2007 Volume 47 Issue 12 Pages 1781-1788
A fine-grained microstructure yields the optimum combination of strength and toughness of steel. Refinement of the ferrite microstructure by formation of fine intra-granular acicular ferrite nucleated on inclusion has been caused wide interesting in weld metallurgy and other metallurgy fields. It is also important to refine the as-cast microstructure during strip casting and thin slab casting process in which the possibility of refining microstructure by rolling technology decreases. In present paper, the possibility to nucleate intra-granular acicular ferrite by sulfide inclusion in steel without special alloy elements during rapid solidification process has been investigated by optical microscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The orientation of the acicular ferrite clusters are also investigated by Electron BackScattered Diffraction (EBSD) analysis. Oxide inclusion and sulfide inclusion are popular in the sample and only sulfide inclusions are observed effective to assistant acicular ferrite nucleation. The possible mechanisms of formation of acicular ferrite in present sample, those are manganese depleted zone and phosphorus rich zone around inclusion in matrix, are discussed in detail.