Advanced High Strength Steel Springback and Sidewall Curl Control Guideline

Low carbon steels are being replaced by advanced high strength steels (AHSS) due to high demand of the future lighter weight vehicle, while still maintaining good or even better crash performance. However, sidewall curl and springback (section opening) have been found to increase as the strength of the sheet metal increases. Experiments have been conducted on the bending under tension (BUT) test to seek an effective control methodology regarding the applications of the advanced high strength steels (AHSS) in this study. Steels that were studied included a low carbon steel (DQSK), two dual phase steels (DP) and a transformation induced plasticity (TRIP) steel. Two different gauges of each AHSS were also included for a gauge sensitivity study. Different processing variables (four different diameter pins combining with five different back tension forces) were applied to the tests, and the springback angle and sidewall curl were measured for bend and bend-unbend areas of the specimen. The results suggest that a higher back tension force (restraining force) reduces springback and sidewall curl, which was expected. A transition point (R/T∼ 3 for thinner gauge AHSS, R/T∼4 for thicker gauge AHSS) was observed regarding the effects of R/T on springback for the AHSS. The springback and sidewall curl increase with R/T until the transition point. After the transition point, the springback and sidewall curl decrease with R/T. A combination of higher back tension force (close to yield strength) and the large R/T, which is associated with lower friction coefficient, is recommended as a control guideline in the development of die design and processing strategies for AHSS.