Fe-Mn-Cフェライト鋼の疲労限度およびコーキシング効果

: Mn-C対の作用に着目して

抄録

In carbon steel, the fatigue limit is determined by non-propagation limit of crack and this non-propagation limit depends on plasticity-induced crack closure and hardness near the crack tip. Especially, in ferritic steel, dynamic strain aging (DSA) is important in terms of hardness near the crack tip. This phenomenon, DSA, may be enhanced by adding Mn in ferritic steel through occurrence of Mn-C coupling. Also, the coaxing effect has been recognized as a phenomenon related to fatigue limit. This phenomenon enhances the fatigue resistance through work hardening and stain-age hardening. To examine the effect of Mn on fatigue behavior of ferritic steels, rotating bending fatigue tests including stepwise stress increases every 10⁷ cycles were carried out in ferrite-cementite Fe-Mn-C steel. The steel showed a higher fatigue limit when arranged in Vickers hardness that contains effect of solid solution strengthening by Mn and C and showed a significant coaxing effect in spite of low solute carbon content. In addition, the crack propagation behavior changed in response to stress amplitude: at high stress amplitudes, the cracks propagated along grain boundaries; at low stress amplitudes, the cracks propagated in the grain interior. These behavior is different from Fe-C ferritic steel. Also, fracture in the coaxing effect test was not resulted by the cracks that initiated in the early stages of the test but caused by the crack that initiated in the final stage of the test. This feature is also different from that of Fe-C ferritic steel and annealed S10C.