This study investigated the twisting phenomenon in curved hat channel products made of dual-phase 980-MPa-class high-tensile-strength steel sheets. The stroke returning deep drawing (SRDD) method was proposed to deal with twisting. In this new method, after the punch reaches the bottom dead point, it returns to a certain drawing height without the blank holder being removed. With the application of the SRDD method, twisting hardly occurred, but sidewall curl increased. A two-step SRDD was then proposed to reduce the sidewall curl of SRDD products. In the two-step SRDD method, a stroke returning process is carried out in two steps under different conditions. The results showed that the two-step SRDD method reduced the sidewall curl and twist simultaneously.
High-strength steel sheets are increasingly applied to automotive bodies for weight reduction in order to meet the needs for reduced environmental impact. However, dimensional defects resulting from springback are serious issues, and effective methods of predicting and reducing such defects are required. In this study, we propose and validate the advantage of the stroke-returning deep-drawing process, in which stroke is returned to the specified depth after drawing to the bottom dead point. The following results were obtained. Firstly, by the stroke-returning deep-drawing process, the twist angle became approximately zero. Secondly, the occurrence of bottom curvature made the sidewall curvature increase. Thirdly, when bottom curvature did not occur, sidewall curvature decreased in accordance with the equation of wide direction stress. Finally, the twist angle and sidewall curvature decreased concurrently upon deeply drawing and the greatly returning stroke process.
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