Root cause analysis of indentation mark defect on Zn coated steel sheet in continuous galvanizing line

Abstract

The coated steel industry is growing as demand of corrosion resistant steels are ever increasing in automotive, construction, appliances, and general engineering segments. The surface requirement of the coated steel is becoming more stringent that demands coated surfaces within specified roughness, waviness, reflectance and free from surface defects. The continuous galvanising line number 2 of Tata Steel Limited, Jamshedpur suffers from a chronic surface defect that appears like a scratch mark in the coated steel. The present study deals with the root cause analysis of the defect focussing on the rolls within annealing furnace in CGL#2, Jamshedpur. The roll profile, roll and strip slippage, roll surface compositions (using profilometer, XRF) are studied in detail. Similarly, the steel strip collected from inside the furnace was also studied for the defect orientation, surface composition (using SEM & EDS), trapped particles etc. The mathematical calculation of slippage along with the characterization evidence confirm the formation of defect through a complex mechanism of particle formation due to non-uniform roll profile, followed by particle coagulation and finally formation of the defect on the strip under tension by those particles. Possible operational strategies to eliminate the defect is also outlined.