Patent ID: 7070664

Claim:
A high strength bolt characterized in that a steel material used for the high strength bolt has a chemical composition containing, by weight %, less than 0.10% of Si and the remainder Fe and unavoidable impurities and that the relation between a tensile strength TS (N/mm 2 ) of the high strength bolt and tempering temperature T (° C.) satisfies the following expression (1), the relation between the tensile strength TS (N/mm 2 ) of the high strength bolt and the carbon equivalent C eq (%) calculated from the chemical composition of the steel material used for the high strength bolt satisfies the following expression (2), and the tensile strength of the bolt is refined to a range of 1426 N/mm 2 to 1600 N/mm 2 by quenching and tempering: TS ≦1.1 T +850 (1) TS ≦550 C eq +1000 (2) where, TS: tensile strength of high strength bolt (N/mm 2 ) T: tempering temperature (° C.) C eq : carbon equivalent (%) where, C eq = C + ( Mn / 6 ) + ( Si / 24 ) + ( Ni / 40 ) + ( Cr / 5 ) + ( Mo / 4 ) + ( V / 14 ) ; and an angle of facing flanks of threads of a thread part cut at an equidistant pitch in the shaft portion is 60° and the valley bottoms of said thread part are formed into arc-shaped curves comprised by the three-arc synthesis method (a) defined by the following conditions and calculation formula: (a) the three-arc synthesis method is the method of forming an arc-shaped curve by setting points of transition between facing flanks of pointed peaks and a valley bottom to (9±1)H/20 from the bottom of the pointed peaks, wherein H is the height of a pointed peak, drawing small contacting circles contacting the flanks at the points of transition and having radii “r” of H/6,drawing a circumcircle of the small contacting circles having a center on a bisector of the acute angle formed by the extensions of the facing flanks and having a radius “R” of at least 2H/3,and superposing the valley bottom side arcs of the small contacting circles and the circumcircle.