EFFECTS OF STRENGTHENING MECHANISMS ON FATIGUE DAMAGE FOR 600 MPA CLASS HOT-ROLLED HIGH-STRENGTH SHEET STEELS

600 MPa class tensile strength 0.9% Si-1.2% Mn dual phase and 0.04% Nb modified precipitation hardened steels were studied by X-ray diffraction, SEM and TEM methods. Si-Mn dual phase steel (DP) with low yield ratio shows higher cyclic stress than Nb containing precipitation hardened steel with high yield to tensile strength ratio. Nb containing steel shows monotonous cyclic softening which provides a decreases in the X-ray half value breadth resulting from sub-boundaries due to the rearrangement of dislocations. DP steel offers initial cyclic hardening followed by cyclic softening with an initial increase followed by a decrease in the X-ray half value breadth resulting from fine cell structures in the ferrite matrix. DP steel shows high resistance to a crack due to the microstructures of soft ferrite and fine martensite.