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.