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Low cycle fatigue properties of two types of formable 60kgf/mm2 class hot-rolled high strength sheet steels for automobile use were studied by the X-ray diffraction and TEM methods. Si-Mn dual phase high strength steel sheets, with duplex microstructures of ferrite and martensite, showed initial cyclic hardening followed by cyclic softening on the stress response curves in the strain-controlled fatigue tests. Nb bearing precipitation hardened steel sheets, however, showed monotonuous cyclic softening. These cyclic properties are explained as follows: (1) The initial increase followed by a decrease in the X-ray half value breadth with increased cycles observed on Si-Mn dual phase steel results from fine cell structures in the ferrite matrix of the soft phase. (2) The monotonuous decrease in the X-ray half value breadth observed on Nb precipitation hardened steel results from subboundaries due to the rearrangement of the dislocations.

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