Article ID: ISIJINT-2018-132
Local formability is one of the most critical properties of Advanced High Strength Steels for successful implementation in automotive applications. However, attaining satisfactory local formability is a challenge for many Advanced High Strength Steels. In the present study, a hot dip galvannealed dual-phase steel with 980 MPa minimum tensile strength was selected as the AHSS and its local formability was evaluated by conducting 90-degree V-bend tests. The bend samples, particularly those that experienced failure during testing, were subjected to intense characterization using Scanning Electron Microscope, Cathodoluminescence Microscope and Thermal Desorption Analyzer. Fractography study indicated that the bend failure is associated with the interaction between inclusions near the steel surface and the diffusible hydrogen in the steel. The same underlying failure mode was also observed in two additional zinc coated AHSS grades. Improved steel cleanliness resulted in reduction of inclusions and diffusible hydrogen amounts, and facilitated enhanced steel bendability. Diffusible hydrogen content in the steel could be reduced by removing the surface zinc coating or by heat treatment; such treatments also led to a significant improvement in bendability.