ATS-55 Stainless Steel for Knife Blades
Hitachi ATS-55 Stainless Steel
ATS-55 is a very good cutlery steel from Japan, and is very similar to ATS-34, but with much less Molybdenum and the addition of Cobalt and Copper. Sal Glesser of Spyderco mentioned that this steel was custom made for Spyderco by Hitachi Metals, and since Spyderco no longer uses this product, it is not seen very much anymore.
It is comparable to 440C in it's performance, takes a very fine edge and cuts very well. It is easier to sharpen than it's big brother, ATS-34, but also holds an edge less longer. This is due to the lack of Moly in the mix.
Since only Spyderco has used this steel in their knives, it is not a widely known product, and since discontinuing it's use in the early 2000's, the steel is not often discussed anymore. However, Spyderco collectors are intimately familiar with this use from it's extensive use in Spyderco's mid-ranged knives of that era.