He's using 5160, a carbon spring steel that is good for making knives...

Here's Joe Talmadge's description of 5160 from KnifeArt.com's Knife Steel FAQ:

5160 A steel popular with forgers, it is extremely popular now and a very high-end steel. It is essentially a simple spring steel with chromium added for hardenability. It has good edge holding, but is known especially for its outstanding toughness (like L-6). Often used for swords (hardened in the low 50s Rc) because of its toughness, and is also used for hard use knives (hardened up near the 60s Rc).

http://www.knifeart.com/steelfaqbyjo.html