Effect of cementite dispersion on void formation process in spheroidize-annealed steels

The effect of cementite dispersion on ductile fracture was investigated by comparing void formations in different spheroidize-annealed steels. Two types of steels with different grain sizes and interparticle distances were used. Tensile tests of plate specimens were performed in longitudinal direction along the rolling direction. After the tensile tests, the fractured specimens were cut in the middle along the width. The cross sections near the fractured surface were observed using a scanning electron microscope. Voids in the microstructure were observed in five thickness reduction areas corresponding to different strain levels. As a result, the number density of voids increased as the strain increased in both steels. The number density of voids in steel containing large cementite grains was higher for all strain values, and the fraction of long voids in this steel was higher with greater applied high strain. Two types of voids were mainly observed in this study: One was decohesion of the interface between bainitic ferrite and cementite, while the other was cracking of the cementite itself. Void formation was evaluated using electron back scatter diffraction analysis with kernel average misorientation (KAM) as parameter. The obtained KAM values (representing misorientation in crystal grains) increased with increasing strain, and the areas with high KAM values were concentrated locally along cementite in the long spacing cementite steel. Therefore, it was revealed that void formation in steels with large cementite grains and long cementite spacings occurred more easily by the stress applied to cementite.