channel strip hot rolling process is simulated by using this model.Compared to the macroscopic empirical physical metallurgy model,the multi-scale calculation method of microstructure evolution during hot working process proposed in this thesis could more clearly reflect the interaction process of each physical metallurgy phenomenon and microstructure evolution characteristics.It is shown that the austenite recrystallization process is affected by the rolling parameters,such as strain rate,strain and deformation temperature and so on.In multi-pass hot rolling process,the microstructure,including recrystallization fraction and average grain size,are dependent on the specific locations since different position in plate experience different rolling history during multi pass hot rolling process.The recrystallization behavior of grain could occur completely after the first two passes and the recrystallization fraction is 1.However,it could not occur completely in each remaining pass because the temperature decreases and the strain rate increases,such as the result of the 7th pass recrystallization was 0.2-0.4.The initial grain size is 190 ?m,and the grain is refined due to the repeated grain recrystallization,and the grain size in surface(17?m)is smaller than that in center(30?m).The simulation results could provide for the optimization of the hot working process.