Effect of cooling-rate during solidification on the structure-property relationship of hot deformed low-carbon steel

The present study investigates the combined effect of cooling-rate during solidification and hot deformation on the microstructure and mechanical properties of low-carbon steel, especially considering the beneficial effect of acicular ferrite. Low-carbon steel was cast in two different moulds to vary the cooling-rate during solidification. The cast ingots were soaked and hot forged at different temperatures. Relatively slower cooling-rate during solidification and deformation at relatively lower temperature were found to be beneficial for the acicular ferrite formation, which increased the hardness and strength without hampering the ductility. The difference in solute contents at the dendrite center and the interdendritic regions of the cast-ingots, as a result of microsegregation, and the compositional homogenization during soaking were predicted. Based on the predicted compositions, the microstructures of the cast and hot forged ingots were explained considering the effect of prior-austenite grain size, austenite conditioning during deformation, fraction of non-metallic inclusions and the segregation of Nb along the prior-austenite grain boundaries on the phase transformation.