Why naval guns need specialised steel

What steel did naval gun barrels use, and why was the British 15-inch Mark I so successful?

Gun barrels require their own specialised steel and construction because firing heat and stress differ from armour loads; the 15-inch Mark I benefited from building on a mature 13.5-inch design.

Gun-barrel metallurgy differs from armour metallurgy because naval steels are made for particular jobs rather than used as interchangeable materials. A barrel must tolerate intense heat and repeated firing stresses, while built-up construction and wire winding help it withstand internal forces. Gun steel is therefore a specialist material rather than ordinary armour or structural plate. The British 15-inch Mark I also evolved from the proven 13.5-inch gun, so much of the design knowledge already existed and the remaining development risk came down to engineering and luck.