Study On Deformation Behavior Of High Nitrogen Austenitic Steel

The high nitrogen austenitic steel is a new type of high speed impact resistance steel with high strain hardening capacity,which is different from traditional martensitic steels.However,the deformation behavior of high nitrogen austenitic steels under different conditions has not been sufficiently clarified.In this work,the deformation behavior of Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel under different conditions of deformation temperatures and strain rates is studied by optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscopy?TEM?,electron back scattering diffraction?EBSD?,and X-ray diffraction?XRD?.Results on the hot deformation behavior of the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel indicate that the experimental steel has high deformation resistance due to strong hindering effect on dislocation moving by nitrogen-induced lattice misfit.The twinning induced plasticity?TWIP?effect is gradually suppressed with the increase of deformation temperature,and high temperature plastic deformation mechanism by twinning is gradually replaced by dislocation planar slip.The deformation resistance of the experimental steel is up to 343 MPa at the deformation temperture of 1000 ^oC and strain rate of 10 s^-1,which is over 100 MPa and 50 MPa higher than those in normal martensitic steels and austenitic steels,respectively.The hot deformation constitutive equation of the experimental steel has been established as ???.The optimum hot working process is obtained as the deformation temperature range of 950¹200 ^oC and strain rate range of 0.01¹0 s^-1,when the deformation resistance is lower than 325 MPa,the deformation activation energy is less than 662.6 k J·mol^-1,the power dissipation efficiency exceeds 0.22,the dynamic recrystallization?DRX?fraction is over 46.1%,and microstructures is without instable characteristics.The pre-deformation behavior of the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel indicates that the deformation mechanism changes with the increase of cold deformation reduction.At critical cold deformation reduction range from 26%to36%,the plastic deformation mechanism by dislocation planar slip and multiplication gradually transits to that by twinning.Grain fragmentation is the dominant deformation mechanism at high cold deformation reduction range from 72%to 87%.With the increase of cold deformation reduction,the strength of the experimental steel gradually increases,and the uniform elongation gradually decreases.Ultimate tensile strength?UST?and yield strength?YS?are increased to 1790 MPaand 1600 MPa,respectively.Deformation mechanism of high nitrogen austenitic steel is related on strain rate and deformation temperature.Dislocation planar slip and twining dominate plastic deformation in the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel during tensile process at strain rate range from 3×10^-6 s^-1 to 1 s^-1.Microstructural observations show that it is changed from dislocation planar slip and twinning to dynamic recovery?DRX?and dynamic recrystallization?DRV?at elevated temperature tensile range from 100 to 900?.Furthermore,the TWIP effct is gradually suppressed with the increase of strain rate and tensile temperature.The plastic deformation mechanism by dislocation planar slip and multiplication gradually transits to twinning during dynamic compression deformation at the strain rate range from 492 to 9843 s^-1 for the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel.Grain fragmentation dominates the plastic deformation when strain rate exceeds7635 s^-1.Meanwhile,the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel exhibits high dynamic strain hardening capacity.With the increase of strain rate,ultimate stress is gradually increased from 1230 MPa to 3360 MPa,the latter being nearly3times of the former.The ultimate stress shows linear rlationship to strain rate as ?_m=1012+0.4???.Results of ductile brittle transition behavior of the Fe-20Mn-19Cr-0.6N high nitrogen austenitic steel show that the ductile brittle transition temperature?DBTT?is determined as-53?(ETT₅₀)and-59?(FATT₅₀),and the upper shelf energy?USE?of impact toughness is as high as 399 J.With the decrease of impact test temperature,the fracture mode of the experimtal steel gradually transits from micro-void coalescence type fracture with entire dimples to cleavage-like fracture.The low-temperature brittle fracture mechanism of the experimental steel is confirmed as slipping off mechanism.