Effects of Finely Dispersed Sulfides on Microstructure and Impact Property of Ferrite-Pearlitic Steel

For the purpose of improving the impact properties of ferrite-pearlitic microalloyed steels through the refinement of structure, the steels with finely dispersed sulfides were produced by P/M process. In these steels, average manganese sulfide size was one-tenth of that in conventional steels (I/M steels) and the number of inclusions was increased about two digits. The ferrite-pearlite grain size was decreased by these inclusions. Although the impact values of the steels were increased by refinement of ferrite-pearlitic structure, they were decreased by the increase of the number of inclusions. Consequently, the impact value of the steels were not improved in comparison with conventional steels.Therefore, it is necessary to decrease the number of sulfides by controlling the sulfides slightly larger than those in the P/M steels. In this case, it is expected that the ferrite-pearlite grain size will become finer by the increase of intragranular ferrite in spite of increasing the prior austenite grain size.