Stan,
BIG DISCLAIMER: My only formal welding education pertains to structural steel. I can tell you how to TIG pipes together but thats about it.
Anyway, most all steel welding is DC. The only AC steel application I regularly see specified is welding heavy plates. The penetration from AC with high current happens to work out better for that application. There are probably other applications where AC is better but, Im not aware of them. And cheap welders. Those are AC only, which is why they sound funny, splatter, and generally suck.
The AC setting on a welder is primarily for aluminum. Maybe you dont need that.
