Effects of sulfur content and sulfide-forming elements addition on impact properties of ferrite-pearlitic microalloyed steels

Longitudinal Charpy impact value of ferrite-pearlitic microalloyed steel was improved by large addition of S (0.05-0.1mass%), however, transverse Charpy impact value was rather deteriorated. This study focused on the sulfide shape, which have large effects on impact properties. The elements of Ca, Mg, Ti, and Zr were added to ferrite-pearlitic microalloyed steels with 0.1 mass% S for sulfide shape control, and their im pact properties were compared. The addition of Ca or Mg to the steels improved the transverse Charpy impact value without changing the longitudinal Charpy impact value, while the addition of Ti or Zr to the steels resulted in deteriorating significantly both longitudinal and transverse Charpy impact values. The crack initiation energy of the Ca or Mg bearing steels improves by enlargement of the minute dimple area ratio in the ductile fracture surface because the elongation of sulfides in the hot forging process are controlled by the addition of Ca or Mg. On the other hand, the reason why the Charpy impact value of Ti or Zr bearing steels are deteriorated is that (Ti, V)C or plate type sulfides promote cleavage fracture and then raise their ductile-to-brittle transition temperature.