JPH0429607A - High-tension bolt excellent in delay breakage resistance characteristic - Google Patents

Abstract

PURPOSE:To enhance aged deterioration resistance, strength and quality reliability by forming the form of a screw by the ellipsoidal contact method to compute an ellipsoid in a specified formula, giving the height of pointing crests of a screw and the height on pointing-crest side of transition points in opposed flanks of adjoining crests and root in a specified formula. CONSTITUTION:Height of pointing rests 1a, 1b is determined to H and height of transition points B1, B2, between opposed flanks 3a, 3b of adjoining crests 2a, 2b and root 4 is determined by formula K.H/20, where K is an arbitrary number between 8 and 10 inclusive. An arc 6a on the root side of an ellipsoid 6 with its major axis (a) being in parallel to the bottom 5, having a locus passing through the transition points B1, B2 0.30<= minor axis (b)/major axis (a) <=0.70, is determined to be an arcuate curved line 7 for the sectional form of a screw. Accordingly, the stress concentration and the plastic deformation are reduced, the workability is good and the fatigue resistance is enhanced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to high-tensile strength bolts used in various industrial fields, particularly high-tensile strength bolts with excellent delayed fracture resistance used in civil engineering, architecture, and construction of marine structures. 6 Related to Bolts [Prior Art] When joining steel materials used in various industrial fields, stupid bolt joints t9 of various welding means are generally used.

In recent years, the pursuit of economic efficiency and advances in technology have promoted the use of high-strength steel materials, and high-tensile bolts have come to be used for joining them. In particular, friction welding means are increasingly used in the civil engineering and construction fields, and the bolts used for this are high-strength hexagonal bolts, hexagonal nuts, and flat washers for friction welding specified in JIS-B-1186 of Japan Industrial Short Circuit. A wide set is adopted.In addition, in the high-strength hexagonal bolts (hereinafter, in the present invention, for convenience of explanation, they will be collectively referred to as high-tensile bolts), due to the delayed fracture phenomenon, currently there is no one that is higher than FI IT. However, as manufacturing technology for steel plates continues to advance, high-strength products at economical prices are now available on the market. High-tensile bolts with excellent delayed fracture characteristics (
In particular, the development of the F15T100 has become strongly desired.

In order to prevent delayed fracture, it is generally known that in addition to using low- to medium-carbon steel with excellent delayed fracture resistance, it is necessary to minimize the stress concentration that occurs in each part of high-tensile bolts. .

By the way, it has been reported in many literatures that this reduction of stress concentration is also very effective for fatigue resistance, and from this point of view, improving the thread shape and reducing stress concentration at the thread root can improve fatigue resistance. A fatigue-resistant screw with increased fatigue strength (hereinafter referred to as invention A) is described in Japanese Patent Publication No. 53-29780, and invention A is thought to be sufficiently effective in resisting delayed fracture. In high-tensile bolts, the parts that cause delayed fracture are mainly the lower part of the neck of the bolt head, the upper part of the thread cut, and the threaded part near the load bearing surface of the nut.These parts have a large concentration of stress, and are often tightened with high axial force. It is known from many studies that delayed fracture occurs starting from these parts because the plastic strain increases. Invention A was devised to improve the shape of the screw thread where delayed fracture occurs most frequently, and the flank angle was set to (70±2)° or (106°).
±5)°, and the radius of each screw root is (0,15
±0.02) p (pitch) or (0,30±0
．． 01) It is formed by an arc of p.

In addition, commercially available high-tensile bolts have a composite structure in the lower part of the neck of the bolt head, in order to avoid concentration of stress in the lower part of the neck of the bolt head, the upper part of the thread cut, and the threaded part near the load bearing surface of the nut. R is adopted, and the R of the root of the thread part is adopted.
There is a bolt that is precisely carved and has a rounded bottom at the top of the thread. However, for this commercially available bolt, it is not clear what kind of theory is used to form the arcuate curve of the thread root, and furthermore, no test data has been published to support its effects.

