EFFECT OF SULFUR-CONTENT ON THE MICROSTRUCTURE AND TOUGHNESS OF SIMULATED HEAT-AFFECTED ZONE IN TI-KILLED STEELS

Four Ti-killed steels were made to study the specific influence of sul fur on the inclusion, microstructure, and toughness of a simulated hea t-affected zone (HAZ). The HAZ toughness was mainly determined by the volume fraction of intragranular acicular ferrite (IAF) which was clos ely related to the supercooling required to initiate austenite to ferr ite transformation. The extent of supercooling was strongly influenced by the composition of grain boundary and inclusions. Sulfur addition up to 102 ppm caused a segregation of sulfur to the grain boundaries a nd a change of inclusion phase from predominantly Ti-oxides to Ti-oxys ulphides and MnS. This behavior, in turn, suppressed the formation of polygonal ferrite and promoted the formation of IAF. Further addition of sulfur elevated transformation temperature and promoted the formati on of polygonal ferrite due to the refinement of grain size and the in crease of grain boundary associated inclusions. A methodology was prop osed to evaluate the intragranular nucleation potential of inclusions, and the results showed that Ti-oxysulphides possessed better nucleati on potential for IAF than Ti-oxides and MnS. With the lowest transform ation temperature and most effective nuclei, the best HAZ toughness ca n be obtained at sulfur content of 102 ppm due to the achievement of t he maximum volume fraction of IAF.