Mechanism of Springback-Free Bending of High-Strength Steel Sheets under Warm Forming Conditions

Tensile loading-unloading tests of high-strength steel sheets in an elevated temperature range are conducted using a 100 kN servo-controlled hydraulic dynamic fatigue testing machine, aiming at clarifying the mechanism of the springback-free phenomenon occurring in warm sheet forming. From stress-strain curves obtained by the tests, it is found that the abrupt decrease in the springback of formed steel sheets at approximately 750 K in isothermal v-bending tests is mainly caused by high-temperature transient creep deformation. Also, from the results obtained by the isothermal v-bending test, bending-unbending deformation observed at temperatures higher than 750 K, as a result of high-temperature creep, was found to have a secondary effect in the springback-free phenomenon.