Hole Expansion Properties of High Strength Automobile Steel Plates with Different Microstructures

Hole expansion tests were conducted for hot-rolled ferrite-bainite dual phase steel FB60, ferrite-martensite dual phase steel DP600 and ferrite-pearlite steel SPFH590. Optical microscope was used to observe formation and propagation of expansion cracks and analyze the hole expansion property and cracking mechanism of the steels. The results show that the FB60 steel exhibits the best hole expansion property, and DP600 steel exhibits the worst hole expansion property. However, the yield ratio of SPFH590 steel was equal to that of FB60 steel, but the hole expansion rate of SPFH590 steel less than that of FB60 steel. The effect of microstructure on hole expansion property was related to the formation and propagation of expansion cracks. The cracks formed not only at ferrite-bainite interfaces but also within ferrite grains in FB60 steel, and propagated along the phase interface between ferrite and bainite and by cutting off ferrite grains. The expansion cracks formed at ferrite-pearlite interfaces, ferrite-cementite interfaces within pearlite and within ferrite grains in SPFH590 steel, and propagated easily along the ferrite-cementite interface. The cracks formed at ferrite-martensite interfaces in DP600 steel, and propagated along the phase interface between ferrite and martensite.