He wasn't invulnerable, he wasn't even strong. Unlike most heroes of the time, he had no swagger to his walk, no false bravado. He spoke with a nervous stammer and a great deal of self-deprecation.
That's him, bottom right. Tin had more heart in him than any other Silver Age character, because he overcame his obvious fears to do the right thing and the brave thing:
As you can probably gather, Tin didn't succeed very often. This does highlight one problem with the Metal Men in general; they might "die" but as robots they could always be brought back to "life".