In order to characterize the solidified structure of low carbon steel, uni-directional solidification experiments have been performed using various commercial grade low carbon steels. Conditions of uni-directional solidification experiment carried out in this study are followings : Temperature gradient : 3.4·10³K/m, Growth velocity : 1.67·10^-5m/s and 8.33·10^-5m/s.
The solid/liquid interfacial morphologies (cellular or dendritic) have been observed on longitudinal and horizontal cross section of the specimens. The primary dendrite arm spacing ( or inter cellular spacing), λ₁, has been measured on the horizontal cross section. These experimental data have been compared with theoretical model, which is based on the marginal stability criteria.
It has been found that it is impossible to describe the solid/liquid interfacial morphology and λ₁ using carbon content in steel when carbon content is low (e.g. C₀ < 0.1mass%). Equivalent carbon content (C₀') for description of solidified structure is newly defined as follows:
C₀'=1.066(%C)-0.202(%C)²+0.019(%Si)+0.0043(%Mn)+1.1775(%P)+1.6740(%S)
The solidification range in quasi Fe-C binary system at C₀' equates with that in real steel. Using C₀' it is possible to characterize solid/liquid interfacial morphology and size in solidification of commercial grade steel when carbon content is less than 0.1mass%.