D2 steel is a tool steel often referred to as “semi-stainless” as it falls just short of the required amount of chromium (13%) to qualify as full stainless yet it still provides a good amount of resistance to corrosion.  On the flip side D2 steel is much harder than other steels in this category such as 154CM or ATS-34 and as a result holds its edge a little better.  That said, it’s not as tough as many other steels and exponentially tougher to sharpen.