Electro-welded steel frame. Heavy but bullet-proof.
As I recall back in the day Schwinn didn't only make their own bike frames - they made the tubes! They bought plate steel, bent them into 3 equal sections around a mandrel, then welded the strips together to make the tubes. Then they cut and fit the tubes to make the frames. I don't know when they ceased all-American production but I'm thinking early 1970's. That's when the French and Swiss parts began appearing on their bikes, with the ever present "Schwinn Approved" label. So I'm thinking your bike is 60's.
there's a website somewhere that allows you to date Schwinn frames by the serial number on them.