Steel core is just a type of bullet... the core is steel and not lead. Not all steel core is designed for piercing armor, that depends on if the steel is hardened, how it is shaped, etc.
Most military ammunition made for the 7.62x39mm was loaded with the Type M43 bullet... a 123gr. steel core bullet, a little over an inch long (some countries, like Yugoslavia, used the flat-based M67 projectile that is lead core). The steel core ammo was not intended for piercing armor- it is just that steel was cheaper to work with and easier to produce.
The U.S.- ever since Olympic made the first 7.62x39mm pistol- has since re-classified the 7.62x39mm round as "pistol capable". This does not affect any other law, except, the law defining Armor Piercing ammunition. "Armor Piercing" ammo is illegal if it can be used in a pistol, or is specifically listed as illegal. This is why you can readily buy .50 BMG steel core rounds, or other types, but not 7.62x39mm. The laws defining what is "armor piercing" are quoted on the ATF's page regarding AP ammo. The link with the full law is below:
http://www.atf.gov/firearms/legal/armor.htm
Anyway... steel core 7.62x39mm ammo was banned in 1994. Anything made before then is legal to own, sell, shoot... but it is a felony to use it in a pistol (it is legal to use in a rifle).
The reason some people- like me- prefer steel core is because of the bullet length. The AK's rifling is designed to stabilize a bullet the length of a steel core round... steel is lighter than lead, so the lead core ammo required by law tends to yaw early. Shoot your AK or SKS 400 yards, you will notice some keyholing. At 500, it is guranteed. This is because the lead is made to weigh 123 grains (or so- Wolf is 122, Uly/Military Classic is 124, and so on), so you can use the original sights, intended for the velocity and drop of a 123 gr. bullet. However, since lead is heavier, the bullets are shorter and not stabilized as well by the fast rate of twist in the AK barrel.
Wolf will NOT cause your gun to rust. I would also not use ammonia... it undergoes a chemical reaction with chrome, and if left in your barrel too long, will cause the chrome plating to flake off. You can use regular Hoppes solvent to clean the gun, the corrosive salts are in the powder fouling... remove the fouling, remove the salts. Of course, if you have an AK with a US made, non chrome lined barrel, or a SKS/AK made by Yugoslavia, then chrome may not be an issue for you.