Remarkable segregation was observed in the modified super clean CrMoV steel forgings for electric power generation applications. Therefore, to make the mechanism clear, effect of such elements as Mn, Ni, Cr and Mo on segregation was studied, using 8 ton sand mold ingots, the solidification time of which corresponds to that of 100 ton ingot.
As a result, we found that factors controlling the segregation are δ-solidification ratio, mean partition coefficient, density difference of molten steels between bulk and segregated liquid at the solidification front and so on. These factors can be calculated from chemical composition of steels.
Then, based on prediction model obtained from the experimental results, chemical composition and shape of ingot were tried to be changed. As for chemical composition, such elements as Mo which is heavy and δ-former were kept low in the specification range. And, as for ingot shape, height to diameter ratio H/D was kept high to shorten the solidification time. The carbon segregation along the axis of ingot was kept relatively low by this ingot design.
The eutectic Nb(C,N) inclusions which give bad effect on the toughness were also investigated. The conditions for the formation of such inclusions were made clear and then predicted and controlled by the calculation from chemical composition.
By this technical development, quality of ingot for HLP rotor forgings was extremely improved.