Modification of hole expansion ratios in Dual Phase Steels by Hole Edge Thermal Treatments

Many of the newly developed AHSS have poor stretch flangeability compared to the older generation autobody steels like IF steels and this can sometimes be a factor preventing their use in automobile stampings. Stretch flangeability is usually measured using the hole expansion test. In this test, a 10 mm diameter hole at the center of a standardized test specimen is expanded using a conical punch until the onset of a through thickness crack. The percentage increase in hole diameter is denoted as the hole expansion ratio (HER). In this study, hole expansion ratios in dual phase (DP) steels were evaluated. Various strategies to improve HER have been discussed. In order to improve the HER, the microstructure and hardness at the circumference (edge) of the hole can be modified using a micro plasma torch heat source using controlled heat inputs. The HER has been correlated with the microstructure and hardness. Strategies to improve stretch flangeability by utilizing stress relaxation during loading have also been reviewed.