JP2000337334A - High structure bolt excellent in delayed fracture resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength bolt excellent in delayed fracture resistance which has a tensile strength of over 1200 N/mm2. SOLUTION: This bolt which is composed of a steel including C: 0.5-1.0% restrains the structure generation of one kind or more than one kind of pro- eutectoid ferrite, pro-eutectoid cementite, bentonite and martensite to make the area rate of pearlite structure with pearlite lamelar interval 200 nm or less to be 80% or above. Then, the high strength wire material which is made to have a tensile strength of over 1200 N/mm2 and excellent delayed fracture resistance by strong extension work is used. The material is cut to a fixed length and both end parts are threaded by thread rolling or cutting.

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 used for automobiles and various industrial machines, and more particularly to a high-strength bolt (tensile strength) of 1200 N / mm ² or more, while having a delayed fracture resistance. It relates to an excellent high-strength high-strength bolt and the like.

[0002]

2. Description of the Related Art As a general high-strength bolt steel, a medium carbon alloy steel (SCM435, SCM440, SCr440) is used.
Etc.) are used, and the necessary strength is ensured by quenching and tempering. However, in general high-strength bolts used for automobiles and various industrial machines, when the tensile strength is in a region exceeding about 1200 N / mm ² , there is a risk that delayed fracture occurs, and there is a restriction on use.

[0003] Delayed destruction can occur in a non-corrosive environment or in a corrosive environment, and it is said that the cause of occurrence is complicated by various factors.
It is generally difficult to identify the above cause. As the control factors affecting the delayed fracture as described above, tempering temperature, structure, material hardness, crystal grain size, various alloying elements and the like have been recognized for some time, but they are effective for preventing delayed fracture. In fact, no means has been established, and only various methods have been proposed by trial and error.

[0004] In order to improve the delayed fracture resistance, for example, Japanese Patent Application Laid-Open Nos. 60-114551, 2-267243 and 3-243745 have been proposed. These techniques are aimed at developing a high-strength bolt steel having excellent delayed fracture resistance even at a tensile strength of 1400 MPa or more by adjusting various main alloying elements. However, these techniques do not completely eliminate the danger of delayed fracture, but their application is limited.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and has as its object to provide a high strength steel having excellent delayed fracture resistance while having a tensile strength of 1200 N / mm ² or more. It is to provide a strength bolt.

[0006]

The high-strength bolt of the present invention, which has achieved the above object, comprises a steel containing C: 0.5 to 1.0% (meaning by mass%, the same applies hereinafter), The formation of one or more structures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite is suppressed, and the area ratio of the pearlite structure having a pearlite lamellar interval of 200 nm or less is set to 80% or more. In addition, a high-strength wire rod having a strength of 1200 N / mm ² or more and excellent delayed fracture resistance by strong wire drawing is used, and after cutting this into a predetermined length, both ends are thread-rolled or It has a gist in that it is threaded by cutting.

[0007] The object of the present invention is to provide C: 0.5 to 1.0.
% Of a pearlite structure having a pearlite lamellar interval of not more than 200 nm by suppressing the formation of one or more structures of proeutectoid ferrite, proeutectoid cementite, bainite, and martensite.
It is the above and 120
Use a high-strength wire rod with a strength of 0 N / mm ² or more and excellent delayed fracture resistance, cut this to a predetermined length, and form a bolt head at one end by warm forging. This is also achieved by employing a high-strength bolt configuration in which the other end is thread-rolled or threaded before or after warm forging.

In the high-strength bolt of the present invention, if necessary, (1) Si: 2.0% or less (excluding 0%) and / or Co: 0.5% or less (excluding 0%) ),
(2) It is also effective to contain Cr: 1.0% or less (excluding 0%).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied from various angles the cause of the inferior delayed fracture resistance of conventional high strength steel for bolts. As a result, in the conventional improvement method, the structure has been tempered martensite, and the delayed fracture resistance has been supplemented by avoiding the tempered embrittlement region, reducing the grain boundary segregation elements, and refining the crystal grains. It has been found that there is a limit in improving the delayed fracture resistance of high-strength steel.

The inventors of the present invention have conducted intensive studies to further improve the delayed fracture resistance. As a result, the structure was changed to a structure mainly composed of pearlite with a certain restriction, and 1200 N / mm ² or more was formed by strong drawing. The present inventors have found that a high-strength bolt exhibiting excellent delayed fracture resistance can be obtained by processing a bolt using a high-strength wire rod having the above strength, and completed the present invention.

