Solidification structure and micro-segregation of unidirectionally solidified steels

Two linepipe steels with carbon mass contents of 0.09 and 0.15% were subjected to steady state unidirectional solidification in a special apparatus. The specimens were positioned in alumina tubes of 15 mm internal diameter, and were pulled downwards at a constant rate, R, from the high temperature furnace. The temperature gradient, G, at the solidification boundary was varied between 5 and 136 K/cm. The dendrite arm spacings measured were quantitatively assessed using the relationship λ = cRmGn. The exponents m and n for the primary arm spacing, λ1, are in agreement with theoretical predictions, at m = -1/4 and n = -1/2. Comparison with other steels containing 0.59% C and 1.48% C showed that λ1 increases with increasing carbon content. The exponents m and n for secondary arm spacing are nearly the same, at -0.4 to -0.5. Correlations with local solidification time, θf, show that λ2 decreases as the carbon content increases. In the δ-phase regime, coarsening of the secondary arms is possible through remelting. Interdendritic segregation peaks are more quickly reduced in the δ-phase regime. This is confirmed by the results of electron beam microprobe analyses for segregation. Reduction of the carbon content of linepipe steels reduces the susceptibility to centreline segregation in the continuously cast slabs.