Effect of Fine Precipitates on Cyclic-Hardening and Softening Behavior in Dual-Phase Steel

Abstract

 The precipitation hardening with fine carbides of nm size has already been applied to various advanced high-strength steels. In the present study, the role of fine TiC precipitates in the ferrite phase on cyclic deformation behavior was investigated for a Ti-added dual-phase steel with a strength of 780 MPa. In particular, the cyclic hardening and softening behavior characterized by the changes in cyclic plastic strain during stress-controlled fatigue testing was discussed from the viewpoints of the evolution of dislocation substructures. The fatigue life of the precipitation-hardened steel with 2 to 5-nm TiC precipitates is remarkably long as compared with that of a Si-added solution-hardened steel with a comparable ultimate tensile strength. For a specimen which eventually fractures before 2×10⁶ cycles at least, the cyclic plastic strain gradually decreases, i.e., the cyclic hardening takes place, at the beginning of cyclic loading. After a certain number of cycles, the cyclic softening starts, i.e., the cyclic plastic strain reaches a minimum and, then, continuously increases, up to fracture. It was confirmed that the cyclic hardening is accompanied by an increase in dislocation density, while cyclic softening is caused by the dislocation rearrangement. The commencement of cyclic softening is remarkably delayed in the steel with fine TiC precipitates. The strong pinning effect by the precipitates, probably due to the Orowan mechanism, not due to the cutting mechanism, was observed by transmission electron microscopy (TEM). Therefore, it is considered that the long fatigue life of the precipitation-hardened steel is led by the fine TiC precipitates which play a role of the obstacles against mobile dislocations.