The best ball bearing steel is 52100; it is very magnetic, and requires corrosion protection.

440C is a stainless steel, nearly as good a bearing material as 52100, it is magnetic, easy to pick up with a strong magnet. Better at not rusting than 52100, but not very good compared to other stainless steels.

Non-magnetic stainless steels do not make very good bearing balls, avoid.