Experimental Study on Bendability of Advanced High Strength Steels

Fracturing in a tight radius during bending is one of the major manufacturing issues in forming Advanced High Strength Steels (AHSS). The study investigated the bendability of AHSS under two forming conditions: bending with and without stretched over the die radius. The bendability was evaluated by conducting modified Bending Under Tension (BUT) test for stretch bending and 90^o v bend test for bending without stretch. The study also examined the effect of material properties on the limiting bend ratio. Various strength high strength steels, range from 420 MPa to 1700 MPa tensile strength, were selected in the study. Results indicated that critical radius-to-thickness ratios between the two tests are different but correlated in a relationship which was depicted in the bendability diagram. The map and constitutive relation curve derived from limiting bend ratios identified the multiple bendability (stretched and no-stretch) of each strength grade of test steel, which is recommended as the guideline for design-evaluate the bendability performance of the AHSS. The determined limiting bending strains could serve as the die radius thinning criterion, while further field validations are needed to verify the robustness.