Abstract：The microstructure of 9Ni steel samples with different impact properties was compared and analyzed. The structure, non-metallic inclusion, austenite grain, low-magnification dendritic crystals and the amount of retained austenite were observed and investigated by scanning electron microscope, metallographic microscope, electronic probe and scanning electron microscope-electron backscatter diffraction (EBSD). The research results indicated that: the fracture of sample with low impact property showed brittle cleavage fracture, the low-magnification dendritic crystal morphology figure indicated columnar crystal was developed, and the austenite grains were bulky and non-uniform. However, the fracture of sample with high impact property showed ductile fracture, the crystalline regions could be well differentiated, and the austenite grains were fine and uniform. The grade of inclusions in two groups of samples had no significant difference, indicating that the difference of impact properties between samples was due to the over-developed columnar crystal caused by improper continuous casting process. The nickel-rich segregated band was formed between as-cast dendritic crystals. The segregation band of nickel could not be fully removed in the following thermal processing and be passed down, resulting in the micro-segregation of nickel element in steel plate. As a result, the impact property of 9Ni steel was reduced due to the non-uniform structure.
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