FREE DEFORMATION OF INITIAL SOLID SHELL OF FE-C ALLOYS

The deformation of initial solid shell has important effects on the fo rmation of surface defects of continuously cast slabs of steel. In ord er to understand the thermal deformation behavior of initial solid she ll of steel, droplets of Fe-C alloys and commercial steels were solidi fied on a chill plate and the bottom shape of solid shell was measured . The solid shell always deforms during solidification and cooling to give a spherical bottom shape. This deformation of solid shell shows a very strong dependence on carbon content. When carbon content is lowe r than about 0.6 mass% or higher than 2.5 mass%, solid shells show pos itive deformation, ia convex towards the chill. When carbon content la ys between 0.6-2.5 mass%, solid shells show negative deformation, ie. concave towards the chill. Two peaks of positive deformation were foun d as carbon contents come near to 0 mass% (pure iron) or 0.12 mass%. T hese two peaks of positive deformation of solid shell correspond to th e reported peaks of frequencies of surface defect occurrence such as l ongitudinal cracks and deep oscillation mark of continuously cast stee l slabs. The deformation of pure iron solid shell was accurately predi cted with a deformation model of initial solid shell previously derive d by the authors. An explanation for the maximum deformation at 0.12 m ass% carbon conent was also made by the model using a numerical heat t ransfer calculation with a coupling of deformation and solid shell-chi ll interface heat transfer coefficient.