It is shown that conventional standards for inclusion rating such as ASTM method and Japanese Industrial Standard (JIS method) are no longer useful for the evaluation of inclusions contained in recent clean steels. A new inclusion rating method based on the statistics of extreme is proposed.
Inclusion ratings on two kind of super-clean bearing steels, SUJ2 (N) and SUJ2 (H) were done by JIS method and the method based on the statistics of extreme and the results were compared. Total oxgen contained in SUJ2 (N) was 8ppm and that in SUJ2 (H) was 5ppm. No clear difference was obtained by JIS method between the inclusion ratings of SUJ2 (N) and SUJ2 (H). If we define the size of the maximum inclusion in a standard inspection area by the square root of projection area, √area_max, the distribution of √area_maxobeys the statistics of extreme.
The method based on statistics of extreme can distinctly discriminate even the slight difference in cleanliness between SUJ2 (N) and SUJ2 (H). Moreover, the method of statistics of extreme enables one to predict the size of the maximum inclusion which is expected to be contained in larger area or volume than inspected area.
The value of √area_max can be also used for the prediction of scatter band of fatigue strength of high strength steels. The agreement of the prediction and experiments on the scatter band of fatigue strength of SUJ2 (N) and SUJ2 (H) is shown.