Effect of microstructure and texture on formability and mechanical properties of hot-dip galvanized steel sheets

Lead content of zinc bath has significant effects on microstructure, texture and as a result on formability and mechanical properties of zinc-coated steel. In this study, microstructures of samples made from hot-dip galvanized sheets were examined by optical and scanning electron microscopy and their chemical compositions were analyzed by energy dispersive spectroscopy (EDS). The crystallographic orientation of the coatings was determined using X-ray diffraction and texture coefficients were calculated. Mechanical properties of galvanized sheets were determined by uniaxial tensile tests and formability of the sheets was evaluated using forming limit diagrams (FLD). From the experimental results, it was concluded that by increasing the lead content of zinc bath the thickness of gamma layer, a brittle intermetallic phase, and the spangle size were increased, and, texture coefficient of basal plane component was decreased. Such variations caused the yield strength of the coated sheet to increase, but ductility was reduced. The value of FLD0s decreased due to reduction in the number of active slip planes; this in turn caused reduction of the formability. The overall evaluation of the results indicated that good formability of hot-dip galvanized steel sheets under press conditions could be attributed to the lubricating effect of zinc as the outermost layer of the coating.