To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

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.