THE FORMATION OF INTRAGRANULAR ACICULAR FERRITE IN SIMULATED HEAT-AFFECTED ZONE

The preferential conditions of intragranular acicular ferrite (IAF) in heat-affected zone were studied by adding sulfur and calcium into a T i-killed steel. Sulfur and calcium addition refined the austenite grai n due to the increase in the number density of inclusions. An intermed iate austenite grain favored the formation of IAF due to the preventio n of Widmanstatten ferrite and polygonal ferrite. The inclusions chang ed from Ti-oxides dominant to Ti-oxysulfides dominant and Ti-Ca-oxysul fides dominant due to small addition of sulfur and calcium. The nuclea tion potential of inclusions was found to increase in the sequence of Ti-oxides, Ti-oxysulfides, and Ti-Ca-oxysulfides. This phenomenon coul d be attributed to the increase of the lattice disregistry between inc lusions and ferrite matrix or the decrease of strain field around incl usions. Both effects prevented the transformation of austenite to ferr ite at elevated temperatures, thereby promoting the formation of IAF. The highest volume fraction of IAF was, therefore, obtained at an inte rmediate sulfur content with small amount of calcium due to the achiev ement of optimum nucleation potential and grain size.