Abstract
The iron containing 0.050.15%C was solidified unidirectionally at a constant rate of 5 mm/min to obtain the critical oxygen concentration for CO macroblowhole formation. This concentration is represented not by a curved line but by a band. At 0.05 %C a large scatter is shown in the critical oxygen concentration. Additionally experiments were done for the ceasing of growth of CO blowholes. It is clarified that the critical oxygen concentration of blowhole formation is higher than that of blowhole growth ceasing.
