Most modern steel cans for food are given a hermetic plastic coating on the interior. Before that steel cans were tin-plated. (I believe all-tin cans haven't been used in over a century.)

If the can is dented then the coating can be compromised and the steel will rust where exposed.

Also, when you open the can you are cutting through the hermetic coating, exposing the steel at least on the edges where you cut, which is why opened cans quickly develop a "metallish" rust smell.

Most modern steel cans for food are given a hermetic plastic coating on the interior. Before that steel cans were tin-plated. (I believe all-tin cans haven't been used in over a century.)

Depending on how hot you got the can when testing it you could have melted or vaporized the plastic coating, exposing the unprotected steel. Also, if the can is dented then the coating can be compromised and the steel will rust where exposed.

Finally, when you open the can you are cutting through the hermetic coating, exposing the steel at least on the edges where you cut, which is why opened cans quickly develop a "metallish" rust smell.