Self Tapping Screws are designed to advance when turned, creating their own threads in the material. They are often used in sheet metal and plastic. Steels are designated by a 4 number SAE Steel grade.  The first two digits indicate the primary materials used to form the steel.  The last 2 digits identify the percentage of carbon for the alloy (in hundredths).  The steel used in these fasteners has a tensile strength ranging from 100,000 to 150,000 psi (pounds per square inch).  Zinc Plating helps to prevent corrosion.Pan head fasteners have a low, large cylindrical head with a high rounded top edge for higher tightening torques - these fasteners are often recommended to replace older head styles such as round, binding, or truss-head where possible. The Phillips drive style was originally designed so that the driver would slip out under extreme torque, preventing over-tightening and damage to the fastener or the material.