JPH11270531A - High strength bolt excellent in delayed fracture characteristics and method of manufacturing the same - Google Patents

Abstract

(57) [Problem] To provide a high-strength bolt excellent in delayed fracture characteristics by adjusting a metal structure, a holding temperature, and a cooling time during quenching or thread rolling, and a manufacturing method thereof. I do. SOLUTION: C, Si, Mn, Al, Ti, B, Cr, Mo, Ni, Cu, V, Nb, Ta, W
In the steel specified, a steel having a structure mainly composed of martensite or tempered martensite or bainite or pearlite is heated to a temperature range of Ac ₃ to Ac ₁ and thread-rolled. By cooling at a cooling rate and then tempering to a temperature range of 200 to 600 ° C., a region from the surface layer to a depth of at least 50 μm is composed of a layered structure of martensite and ferrite,
Further, the present invention provides a high-strength bolt excellent in delayed fracture characteristics, having a tensile strength of 1300 MPa or more, and a method for producing the same.

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 having a tensile strength of not less than 1300 MPa and excellent in delayed fracture resistance (hereinafter referred to as delayed fracture characteristic) and a method for producing the same.

[0002]

2. Description of the Related Art High-strength bolts widely used in machines, automobiles, bridges and buildings are, for example, JIS G410.
4, SCr, S specified in JIS G 4105
It is manufactured by quenching and tempering using medium carbon steel having a C content of 0.20 to 0.35% such as CM. However, the tensile strength was 13
It is well known that when the pressure exceeds 00 MPa, the risk of delayed fracture increases. For example, at present, the strength of building bolts currently used is limited to 1150 MPa class.

As a conventional finding for improving the delayed fracture characteristics of high strength bolts, for example, Japanese Patent Publication No. 3-243744 proposes that it is effective to make old austenite grains finer and to make the structure bainite. doing. Certainly, the bainite structure is effective against delayed fracture, but the bainite treatment increases the manufacturing cost.

Regarding the refinement of old austenite grains,
Japanese Patent Publication No. 64-4566 and Japanese Patent Publication No. 3-243745.
It is also proposed in the official gazette. In addition, Japanese Patent Publication No. 61-648
No. 15 proposes to add Ca. However, none of the proposals has led to a significant improvement in delayed fracture characteristics in the tests of the present inventors.

As described above, in the prior art, there is a limit in manufacturing a high-strength bolt with drastically improved delayed fracture characteristics.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above situation, and provides a high-strength bolt having good delayed fracture characteristics and a strength of 1300 MPa or more, and a method of manufacturing the same. The purpose is to provide.

[0007]

Means for Solving the Problems The present inventors first analyzed in detail the delayed fracture behavior using bolt steels of various strength levels manufactured by quenching and tempering. Since it is already clear that delayed fracture has occurred due to hydrogen in the steel material, the delayed fracture characteristics were evaluated by determining the "critical diffusible hydrogen amount" at which delayed fracture does not occur. In this method, various levels of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from leaking from the sample to the atmosphere during the delayed fracture test, and then the air is charged in the atmosphere. A predetermined load is applied to evaluate the diffusible hydrogen amount at which delayed fracture does not occur.

FIG. 1 shows an example in which the relationship between the amount of diffusible hydrogen and the rupture time until delayed fracture is analyzed. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture increases, and when the amount of diffusible hydrogen is less than a certain value, delayed fracture does not occur. This amount of hydrogen is defined as “critical diffusible hydrogen amount”. The higher the critical diffusible hydrogen content, the better the delayed fracture resistance properties of the steel material, which is a value specific to the steel material determined by the composition of the steel material and manufacturing conditions such as heat treatment. The amount of diffusible hydrogen in a sample can be easily measured by gas chromatography.

Therefore, in order to increase the critical diffusible hydrogen content of the high-strength bolt, that is, to increase the delayed fracture characteristics, the inventors repeated studies on the austenitic crystal grain size and the effects of quenching and tempering conditions. As a result, it was found that the delayed fracture characteristics could not be significantly improved even if any of the above factors was greatly changed. Since delayed fracture is grain boundary cracking along the former austenite grain boundary, it was concluded that prevention of grain boundary cracking is important to achieve significant improvement in delayed fracture characteristics. did.

