Intragranular ferrite formation mechanism and mechanical properties of non-quenched-and-tempered medium carbon steels

The type and size distribution of inclusions in non quenched-and-tempered medium carbon steels were investigated quantitatively. The effect of Ti containing complex inclusions on the formation of intragranular ferrite was studied. The continuous cooling transformation (CCT) diagrams of the tested steels were obtained for determining the cooling rate range of the formation of intragranular ferrite. The mechanical properties of the tested steels were determined at room temperature. The results show that with increasing Ti content, the fraction of Ti containing complex inclusions, which could act as nuclei for intragranular ferrite, increased and the inclusion size became smaller. In the cooling rate range of 0.5 similar to 2.5 degrees C/s, plenty of intragranular ferrite formed, while at the rate of 2 degrees C/s, the microstructure was mainly acicular ferrite. With the formation of intragranular ferrite, the toughness was enhanced by about 50% keeping the same strength level for the studied steels.