The high-strength wire used as a material in the present invention is:
As described above, the formation of one or more of microstructures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite is suppressed, and the pearlite lamellar interval is 200 nm.
The area ratio of the following pearlite structure needs to be 80% or more. When a large amount of pro-eutectoid ferrite and pro-eutectoid cementite is formed in the above structure, a longitudinal crack occurs at the time of wire drawing and the wire cannot be drawn.
/ Mm ² or more. In addition, proeutectoid cementite and martensite cause disconnection during wire drawing, and thus need to be reduced. Further, since the amount of work hardening of bainite is smaller than that of pearlite, it is not possible to expect an increase in strength due to strong wire drawing.

On the other hand, the remaining pearlite structure has the effect of trapping hydrogen at the interface between cementite and ferrite and reducing the amount of hydrogen accumulated at grain boundaries. From these facts, one of proeutectoid ferrite, proeutectoid cementite, bainite and martensite
The formation of the seed or two or more kinds of tissues is suppressed as much as possible (ie, less than 20%), and the area ratio of the pearlite structure is reduced to 8%.
Must be 0% or more. That is, the area ratio of the pearlite structure is set to 80% or more so that at least one of the structures such as proeutectoid ferrite, proeutectoid cementite, bainite and martensite is reduced as much as possible so that the total area ratio is less than 20%. Need to be The area ratio of the pearlite structure is preferably 90% or more, and more preferably 100% pearlite structure.

The pearlite structure needs to have a pearlite lamellar interval of 200 nm or less. Refinement of the pearlite lamellar spacing is effective for increasing the strength of the steel material. As described above, the interface between cementite and ferrite is increased, and the hydrogen trapping effect is promoted. In order to sufficiently exhibit such effects, the pearlite lamellar interval needs to be 200 nm or less. The preferred pearlite lamellar interval is 150 nm or less, more preferably 100 nm or less, and even more preferably 7 nm or less.
5 nm or less.

In the wire rod used as a raw material in the present invention, required dimensional accuracy cannot be obtained as it is as rolled or forged, and it becomes difficult to obtain a strength of 1200 N / mm ² or more. Is required. In addition, cementite in some pearlite is finely dispersed by strong wire drawing processing, and while improving the hydrogen trapping ability,
The arrangement of the structures along the drawing direction provides resistance to crack propagation (the crack propagation direction is perpendicular to the drawing direction).

In the high-strength bolt of the present invention, C is set to 0.5 to
Medium carbon steel containing 1.0% is assumed, but the reason for limiting the range of the C content is as follows.

C: 0.5-1.0% C is a necessary and economical element for securing the strength of steel.
The strength increases as the C content increases. In order to secure the target strength, C needs to be contained at 0.5% or more. However, when the C content exceeds 1.0%, the precipitation amount of proeutectoid cementite increases, so that the toughness and ductility are significantly reduced and the wire drawing workability is deteriorated. A preferred lower limit of the C content is 0.65%, more preferably 0.1%.
7%. A preferable upper limit of the C content is 0.95.
%, More preferably 0.9%.

In the high-strength bolt of the present invention, various elements (Si, Co, Mn, Cu, Ni, Cr, M
o, Ti, Nb, V, W, Al, B, etc.). Of course, the addition of a predetermined amount of Si or Co is effective in suppressing the precipitation of proeutectoid cementite. Cr is effective for reducing the lamella spacing of pearlite to increase the strength and drawability of the wire. The reasons for limiting each element to be added as necessary are as follows.

Si: 2.0% or less (excluding 0%) Si has the effect of improving the hardenability of steel wire and suppressing the precipitation of proeutectoid cementite. In addition, it is expected to act as a deoxidizing agent, and exhibits a remarkable solid solution strengthening effect by being dissolved in ferrite. These effects increase as the content increases, but an excessive Si content reduces the ductility of the drawn steel wire, so the upper limit is 2.0%. The preferred upper limit of the Si content is 1.0%, more preferably 0.5%.

Co: 0.5% or less (excluding 0%) Co has an effect of suppressing the precipitation of pro-eutectoid cementite like Si, and is added to the high-strength bolt of the present invention for reducing pro-eutectoid cementite. It is particularly effective as a component.
These effects increase as the content increases, but if the content exceeds 0.5%, the effect is saturated and uneconomical. Therefore, the upper limit is set to 0.5%. Co
The preferred range of the content is 0.05 to 0.3%, more preferably the lower limit is 0.1%, and the upper limit is 0.2%.
Good to be.

