Formability of galvanized interstitial-free steel sheets

Hot dip galvanized steel sheets are being widely used in automotive applications for better corrosion protection. Formability of hot dip galvanized steel sheets has been one of the key issues in the automotive industryʹs transition to more corrosion resistant vehicle parts. In the present work, formability of hot dip galvanized interstitial-free (IF) steel sheets has been characterized. Forming limit diagrams (FLDs) and strain distribution have been evaluated experimentally by stretch-forming sheet samples over a hemispherical punch. Coefficient of friction at the punch–sheet interface during deformation has been determined. Microstructural studies at the steel–zinc interface and fractographic studies on the deformed samples have been carried out. The effect of coating thickness on formability has been analyzed by using hot dip galvanized sheets with three different coating thicknesses and a comparison has been made with an uncoated interstitial free steel sheet. The presence of brittle inter-metallic phases (which reduce formability) at zinc–steel interface layer in the coated sheets appears to have a more dominant role than the beneficial effect of reduced friction due to the presence of Zn coating at the punch–sheet interface during deformation. The combined effect of these two resulted in limit strains of the coated sheets, which are marginally lower than that of the uncoated sheet.