PURPOSE: To prevent the penetration of the phosphorus component of phosphate films formed by a lubricating film treatment into steel in production of the steel bolts by forming utilizing plastic deformation.
CONSTITUTION: The phosphate films are formed on the surfaces of a blank material for the steel bolts and a lubricant is held in these phosphate films in a lubricating film treatment stage 3 and thereafter, the blank material is passed through dies and is drawn in a drawing stage 4 and is subjected to forming of threaded parts by rolling in a bolt forming stage 5. The blank material is then hardened in a hardening stage 6 and its surface is subjected to galvanizing at 25 to 35μm material thickness in a plating stage 7; following which the blank material is heat treated for 60 to 90 minutes treating time at 350 to 400°C in a heat treating stage 8. As a result, the phosphorus component reacts with zinc and is trapped in the form of zinc phosphate, by which the penetration of phosphorus is prevented and the delayed crack resistance is improved.
TAKATO MASARU
MATSUI KEIKO
MAKIMAE TATSUMI