Cr: 1.0% or less (excluding 0%) Cr has the effect of reducing the lamella spacing of pearlite and increasing the strength and drawability of the wire. However, C
If the r content is excessive, the transformation end temperature is similarly too long, which leads to oversize of the equipment and deterioration of productivity.
1.0% is the upper limit. Note that a preferable lower limit of the Cr content is 0.05%, and more preferably 0.1%. The preferable upper limit of the Cr content is 0.5%, more preferably 0.3%.

Mn: 0.2 to 1.0% Mn exerts an effect as a deoxidizing agent and an effect of improving the hardenability of the steel wire and improving the uniformity of the structure of the steel wire. In order to exert these effects, it is necessary to contain 0.2% or more. However, when the Mn content becomes excessive, M
Since a supercooled structure such as martensite or bainite is formed in the segregated portion of n to deteriorate the drawability, the upper limit is 1.0%. The preferred lower limit of the Mn content is 0.40%.
And more preferably 0.45%. Further, a preferable upper limit of the Mn content is 0.70%, and more preferably 0.55%.

Cu: 0.5% or less (excluding 0%) Cu is an element that contributes to increasing the strength of a steel wire by a precipitation hardening action. However, if added excessively, it causes grain boundary embrittlement and deteriorates delayed fracture resistance.
The upper limit is 0.5%. The lower limit of the Cu content is preferably 0.05%, and more preferably 0.1%. The preferable upper limit of the Cu content is 0.3%, more preferably 0.2%.

Ni: 1.0% or less (excluding 0%) Ni does not contribute much to the increase in the strength of the steel wire, but has the effect of increasing the toughness of the drawn wire. However, when the Ni content is excessive, the transformation end temperature becomes too long as in the case of Cr, which leads to oversize of equipment and deterioration of productivity.
1.0% is the upper limit. The lower limit of the Ni content is preferably 0.05%, and more preferably 0.1%. The preferable upper limit of the Ni content is 0.5%, and more preferably 0.3%.

A group consisting of Mo, Ti, Nb, V and W
At least one element selected from the group consisting of: 0.01 to 0.5% in total These elements form fine carbon / nitride and contribute to improvement in delayed fracture resistance. These carbides and nitrides are also effective for refining crystal grains. In order to exert such effects, it is necessary to contain a total of 0.01% or more, but if it is contained excessively, the delayed fracture resistance and toughness are impaired, so it is necessary to make the total 0.5% or less. . Incidentally, the preferable lower limit of the content of these elements is 0.1 in total.
02%, and more preferably 0.03%. A preferable upper limit is 0.3% in total, and more preferably 0.1%.

Al: 0.01% to 0.05% Al captures N in steel to form AlN, and contributes to the improvement of delayed fracture resistance by refining crystal grains.
For that purpose, it is necessary to contain 0.01% or more,
If the content exceeds 0.05%, nitride-based inclusions and oxide-based inclusions are generated, and the wire drawing property is reduced. The preferred lower limit of the Al content is 0.02
5%, and a preferable upper limit is 0.035%.

B: 0.0005% to 0.003% B is added for improving the hardenability of steel, but in order to exert its effect, it is necessary to contain 0.0005% or more. However, if the content exceeds 0.003%, the toughness is rather hindered. The preferred lower limit of the B content is 0.0010%, and the preferred upper limit is 0.0010%.
25%.

N: 0.015% or less (excluding 0%) N forms a nitride such as AlN or TiN, and thus has a favorable effect on the refinement of crystal grains and the improvement in delayed fracture resistance. However, if the N content is excessive, not only does the nitride excessively increase and adversely affect drawability, but also solute N promotes aging during drawing, so
Must be 5% or less. The upper limit of the N content is preferably 0.007%, and more preferably 0.005% or less.

In the high-strength wire rod of the present invention, other than the above components (the rest) is basically made of iron, but may contain trace components in addition to these components. It is included in the technical scope of the present invention. From the viewpoint of further improving the characteristics, P, S, and O are preferably suppressed as follows. Furthermore, although the high-strength wire of the present invention inevitably contains impurities, they are permissible as long as the effects of the present invention are not impaired.

