In order to develop hot-roiled sheet steel with excellent formability and fatigue properties, the effects of strengthening mechanisms on the fatigue properties of ferrite-pearlite hot-rolled sheet steels were studied. Tests were conducted on the hot-rolled steels having tensile strength 500 to 600 MPa which were strengthened by solid solution, precipitation, increasing dislocation density and pearlite volume fraction and grain-refining. The following results are obtained: (1)Fatigue limit and endurance ratio depend on strengthening mechanism. Endurance ratio is increased by solid-solution and precipitation strengthening, while not by increasing dislocation density and increasing pearlite volume fraction. (2)Fatigue limit of the strengthened material can be related with the fatigue limit of base material, strengthening mechanism, and the increment of tensile strength by each strengthening mechanism. (3)Though the stress amplitude at strain-controlled fatigue test, that is the resistance of material against cyclic deformation, is dependent on strengthening mechanisms, it is linearly related with fatigue limit independent of strengthening mechanisms. (4)This fact suggests that the resistance of ferrite phase against crack initiation is proportional to its resistance against cyclic movement of dislocation, and that, therefore it is more effective to use the strengthening mechanisms which prevent the dislocation movement in ferrite phase under cyclic deformation for improving fatigue strength.
