PURPOSE: To provide the production method of a high strength bolt excellent in delayed fracture resistance by using the steel of specified component and adjusting harm forging after annealing and tempering temp.
CONSTITUTION: After the steel, in which C, Si, Mn, P, S, Cr, Mo, Al, V, Ti, Nb, N are specified, is subjected to supheroidizing, it is subjected to bolt forming with heating to 200-400°C and at a working speed of ≥200mm/sec, at executing subsequent quenching-tempering, the tempering temp. is ≥400C. Further, in the case of forging with heating to ≥400°C, the punch having a surface temp. of ≤100°C is used, further after heat is withdrawn so that a steel surface temp. is ≤100°C and a temp. at 1/10 of stock diameter from the surface layer is ≥250°C, it is subjected to bolt forming at a working speed of average ≥200mm/ sec, the tempering temp. at subsequent quenching.tempering is ≥400°C.
ISHIKAWA FUSAO
KANISAWA HIDEO
