Effect of the state of carbon on ductility in Fe-0.017mass%C ferritic steel

The influence of the state of carbon on ductility in Fe-0.017mass%C alloy was systematically investigated. Moreover, the changes in dislocation density, dislocation substructure, and dislocation character with tensile strain were evaluated using X-ray line profile analysis together with transmission electron microscopy. The solute carbon maintains uniform elongation while simultaneously increasing the ultimate tensile strength by increasing the work-hardening rate; however, post-uniform elongation is significantly impaired by the occurrence of dynamic strain aging (DSA). The coarse intergranular cementite leads to superior uniform elongation; however, post-uniform elongation deteriorates presumably owing to the void formation in the cementite/matrix interface or the fracture of cementite. Meanwhile, the fine transgranular cementite increases the yield strength, thus impairing the uniform elongation; however, it maintains post-uniform elongation. This is because small cementites less than about 1 µm in size do not deteriorate the post-uniform elongation. The solute carbon prominently increases the dislocation density, particularly the edge dislocation density, with tensile strain owing to DSA, leading to the prohibition of dislocation cell structure formation.