What type of steel used in bulletproof vest?

So I know that Level III and IV ballistic vests and mostly either steel or ceramic plates. I am trying to figure out what exactly the composition of the plates are. For example steel can be made to either bend or shatter when enough force is applied to it, depending on the amount of copper, zinc, and other soft metals in the steel. Does anyone know where I can find this information. So far my searches have turned up and big fat goose egg.