Lager forgings usually used to significant carrier and large-scale structural components. With the rapid development of our country economy, electric power consumption growth with each passing day. Nuclear power as a kind of economic, clean, reliable substitute energy has been recognized in the world. Lager forgings as significant structural components used in nuclear power equipment, the demand will be getting bigger and bigger. But many lager forgings used in nuclear power depend on importation, for researches of lager forgings begin late in our country, the capacity of equipment that manufacture lager forgings can’t meet the requirement. In this way, not only the cost is very high, but also be controlled by foreigners, even terribly delayed the development of our country nuclear power. Therefore, improving manufacturing capability of lager forgings will be one of very urgent and important missions to the development of our country nuclear power, which has important significance.

　　In this paper, we investigated the flow stress curve of 20MnNiMo at different deformation conditions, the steel used in lager nuclear power head. Combined with methods of mathematical analysis, established flow stress models in high temperature and microstructure evolution models, which can precisely indicate material’s thermoplastic deforming behavior. Meanwhile, the workability of 20MnNiMo in different deformation areas is researched based on dynamic material model. On the basis of research, we show the numerical simulation that combined lager nuclear power head forging deformation and microstructure evolution, which will provide reference value and spot guidance for practical production of lager nuclear power head.

　　The true stress-strain curves of 20MnNiMo steel samples at different deformation conditions have been obtained by high temperature compression tests on the Gleeble-1500 thermal simulation machine. Two different flow stress models have been built on the basis of Sellars model, the parameters variation Arrhenius and Laasraoui with two segment flow stress models. Compared the models, we found that, the curve calculated from the model with two segment agree well with those of experiments. The maximum relative error is less than 10%. That is to say, the model can truly reflect the flow stress of 20MnNiMo at high temperature. Based on the processing map theory of the dynamic materials model, using 20MnNiMo steel’s true stress-strain data as the basic material model to calculate the strain rate sensitivity, the energy dissipation efficiency, the instability criterion. Finally, the thermal processing map including strain of 20MnNiMo is drawed out. The results show that: with the strain increase, the thermal processing security region increase too. It illustrate that when the strain is large, the deformation temperature and strain rate won’t occur flow instabilities in wide range. So the heating times decrease, the productivity improve. In the end, the DEFORM-3D numerical simulation software was used to render lager nuclear power integrated head gradually rotating cycle forging forming process.

　　The results show that: when using small deformation gradually rotating forming process, forgings deform uniformly during the whole cyclic process of forging, the inner spherical of the forgings is in a state of three-direction compressed stress, the region that occurs the full dynamic recrystallization can up to 95.4% account for whole forgings. The average grain size belows 10m at the inner spherical of the forgings, which can basically reach the requirement of grain size of the product’s microstructure. According to the actual production test, the integral forming condition of forgings is well, shape and size can reach the basic drawing requirement, the surface crack or folding defect not be appeared, uniform deformation within forgings, microstructure and property uniform, which can reach each technical indicators of lager nuclear power head.