The outer surface of the spiral groove of the cylindrical cut or cut with a conical spiral groove cone. The screw has a different head, the head of the picture is called the outer six angle screw. There are other, such as flat screw, inner six angle screw etc..
By the extrusion process, the screw is in high temperature, corrosion, strong wear and tear, and therefore, the screw must: high temperature, high temperature deformation; wear resistance, long life; corrosion resistance, material with high strength, can bear large torque, high speed, with good machining performance; heat treatment, residual stress, thermal deformation, etc.
No. 45 steel is cheap, good processing performance, but poor wear resistance.