Advanced Characterization of Precipitation and Microstructure Heterogeneity in X70 Steel

The spatial distribution of precipitates and microstructure heterogeneity in a vacuum induction melted X70 steel rolled at a pilot-scale facility were investigated using advanced characterization techniques. Electron Backscatter Diffraction was used to identify areas with low and high Kernel Average Misorientation (KAM). A newly developed procedure utilizing a Plasma Focused Ion Beam microscope was used to lift-out plane view sections of small and large grains within the areas of low and high KAM values. Quantitative analysis by transmission electron microscopy (TEM) revealed a uniformly dispersed set of core-cap structure precipitates varying between 30 and 70 nm in diameter. The core was enriched in Ti and N, while the shell typically contained Nb and C. Strain-induced precipitation of fine (5 to 20 nm) NbC particles on dislocations was not observed by TEM and 3D atom probe tomography (APT). The absence of strain-induced precipitation is believed to be due to the depletion of Nb from solid solution as the result of the core-cap structure of NbC on pre-existing TiN particles. The similarity of precipitates in each location suggests that the local features (strain, grain size) in the final microstructure arise from phase transformations during cooling.

采用先进的表征技术研究了在中试规模设备上轧制的真空感应熔炼X70钢中析出物的空间分布和微观结构的不均匀性。利用电子背散射衍射来识别具有低和高核平均取向差（KAM）的区域。采用一种新开发的利用等离子聚焦离子束显微镜的方法，在低和高KAM值区域内提取大小晶粒的平面视图切片。通过透射电子显微镜（TEM）进行的定量分析显示，一组均匀分散的核 - 帽结构析出物，其直径在30到70纳米之间变化。核心富含Ti和N，而外壳通常含有Nb和C。TEM和三维原子探针断层扫描（APT）未观察到位错上应变诱导析出的细小（5到20纳米）NbC颗粒。据信，应变诱导析出的缺失是由于预先存在的TiN颗粒上NbC的核 - 帽结构导致Nb从固溶体中贫化。每个位置析出物的相似性表明，最终微观结构中的局部特征（应变、晶粒尺寸）源于冷却过程中的相变。