The influence of sulfide inclusion morphology on the directionality of impact properties and transition temperature was studied in hot rolled C-Mn steels with 0.02%S level. The sulfide shape was controlled by adding rare earth metals. The addition ratio of rare earth metals to sulfur(RE/S) was varied from 0 to 10.
As RE/S addition ratio increased, the amount of globular RE-sulfides increased and the amount of elongated MnS inclusions decreased. But the total inclusion content remained nearly constant.
The degree of anisotropy of impact properties reduced as the amount of elongated inclusions decreased and the amount of globular RE-sulfides increased.
The effect of sulfide shape control revealed great difference depending on both transition temperature criteria and specimen-notch orientation. It is suggested that 50% FATT is appropriate for transition temperature criterion under the condition of the same specimen-notch orientation in steels with the same level of inclusion content, size and morphology. Otherwise energy or lateral expansion criteria is more suitable.
Experimental evidence indicates that the microfissuring due to elongated inclusions had great influence on the impact energy and transition temperature by the crack arresting or dividing effects.
