Study On Constitutive Modeling Of Low Activation Martensitic Steel Based On Phase Composition

Low activation martensitic steel is considered as the preferred structural material for the blanket of fusion demonstration reactor and fusion power reactor in the future due to its good geometric dimensional stability,low irradiated swelling rate and thermal expansion coefficient,high thermal conductivity and excellent mechanical properties.As one of the effective methods to improve the comprehensive properties of materials,the large plastic deformation technology is applied to low activation martensitic steel to obtain the ultra-fine grain structure,so as to improve the properties and extend the service life effectively in the harsh working environment of the fusion reactor.In order to control the microstructure and properties of materials accurately in the process of plastic deformation,simulation research method is usually used to optimize the deformation parameters,among which the first task is to establish a reliable constitutive model.For the target material,high temperature forming is usually used because the low plasticity limits the room temperature forming ability.In the process of thermal deformation,the phase composition is easy to change due to the small phase transition temperature range,and then the plastic rheological behavior is affected.Therefore,a unified constitutive equation of low activation martensitic steel is established in this paper on the premise of considering the phase composition.The main research contents and results are as follows:The thermal compression simulation experiment of low activation martensitic steel was carried out at temperatures ranging from 600?to 950?and strain rates ranging from 0.001s^-1 to 1s^-1.The rheological curves under various deformation conditions were obtained,and plastic rheological behavior,the effect of thermal deformation parameters on the mechanical response was studied.The flow stress tends to be constant after reaching the peak at high temperature(850-950?)and high strain rate(1s^-1),while decreases to varying degrees at low temperature(600-800?),high temperature and low strain rate(0.001-0.1s^-1).At the same strain rate,the peak stress decreases basically with the increase of temperature,and the steady-state stress also shows the same trend of change in the low temperature region(600-800?)and the high temperature region(900-950?).However,the steady-state stress in the intercritical region(850?)presents an abnormal phenomenon,that is,the steady-state stress at 850?is greater than 800?.At the same deformation temperature,the peak stress and steady-state stress increase with the increase of strain rate.The microstructure after thermal compression deformation and quenching at the same temperature of low activation martensitic steel were characterized.The phase composition before deformation was obtained by analyzing the microstructure of different quenching temperatures.The deformation mechanism was revealed by comparing the deformed microstructure with the quenched microstructure,and the influence of thermal deformation parameters on the microstructure evolution behavior was analyzed.The results show that the experimental material is single ferrite phase in the temperature range of 600-800?,ferrite phase and austenite phase coexist at 850?,and is austenite single phase in the range of 900-950?.Dynamic recrystallization occurred under all the experimental conditions,but the degree of the process was different.Partial dynamic recrystallization of ferrite phase occurred in the low temperature region,partial dynamic recrystallization of ferrite and austenite occurred simultaneously and the dynamic recrystallization characteristics of ferrite was more obvious in the intercritical temperature region,and partial or complete dynamic recrystallization of austenite occurred in the high temperature region.The degree of dynamic recrystallization increases with the increase of deformation temperature and the decrease of strain rate,and the volume fraction and grain size of recrystallization also increase.Firstly,a single-phase constitutive equation was established based on the effect of microstructure evolution mechanism and macroscopic deformation parameters on the mechanical behavior of low activation martensitic steel.And then,a constitutive equation suitable for the thermal deformation of dual phase in the intercritical temperature region was established by introducing a correction coefficient W of strain compensation to modify the equal strain mixture law.Finally,based on the phase composition of different temperature ranges,a unified constitutive equation was proposed to reflect the flow characteristics of each stage.The reliability of the model was verified by comparing the predicted stress and the experimental stress under various deformation conditions.