P: 0.03% or less (including 0%) P is an element that causes grain boundary segregation and deteriorates delayed fracture resistance. Therefore, by setting the P content to 0.03% or less, delayed fracture resistance can be improved. The P content is
It is preferably reduced to 0.015% or less, and more preferably 0.005% or less.

S: 0.03% or less (including 0%) S forms MnS in steel, and when stress is applied, M
nS is the stress concentration point. Therefore, in order to improve the delayed fracture resistance, it is necessary to reduce the S content as much as possible, and the S content is preferably set to 0.03% or less. The S content is
It is preferably reduced to 0.01% or less, more preferably 0.005% or less.

O: 0.005% or less (including 0%) O hardly forms a solid solution in steel at room temperature, exists as hard oxide-based inclusions, and causes a disconnection of copper during wire drawing. Therefore, the O content should be as low as possible, and should be suppressed to at least 0.005% or less.
The O content is preferably reduced to 0.003% or less, and more preferably to 0.002% or less.

The high-strength wire used as a material in the present invention is:
The organization can be adjusted by various methods. A typical method will be described. As one of the methods, first, using a steel material having the above chemical composition, hot rolling or hot forging is performed so that the end temperature of rolling or forging the steel material is 800 ° C. or higher, and then the average cooling rate V (° C./second) is set so as to satisfy the following equation (1).
A method of continuously cooling to 00 ° C. and subsequently allowing the mixture to cool. 166 × (wire diameter) ^-1.4 ≦ V ≦ 288 × (wire diameter) ^-1.4 … (1)

According to this step, a more uniform pearlite structure can be obtained than in a normal rolled material, and the strength before drawing can be increased. If the end temperature of rolling or forging is too low, austenitization becomes insufficient and a uniform pearlite structure cannot be obtained, so the end temperature needs to be 800 ° C. or higher. The preferred range of this temperature is about 850 to 950 ° C, more preferably about 850 to 900 ° C.

The average cooling rate V is 166 × (wire diameter)
^When it is smaller than ^-1.4, not only a homogeneous pearlite structure cannot be obtained, but also proeutectoid ferrite and proeutectoid cementite are easily formed. The average cooling rate V is 288 ×
(Wire diameter) When it is larger than ^-1.4 , bainite and martensite are easily generated.

For the high-strength wire used in the present invention, a steel having the chemical composition as described above is used.
After heating to 0 ° C. or higher, the steel is rapidly cooled to a temperature of 520 to 650 ° C., and is maintained at a constant temperature (patenting treatment). It can rise.

In this method, the specified range of the steel material heating temperature must be 800 ° C. or higher for the same reason as the above-mentioned rolling or forging end temperature. The preferred range of the heating temperature is the same as described above. In the patenting process, it is desirable to use a salt bath, lead, a fluidized bed, or the like to rapidly cool the heated wire at a cooling rate as fast as possible. In order to obtain a homogeneous perlite structure, 520 to 650
It is necessary to carry out isothermal transformation at ℃. The preferred temperature range of the isothermal transformation temperature is 550 to 600 ° C., and the most preferred isothermal holding temperature is a temperature near the pearlite nose in the TTT diagram.

On the other hand, after the rolling or forging of the steel material, hot rolling or hot forging is performed so that the temperature becomes 800 ° C. or more, and then the steel is cooled to a temperature of 520 to 750 ° C. at an average cooling rate of 5 ° C./sec or more. By holding at an average cooling rate of 1.0 ° C./sec or less from the temperature for 200 seconds or more and then allowing to cool, a pearlite structure more uniform than that of a normal rolled material is obtained, and the strength before drawing is increased. Can be achieved. The operation in each step when such a method is adopted is as follows.

First, the specified range of the temperature after the end of the rolling or forging is set to 800 ° C. for the same reason as the heating temperature of the steel material.
It was decided above. The preferred range of this temperature is the same as described above. If the cooling rate after hot rolling or hot forging is too slow, ferrite transformation may occur during cooling, and it is preferable to cool at a cooling rate as high as possible. Therefore, the cooling rate at this time was specified to be 5 ° C./sec or more. The preferred range of the cooling rate is 10 ° C./sec or more, more preferably 30 ° C./sec or more. Although it is necessary to cool to 520 to 750 ° C. by this cooling, when the cooling end temperature is lower than 520 ° C. or higher than 750 ° C., a structure other than pearlite is easily generated by subsequent slow cooling.

