高強度鋼の疲労強度に及ぼす非金属介在物の化学組成の影響

抄録

It is well known that the fatigue strength of high strength steels are strongly influenced by the existence of nonmetallic inclusions. It is said that the effects of nonmetallic inclusions on the fatigue strength of high strength steels depend on their size, shape, chemical composition and locations where they exist. Therefore, nonmetallic inclusions are likely to cause a large scatter in fatigue strength.
A firm opinion that hard inclusions are more detrimental than soft inclusions for fatigue strength, has prevailed for long years.
In the present paper, this opinion is proved incorrect by detailed investigations of the chemical composition and size of inclusions observed at fracture origin, i. e., the chemical composition of inclusions is not crucial factor controlling fatigue limit, even if the chemical composition influences the rigidity of inclusions and the residual stresses around inclusions.
On the contrary, it is verified that two crucial factors which control the fatigue strength are the Vickers hardness of matrix (HV) and the square root of projection area of inclusion (√area). It is shown that the lower bound of large scatter in fatigue strength can be successfully predicted by the statistics of extreme values (√area _max) of inclusions contained in many specimens.