As-cast Microstructure And Hot Deformation Behaviors Of Modified Ultra High Nitrogen Austenitic Steel For Retaining Rings

High nitrogen CrMn austenitic steels such as P900（18Mn18Cr0.5N） and P2000（16Cr13Mn3Mo0.9N） are widely used in manufacturing of large generator rotor retaining ring. Free surface cracking, grain coarsening and mixed grain structure are usually formed,which significantly restrain the development of the retaining rings. The properties of steels can be improved through adding microalloying elements to form carbide/nitride during cast, heat treatment and hot deformation. However, whether the effect of hot processing,corrosion resistance and mechanical properties on microalloying to high nitrogen steels is as same as low alloying steels is not clear.In this thesis, the pseudo-binary equilibrium phase diagram, the mole fraction of each phase and elemental composition of the alloy at different temperatures for a ultra-high nitrogen austenitic steels containing Nb and V were studied by the use of thermodynamic calculation. The as-cast microstructure characteristics and microstructure evolution during hot deformation of the steel were examined by using differential scanning calorimetry analysis（DSC）, optical microscopy（OM）, X-ray diffraction（XRD）, scanning electron microscopy（SEM）, energy diffraction spectrum（EDS） as well as electrolytic extraction,etc. The main research results are as follows:The pseudo binary equilibrium phase diagram of the high nitrogen austenitic steel containing Nb and V was obtained, the liquidus and solidus temperature are 1440℃ and1310℃, respectively; MX-type carbonitride starts to precipitate from liquid, δ ferrite and austenite at 1440℃, 1310℃�'� 1250℃, respectively; Cr₂N、σ and M₂₃C₆ start to precipitate at 1080℃, 931℃ and 907℃, respectively. The equilibrium microstructures of the steel at room temperature are the mixture of(γ+MX+ε+M₂₃C₆+σ). The measured precipitation temperature of δ ferrite is 1265℃, 25℃ higher than the result of calculated value; the austenite begins to dissolve at 1345℃, 25℃lower than the result of calculated value. The liquidus temperature and the measured values of MX type precipitate are consistent with the theoretically calculated values.The relationship between temperature and equilibrium phase mole fraction of the steel, and the alloying element composition in each phase under equilibrium conditions were obtained also.The as-cast microstructure of the high nitrogen austenitic steel containing Nb and V consists of austenite and（Nb,V）N.（Nb,V）N is distributed in austenite grain boundaries and the junction of three grain boundaries with irregular strip and flocculent mass, and dispersed in the intracrystalline with spherical, square or triangle.The hot deformation equation of the steel at 1000 1200℃ and 0.001 10s_-1 can be expressed ε=3.34×10²²[sinh（0.00573σ_ρ）]^4.79exp（-631000/RT）. In the modified steel, the quantitative relationship between peak stress and Z parameter is:σ_p=17.5lnZ-612.8（MPa）.The dynamic recrystallization（DRX） during the hot deformation of the tested steel mainly consists of classical DRX and necklaces DRX. The necklaces DRX is more likely to happen with high strain rate. The classic DRX is more likely to happen and the DRX grain is bigger with the higher temperature and the lower strain rate. The relationship between deformation condition and DRX grain size was given as:D=2.164×（Z/A）^-0.081（μm）. The primary instability of the ultra-high nitrogen steel during hot deformation is local instability.Many small particles（Nb,V）N were observed at grain boundaries and in the deformed matrix of the ultra-high nitrogen austenitic steel. The intragranular precipitate phase presents round ellipse with the size of 0.22 μm, and the intergranular precipitates are square or irregular with the size of 0.52 μm.The dynamic microstructure state diagram and thermal processing maps at different strain of the steel were obtained. Power dissipation rate increases with the decrease of strain rate and the increase of temperature. When the true strain is 0.8, deformation temperature is 1200℃, strain rate is 0.001s_-1, the power dissipation reaches the maximum percentage of 60%.When reheating temperature of the ultra-high nitrogen austenitic steel containing V and Nb is 1200℃, the suggested hot working window is 1050 1150℃ and 0.01 1 s_-1.