Study on stretch-flangeability of steel sheets: microstructural influence and prediction using uniaxial tensile properties

DoctorThe present dissertation aimed to clarify the effect of microstructure on stretch-flangeability of various steel sheets, and to establish a model for predicting the hole expansion ratio (a typical term to evaluate stretch-flangeability) using uniaxial tensile properties. The dissertation consists of 3 topics. First, the stretch-flangeability of high carbon steel was investigated for hot-rolled high carbon steel sheets with various hot-rolled microstructures. Due to the high driving force for spheroidization of carbides after cold rolling, all the specimens which were cold rolled and annealed showed similar microstructures and uniaxial tensile properties. However, it was found that hole expansion ratio of punched hole was closely related with the sheared area fraction of punched hole edge. On the other hand, hole expansion ratio of machined hole was largely dependent on the uniformity of microstructure. Second, an investigation was made to correlate hole expansion ratio (HER) of steel sheet and its uniaxial tensile properties. It was found that circumferential strain at the hole edge had a linear relation with the radial strain at the hole edge. The radial strain at the hole edge can be replaced by the width strain of uniaxial tensile specimen since the stress state of the hole edge during hole expansion is equivalent to the stress state of uniaxial tension. From the definition of normal anisotropy, the width strain was calculated using average normal anisotropy and total elongation, and it showed a good linear relationship with HER of machined hole. The HER of the punched hole was also well predicted using the similar approach, which reflected only the portion of post uniform elongation. The physical meaning drawn by our approach successfully explained the poor HER of austenitic steels despite their considerable elongation. Finally, mechanical properties of Fe-Mn-Al-C dual phase lightweight steels was investigated with the variation of Al content and annealing temperature. Fe-Mn-Al-C dual phase lightweight steels were developed for the purpose of improvement of yield strength (YS) and HER of punched hole (HERP) of high Mn austenitic TWIP steels. Strain hardening behavior of developed steels was also analyzed by use of modified Crussard-Jaoul (C-J) analysis. Hole expansion properties of dual phase lightweight steels were compared with those of advanced high strength steels (AHSSs) such as dual-phase steel, transformation induced plasticity (TRIP) steel, and twinning induced plasticity (TWIP) steel. Developed steels showed good YS-HER among the AHSSs. Our results show a possibility to develop multi-phase advanced high strength steels with excellent stretch-flangeability if post-necking elongation can be increased