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Compact hot extrusion (CHE) process of heavy caliber thick-wall pipe is a new material-saving production process.In order to reveal the optimum hot extrusion parameters in CHE process,the effects of the extrusion parameters on the microstructural evolution are investigated systematically.The metadynamic recrystallization (MDRX) kinetic models and grain size models of as-cast P91 steel are established for the first time according to the hot compression tests performed on the Gleeble-3500 thermal-simulation machine.Then a thermal-mechanical and micro-macro coupled hot extrusion finite element (FE) model is established and further developed in DEFORM software.The results indicated that the grain size of the extruded pipe increases with the increasing of initial temperature and extrusion speed,decreases when extrusion ratio increases.Moreover,the grain size is more sensitive to the initial temperature and the extrusion ratio.The optimum hot extrusion parameters are including that,the initial extrusion temperature of 1250 ℃,the extrusion ratio of 9 and the extrusion speed of 50 mm/s.Furthermore,in order to verifythe simulation precisions,hot extrusion experiment verification on the heavy caliber thick-wall pipe is carried out on the 500 MN vertical hot extrusion equipment.The load-displacement curve of the extrusion process and the grain sizes of the middle part extruded pipe are in good accuracy with the simulation results,which confirms that the hot extrusion FE models of as-cast P91 steel could estimate the hot extrusion behaviors.The proposed hot extrusion FE model can be used to guide the industrial production research of CHE process.