2019 Volume 60 Issue 699 Pages 108-114
Automobile body parts produced by the hot stamping process show excellent shape fixability with an ultrahigh tensile strength of 1.5 GPa. In the present study, we investigated the effect of forming conditions on stretch formability in the hot stamping process. The stretch formability of a hot-stamped steel sheet was much better than that of a cold- stamped sheet of 1470 MPa class, and equivalent to that of a cold-stamped sheet of the 270 - 440 MPa class. The maximum thinning point was the area not in contact with the tool, where the temperature was higher than that of other areas because there was no heat transfer between the sheet and the tool. Strain concentration was considered to occur in this area because the higher temperature leads to lower deformation resistance. The strain distribution, which is a dominant factor controlling stretch formability, was greatly influenced by tool shape, whereas the influences of the temperature at the start of forming and forming speed were relatively small. On the other hand, the deformation limit, which is another dominant factor controlling stretch formability, was greatly influenced by forming speed, which strongly affected the temperature of the blank. In addition, the heat insulation of the punch greatly contributed to the improvement of formability because strain uniformity was increased.
