Shunji Hiwatashi

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

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.

ADVISABLE GRADES OF HIGH-STRENGTH STEEL SHEETS AND WELDING CONDITIONS FOR SPOT-WELD STRENGTH IN AUTOMOTIVE BODY APPLICATION

Experimental study on static strength of spot-welds for high-strength steel sheets is summarized, focusing on thicker and coated sheets. Tensile shear strength (TSS) increases with sheet strength and thickness. Cross tensile strength (CTS) also increases, but the variation with sheet strength is small. Welding with expulsion deteriorates CTS, especially for thicker or coated sheets with higher strength or carbon content. Blow-holes and shrinkage cavities are found frequently at thicker sheet welds, and their existence near nugget edge reduces CTS. Above-mentioned knowledge leads developments of galvannealed sheets with carbon content less than 0.1% for tensile strength up to 180MPa. (A) For the covering abstract see ITRD E121867.