In order to produce large ingots of sound inner quality, important factors which influenced the loose structure formation were investigated by employing the ultrasonic testing method.
The loose structure was found to differ both in its formation mechanism and solidifying condition depending on the location under investigation.
The loose structure which appeared in the axial region varied depending on the ingot shape. The degree of looseness could be evaluated by the shrinkage pipe index, α ; the looseness decreased as the index α increased. The index is defined as a function of three parameters on ingot shape:
α=ψ-35.3 [(L/D)/P]+7.7
where, ψ: taper of wide face (%),
L/D: height to thickness ratio (-),
P: hot top ratio (%).
On the other hand, the loose structure which appeared in areas other than the axial region varied depending on the hydrogen content in molten steel. It was found to be essential to reduce the hydrogen content in order to decrease the degree of looseness. Its reduction was remarkably effective for the structure especially in the region where inverse V-segregation appeared.
