Springback-free mechanism in hot stamping of ultra-high-strength steel parts and deformation behaviour and quenchability for thin sheet

The springback-free mechanism in hot stamping of ultra-high-strength steel parts was clarified by the mechanical, thermal and transformation viewpoints. In hot stamping, the effects of elastic recovery during unloading and thermal shrinkage on the springback are comparatively small, but the effect of the phase transformation is critical. Volume expansion occurs primarily upon the start of the martensitic transformation, and plastic deformation is induced by the volume expansion during holding at the bottom dead centre, causing the springback including the post-stamping deformation to disappear. It was observed from well-organised experiments that holding at the bottom dead centre until the martensite finish temperature prevents the springback, and the springback-free mechanism in hot stamping of ultra-high-strength steel parts was clarified from the observation. The springback behaviour in hot stamping of a thin steel sheet with 0.6 mm thickness was explained from the above mechanism, and the deformation behaviour and quenchability for the thin sheet were examined. A sufficient holding time at the bottom dead centre was more closely associated with the prevention of springback rather than sufficient hardening. Additionally, local thinning around the bottom corner of a bent thin sheet was prevented by optimising the transfer time from the furnace.