1995 Volume 81 Issue 11 Pages 1055-1060
In order to investigate initial solidification of ultra low carbon steel, dip tests of chill block have been carried out. Large irregularity of shell thickness is obtained in ultra low carbon steel. Deformation of solidified shell increases the irregularity by forming air gap between shell and chill. However, irregular growth is also observed before the shell defomiation.
Detail analyses of solidified microstructure reveal the characteristic of the irregular growth in ultra low carbon steel. Fine cellular structure exists near the surface of shell and induces the irregular growth. Number of nuclei in ultra low carbon steel is smaller than that in middle carbon steel.
Furthermore, cooling curve on the onset of solidification is measured using very fine thermocouples. There exists large undercooling for nucleation in the case of ultra low carbon steel.
Based on these microscopic analyses, the irregular growth originates from low nucleation rate in ultra low carbon steel. Fine cellular structure which is due to large undercooling for nucleation, grows from nuclei with high velocity, and results in the irregularity of shell thickness. Then the irregularity is amplified by the deflection of thin shell part.
