Search Results

Bake-hardening steels used for exposed auto-body panels provide low yield strengths before forming, and increased strength and dent resistance after the forming and paint-baking processes. Room temperature aging can alter the sheet properties before forming, after forming, or after baking. Knowledge of the evolution of mechanical properties is important, and the effects of room temperature and simulated room temperature aging (at 50°C and 100°C) on the yielding behavior and the bake hardening response of two different bake-hardening steel grades were studied. The steels included a low strength ultra-low carbon steel and a dual-phase steel with higher strength and greater bake-hardening index. Neither steel exhibited a substantial response to aging prior to tensile pre-straining, although both steels exhibited strength increases after either aging or baking following straining.

The dent resistance of AKDQ, bake-hardenable, and IF Re-phosphorized sheet steels in both the as-received and biaxially prestrained conditions were evaluated. A laboratory dent test system for evaluating flat 76.2 mm diameter samples was designed and adapted to a standard universal test frame. Dent testing consisted of measuring the loads and displacements associated with the incremental quasistatic penetration and withdrawal of an indenter into the center of a circular sheet sample mechanically restrained at its perimeter. Dent resistance was characterized by a critical amount of residual displacement. Both experimental measurements and theoretical predictions demonstrate an increase in dent resistance with prestrain up to a maximum followed by a decrease due to thinning. Correlations between dent resistance, tensile properties, and the strain within the dented region indicate that the dent performance depends on the low strain, strain hardening behavior of the material.