Effect of Microstructure on Stretch Flangeability and Fatigue Property in High Strength Hot-Rolled Steel

摘要

Recent application of high strength steel sheets to automobile part have been considered by automaker in Korea. High strength steel sheets have been required to have high strength, high formability, high fatigue properties and weight reduction. Among high strength steel sheets, 440 and 590MPa grade hot rolled steel sheets have been applied to automobile part such as suspension and wheel disc. That automobile parts are generally required to have high stretch flangeability and high fatigue properties with high strength and ductility [1-3]. To improve stretch flangeability, microstructures are controlled as multi-phase microstructure, which consists of a polygonal ferrite as matrix and hard phase as strengthening. It is well known that hardness difference between matrix and hard second phase affect stretch flangeability and micro-void nucleation and coalescence during stretch flanging process can be promoted at boundary between matrix and harder second phase. Many papers have shown that steel sheets with bainite as second phase exhibits superior stretch flangeability than martensite and pearlite as second phases[1-13].