Anisotropic Increase in Yield Strength by Straining and Baking of Bake-Hardenable Steel

Abstract

The yield strength was investigated in a prestrained and baked bake-hardenable sheet steel, which can increase the dent-resistance of exterior automotive body panels. Instead of a standard bake-hardenability test in which the final tensile test is in the same direction as prestrain, the strain-path effect in denting of stamped and paint-baked panels was examined by changing the direction of final tensile tests in uniaxially prestrained and baked samples. The prestrained samples showed the maximum of 0.2% proof stress at 45° to the prestrain axis and the minimum at 90°. Baking raised the minimums at 0° and 90° so that the anisotropy decreases. Furthermore, the strain path changes were characterized by a scalar product of previous and current strain-rate mode tensors. By regarding this parameter as the degree of reactivation of slip systems, the anisotropy observed after prestraining and baking was microscopically explained from the viewpoints of residual stress, crystal plasticity, and dislocation pinning.

Mechanisms and Modeling of Bake-Hardening Steels: Part II. Complex Loading Paths

“…Figure 7 displays the difference between the yield stresses of aged and unaged material DYS, as a function of cos h. Simulations of the influence of aging on strain-path changes are compared with experimental data. A good agreement with experiments and literature [4][5][6] can be noticed. Particularly, the micromechanical approach reproduces well the important effect of aging during continuous and Bauschinger tests, but also the smaller influence on orthogonal sequences.…”

“…This absence of plateau after aging is confirmed by literature for complex strain-path tests. [4][5][6] Besides, the effect of aging on yield strength is strongly dependent of the strain path. The yield strength is higher for aged materials than for the as-received one when cos h is close to 1 or À1.…”

“…Experimental results, but also literature, [5] show that DYS can be negative for orthogonal sequences. This finding means that the yield strength after aging is lower than the one for the unaged material.…”

“…Except for continuous loading, experiments showed a disappearance of the sharp yield point. However, results are contradictory regarding the aging influence on yield strength: Jun et al [4] found an increase of yield strength whatever the angle between directions of loading, whereas Hiwatashi et al [5] noticed an increase or a slight decrease depending on the angle. Yet, these conclusions have to be put into perspective because the considered materials were not exactly the same, nor were the aging treatment and prestrain level.…”

“…They found that, for Bauschinger tests, the sharp yield point returns very slowly, if at all, and the Bauschinger effect is reduced by the aging treatment. Other particular tensile tests were performed [4,5] with prestrain in uniaxial tension, baking, and then tensile tests but with an angle from 0 to 90 deg between directions of prestrain and restrain. Except for continuous loading, experiments showed a disappearance of the sharp yield point.…”