Effect of Microstructural Transformation Upon Edge Heat Treatment on Stretch Flangeability of Ultrahigh-Strength Steel with Sheared Edge

Fracture at sheared edges upon stretch flanging is a severe problem for ultrahigh-strength steel (UHSS). This study was focused on the improvement of stretch flangeability by heat treatment of the sheared edge of UHSS samples. The materials studied were dual-phase steel and full martensite steel. The steel samples were pierced with 10 mm over bar holes and then heat-treated by whole-area or partial-area heating. The heated materials were tested by the hole expansion test to determine the hole expansion ratio (A). The mechanical properties and microstructural transformation depending on the heat-treatment temperature were also investigated. The experimental results of whole-area heating showed that A of the steels improved with heat-treatment temperature up to Ac1 (the temperature at which ferrite starts to transform to austenite). The A significantly decreased with heat treatment between Ac1 and Ac3 (the temperature at which ferrite has transformed completely to austenite). The A increased again with heat treatment over Ac3. The A dependence on the heat-treatment temperature was explained by the recovery of work hardening and microstructural transformation. The experimental results of partial-area heating showed that short-time and partial-area heat treatment was sufficient to improve A.