Many military ammunition procurement specifications include alternate specs for steel cartridge cases. This includes the US Military which primarily uses brass cases, but during WWII and the Korean Conflict, steel was used for .45 ACP, .30 US M1 Carbine, .30-06, .50 BMG, and 20mm cannon, as well as artillery shells.
The former Soviet Union has used steel small arms ammunition cases almost exclusively for generations. This also applies to their former client states (China, north africa, the middle east, etc.)
Cartridge brass, copper, tin, and zinc are "strategic metals", and during wartime shortages, steel is used instead. It is actually more expensive to manufacture steel cases, because more annealing operations are needed, as well as protective coatings to prevent them from rusting.
HTH
John