Wiki-ZDP189

Hitachi Metals' strongest powder steel boasts a hardness of 67 or higher, the highest of any known steel for cutting tools, and the ultimate combination of hardness, toughness, wear resistance, and corrosion resistance. However, due to its extreme hardness, it is difficult to sharpen with regular sharpening stones and is quite expensive.
*Production has been discontinued.

Related websites
ZDP189 Japanese Knives: Unmatched Sharpness & Durability | KIREAJI