After the above cooling, from the viewpoint of obtaining a homogeneous pearlite structure, the temperature (520 to 750 ° C.)
(Temperature: slow cooling start temperature), it is necessary to hold at least 200 seconds while cooling (slowly cooling) at an average cooling rate of 1.0 ° C./sec or less. If the average cooling rate at this time is higher than 1.0 ° C./sec or the holding time is less than 200 seconds, it is left to cool before transforming to a pearlite structure, and bainite and martensite are easily generated. The preferred range of the cooling rate is 0.5 ° C./sec or less, more preferably 0.2 ° C./sec or less. Further, a preferable range of the holding time is 300 seconds or more, and more preferably, 600 seconds or more. It is most preferable to keep the temperature near the pearlite nose in the TTT diagram for a long time.

After using the high-strength wire rod obtained as described above and cutting it into a predetermined length, (1) threading or threading the both ends by threading or cutting (to form stud bolts); Or (2) a bolt head is formed at one end by warm forging, and the other end is threaded by thread rolling or cutting before or after warm forging, and the like, so that excellent delayed fracture resistance and A bolt exhibiting strength is obtained.
In the above method (2), the use of the warm forging method when forming the bolt head is because the strength of the wire is high and it is difficult to form the bolt into a predetermined bolt shape by ordinary cold forging. It is.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design changes in the spirit of the present invention will be described. Included in the technical scope.

[0042]

Example 1 Using a test steel having a chemical composition shown in Table 1 below, the rolling end temperature was about 93 to a wire diameter of 11 mmφ and 14 mmφ.
After hot rolling to 0 ° C., the average cooling rate was 4.2
The blast cooling was performed in the range of １12.1 ° C./sec (Table 2 below). Thereafter, the wire was drawn to a wire diameter of 7.06 mm (drawing ratio: 59%, 75%).

[0043]

[Table 1]

Using the obtained various wire rods, M8 shown in FIG.
A stud bolt of × P1.25 was prepared, and a delayed fracture test was performed. In the delayed fracture test, a bolt was immersed in an acid (15% HCl × 30 minutes), washed with water and dried, and subjected to a stress load in air (load stress was 90% of the tensile strength). The presence or absence was evaluated. In addition, pro-eutectoid ferrite, pro-eutectoid cementite, bainite, martensite or pearlite structures were classified by the following method to determine the area ratio of each structure. Further, the lamellar interval of pearlite was measured by the following method. At this time, for the purpose of comparison, a delayed fracture test was performed on some of the alloys that had been quenched and tempered to have a 100% tempered martensite structure (No. 13 in Table 2 below).

(Method of Classifying Each Tissue) The cross section of the wire was embedded, polished, and then immersed in a 5% picric acid alcohol solution.
After immersion for 30 seconds to corrode, the structure of D / 4 (D: diameter) was observed by scanning electron microscope (SEM). After photographing 5 to 10 visual fields at 1000 to 3000 times and determining the pearlite tissue portion, the area ratio of each tissue was determined by an image analyzer. For the bainite structure and the proeutectoid ferrite structure, which are hard to distinguish from the pearlite structure, the structure shown in FIG. 2 (micrograph as a substitute for a drawing) is defined as a bainite structure, and the structure as shown in FIG. Was determined to be a proeutectoid ferrite structure. As a tendency of these structures, proeutectoid ferrite and proeutectoid cementite are:
Needle-like precipitates were formed along the former austenite crystal grain boundaries, and martensite was precipitated in bulk.

(Measurement method of perlite-lamellar interval) After embedding the cross section of the wire, polishing and immersing it in a 5% alcoholic picrate solution for 15 to 30 seconds to corrode it, using a scanning electron microscope (SEM) D / 4 (D is diameter)
The structure of the part was observed. In the pearlite structure near D / 4 part, the part considered to be the narrowest in the lamellar interval was 5,000 to 1
Ten visual fields were photographed at a magnification of 00000, and the length of a line perpendicularly crossing each lamellar was determined to measure the lamellar interval. The average value of 10 visual fields was defined as the average pearlite lamella interval.

The structure of each wire is shown in Table 2 below together with the average cooling rate V, and the results of the delayed fracture test are shown in Table 3 below together with the drawing conditions and mechanical properties. The appropriate range of the average cooling rate [the range that satisfies the formula (1)] is as follows.
4.12 ≦ V ≦ 7.16 (° C./second) when the wire diameter is 11 mm and 5.78 ≦ V ≦ 10.03 (° C./second) when the wire diameter is 11 mm.
Seconds).

