Galvanised spangle products from the continuous galvanising line-2 (CGL-2) at Tata Steel used to exhibit significantly lower ratio of bright to dull spangles, compared to CGL-1 products. The solute enriched surfaces of dull areas are more susceptible to premature darkening on external exposure. An effort was made here to understand this problem and improve the surface quality of spangle products from CGL-2, as well as to predict the end of zinc solidification i.e. the spangle formation time on steel sheets through heat balance study.
Attention was paid on a few parameters such as strip temperature before entry to zinc bath, bath temperature and line speed. A comparative heat balance calculation for zinc solidification revealed that the total heat required to be removed was more in case of CGL-2, whereas the amount of cooling after galvanising was less as compared to CGL-1. This results in extended solidification in CGL-2 during which more solute rejection may occur producing more dull areas. Based on these observations plant trials were carried out at lower temperature of zinc bath, and the bright areas from coil samples were measured. An improvement in bright areas from 25-30% to 35-40% was observed. The surface brightness improved up to 56% at lower line speed. The heat balance calculation also predicted the approximate positions of the formation of spangles at the end of zinc solidification, which were close with actual observations in the plant.