2017 Volume 58 Issue 12 Pages 1708-1714
Steel plate with tensile strengths of from 440 MPa to 980 MPa is increasingly being adopted for the structural elements of automobile bodies to both reduce weight and increase collision safety. However, the use of such material brings defects in press forming and other problems. The research reported here focuses on the form-forming method, which is effective for forming high tensile strength steel plate. To clarify the features of form-forming, we used eight types of steel plate having tensile strengths of from 270 MPa to 980 MPa in three forming processes, draw-forming, one-step form-forming, and two-step form-forming, to evaluate the shape freezing property. The results showed that the shape freezing property depends on the mechanical properties of the material; specifically, a correlation with tensile strength, 0.2% proof stress, and elongation was seen. The effectiveness of form-forming for materials of higher strength was confirmed. Improved shape freezing by dividing the form-forming process into two steps was also confirmed. For draw-forming, the entire shaped surface was affected by unbending, so form-forming effectively suppresses unbending. The spatial distribution of unbending was evaluated quantitatively by measuring the residual stress distribution, confirming that residual stress serves as a useful index for clarifying the forming mechanism.
