High strength bolt superior in delayed fracture resistant property and steel material for the same
A high strength bold superior in delayed fracture resistant property has a bolt tensile strength of 1200 N/mm 2 . A high strength bolt comprised of a head portion and shaft portion formed by a steel material, wherein the relation between a bolt tensile strength TS (N/mm 2 ) and tempering temperature T (° C.) satisfies the following expression (1), the relation between the bolt tensile strength TS (N/mm 2 ) 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 1200 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, C eq = C + ( Mn / 6 ) + ( Si / 24 ) + ( Ni / 40 ) + ( Cr / 5 ) + ( Mo / 4 ) + ( V / 14 ) .
1. 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.
2. A high strength bolt as set forth in claim 1 , characterized in that said steel material contains, by wt %,
C: 0.30 to 0.45%,
Mn: more than 0.40% to less than 1.00%,
P: less than 0.010%,
S: not more than 0.010%,
Cr: 0.5% to less than 1.5%,
Mo: more than 0.35% to less than 1.5%,
V: more than 0.30% to 1.0%.
3. A high strength bolt as set forth in claim 1 or 2 , characterized by said steel material further containing, by wt %
Al: 0.010 to 0.100%.
4. A high strength bolt as set forth in claim 1 or 2 , characterized by said steel material further containing, by wt %
Nb: 0.005 to 0.030% and
Ti: 0.005 to 0.030%.