The Mechanism of Microstructural Transformation upon Edge Heat Treatment on Stretch Flangeability and Fracture of Ultrahigh-Strength Steel with Sheared Edge

Fracture at sheared edges upon stretch flanging is a common problem for ultrahigh-strength steel (UHSS). We focused on the improvement of stretch flangeability by heat treatment of the sheared edge of UHSS samples in this study. The steel sheet samples were pierced and then heat-treated by whole-area heating. The heated materials were tested by the hole expansion test to determine the hole expansion ratio (denoted as X) and to investigate the X improvement mechanism. The microstructural transformation depending on the heat-treatment temperature were also investigated. The experimental results of whole-area heating showed that X of the steels improved with heat-treatment temperature up to Ac1 (the temperature at which ferrite starts to transform to austenite). The X significantly decreased with heat treatment between Ac1 and Ac3 (the temperature at which ferrite has transformed completely to austenite). The X increased again with heat treatment over Ac3. The X dependence on the heat-treatment temperature was explained by work hardening recovery and microstructural transformation on the sheared edge. We added observation and discussion about fracture mechanism of sheared and heated edge.