Study On Hot Deformation Behavior And Microstructure Properties Of A V And Nb Added High Nitrogen CrMn Austenitic Stainless Steel

As a typical kind of high nitrogen CrMn austenitic steel,18Mn18CrO.5N has been widely used in large generator retaining ring. However, such retaining ring always have a problem of forging surface crack or mixed grain structure and other issues. For decades, the country has not made substantial breakthroughs in how to improve the yield of the steel retaining ring. The use of microalloying elements in the field of high-strength low-alloy steels and non-quenched and temperature steels is wide, but its application in the area of high-alloy steel such as high nitrogen CrMn austenitic steel is rarely reported.In this study, physical simulation of hot compression deformation is used. The mechanical behavior is study by the analysis of flow curves and the building of thermal deformation equation, and the processing map. The type and composition of precipitates is studied by using the XRD and EDS. The characteristic of microstructure has been analyzed by OM and SEM. The main results obtained are as follows:The thermal deformation equation of the V and Nb added high Nitrogen CrMn austenitic stainless steel in the temperature rang of800-1200℃and strain rate range of0.001-10s-1is ε=9.7×1025[sinh(0.013365σ)]4.06exp(-767000/RT). The relationships between Z value and peak stress is σp=17.01nZ-944.3(MPa).The microstructure diagram of testing steels is obtained and the processing map of tested steel under different strain is obtained. The maximum power dissipation is89%when the condition is1200℃and0.001s-1on the true strain of0.8. Power dissipation rate is reduced with increasing strain rate and temperature. The instability zones reduce to the low temperature and low strain rate zone as the increasing of strain. The instability zones is mainly concentrate on the low temperature, it shrink with increasing of temperature.The forms of dynamic recrystallization in the hot deformation of test steel are mainly include classical dynamic recrystallization and necklaces dynamic recrystallization. The classic dynamic recrystallization is more likely to happen and the dynamic recrystallization grain is bigger with the higher of the temperature and the lower of the strain rate. The quantitative relationship among the grain size of dynamic recrystallization, Z and A is obtained:D=3.34×(Z/A)-0.11(μm).The primary instability forms of the V and Nb added high Nitrogen CrMn austenitic stainless steel during hot deformation are surfacecracking and local instability.The precipitates existed in the V and Nb added high Nitrogen CrMn austeniticstainless steel is just Nb4N3.38. The size of precipitation in the steel before deformation isbetween1-9μm, content in0.68%or so, and about90%of it concentrated in4μm or lessafter the hot deformation.With the adding of micro-alloy elements, the apparent activation energy for hotdeformation of high Nitrogen CrMn austenitic stainless steel increase, and the hotdeformation instability region become bigger. The recrystallized grain size of V and Nbadded high nitrogen stainless steel is smaller in the corresponding deformation condition.