Grain Revolution Study And Numerical Simulation Of A Large Section Plastic Mold Steel During Free Forging

In order to realize the controllability,microstructure prediction and technology optimization of a large section plastic mold steel during free forging,the plastic deformation behaviors of SDP1 steel at high temperature are studied by experiments,constitution analysis and computer simulation in this thesis.The influence of deformation conditions on recrystallization is investigated and the material data of recrystallization kinetics is obtained by regression analysis.The equal proportion FEM and CA model are established to research the influence of deformation conditions on free forging and heat treatment.The technology optimization is provided and the results show that:(1)The peak stress in flow stress curve is higher at lower temperature or higher strain rate.The strain at a peak stress tend to decrease with the increase of temperature.The peak strains and DRX grain size under different deformation conditions can be given by equation ?p=0.0011Z0.22 and DDRX=21400Z-0.26 respectively.The relationship between temperatures,stress and strain rate can be predicted by a hyperbolic-sine constitutive law,A[sinh(??)]n ?(?)exp(Qact/RT),with Qact=257 kJ/mol,?=0.0094MPa-1 and n=7.51;(2)To research the recrystallization behavior during interval time of hot deformation,double-pass compression and grain growth tests were carried out by thermal mechanical simulator.The Avrami equations for describing the dynamical behavior of static,metadynamic recrystallization and grain growth were established.The activation energy,strain factor and strain rate factor of SRX are Q=152 kJ/mol,n=-1.27 and m=-0.25,respectively.The activation energy,coefficient of deformation and strain rate factor of MRX are Q=126 kJ/mol,?=1.22e-6 and m=-0.41,respectively.The activation energy,coefficient of deformation and strain rate factor of grain growth are Q=800 kJ/mol,Ag=6.03 and m=-0.41.(3)The FEM model of hot compression test were established to testify the accuracy of SDP1 material data.An equal proportion FEM model of free forging of large section mold steel,which concluding upsetting,drawing out and dressing,is built to get the stress-strain and grain evolution data.Based on the simulated results,the author provided the optimize technology:upsetting temperature is 1100 ?,compression reduction is 50%,drawing out is 1050 ?>compression reduction is 30%and feed value is 50%.(4)A DRX-CA model is employed to simulate the deformation process and to provide necessary data for the analysis of recrystallization kinetics.In comparison with the peak stresses and peak strains in the flow curves,the simulated results agree reasonably well with the experimental findings,which demonstrate that the current CA model captures the main features of flow behavior during compression.Once the dislocation density in the primary grains reaches a critical value,dynamic recrystallization occurs.As a result,the average size of all grains decreases rapidly and then tends slowly to a steady value.At the same time,the average size of primary grains decreases linearly until all the primary grains are consumed and the average size of recrystallized grains increases slowly to a peak value and then decreases to a steady value.(5)CCT curves of three kinds of grain size were drew by using thermal dilatometer and the influence of grain size to microstructure and hardness was analyzed.It can be discovered that the hardness of smaller grain specimen is less than bigger grain's under the lowest cooling rate but opposite under higher cooling rate.The hardness uniformity of SDP1 steel improved with grain size increasing.(6)The functional(corrosion resistance,polishing)and machining properties of two kinds of grain size specimen were contrasted.The machining properties,corrosion resistance and etching of smaller grain specimen is obviously superior to bigger ones.