抄録
The effect of Ca addition on fracture toughness has been examined by means of the analysis of fracture mechanics and tensile tests together with micro-structural observation. The Ca addition results in the spheroidization and reduction of the number of non-metallic inclusions, while the matrix structure does not show significant changes. Fracture mechanics test in terms of J-integral using a three point bending test shows the increase in the slope of the R-curve, indicating enhanced increase in the stable crack growth resistance. The dimple morphology on the ductile fracture surface is classified into two types, i.e. primary and secondary dimples. By Ca addition, the depth of the primary dimple and local shear area on the fracture surface increase. It implies that Ca addition causes increased local plastic deformation in the course of stable crack extension, resulting in the improvement of fracture toughness. On the other hand, effect of Ca addition does not appear in tensile test. It suggests that the effect of micro-structural factors on mechanical properties depends on the testing methods.
