Macro-micro Coupled Modelling And Simulation Of The 300M Steel Big Part In The Whole Die Forging Process

As a low alloyed ultra-high strength steel,the 300 M steel is widely used in manufacturing of large structural parts in the field of aviation,aerospace,shipping,powerplant,and so on.To ensure high service performance,these parts are usually hot forged.But the hot forging process is very complicated,and repeated heatings,forgings,and coolings are needed,resulting in the difficulty of microstructure controlling.Therefore,we focus on the die forging process of 300 M steel,and the microstructure evolution of300 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 behavior in heating was investigated by in-situ experiments,and the influences of initial grain size,initial structure,holding temperature,and holding time on grain growth of 300 M steel were investigated experimentally.Results 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.Thegrain growth mechanism was discussed based on experiment results.A grain evolution model considering the grain boundary migration was established.Secondly,to reveal the microstructure evolution law of 300 M steel in the 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 Kocks-Mecking 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 of 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 cylinder with the diameter of 75 mm,the model parameters for temperature 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 4points were obtained.The temperature curves of these points was selected to import into the high temperature laser confocal microscope,and the microstructure evolution 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 evolution in the whole die forging process,we have integrated the established models in the Deform platform,and the microstructure simulations of aircraft landing gear in the whole die forging processwas realized.The FEM simulation results showed that the material flow was reasonable,the materials were not folded,and the microstructure distribution was acceptable.The test results also showed that the deviation of the predicted value of average grain size was8.5%.Therefore,the simulation system was accurate for the die forging process,which has laid a foundation for the forging process optimization of 300 M steel parts.