Abstract: The forming difficulty of ultra high strength steel is greater than that of ordinary steel because of the high strength and poor plasticity of ultra high strength steel. Further, ultra high strength steel is prone to springback, which is an important factor influencing the roll forming accuracy. An investigation of the springback law and its production mechanism using experiments and finite element simulation is introduced. Three sets of roll forming experiments with different numbers of stands are designed. The variation law of the springback angle is elucidated in terms of the forming angle, number of stages, increment angle, and so on. The finite element simulation model of roll forming of ultra high strength steel is established. The forming characteristics of V-shaped sheets are analysed. The distribution of the total strain, plastic strain, and elastic strain in the transverse direction of the sheet are analysed. The mechanism of the variation of the springback angle in roll forming is discussed. The results show that for single-stand roll forming, the springback angle first increases and then decreases with increasing forming angle. The change in the elastic strain peak is the main reason for the variation of the springback angle. For multi-stand roll forming, increasing the number of stands is conducive to increasing the accumulation of plastic deformation and reducing the elastic strain peak. This multi-stand incremental forming is a unique process characteristic of roll forming; increasing the increment angle reduces the thickness of the sheet, promotes material flow in the forming zone, and effectively reduces the springback. The systematic investigation of the springback law of multi-stand roll forming provides a theoretical basis for reducing the springback and improving the forming accuracy.

Key words: roll forming, springback, ultra high strength steel