The mysterious outer-space element of "
K-Metal" along with the phrases, "I'll never give 'em
the secret of the k-metal!" and "You will be compelled to yield
the secret of the metal that energizes!" appear in the novella,
The Copper-Clad World, by Harl Vincent; published in the September, 1931 issue of
Astounding Stories magazine:

Jerry Siegel was nearly 17 years old when this issue hit the stands.
Check out the new
Quick Timeline to see where this fits in the overall continuity:
https://k-metal.art/secret-of-the-k-metal/timeline.php