Abstract
Strain-controlled low cycle fatigue damage has been studied by means of X-ray diffraction and AE (acoustic emission) methods in addition to metallographic technique for the following two types of formable 60kgf/mm2 class hot-rolled high strength sheet steels; a 0.9%Si-1.2%Mn dual phase (DP) hardened steel with duplex microstructures of ferrite and martensite and a 0.04%Nb added precipitation hardened fine grain steel. The following results were obtained.
(1) The Nb added steel showed monotonic cyclic softening, causing a decrease in X-ray half value breadth as a result of sub-boundaries due to the rearrangement of dislocation during the repeated cyclic strain. On the other hand, the Si-Mn DP steel showed cyclic hardening at the early stage followed by softening, which corresponds to an increase followed by a slight decrease in X-ray half value breadth caused by fine cell-structures in the ferrite matrix, which were essentially stable even in the cyclic softening stage.
(2) The Si-Mn DP steel gave more and smaller AE waves in the early and the middle stages than the Nb added steel. The phenomenon in the DP steel might be the result of the continuous release of elastic energy due to the formation and annihilation of persistent slipbands in the ferrite matrix, and the initiation of small cracks at ferrite/martensite boundaries. The Si-Mn DP steel showed consequently higher registance to the fatal cracking than the other.
