High Cycle Fatigue Behavior of High Tensile Steel Sheet for Hot Press Use
Masaki NAKAJIMA, Yuki NAKAMURA, Toshihiro SHIMIZU and Yoshifumi KAMIYA
Abstract:This paper presents the fatigue behavior of a high tensile steel, NHPA-1500. This steel is quenched in press-die and used as pressworked car component without tempering. Axial loading fatigue tests have been performed in laboratory air and the results obtained were discussed through the comparison with those of a high tensile steel, SPC1470. NHPA-1500 after quenching showed very similar static strengths to SPC1470, especially in tensile strength, elongation and Vickers hardness, while fatigue behavior was so different. In NHPA-1500, the S-N curve showed a definite fatigue limit, that is, no fatigue failure occurred until 108 cycles. On the other hand, SPC1470 exhibited a step-wise S-N curve and fatigue failure occurred at stress levels below the conventional fatigue limit of 900 MPa. Fracture mechanism operated in low stress region below 900 MPa was subsurface crack initiation with a fish-eye. In both steels, the sizes of the maximum nonmetallic inclusions in a standard area were measured using SEM. The results obtained were plotted on an extreme value probability paper, where the sizes of nonmetallic inclusions observed in SPC1470 were statistically larger than those in NHPA-1500. It is considered that the difference in the high cycle fatigue behavior between both steels was due to the size of non-metallic inclusions. Key Words:High tensile steel, Step-wise S-N curve, Fish-eye, Nonmetallic inclusion, Extreme value statistics