Black-line surface defects in aluminium-killed steels

Abstract

Cold-rolled, low-carbon aluminium-killed steels, which possess both good drawability and internal cleanliness, frequently exhibit objectionable surface defects termed ‘black lines’, ‘pencil lines’, ‘tight seams’, etc. Because of economic incentives, aprogramme was undertaken to determine the nature and origin of these defects and to assess the effect of various steelmaking, teeming, and scarfing practices on their occurrence. Structural studies of cold-band, hot-band, slab, and ingot samples revealed that the defects originate in the ingot as inclusion clusters containing alumina and sometimes galaxite, hercynite, and iron oxide. These clusters are present well below the slab and ingot surface and therefore extensive scarfing does not provide a practical solution. Results of mill trials to evaluate the effect of mould geometry (big-end-up v. big-end-down moulds) and teeming practice (protectionfrom reoxidation) on thefrequency of black lines are presented. Using big-end-up (BEU) instead of big-end-down (BED) moulds produced a 42–51% decrease in defect frequency in the finished product. Combining the BEU practice with teeming stream protection from reoxidation achieved a 63% reduction in defect frequency.