The material choice of American manufactures for AK Blanks Receivers and Flats has always been 4130. It is an excellent alloy steel its incredibly strong and relativley lightwieght. The 4130 name is denoted from the specific formulation of the steel as is the case in most steel blends. 8620 alloy is the alloy used for the Polish Blanks that are commonly availble from a varitety of sources and presumably most other original ak receivers used this material as well. 8620 is also a very, very, very strong and lightwieght material as well.
OK so they're both plenty strong, however 4130 does have the edge in strength and also a bigger reason why 4130 is used so commonly is its ease in heat treatment. 4130 has a high carbon content and is designed to be through hardened. This means that when it is heat treated the material is completely hardened all the way through inside, outside, center, everywhere. 8620 has a lower carbon content and is meant to be case hardened. Case hardening results in a work piece which has only the outer surface hardened and the middle stays soft. However on a 1mm thick piece of steel the case hardening is basically the same as through hardening because the penetration of hardened layer would be atleast .02 per side making it fully hardened anyway. The worst part about low carbon steel is they need the addition of carbon to even become hard during heat treatment. One method to do this is the use of products like Kasenit. These add carbon to the surface of the part being treated. This process is know as carburization. In studies carburized parts do not have as much as great of impact resistance as through hardened parts so once again 4130 comes out on top.
Above: This 4130 Blank was heat treated, the black material on the surface is the carbon that leached out to the surface when it reached its austenitizing temperature of 1600 deg F. Low carbon steel do not have the carbon content to produce such a result.
If 4130 is superior in strength and ease of heat treatment why were'nt all AK's originally built out of it? I guess for the same reason ford is making its new F150's out of aluminum. Cost. Nobody changes until they have to.