Ultrafine Grains Of 09MnNiD Steel Produced By Warm Rolling Of Martensite And Annealing And Their Thermal Stability

In this paper, the ultra fine grained and the nano-grained steel sheets were prepared by a new process of severe multi-pass warm rolling(accumulated reduction of 70 %) and low temperature annealing of 09MnNiD steel with starting structure of lath martensite. And the effects of the annealing temperature and duracing time on the microstructure of the severe warm-rolled samples of lath martensite were studied by using optical microscopy(OM), transmission electron microscopy(TEM), X-ray diffraction (XRD), Scanning electron microscopy(SEM), and the mechanical property were studied by microhardness test and tensile tests.The microstructure observation indicates that, the results of XRD scheer method are in good agreement with that of TEM dark field technique. And, by adjusting annealing temperature and duracing time, the ultra fine grained and the nano-grained steel sheets with average grain size from 20 nm to 1.8μm were obtained.The tests of microhardness and stretch at the room temperature show that the annealing temperature and the duracing time have limited effect on the specimen annealing at 300-400℃, and notable effect while annealing at 400-600℃. SEM observation of the tensile fracture indicated that, when annealed below 550℃, tensile fracture showed delamination, when annealed at 600℃, the delamination character disappeared. Then recrystalization dynamics of warm-rolled lath martensite was studied by the method of remained work hardening. In addition, the mechanisms of the refining and the thermal stablity of the ultra fine grains and the nano-grains was also discussed. And it shows that the major factor of the recrystal grains growth inhibition is the drag-like effect of the growth of the precipitated carbide .