Steel Alloys:

At its most simple, steel is iron with carbon in it. Other alloys are added to make the steel perform differently.
Carbon steels can be differentially tempered, to give a hard edge-holding edge and a tough springy back. Carbon steel get rust faster than stainless steels.

D2 Tool Steel

This material is a very high carbon steel (1.5%) that has superb edge holding ability. D2 is sometimes called a "semi-stainless". It has a fairly high chrome content (12%), but not high enough to classify it as stainless. It is more stain resistant than the carbon steels, it is excellent steel with high carbon and high chrome contents of 64 Hrc maximum hardness. Although it is quite resistant to rust, it is not free from the rust.  This material is desired by consumers because of its high hardness so it is not easy to get dull and it is not easily to be re-sharpened. Its composition is C = 1.55%; Si = 0.25 ; Mn = 0.35% ; Cr = 11.8% ; Mo = 0.80% ; V = 0.95%

440C

440 C is stainless steel with high enough carbon and high chrome contents 60 Hrc maximum hardness.  It is resistant  to rust, but when it gets dull, this material is difficult to re-sharpened This is a stainless steel that is used on many factory manufactured knife blades. It is very tough and extremely stain resistant. It holds an edge well and is fairly easy to sharpen. It is generally hardened to between 56 and 59 Rc., and makes a great all around blade. 440C is far superior to most other 400 series steels. 440A and 440B are not as good.

ATS-34 Stainless Steel

Currently, this is the most excellent steel with high carbon and high chrome contents 60 to 61 Hrc maximum hardness and free from rust. It is high recommended for knife manufacturer although it is not so easy to be re-sharpened. Its chemical composition is C= 1.03% ; Si = 0.25% ; Mn = 0.41% ; Cr = 13.74% ; Mo =  3.56% ; P = 0.001%. This is a stainless steel that comes from Japan and is very similar to The U.S.'s 154CM. It became very popular with knife makers in the 1990's as a higher quality alternative to 440C. The steel is not as rust resistant as 440C, but holds a better edge. The carbon content is 1.04%. It works best when hardened to between 59 Rc to 61 Rc.

Damascus Steel

It is a type of steel that produced through dying of layers hight carbon steel, low carbon steel and Nickel, with different color, and then folded, heated and hammered repeatedly and folded up to 64 in normal blades and 4098 in very high quality ones to obtain intended pattern. After that the blades must be polished using different grain stones and carefully tempered. In the final phase the Pamor pattern is revealed with a solution of lime and arsenic powder.  The end result is a tough blade with good edge quality. It is quite expensive and used mostly for its artistic nature.