Control of Surface Quality of 0.08% < C < 0.12 % Steel Slabs in Continuous Casting

Abstract

The present study summarizes the work done in collaboration between Sollac Fos sur Mer, Dunkerque and Irsid, in order to propose and test industrial solutions that can significantly reduce longitudinal cracks on so-called “middle carbon steels”. A mathematical model of the volumetric contraction of the solidified shell shows that, the maximum contraction takes place at about 45 mm from meniscus, for ~0.1 % C steels. A mechanism for longitudinal crack formation is thus proposed in three steps: 1) separation of the shell from the mould due to δ→? transformation, 2) deformation of the uneven solidified shell of the slab as it passes through support rolls; and 3) liquid steel level variations in the mold, amplified by the above deformations imposed on the slab. Three main actions have been tested and have proved efficient on industrial casters: 1) The use of a “crystallizing” powder reduces the extracted heat flux by 15% and the longitudinal crack ratio by 2 times. 2) A grooved hot surface in the Cu mould plate reduces the heat flux by about 10%; and 3) The prevention of large liquid steel level variations. Some additional work is being done to control the thermal state of the nozzle area where these cracks are still mainly located. © 1996, The Iron and Steel Institute of Japan. All rights reserved.