Effect of Si on the Yield Strength of Medium-Carbon Steels Consisting of Ferrite and Pearlite

To enhance the intrinsic yield strength of the ferrite-pearlite microstructure that inevitably forms in a non-quenched-and-tempered steel for large structural components, this study analyzed the effect of adding Si. The resulting ferrite-pearlite microstructure in medium-carbon steels was examined, and the resulting increase in yield strength was interpreted based on strengthening mechanisms. Hot-rolled sheets of 0.3C-1.5Mn steels were fabricated with Si ranging from 0.22 to 2.21 wt. %. Austenitization was conducted at 880°C followed by slow continuous cooling, and ferrite-pearlite microstructures were formed. With increasing Si, the yield strength increased from 409.1 to 573.6 MPa. Although there was a negligible change in the austenitic grain size, the mean diameter of the ferritic grains decreased from 8.8 to 5.0 μm, while the mean interlamellar spacing of the pearlites decreased from 196.6 to 109.9 nm. Using these quantitative data about microstructural features and considering possible strengthening mechanisms, a model to predict yield strength was derived via a constrained optimization method. The resulting model indicated that the major mechanisms are the refinement of pearlitic interlamellar spacing and the solid solution strengthening of ferrite by Si. Their contribution to the increment in yield strength was over 75 %, while others such as ferritic grain refinement and increased pearlitic volume fraction have limited influence.