Research On Hot Deformation Behavior And Forging Process Optimization Of 55NiCrMoV7 Large Die Steel Forgings

55NiCrMoV7 hot-working die steel is a new type of die steel.Because of its excellent thermal stability,dimensional stability and other comprehensive mechanical properties,it has attracted wide attention in the production of large-scale casting and forgings and has been widely used in aerospace,ship,nuclear power and other fields.However,55NiCrMoV7 hot-work die steel often faces various harsh working environments,such as impact,high pressure,quench and abrupt heat during actual production,especially in the production of forgings of large-size 55NiCrMoV7hot-work die steel.If the forging process of large-size 55NiCrMoV7 hot-work die steel is not designed properly during the forging process,segregation,segregation and abrasion will occur.Severe problems,such as coarse grain and internal hole defects in large forgings,among which hole defects will seriously affect the comprehensive performance of shaped forgings,destroy the continuity of internal metal structure of large forgings,and easily lead to stress concentration,cracks and low service life.In the process of large forgings production,a large amount of economic and time cost is wasted.Therefore,by studying the high-temperature hot deformation behavior and microstructure evolution process of 55NiCrMoV7 hot-work die steel,a numerical simulation platform for closing defects of internal holes in forgings of large-scale die steel is established,which reveals the influence mechanism of forging process on large-size forgings,and further proves the evolution law of holes defects in forgings during forging process,and optimizes forging process,so as to reduce production costs.It is of great guiding significance to improve the comprehensive performance of large-size hot-working die steel and guide the actual production.The main contents and achievements of this paper are as follows:(1)The stress-strain curves of 55NiCrMoV7 hot-working die steel at 800?1200?and strain rate of 0.01?5s^-1 were obtained by thermal compression test,and the Arrhenius constitutive model of the material was established.The accuracy of the constitutive model was verified by introducing correlation coefficient and average relative error to compare the experimental and predicted values.Based on the power dissipation diagram and destabilization diagram,the thermal processing diagram of the material is constructed,and the optimum processing intervals are obtained as follows:temperature 875?900?,strain rate 0.01?0.135s^-1;Temperature 1045?1125?,strain rate 0.01?0.223s^-1;Temperature 1125?1175?,strain rate 0.01?1.65s^-1.(2)Observing the metallographic structure of 55NiCrMoV7 hot-working die steel,the initial grain size of the material is 40.1?m.The dynamic recrystallization model of the material was established and the accuracy of the microstructure evolution model was verified by comparing the finite element numerical simulation with the experimental results.(3)Based on the secondary development of numerical analysis software and hydrostatic stress criterion method,a numerical simulation platform for closure of internal void defects in large-scale die steel forgings was established.The accuracy of the simulation platform was verified by comparing experiments with simulation,and the evolution law of internal void defects in forgings was studied.The important factors influencing the closure of internal void defects in large forgings are obtained as follows:friction coefficient,strain rate,location of defects and ratio of height to diameter of defects.Simulated forging was carried out for large forgings,the main causes leading to fracture of large forgings were explored,and the existing forging process was optimized.