When was Superman first referred to as the “Man of Steel”?

Everyone knows Superman as the Man of Steel, but when was the nickname first used?

The “Man of Steel” moniker was first used in reference to Superman within the pages of “Action Comics #6” (November 1938).

The nickname appeared for the first time in the front page headline of the Daily Star newspaper, reading “Mystery Man of Steel re-appears”.

Action Comics #6

It only took a few months for the phrase to catch on, with a panel at the end of “Action Comics #11” (April 1939) appearing in a small one-panel advertisement reading “More Startling Adventures of Superman (Man of Steel) Action Comics”.

Action Comics #11
Man of Steel