Improvement of Hole Expansion Properties of Microalloyed Dual-Phase Steel

摘要

The stretch-flangeability of dual-phase (DP) steels containing different microalloying elements was investigated in order to overcome the poor hole expandability of AHSS. The hole expansion ratio (HER), i.e. the ratio of the diameter increase to the initial diameter in the HE test, was evaluated for various factors which are known to affect the test results, such as the blanking method, the addition of microalloying elements, and the morphology of the second phase. The amounts of ferrite and martensite formed during continuous annealing were analyzed by dilatometry combined with Transmission electron microscope (TEM) observations. Detailed high resolution energy dispersive spectroscopy (EDS) analysis was carried out to distinguish the epitaxial ferrite formed during "soft" cooling stage after intercritical annealing. It was found that an increased amount of epitaxial ferrite contributed to the improvement of HER and total elongation while the amount of martensite remained unchanged.