Abstract
The purpose of this study is to clarify the effects of strengthening mechanism on the fatigue properties of ferrite plus pearlite sheet steel. Ferrite plus pearlite sheet steels having the tensile strength of 400 to 600 MPa grade were prepared with being strengthened by solid solution, precipitation, increased dislocation density, grain-refinement or increased pearlite volume fraction. Furthermore, a ferrite plus bainite steel was prepared to investigate the effect of replacement of the second phase pearlite with harder phase, bainite. Load-controlled fatigue test and strain-controlled fatigue test were carried out ot examine fatigue limits and obtain cyclic stress response curves, respectively.
Main results are as follows: (1) The ratio of an increase in fatigue limit to an increase in tensile strength (Δσw/ΔσB) heavily depended on strengthening mechanism. The ratio was higher for both solid solution and precipitation strengthenings than bainite or grain-refinement strengthenings, while lower for dislocation and pearlite strengthenings. (2) Stress amplitude obtained from cyclic stress response curve related more closely to fatigue limit than to tensile strength. (3) The initial fatigue cracks were observed only in the ferrite matrix; but not in the second phase. (4) It was concluded from these results that it is of vital importance to strengthen the ferrite matrix itself by solid solution or precipitation in order to obtain higher ratio of Δσw/ΔσB.
