Modelling And Simulation Of Microstructure Evolution Of 300M Steel Heavy Forging In The Whole Process Of Die Forging

As a low alloyed ultra-high strength steel,the 300 M steel is widely used in manufacturing of the large structural parts in the field of aviation,aerospace,ship,powerplant,and so on.To ensure high service performance,these parts are usually hot forged.But the hot forging process is very complecated,and repeated heatings,forgings,and coolings are needed,resulting in the difficulty in the microstructure controlling.Therefore,we focus on the die forging process of 300 M steel,and the microstructure evolutions of 300 M steel in the whole process of die forging is systematically and deeply investigated,which lays a foundation for the shape and property controlling of 300 M steel parts.Firstly,the grain growth behaviors in heatings and holdings were investigated by insitu experiments,and the influences of initial grain size,initial structure,holding temperature,and holding time on grain growth of 300 M steel was investigated experimentally.Resluts showed that the grain refinement by austenization of 300 M steel was followed by a dramatic grain growth in the initial stage of holding(? ~600 s),and with increasing time(~600-7200 s)the average grain size appeared to have a limit value at specific temperatures.The mechanisms of grain growth were further discussed based on experiment results.A grain evolution model considering the grain boundary migration was established.Secondly,in order to reveal the microstructure evolution law of 300 M steel in thermal compression process,thermal compressions were carried out on Gleeble 3500.A new flow stress model considering the complex interactions between dislocation motion,dynamic recrystallization,grain size evolution,and flow stress was proposed based on the KocksMecking model.The model could accurately describe the flow behaviors in dynamic recrystallization,and it was unified both in the work-hardening stage and in the dynamic recrystallization stage.Thirdly,in order to reveal the microstructure evolution law of 300 M steel in the cooling process,a new experimental method to investigate the continuous cooling behaviors was proposed,and the continuous cooling transition behaviors of 300 M steel was investigated by in situ observation at various cooling rates.Results showed that the pearlite formed in a way that the newly formed lamella densified the lamella formed at higher temperatures,which can be explained by the carbon diffusion under the driving force of the reduction of free energy in super cooled austenite.The cooling rate ranges of pearlite,bainite and martensite were determined.The quantitative relationship between the Vickers hardness,retained austenite content,and cooling rate was modelled.The CCT map and the microstructure transformation model of bainite was constructed based on in situ results.Based on the above results,in order to apply the established model to the actual die forging process,the local temperature control method of large forgings was investigated.Starting from the heating experiment of a small cylinder,the finite element model parameters for temperature field simulation were calibrated.A local temperature control method of large forgings was proposed.The temperature field of the landing gear was simulated under local temperature controlling of the rod by asbestos,and the temperature curves of 4 points were obtained.The temperature curves of 3 points was selected to import into the high temperature laser confocal microscope,and the microstructure evolutions of these points was in situ observed.Results showed that the local temperature control method was adaptive in controlling the temperature and microstructure distribution of large forgings,for example,the aircraft landing gear.Finally,in order to accurately predict the microstructure evolutions in the whole die forging process,we have integrated the established models in the Deform platform,and the microstructure simulations of aircraft landing gearing in the whole die forging process was realized.The FEM simulation results showed that the material flow was reasonable,the materials were not folded,and the microstructure distribution was reasonable.The test results also showed that the deviation of the grain size prediction was 8.5%.Therefore,this system was accurate in prediction of the microstructure evolution of the 300 M steel aircraft landing gear in the whole process die forging,and this investigation has laid a foundation for the forging process optimization of 300 M steel parts.