Influence of constant and variable restraining force on springback of TRIP700 and stainless steel grades

Abstract

Newly developed high strength steels (HSS) like dual phase (DP) and transformation induced plasticity (TRIP) grades as well as austenitic grades like AISI 304 and AISI 301 show superior strength compared to the microalloyed grades with the same formability (e.g. HSLA340). However, due to the multiphase microstructure and the austenite martensite transformation during forming, a higher springback will appear.In this paper the influence of different process parameters of six high strength steels on springback after stamping is investigated. The material properties were determined with uniaxial tensile tests. A modified Duncan Shabel test was used to draw U‐profiles with a controlled restraining force. To investigate different process parameters, constant restraining forces of 1 and 5kN and die radii of 3, 6, 9, and 12 mm were applied. In a second step, a shape set process was used. A constant restraining force of 1kN was used until a draw depth of 80 % was reached. Then the restraining was increased to 5.5 kN. The springback and the sidewall force were measured and analysed. An increased restraining force and a reduced die radius increases the sidewall force and reduces the springback. This resulted in a significant decrease in springback. The tests with variable restraining forces, also known as shape set process have shown that it combines a good formability with a reduced springback.