FINAL SOLIDIFICATION, CENTER SEGREGATION AND PRECIPITATION PHENOMENA IN CONTINUOUSLY CAST ROUND BILLETS

Archimedes' principle was applied to strand lengths of up to 14 m of r ound 177 mm diameter billets to determine cavity volume V (in cm(3)/m) for a series of tubemaking steels. Under steady state casting conditi ons, V depends primarily on the chemical composition and solidificatio n behaviour of the steels. Averages ranged up to (V) over bar = 24.1 /- 3.1 cm(3)/m in an extreme case (bearing steel; v(C) = 2.0 m/min). S topping the strand and abrupt lowering of withdrawal speed made it pos sible to artificially induce individual shrinkage cavities with peak v alues of V = 45 cm(3)/m. In the round mould, subsequent necking of the liquid crater is initiated in this process. Whisker-like and icicle-l ike dendrites can be observed in horizontally formed macroscopic shrin kage cavities. The liquid crater is broken off by periodic bridging (t he mini-lingot model). As permeability decreases, the bridges deflect downward in a U-shape under the ferrostatic load. The residual inter-d endritic melt is squeezed out in this plastic deformation process. Som e bridges ultimately break. This interpretation is based on comprehens ive CMA studies and concentration maps. Cores taken from the centre of a strand of bearing steel were submitted to various heat treatments b etween 1100 and 1350 degrees C. Homogenization occurs below solidus (T < 1225 degrees C). Carbides dissolve and sulfides become globular. Sp atial segregation, on the other hand, increases, above solidus. In the extreme case at 1350 degrees C/30 h, the residual melt at the grain b oundaries migrates practically completely from the surface zone to the centre of the cylindrical sample (40 mm diameter). The propellent for ces for this macro-heterogenization are provided by the activity gradi ent from the cylindrical surface to the centre and minimization of int erface energy gamma/l.