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Frictional Behaviour of Lubricants during Deep Drawing and Ironing Processes for Zinc-Coated Steel Sheets

Frictional behaviour of lubricants during the deep drawing and the ironing processes was studied for two types of zinc-coated steel sheets at low speeds. Deep drawability of galvannealed steel sheet (GA) was significantly improved by an increase in viscosity as well as by addition of the friction modifiers, which was due to reduction in frictional forces in the flange zone and around the die radius. In contrast to the results in the drawing test, the frictional characteristics in the ironing test were little affected by the viscosity because the ironing process was primarily performed in the boundary lubrication regime. As to workpiece materials, a double-layer steel (GAE) coated with an electrogalvanized iron-rich alloy layer on the surface of GA was superior to GA in terms of deep drawability due to the lower contact ratio based on high hardness of the top coating layer and probably due to a low coefficient of friction in the boundary contact area. When compared with the effects of the friction modifiers, active sulphur EP (extreme pressure) additive showed the most significant effect on formability and the lowest friction in the ironing process. On the other hand calcium sulphonate raised formability in the deep drawing, but was ineffective for reducing friction in the ironing process.