钢铁 (gāng tiě) is the noun for 'steel', the alloy, and as an adjective it figuratively describes something strong, firm, and unyielding (e.g., 钢铁意志 'will of steel'). Compared to the sibling 金属 ('metal'), which is the general category, 钢铁 is a specific alloy. The sibling 铁 ('iron') is a different metal and in figurative use means 'tough' or 'close' (slang), while 钢铁's figurative sense is more formal and emphasizes unyielding strength.