Abstract：The fracture morphology observation and composition analysis of head and tail hole-expansion samples from hot rolled FB780 high hole-expansion steel were performed by scanning electron microscope (SEM) and energy dispersive spectrum (EDS). The results showed that, as the preferential nucleation sites of microvoid, the inclusions were disadvantageous to the hole-expansion property of material. INCA Feature inclusion automatic analysis method was used for the statistics of composition, dimension and area of sectional inclusions in two samples. The results indicated that both the dimension and area fraction of inclusions in head sample were higher than those in tail sample due to the difference of cast process. These inclusions were the crack initiation source in hole-expansion process, thus influencing the hole-expansion property of material. The distribution statistics of ferrite phase and bainite phase in two samples were conducted by electron back scattering diffraction (EBSD) technique. The results showed that the content of ferrite in tail sample was higher and the structure distribution was more uniform. Therefore, the plasticity, toughness and hole-expansion property of material were improved.
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