Iron is not steel and was in no way rare. Steel is carbonized iron (coking process) and the technique wasn't perfected until after the middle ages.
The Romans didn't have any difficulty equipping their legions with steel weapons and armor.
The armor was iron not steel.
Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the production of blister steel and then crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began. This was followed by the Siemens-Martin process and then the Gilchrist-Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron.
The Romans had steel in their weapons and armor. You don't have to like that fact.