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 °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 °C during hot forming process.

描述了氮含量和热成形温度对用于规范低温压力容器钢的先前奥氏体晶粒尺寸的影响。分析了热成型热循环过程中V（C，N）析出物的演变。光学显微镜显示，随着氮含量从0.0094增至0.0198 wt。％，在每个870、910和950°C的热成形温度下都可以观察到奥氏体晶粒的细化。透射电子显微镜显示，在氮含量为0.0198 wt。％的样品中发现的V（C，N）沉淀物比氮含量为0.0094 wt。％的样品要细得多。Thermo-Calc计算表明，氮含量的增加使V（C，N）的析出温度升高。通过增加氮含量可以降低V（C，N）的奥斯特瓦尔德熟化速率。TEM结果和Thermo-Calc计算表明，增加N含量可增强V（C，N）沉淀物的热稳定性。稳定的更细的V（C，N）析出物即使在热成型过程中甚至在950°C的较高温度下也能提供更细的先前奥氏体晶粒和良好的机械性能，而样品中的N含量为0.0198 wt。％。