There are various choice for hard plate body armor. The three most common materials because of their harness and weight are UHMWPE, Aluminum Oxide, Boron Carbide, and Silicone Carbide.
Hard plate armor is the strongest type of personal ballistic body armor currently available. It can stop rifle rounds up to 0.30cal Armor Piercing as well as almost all types of explosive shrapnel.
Steel plate fell out of favor in the 17th century when firearms became too powerful. But, ballistic plates from different modern materials are still used to this day.