Coarse Grain Refinement Mechanism During Heavy Forgings Forming Process

The deformation mechanism of the microstructure during forging has an important influence on the forming precision and the control of mechanical properties.Heavy foging is a process consisting of multi-pass and multi-step,so the deformation mechanism of grain refinement is affected by the deformation temperature,stain rate,stain,initial austenite grain size and interpass time.It is highly necessary to research the effect of the deformation condition and the initial microstructure on the mechanism of the grain refinement.In this research,the 30Cr2Ni4MoV steel sampled from 600t ingot and the 25CrMo4 steel taken from the mirror plate billet are compressed at high temperature.Combining with the microstructure observation,the grain refinement mechanism can supply the theory basis for the processing control of the heavy forging.Following is the main research content:The coarse grain structure of the 30Cr2Ni4MoV steel at room termperature,which is composed of dendrite and mixed grain,is investigated.The single compression deformation is carried out in the Gleeble-1500 thermo-mechanical simulator and the flow curves under different defomation temperatures and strain rates are obtained.The dynamic recrystallization?DRX?behavior of the 30Cr2Ni4MoV steel and 25CrMo4steel is investigated.Based on the experimental flow curves and the microstructure,the DRX kinetics model and DRX grain model are established.With the help of the DRX kinetics model,the flow stress is expressed as a function of deformation temperature,strain rate and stain.The double hit compression deformation of 25CrMo4 steel is carried out in the Gleeble-1500 thermo-mechanical simulator and the flow curves under different deformation conditions and interpass times are obtained.Based on the flow curves,the volume fraction of the static recrystallization?SRX?and metadynamic recrystallization?MDRX?is calculated.The effects of the deformation temperature,strain rate,strain and initial grain size on the recrystallization rate and microstructure of the SRX and MDRX are discussed.Furthermore,the kinetics models of SRX and MDRX are constructed and the predicted results agree well with the experimental results.Based on the obtained flow curves,the power dissipation maps of the30Cr2Ni4MoV steel and 25CrMo4 steel at different strains are developed and the effect of the strain on the efficiency of power dissipation is discussed in detail.The processing maps at different strains are obtained by superimposing the instability maps on the power dissipation maps.When the strain is 0.7,according to the processing map and the metallographic observation,the optimum domain of hot deformation for 30Cr2Ni4MoV steel is in the temperature range of 950¹200?C and strain rate range of 0.03⁰.5s^-1,with a peak efficiency of 0.41 at 1100?C and 0.25s^-1 which are the optimum hot working parameters.For the 25CrMo4 steel,the domain occurs at the temperature in the range1100¹200?C and strain rate in the range 0.03⁰.5s^-1 is recommended for the blooming process.In view of the control of the grain size for the final product,the optimum parameters for the finish forging process should be located at the temperature in the range 1000¹100?C and strain rate in the range 0.03⁰.5s^-1.