This appears to be a heavy-duty D-ring, which is a type of lashing point used to secure cargo. The markings "BL 36T" indicate its breaking load (BL), which is 36 metric tons, or 360 kN. This type of D-ring is commonly used in marine and industrial applications, such as on the decks of cargo ships, Ro-Ro vessels, and trucks, to provide a secure anchor point for chains or straps.