JPH08177831A - High strength bolt which excels in delayed fracture resistance characteristic - Google Patents

Abstract

PURPOSE: To provide a high strength bolt which has the strength of not less than a specific value and at the same time excels in a delayed fracture resistance characteristic. CONSTITUTION: This high strength bolt has the tensile strength of not less than 1300N/m<2> and excels in a delayed fracture resistance characteristic and has an asymmetrical thread form in which the left and right flanks of a thread are different, and the flank angle of a pressure side flank surface 2a is 40 to 60 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength bolt for friction welding used in various structures in the fields of construction and civil engineering,
The present invention relates to a high strength bolt having a tensile strength of 1300 N / mm ² or more and excellent in delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength bolts are used for bridges, buildings, automobiles,
It is widely used in machinery, but has a tensile strength of 1250N.
If it exceeds / mm ² , delayed fracture is likely to occur. Therefore, according to JIS / B1186 and JIS / B1051,
F10T grade and 12.9 grade are respectively defined as the upper limit strength.

However, in every field, there is a strong need for higher strength, and improvement in delayed fracture resistance of high strength bolts is strongly desired. Delayed fracture of high-strength bolts often occurs from thread troughs where stress concentration is likely to occur. Therefore, in normal triangular threads, the radius R of the thread trough is set to be large and the stress concentration coefficient is reduced to make delayed fracture resistance Is generally known to improve. In addition, JP-A-4-29
Japanese Patent No. 607 discloses a high-strength bolt having a thread trough having an elliptic arc whose major axis is parallel to the axial direction of the bolt.

[0004]

However, it is not possible to compensate for the decrease in delayed fracture susceptibility associated with the increase in strength of bolts only by reducing the stress concentration factor by changing the screw bottom R in a normal triangular screw. If it is 1300 N / mm ² or more, delayed fracture will occur.

In the high-strength bolt disclosed in Japanese Patent Laid-Open No. 4-29607, stress concentration is further reduced,
No experimental example is shown for delayed fracture resistance,
If the strength is 1300 N / mm ² or more, delayed fracture may occur in actual use.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength bolt having a strength of 1300 N / mm ² or more and excellent in delayed fracture resistance.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted extensive studies on the relationship between the delayed fracture resistance of high strength bolts and the screw shape, and as a result, at high strengths of 1300 N / mm ² or higher, bolt threads It was found that the delayed fracture resistance can be significantly improved by making the shape asymmetric.

The present invention has been made on the basis of such knowledge, and is a bolt having an asymmetrical thread shape in which the left and right flanks of the thread are different, and the flank angle of the pressure side flank surface is 40 to 60 °. And the tensile strength is 130
A high-strength bolt excellent in delayed fracture resistance, which is characterized by being 0 N / mm ² or more.

[0009]

1 is a diagram schematically showing the thread shape of the high-strength bolt according to the present invention. In FIG. 1, reference numerals 1a, 1
b is a point, 2a is a flank on the pressure side, 2b is a flank on the idle side, 3 is the bottom of the valley, α is the angle of the thread, α1
Is the flank angle of the pressure side flank surface, and α2 is the flank angle of the play side flank surface.

As shown in this figure, the high-strength bolt of the present invention has an asymmetrical screw shape in which the flank angle α1 of the pressure side flank surface 2a and the flank angle 2α of the play side flank surface 2b are different. And the flank angle on the pressure side is 40 to 60 °.

The reason why the screw shape is asymmetric and the flank angle on the pressure side is 40 to 60 ° in the present invention is as follows. That is, the pressure-side flank surface is a portion to which a load is directly applied at the time of bolt fastening, and the larger the flank angle of this surface is, the more the delayed fracture resistance is improved. Therefore,
Use an asymmetrical screw shape with a large flank angle on the pressure side. In this case, if the pressure side flank angle is less than 40 °, the delayed fracture resistance is insufficient, and if it exceeds 60 °, the nut may come off during bolt fastening. It is defined in the range of 60 °.

