改良オースフォームを適用したSCM440鋼の高サイクル疲労特性(第1報,1600MPa級焼入れ焼戻し材と改良オースフォーム材の疲労特性)

抄録

Modified-ausforming have been proved effective for improvement of high-cycle fatigue properties in high strength steels. Three SCM 440 steels (heats B, C and D) with the tensile strength of 1600 MPa were supplied for fatigue tests. The heat D included larger Al₂O₃ inclusions than heats B and C. Heat B was ausformed and induction tempered, whereas heats C and D were normally quenched and furnace tempered. AFM microstructure observation showed that martensite blocks and carbides of the ausformed heat B were smaller than those of normally quenched heats C and D. Fatigue limits at 10⁸ cycles were 890 MPa for ausformed heat B and 840 and 750 MPa for quenched heat C and D. Fish-eye type fracture, which occurred in the quenched heats, disappeared in ausformed heat B. Origins of fish-eye type fracture were TiN inclusions in quenched heat C and Al₂O₃ inclusions in quenched heat D. Large and clear ODA, i.e. optically dark area, was observed on the fracture surface around the Al₂O₃ inclusion. On the other hand, ODA around the TiN inclusion was obscure. The fact that fish-eye type fracture disappeared and a higher fatigue limit was achieved in the ausformed heat B have shown the good performance of ausforming in fatigue properties of high strength steels.