Effects of Nitrogen Content and Hot Forming Temperature on Prior Austenite Grain Size and Mechanical Properties for Normalizing Cryogenic Pressure-Vessel Steels

摘要

The effects of nitrogen content and hot forming temperature on prior austenite grain size for normalizing cryogenic pressure-vessel steels were described. The evolution of V(C,N) precipitates during thermal cycle of hot forming was analyzed. The optical microscopy displays that as increasing N content from 0.0094 to 0.0198 wt.% the refinement of prior austenite grain was seen at each hot forming temperature of 870, 910 and 950 degrees C. The transmission electron microscopy presents that much finer V(C,N) precipitates were found in sample with 0.0198 wt.% N content than in sample with 0.0094 wt.% N content. Thermo-Calc calculation reveals the precipitation temperature of V(C,N) was increased by increasing N content. While the Ostwald ripening rate of V(C,N) was decreased by increasing N content. TEM result and Thermo-Calc calculation indicate that increasing N content enhanced the thermal stability of V(C,N) precipitates. The stable finer V(C,N) precipitates contribute to the finer prior austenite grain and good mechanical properties for sample with 0.0198 wt.% N content even at higher temperature of 950 degrees C during hot forming process.