Abstract
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 mmΦ holes 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 (λ). The mechanical properties and microstructural transformation depending on the heat-treatment temperature were also investigated. The experimental results of whole-area heating showed that the λ of the steels improved with heat-treatment temperature up to Ac1 (the temperature at which austenite starts to transform from ferrite). The λ significantly decreased with heat treatment between Ac1 and Ac3 (the temperature at which austenite is completely transformed from ferrite). The λ increased again with heat treatment over Ac3. The λ dependence on the heat-treatment temperature was explained by the alleviation of work hardening and microstructural transformation. The experimental results of partial-area heating showed that instant and partial-area heat treatment was sufficient to improve λ.
