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In this study, hat-shaped bending experiments have been conducted on MART1200 and MART1400 Ultra-High-Strength Steels (UHSS). To investigate the influence of different lubrication conditions, on the spring-back, three different lubricant types (Graphite, Teflon film and Teflon film + Graphite) have been used in the experiments. A constant 6.5 tons of bottoming force has been applied at the end of the bending stroke and the force has been held on the samples for 0, 10s and 60s to observe the effect of force holding time on the spring-back. In addition, the effect of blankholder force on the spring-back has also been investigated for MART1400 steel. It has been found that the decrease of blankholder force and the friction between the die and sheet interfaces has considerably reduced the spring-back in the sidewalls and flange regions and also lowered the side wall curl. It has been revealed through finite element analysis that the sheet bottom has suffered a larger deformation in lower friction conditions and blankholder forces and caused in the reduction of spring-back.

Investigation of the effects of lubrication type, blankholder force and force holding time on the spring-back behaviour for hat-shaped MART steels