A NOTE ON SPALL: Spall is defined as fragments resulting from an impact. When discussing rifle plates, spall is a major concern in all except UHMWPE. When a round strikes a hard face plate, it usually will come apart. spraying lead and jacket fragments in a conoid trajectory (like a drop of water hitting the ground). This will spray fragments parallel to the surface of the plate, including up and into your face and throat. To mitigate spall, it is advised you run spall guards. Tests have shown that layering (BUT NOT GLUEING) 5-10 layers of woven Kevlar to the surface of the plate can be moderately effective. Special forms of Kevlar are even more effective, stopping the majority of spalling with a single layer. Applying coatings such as Line-X and Rhino Lining can also be effective, but require much thicker layers, and after about 5 rounds are rendered ineffective. The advantage of steel plates is their thin profile and multi-hit capacity, so coatings are a stopgap at best. Engineered spall guards are optimal, as they can be replaced easily, continue to be effective after multiple hits, and keep the plate profile thin, as it should be.
*NEW INFORMATION ADDED*Super Bainite Steel- This is a new twist on an old material. Bainite steel is a form of crystalline structure that steel forms when processed in a special way. It is EXTREMELY tough, consisting of iron particles in a carbon matrix. While being very hard, it is tough enough to resist shattering like glass, which is what happens when steel is hardened past a certain point. Currently, it is the only way to get level IV (AP) multi-hit protection out of a steel plate. The only source I know of right now is ebay (look up flashbainite). All the other advice about steel plates apply.