To the best of the applicant's knowledge, the invention A has not been put to practical use in high tensile strength bolts, particularly in the civil engineering and construction fields, and has not been put to practical use in high tensile strength bolts, especially in the fields of civil engineering and construction, and has not been put to practical use in high tensile strength bolts, especially in the fields of civil engineering and construction.
There is no example of a high tensile strength bolt with a novel shape that reduces stress concentration and is widely provided in fabrics. Therefore, as a result of research on high-tensile bolts with excellent delayed fracture resistance, the present inventors found that the composition is low to medium carbon steel, the thread flank angle is 60°, and the bolts are carved at equal pitches. A high-tensile bolt with excellent delayed fracture resistance (
(hereinafter referred to as Invention B). In the three-arc synthesis method, the height of the peak is H, and the transition point B1. B1 is set to (9±1)H/20 from the bottom of the peak, and at each of the transition points B+ and Ba, draw a contact small circle with a radius r of H/6 that contacts the flank surface, and then The radius R is centered at an arbitrary position of 0,976H or more in the height direction perpendicular to the base from the intersection of the extension line of the surface and the base, and the radius R is 2.
A method for forming an arcuate curve by drawing a large circle of /3H or more and superimposing the contact small circle and the arc on the root side of the large circle, the method comprising: a high tension bolt having a screw root formed in an arcuate curve by the method; has very small stress concentration and plastic strain;
Excellent delayed fracture resistance.

[Problem to be solved by the invention]

Bolts, which are the most important components that connect structures, have not only mechanical strength but also high quality reliability and resistance to aging, as well as excellent workability and productivity. It goes without saying that a good quality and low price are conditions for widespread adoption in the market, but the reason why Invention A is not put into practical use in the market, regardless of its excellent characteristics, is that some of the above conditions are not met. It is presumed that this is because they are not satisfied.

The present inventors have developed high-tensile bolts in the civil engineering and architectural fields,
In particular, as a result of research into new shapes with less stress concentration for things beyond FI IT, we found that bolts with new flank angles and threads with unequal pitches confirmed reliability and improved productivity. He developed Invention B, knowing that it would require a huge amount of experimentation to confirm it, it would be extremely difficult economically, and it would take a long time to supply it to the market.
In order to conduct further research and create a high-tensile bolt comparable to Invention B in terms of equipment costs and processing costs, the arcuate curve of the thread root should always be a simple and easy-to-process curve as long as it does not deviate from the purpose. This is desirable.

The purpose of the present invention is to have high reliability in mechanical strength and quality, high resistance to aging, excellent workability and productivity, easy processing, low price, and delayed fracture resistance. Our goal is to provide superior high tensile strength bolts.

[Means for Solving the Problems 1] The present invention solves the above problems and achieves the objects. , and are engraved at equal pitches, and the valley bottom is formed into an arcuate curve formed by the elliptical arc abutment method defined by the following conditions and formulas, making it a high-tensile bolt with excellent delayed fracture resistance.

In the elliptical arc contact method, the height of the pointed crest is H, and the transition points B, , B, between the flank surface of the opposing screw thread and the root are taken from the crest side and set at the height specified in Equation 11 below. Further, a method of defining the bottom-side arc of the ellipse defined by the following equation (2), which has a locus passing through each of the transition points BB2 and whose long axis is parallel to the base as the bottom arc-shaped curve.

K-H/20・・・・・・・・・・・・・・・
・・・・・・・・・ (1) However, = any number from 8 to 10 0.30≦b/a≦0.70 ・・・・・・・・・
(2) However, a is the long axis, and b is the short axis. Furthermore, it is preferable to use low/medium carbon steel with excellent delayed fracture resistance.

[Function] The high-tensile bolt of the present invention uses low/medium carbon steel that has high mechanical strength and is suitable for processing and heat treatment, so it has good productivity, can be mass-produced, and can be produced at a reasonable price. can be supplied to the market. In addition, in the present invention, low/medium carbon steel is defined as having a carbon content of 0.0% by weight. Refers to carbon steel used for manufacturing bolts with lθ% to 0.40%, that is, if C is less than 0.10%, the desired strength cannot be obtained;
．． This is because if it exceeds 40%, the hardness becomes too high and the purpose cannot be achieved.

Next, the high tensile strength bolt of the present invention has a flank angle of 60"
It adopts threads carved at regular pitches, and is the same as the flank angle and pitch of general bolts specified in JIS and ISO standards, so many of the problems associated with conventional bolts can be avoided. Data on characteristics research, experiments, and implementation can be used, reducing research costs and making it possible to supply highly reliable high-tensile bolts to the market at an economical price.

Furthermore, since the root of the root is formed into an arcuate curve formed by the elliptical arc abutment method defined by the above conditions and formula, the stress concentration at the root of the thread, which was considered the most problematic in the past, is significantly reduced, exceeding FI IT. Especially F15T1
It is possible to realize a high tensile strength bolt without delayed fracture for 000 applications.

Next, regarding one embodiment, drawings according to the elliptical arc abutment method and its effects regarding the formation of the valley bottom will be explained with reference to the drawings.