In the present invention, the size of the flank angle α2 of the play-side flank surface does not affect the delayed fracture resistance, so its size is not specified, but the bending of the thread and the nut slippage are not specified. In order to prevent this, it is desirable that the flank angle of the play side flank surface is 10 to 20 ° and the angle α of the screw thread is 50 to 70 °.

FIG. 2 shows a conventional bolt having a symmetrical screw shape, that is, the pressure side flank angle α1 and the idle side flank angle α2 have the same magnitude. In this case, the desired thread angle α is In the range of 50 to 70 °, the flank angle α1 on the pressure side can be only 35 ° at the maximum, and the delayed fracture resistance is insufficient.

[0014]

Embodiments of the present invention will be described below. Table 1 shows the composition of the sample steel in this example. In Table 1, I is steel specified in JIS SCM435,
II is a steel with improved delayed fracture resistance containing reduced P and S and added with Ni and Ti. Using these test steels, 22
M22 by rolling into a mmφ round bar and annealing after cold forging
Is formed on the bolt, by tempering even Quenched, the tensile strength was at 1360N / mm ^2, II and 1430N / mm ² at I.

As the screw shape of the bolt, the asymmetric screw shown in FIG. 1 and the triangular screw shown in FIG. 2 were used, and the detailed dimensions thereof are shown in Table 2. In these figures, reference numeral 1
a, 1b, 2a, 2b, 3, α are as described above,
In addition, r1 and r2 indicate the radius of curvature of the valley bottom, H indicates the height of the sharp peak, and p indicates the pitch.

[0016]

[Table 1]

[0017]

[Table 2]

A delayed fracture test was performed on a bolt having such a screw formed therein. In the delayed fracture test, tighten 10 bolts on each steel plate by the nut rotation angle method, 3.5%
A test of repeated dry and wet with a saline solution was carried out for 6 months to examine the presence or absence of breakage. Table 3 shows the results.

[0019]

[Table 3]

As shown in Table 3, none of the bolts A, B and C having the screw shape of the present invention caused delayed fracture. On the other hand, although D has an asymmetrical screw shape, delayed fracture occurred because the flank angle on the pressure side was small. Further, E is a conventional triangular screw, and delayed fracture has occurred. F is a conventional triangular screw whose roundness of the screw bottom is larger than that of E, but delayed fracture also occurred. Therefore, even if such measures are taken, 1300
It was confirmed that delayed fracture cannot be prevented with a strength of N / mm ² or more.

As described above, by adopting the thread shape defined by the present invention, a high strength bolt of 1300 N / mm ² class having excellent delayed fracture resistance can be obtained even with the conventional steel SCM435 steel, and delayed fracture resistance is obtained. 1400 N / m for property-improving steel
It was confirmed that a high-strength bolt having a strength of m ² class or higher and excellent in delayed fracture resistance can be obtained.

[0022]

As described above, according to the present invention,
It is possible to obtain a high-strength bolt excellent in delayed fracture resistance while having a strength of 1300 N / mm ² or more, which has an extremely great industrial significance.

[Brief description of drawings]

FIG. 1 is a diagram showing a screw shape of an asymmetric screw applied to a high-strength bolt according to the present invention.

FIG. 2 is a view showing a screw shape of a triangular screw applied to a conventional high strength bolt.

[Explanation of symbols]

1a, 1b ... pointed ridges, 2a ... pressure side flank surface,
2b: Play-side flank surface, 3 ... Valley bottom, α: Thread angle, α1: Pressure-side flank angle, α2
…… Frank angle of the flank on the play side, H …… Height of the point, p …… Pitch

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Akira Ebata Akira Marunouchi 1-2-2 Japan, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Moriyuki Ishiguro 1-2-1 Marunouchi Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Tetsuyuki Ishizaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Masao Kamei, 4-16 Midoriki, Suminoe-ku, Osaka, Osaka Prefecture Samsung Sangyo Osaka Factory Co., Ltd.

Claims (1)

[Claims] 1. A bolt having an asymmetrical thread shape in which left and right flanks of a screw thread are different, wherein a flank angle of a pressure side flank is 40 to 60 °, and a tensile strength is 1300.
High-strength bolt with excellent delayed fracture resistance characterized by N / mm ² or more.