Precipitation of Titanium and Zirconium Sulfides during Uni-directional Solidification

Abstract

Morphologies, chemical compositions and distribution of sulfide inclusions in steel have been examined on uni-directionally solidified iron alloy containing about 0.1% sulfur and one of sulfide forming elements; Mn about 1%, Ti about 0.5 or 1%, and Zr about 0.1 or 0.3%.
The results obtained are summarized as follows;
(1) Almost all of titanium and zirconium sulfides are observed in an interdendritic space in the morphology of Type II. The number density and mean diameter of these sulfides are approximately same as those for MnS.
(2) In titanium added alloys many sulfide colonies consisting of rod- and/or plate-like TiS are observed, whose area fraction amounts to 0.2.
(3) Rod- and plate-like inclusions observed in zirconium dded alloys make solid solution with manganese sulfide to form (Zr, Mn)3S4(Zr/Mn molar ratio: 0.4-3.5) and approach to a spherical form in the existence of manganese with about 1%.
(4) The supersaturation for these sulfides to precipitate in the solidification sequence has been evaluated by use of phosphorus content in the steel matrix adjacent to a given sulfide precipitate. The supersaturation for sulfide precipitation is evaluated as follows;
MnS : 1.0-1.7, TiS : 1.0-1.7, Zr3S4: 3.0-10 where the equilibrium relations between sulfur and sulfide forming elements determined by authors, and solute enrichment during solidification are taken into account.