Press formability of newly developed high lubricity hot-dip galvanized steel sheets

Surface modification technology was applied to hot-dip galvanized steel sheets (HDG) in order to improve their press formability. The frictional behavior and press formability of surface modified HDG were investigated in comparison with those of conventional HDG, conventional electrogalvanized steel sheets (EG) and pre-phosphate treated EG (EG pre-phos). Specimens of these coated mild steel sheets with various mechanical properties were used as test materials. The friction coefficient was measured by a flat sliding test, and press formability was evaluated by a deep drawing test and a stretch forming test. As a result, surface modified HDG showed a lower friction coefficient and higher press formability than conventional HDG and conventional EG. The friction coefficient and press formability of surface modified HDG were similar to those of EG pre-phos. In deep drawing, the inflow of material from the flange area to the cracking area becomes easier due to the lower friction coefficient of surface modified HDG, which leads to improvement of deep drawability. In stretch forming, the outflows of material from the sheet/punch contact area to the cracking area become easier due to the lower friction coefficient of surface modified HDG, leading to improvement of stretch formability. These results indicate that surface modified HDG is a promising material for automotive use. (paper)