In Fig. 1, the heights of the peaks 1a and lb are set as H, and the transition points B, B2 between the flank surfaces 3a and 3b of the opposing threads 2a and 2b and the root 4 are set according to the following equation (1). 10H/20, that is, 1/2 H
The bottom side of an ellipse 6, which is set to a height of It is a thread-shaped cross-sectional schematic diagram in which the circular arc 6a is defined as a root arc shape l117.

K・H/20・・・・・・・・・・・・Old・
...fil = any number from 8 to lO 0, 30≦b/as; 0.70 ...
(2) However, a is the long axis, and b is the short axis. In the present invention, as described above, the transition points B3 and B2 are set at the height defined by the above-mentioned formula [11] from the bottom side 5 of the pointed mountain. The reason is that when forming the arcuate surface [7] that satisfies the object of the present invention, an appropriate hooking rate that prevents screws from coming out is maintained. This is because it is impossible to form a satisfactory arcuate curve, and it is difficult to maintain an appropriate catching rate at an upper limit of 10/20.8 or more.

Further, in the present invention, as described above, the valley bottom arcuate curve is defined, but at this time, the ratio b/a of the major axis a to the minor axis is set to 0.
．． The reason for setting the value to be 30 or more and 0.70 or less is that transition points B, ,B
These are the results of simulations carried out on a large number of ellipses 6 whose long axes are parallel to the base 5 and whose loci pass through This is because it is not possible to obtain an appropriate valley bottom arcuate curve. This will be explained using the example shown in FIG. Figure 2 shows major axis radius a
, ellipses 8 to 11 whose minor axis radius is determined as shown in Table 1.
The valley bottom side circular arcs 8a to lla are the valley bottom arcuate curves 8a+ to fl
It is a screw-shaped cross-sectional schematic diagram drawn as at.

Table 1 However, H = 2.1651 mm M 22 The fact that the thread root formed according to the root arc curves 8a+ to 11a+ obtained as described above has the lowest degree of stress concentration is explained with reference to Figs. 3 to 10. do.

Figures 3 to 10 show M2 manufactured into various thread shapes using low/medium carbon steel having the components shown in Table 2.
2 bolts (Architectural Institute of Japan/Architectural Work Standard Specifications JASS
6, but the pitch P is 2.5 mm), a uniform tensile load is applied in the axial direction, and an axisymmetric finite element analysis (8-node isoparametric method) is performed. Figures to Figures 6 are calculated stress concentration degrees during elastic loading, and Figures 7 to 1O are for F15.
The bolt axial force (30,77o
The magnitude of the plastic strain when pulled in a - direction is calculated by n), and FIGS. 3 to 6 correspond to the valley bottom arc shapes 1#18a1 to 11a+, respectively.

As is clear from Table 2, Figures 3 to 6, the stress concentration coefficients at the thread root are respectively 1.68 to 1.46 at their maximum values, and it can be seen that results comparable to Invention B can be expected. , is clearly far superior to 1.98 of invention A.

Furthermore, regarding the above-mentioned example, a comparison will be made with respect to FIGS. 7 to 1O with regard to plastic strain. 7 to 10 correspond to the valley bottom arcuate curves 8a+ to 11a+, respectively. As shown in Figures 7 to 10, the plastic strain is 2490μ
~217μ, whereas that of the fatigue-resistant screw of the A invention is 2500μ, and for the ISO metric screw it is 7
300μ, and the difference is so large that it cannot be compared.

Traditionally, it has been difficult to evaluate delayed fracture properties, but a method for evaluating hydrogen embrittlement susceptibility has been developed, and based on this evaluation method, high-strength steels made of low and medium carbon steels with excellent delayed fracture properties have been proposed. For example, it is known that low- and medium-carbon steels having the components shown in Table 3 are suitable for high-tensile bolts.

Accordingly, in the present invention, if these well-known high-strength steels made of low- and medium-carbon steels with excellent delayed fracture resistance are used as high-tensile bolt materials, a synergistic effect on delayed fracture resistance can be expected. I can do it.

In addition, for the high-tensile bolt of the present invention, a transition part whose length is four times or more the thread pitch is provided from the shaft part to the thread part, and the boundary part between the transition part and the thread part is larger than the radius of the thread bottom. Rounded, and the rounded part under the bolt neck is JASS6
According to the findings of the present inventors, these improvements have a considerable effect because their positions are delayed and the rate of fracture starting points is significantly reduced. Recognized as having cattle.

[Example] Next, the high-tensile bolt according to the present invention (hereinafter referred to as B-port) and the high-tensile-strength bolt manufactured in accordance with JASS6 (hereinafter referred to as C-bolt) were tested to reach failure using the nut rotation method. We will explain the results of comparing the fracture resistance properties of bolt axial force application tests up to the point where the bolts were applied.

