Abstract An extraordinarily large gas pocket formed during the rolling of a rimming steel slab wasstudied. Results of investigation indicated that the serious lamination defects that often occurredin such slabs were due to the presence in large rimming steel ingots of central cavities that (1) con-tained gases at high pressure, (2) were surrounded by heavily segregated metal, and (3) wereloaded at their surfaces with non-metallic substances. During the rolling operation these cavitiesfailed to weld up. On the contrary, the solid walls dividing neighbouring cavities were pushedopen and, as a result, a large discontinuity remained in the rolled slab. It was found that aftershearing the slab and thus exposing the discontinuity to atmosphere, in most cases such discon-tinuities could weld up during subsequent rolling into plates. Lower tapping temperature and lower pouring temperature (speed) were found effective tolessen lamination rejects.
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