過飽和固溶炭素を含むフェライト鋼の疲労き裂発生挙動および疲労特性におよぼすMnの影響

抄録

Solute carbon plays an important role in fatigue phenomena of Fe–C ferritic steels. For example, strain aging owing to solute carbon hardens the crack tip regions, resulting in a higher non-propagation limit of the fatigue crack. However, the low solute carbon content regions are formed near grain boundaries during quenching process from ferrite single phase region because of the segregation of carbon at grain boundaries. As a result, the relatively low resistance to plastic deformation in the weak regions is a primary factor causing intergranular cracks. When these cracks coalesce and lengthen, these cracks continue to propagate until fracture. That is, in the Fe–C ferritic steels, a suppression of crack initiations is also required. In this study, we try to improve the fatigue properties of ferritic steel from a viewpoint of the suppression of crack initiations. We used a 1.9 mass% Mn-added Fe–C–Mn steel with a ferrite-cementite structure based on a consideration for the fact that the carbon diffusion coefficient is reduced by Mn addition. Rotating bending fatigue test was performed at ambient temperature and at 50 Hz, and a replica method was used to observe the fatigue crack initiation behaviors. The Mn-added steel showed the comparable fatigue limit as Fe–C ferritic steels when compared at the same Vickers hardness in spite of low solute carbon content (0.0035 mass%). Also, the crack initiation sites near fatigue limit were different from Fe–C ferritic steels, namely, transgranular cracks were observed in Fe–C–Mn steel and intergranular cracks were observed in Fe–C ferritic steels.