Table 4 shows the components of the carbon steel used in the test.

Table 4: The bolt body is made by cold forging a bar material to form the head and shaft portion, cutting the threaded portion, water quenching at 900℃, and then tempering at 430℃.
The nut and washer were manufactured to have a strength of 1/2 mm, but the explanation of the nut and washer will be omitted.

The dimensions of the bolt are M22, the length under the neck is 90 mm, and the thread pitch is 2.5 mm.

The pilot hole diameter of the nut used for the B bolt was machined to be 0.8 mm larger than that normally used for the C bolt.

FIG. 11 is a diagram showing the results of the bolt axial force application test,
The horizontal axis is the nut rotation angle (degrees), and the vertical axis is the bolt axial force (Ton).
) is shown.

After applying an initial torque of 15 kgf-m, the nuts were tightened, but the B-bolt broke at the play thread at about 900 degrees, and the C-bolt, which has a well-known spiral shape, broke at about 600 degrees. It broke at the play screw.

As is clear from FIG. 11, the high tensile strength bolt according to the present invention has significantly improved fracture resistance of the threaded portion as compared to the conventional well-known high tensile strength bolt. Furthermore, as is clear from FIG. 11, the maximum axial force of the B bolt is about 10% larger than that of the C bolt.

This indicates that the thread shape of the present invention has an effective cross-sectional area of the bolt approximately 10% larger than that of the conventional JIS metric coarse thread, and apparently has a higher strength of about 10%. You can expect the same effect as using -.

[Effect of the invention 1 As explained in detail above, the high-tensile strength bolt according to the present invention has a very small stress concentration and a very small amount of plastic strain compared to conventional high-tensile strength bolts, and has excellent delayed fracture resistance. Therefore, it is possible to reduce the number of steel plates used when joining steel plates, and also to improve the joining speed and reduce construction costs.The joints can also be compact, so extremely large economic effects can be expected.

In addition, the flank angle is structurally the same as JIS metric threads, and the same pitch as JIS metric coarse threads can be used, so there is no discomfort during work.
It has the advantage of being easy to accept at work sites due to its good workability, and in terms of reliability, it is possible to use test data and actual values for well-known JIS metric screws because the change in friction coefficient value (k+ is small) As a result, research costs, production planning costs, equipment costs, etc. can be significantly saved, and furthermore, as already mentioned, reducing stress concentration is very effective in improving fatigue resistance. High tensile strength bolts can also be expected to have excellent effects as fatigue resistant bolts.

[Brief explanation of drawings]

Fig. 1 is a diagram showing how to determine the shape of the thread and root according to the present invention, Fig. 2 is a schematic cross-sectional view of the thread shape regarding the high-tensile bolt according to the present invention, and Figs. FIGS. 7 to 10 are diagrams showing the stress concentration coefficient, FIGS. 7 to 10 are diagrams showing the plastic strain regarding high-tensile bolts, and FIG. 11 is a diagram showing the results of the bolt axial force application test. 1a, ■b...Pointed thread, 2a, 2b...Screw thread,
3a, 3b... flank surface, 4... valley bottom, B+, B
t...Transition point, 5...Base, 6...Ellipse, 6a.
... Valley bottom side circular arc, 7... Valley bottom arc-shaped curve, 88 ~ lla
... Valley bottom side circular arc, 8 a + - 11a + ... Valley bottom arc-shaped curve.

Claims (2)

[Claims] (1) The composition is low/medium carbon steel, the flank angle of the thread is 60°, the grooves are carved at equal pitches, and the root is formed by the elliptical arc contact method specified by the following conditions and formula. A high-tensile bolt with excellent delayed fracture resistance that is formed into an arcuate curve. In the elliptical arc contact method, the height of the peak is H, and the transition points B_1 and B_2 between the flank surface and the root of the opposing screw thread are
is set at a height defined by the following equation (1) from the base of the peak, and is an ellipse defined by the following equation (2), and has a locus passing through each of the transition points B_1 and B_2, and the major axis is the above-mentioned. A method of determining the bottom arc of an ellipse parallel to the bottom as the bottom arc curve. K・H/20・・・・・・・・・・・・・・・・・・
...(1) However, K = any number from 8 to 10, 0.30≦b/a≦0.70...
...(2) However, a is the long axis and b is the short axis 2. The high tensile strength bolt according to claim 1, wherein the component (2) is a low/medium carbon steel having delayed fracture resistance.