Solute segregation, constitution and distribution of sulfides in Ca-treated compared with RE-treated HSLA steel ingots have been investigated as a function of the concentration of Ca, RE, S and O. Characteristic S-segregation pattern including bottom cone sedimentation of sulfides is found to disappear at [%Ca] · [%S] ^0.28≤1×10^-3 in Ca-treated ingots and at [%RE] · [%S] ≤1×10^-4 in RE-treated ingots. The disappearence is named “S-reversal”. These features are reasonably interpreted on the basis of the model proposed earlier to account for the “S-reversal” in RE-treated ingots.
The degree of sulfide shape controlling with Ca or RE can favourably be evaluated in terms of atomic concentration ratio, ACR, of effective Ca or RE to S in the melt in mold. Sulfide shape control is in complete throughout the ingots at ACR of 0.2, satisfactory at 0.4 except for A- and V-segregates, then becomes complete even in the segregates at 1.8. By controlling the concentration of Ca or RE and S to fall within the area bound by ACR≥1.8 and [%Ca] · [%S] ^0.28≤1×10^-3 or [%RE] · [%S] ≤1×10^-4 on Ca or RE vs S coordinates, HSLA steels that are totally free from hydrogen induced cracks have been produced for sour gas service.