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 struc
ture does not show significant changes. Fracture mechanics test in terms of
J-integral using a three point bending rest shows the increase in the slop
e of the R-curve, indicating enhanced increase in the stable crack growth r
esistance. The dimple morphology on the ductile fracture surface is classif
ied 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 i
ncrease. It implies that Ca addition causes increased local plastic deforma
tion in the course of stable crack extension, resulting in the improvement
of fracture toughness. On the other hand effect of Ca addition does not app
ear in tensile test. It suggests that the effect of micro-structural factor
s on mechanical properties depends on the testing methods.