[0048]

[Table 2]

[0049]

[Table 3]

Example 2 Using the test steel C shown in Table 1 above, hot rolling was performed so that the rolling end temperature was about 930 ° C. to a wire diameter of 11 mmφ, followed by rapid cooling. Patenting treatment (heating temperature: 750-935 ° C., constant temperature transformation: 495-6)
70 ° C. × 4 minutes). Thereafter, the wire was drawn to a wire diameter of 7.06 mm (drawing ratio: 59%).

[0051]

[Table 4]

Using the obtained various wires, stud bolts of M8 × P1.25 shown in FIG. 1 were produced, and a delayed fracture test was performed in the same manner as in Example 1. The structure of each wire is also shown in Table 4 above, and the results of the delayed fracture test are shown in Table 5 below together with the drawing conditions and mechanical properties.

[0053]

[Table 5]

Example 3 The test steel C shown in Table 1 was hot-rolled to a wire diameter of 11 mmφ under the rolling conditions shown in Table 6 below. afterwards,
The wire was drawn to a diameter of 7.06 mm (drawing ratio: 59%).

[0055]

[Table 6]

Using the obtained various wire rods, an M8 × P1.25 stud bolt shown in FIG. 1 was produced, and a delayed fracture test was performed in the same manner as in Example 1. The structure of each wire is shown in Table 7 below, and the results of the delayed fracture test are shown in Table 8 below together with the drawing conditions and mechanical properties.

[0057]

[Table 7]

[0058]

[Table 8]

As is apparent from these results, a bolt satisfying the requirements specified in the present invention has a tensile strength of 120.
It can be seen that even at 0 N / mm ² or more, it has excellent delayed fracture resistance.

[0060]

According to the present invention, a high-strength bolt excellent in delayed fracture resistance while having a tensile strength of 1200 N / mm ² or more can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a shape of a bolt subjected to a delayed fracture test in an example.

FIG. 2 is a drawing-substituting micrograph showing a bainite structure.

FIG. 3 is a drawing-substituting micrograph showing a proeutectoid ferrite structure.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yuichi Namimura 2nd Nadahama-Higashi-cho, Nada-ku, Kobe Inside Kobe Steel, Ltd. Kobe Steel Works (72) Inventor Koichi Makii 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Co., Ltd. (72) Inventor Hiroshi Ieguchi 1-5-5 Takatsukadai, Nishi-ku, Kobe City No. 5 Inside Kobe Research Institute, Kobe Steel, Ltd.

Claims (6)

[Claims] C: 0.5 to 1.0% (meaning by mass%,
The same shall apply hereinafter), the formation of one or two or more microstructures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite is suppressed, and the area ratio of the pearlite structure having a pearlite lamellar interval of 200 nm or less is reduced. A high-strength wire having a strength of not less than 80% and having a strength of 1200 N / mm ² or more and excellent delayed fracture resistance by strong wire drawing was used and cut into a predetermined length. A high-strength bolt excellent in delayed fracture resistance, characterized in that both ends are threaded by thread rolling or cutting. 2. Si: 2.0% or less (excluding 0%)
And / or Co: 0.5% or less (excluding 0%)
The high-strength bolt according to claim 1, wherein a high-strength wire rod containing 3. Cr: 1.0% or less (excluding 0%)
The high-strength bolt according to claim 1 or 2, wherein a high-strength wire rod containing: 4. A pearlite lamellar made of a steel containing C: 0.5 to 1.0%, suppressing formation of one or more types of microstructures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite. A high-strength wire rod having an area ratio of a pearlite structure having an interval of not more than 200 nm of not less than 80% and having a strength of not less than 1200 N / mm ² and excellent delayed fracture resistance by strong wire drawing. Used, after cutting this to a predetermined length, a bolt head is formed at one end by warm forging, and the other end is thread rolled or threaded before or after warm forging. High-strength bolt with excellent delayed fracture resistance. 5. Si: 2.0% or less (excluding 0%)
And / or Co: 0.5% or less (excluding 0%)
The high-strength bolt according to claim 4, wherein a high-strength wire rod containing: 6. Cr: 1.0% or less (excluding 0%)
The high-strength bolt according to claim 4 or 5, wherein a high-strength wire rod containing: