A Methodology to Characterize the Influence of Paint Baking and Pre-Straining on the Tensile Properties and Sheared Edge Sensitivity of Third Generation Advanced High Strength Steels

The existing ASTM A653 standard for characterization of the bake hardening index (BHI) involves application of a pre-strain followed by bake hardening and subsequent testing of an ASTM-E8 tensile coupon that can lead to premature fracture in steels with high strengths and lower elongations. To this end, a new test procedure is proposed with application to 980 and 1180 MPa third generation advanced high strength steels (3G-AHSS). The proposed methodology involves pre-straining over-sized tensile specimens using the so-called KS-1B geometry, which is followed by the extraction of an ASTM E8 sample from the gauge region on which the baking and secondary tensile testing are conducted. Various pre-strain levels from 2% to 10% were considered with all tests performed using the proposed methodology avoiding premature fracture compared to the ASTM A653 method that was evaluated for pre-strain levels of 2% and 8%. Finally, to characterize the influence of paint baking on regions with significant pre-strain induced during forming, the sheared edge conical hole expansion test was employed. The change in edge sensitivity with paint baking was assessed for both a machined edge that has a marginal pre-strain, as well as a pierced hole cut with 12% clearance by measuring the change in the hardness of the shear-affected zone (SAZ). The considered 980GEN3 and 1180GEN3 steels exhibited a significant sensitivity to both the applied pre-strain and the bake hardening treatment.