The solidification of rimmed steel and the segregation of the elements in the solidified steel are described and discussed.
Experimental results show that the primary blowholes created in the solidifing ingot are very small compared with normal rimmed steel ingot. It will be due to the fact that molten steel of this work are poured more slowly than in ordinary operation. During the solidification with rimming, carbon and sulphur are most remarkably segregated, and phosphorus, not so much as them, white manganese is hardly segregated.
From the autoradiography of 198Au on the cross-section of ingots, it is observed that the solid-liquid interface during the rimming period is very smooth and solidification velocity of liquid steel is reduced from the bottom to the top of ingot. And also the solidification velocity is decreased by the existence of blowholes too. The experimental equations of the solidification velocity are as follows:
Symbols A, B, C and D show the portions corresponding to 1/5, 2/5, 3/5 and 4/5 in the height of 10 t flat steel ingot (1800mm) respectively, and subscripts u, m and l show upper, middle and lower parts in each portion respectively, x is the thickness of solidified steel (cm) and t, time (min). The thickness of the impure layer ahead of the solid-liquid interface, which, in generally, depends on the solidification velocity of liquid steel, is decreased from the bottom to the top of ingots where the rimming action occurs more violently.
Moreover, by the fundamental consideration based on “BURTON'S theory”, it is clarified that the CO evolution from the rim-layer during solidification is more intense in low-manganese content ingot than in high-manganese one, and also that the amount of CO evolved is, in either kinds of ingot, much more at the top of ingot than the bottom. This result is very interesting in relating with the powerfulness of rimming action, the degree of deoxidation and the static pressure of liquid steel.