"Copper is so much more malleable than steel I don't know why they would use any steel unless special application for penetration or form retention."
Copper alloy coated steel is a very common bullet jacket material and has been used for military ammunition since WWI. Maybe even longer. Steel is much cheaper than copper. Much of the 150 grain .30 M2, .50 BMG, .45 ACP, and some .30 Carbine ammunition made during WWII used steel-jacketed bullets. And later, so did some 5.56mm and 7.62mm ammunition. The Lake City Product Guide shows many cartridges specified with steel-jacketed bullets.