Formation of CO and Macrosegregation during Uni-directional Solidification of Iron

Abstract

Experimental studies have been made of the uni-directional solidification of iron with a view of studying the mechanism of solidification with the formation of CO. From the experiments at the solidification velocity of 5mm/min and [%C] _L≈0.1, it is shown that oxygen contents of 0.003 to 0.006% correspond to the transitional concentrations for the formation of CO bubbles and macrosegregation. In this concentration range elongated blowholes are formed, resulting in lowering k_c* and k_p*. At oxygen contents higher than 0.006%, k_p* is unaffected by the increase of oxygen concentration of the melt, whereas k_c* tends to decrease at high oxygen contents.
An expression has been derived to predict the onset of constitutional supercooling for the case of solidification without the formation of CO. From the calculation, it is suggested that the solidification morphology at oxygen contents below 0.003% is dendritic structure.
The solute distribution at solid-liquid fronts growing with bubble formation is explained on the bases of the model that the enriched liquid is being pushed to the liquid ahead of the growth front. Due to the consumption of carbon in the reaction with oxygen k_c* is lowered more than k_p*.
It is shown that the present data give a satisfactory explanation to the phenomena occurring during solidification of a rimming steel.