[0010] Then, as a result of various studies on means for preventing austenite grain boundary cracking, the layered structure of martensite or tempered martensite and ferrite, or the major axis direction of each structure is oriented in the axial direction of the bolt. When a layered structure of martensite or tempered martensite and ferrite is formed, 130
Knowledge that austenite intergranular cracking can be prevented even in a high-strength region exceeding 0 MPa, that is, since the fracture mode is intragranular, the critical diffusible hydrogen content is greatly increased and the delayed fracture resistance is significantly improved. Was found.

[0011] Further, it is possible to obtain a layered structure of martensite and ferrite by selecting heat treatment conditions and a steel structure before heat treatment, and to select a rolling temperature condition and a steel structure before rolling. As a result, a technique was established that could form a layered structure of martensite and ferrite oriented in the axial direction of the bolt.

On the basis of the above examination results, it was concluded that a high-strength bolt excellent in delayed fracture characteristics can be realized by optimally selecting a steel material composition, a structural form, heat treatment conditions or thread rolling conditions. It has been achieved.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) A region from the surface layer to a depth of at least 50 μm is composed of a layered structure of martensite and ferrite,
Further, a high-strength bolt excellent in delayed fracture characteristics, having a tensile strength of 1300 MPa or more. (2) A high-strength bolt excellent in delayed fracture characteristics, characterized in that a region from the surface layer to a depth of at least 50 μm is composed of a layered structure of tempered martensite and ferrite and has a tensile strength of 1300 MPa or more.

(3) Delayed fracture characterized in that a region from the surface layer to a depth of at least 50 μm is composed of a layered structure of martensite and ferrite oriented in the axial direction of the bolt, and has a tensile strength of 1300 MPa or more. High strength bolt with excellent properties. (4) A region from the surface layer to a depth of at least 50 μm is composed of a layered structure of tempered martensite and ferrite oriented in the axial direction of the bolt, and further has a tensile strength of 1%.
A high-strength bolt excellent in delayed fracture characteristics characterized by being 300 MPa or more.

(5) In the layered structure of martensite or tempered martensite and ferrite, the distance between martensite or tempered martensite is 10 μm or less, wherein (1) to (4). A high strength bolt excellent in delayed fracture characteristics according to any one of the above. (6) C: 0.15 to 0.50% by weight, S
i: 0.05 to 2.0%, Mn: 0.2 to 2.0%,
Al: 0.005 to 0.1%, the balance being Fe
And a high-strength bolt excellent in delayed fracture characteristics according to any one of the above items (1) to (5), wherein the bolt is made of unavoidable impurities.

(7) C: 0.15 to 0.5% by weight
0%, Si: 0.05 to 2.0%, Mn: 0.2 to
2.0%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.05%, B: 0.
0003-0.0050%, Cr: 0.05-2.0
%, Mo: 0.05-1.0%, Ni: 0.05-
5.0%, Cu: 0.05-1.0%, V: 0.
05-0.5%, Nb: 0.005-0.2%, T
a: 0.005 to 0.5%, W: 0.05 to 0.5
%, One or more kinds of the high-strength bolts having excellent delayed fracture characteristics as described in any one of the above items (1) to (5). (8) By weight%, C: 0.15 to 0.50%, S
i: 0.05 to 2.0%, Mn: 0.2 to 2.0%,
Al: 0.005 to 0.1%, the balance being Fe
Ac ₃ to Ac ₁ is a steel made of martensite, tempered martensite, or bainite or pearlite.
And then cooling at a cooling rate of 10 ° C./sec or more.

(9) C: 0.15 to 0.5% by weight
0%, Si: 0.05 to 2.0%, Mn: 0.2 to
Ac is a steel containing 2.0%, Al: 0.005 to 0.1%, the balance being Fe and unavoidable impurities and having a structure mainly composed of martensite or tempered martensite or bainite or pearlite. ₃ after heating to the temperature range of to Ac _1, cooled at a cooling rate of more than 10 ° C. / sec, then 200 to 600 of the delayed fracture, characterized in that tempering in the temperature range of ° C. excellent high strength bolts Production method. (10) By weight%, C: 0.15 to 0.50%, S
i: 0.05 to 2.0%, Mn: 0.2 to 2.0%,
Al: 0.005 to 0.1%, the balance being F
consists e and unavoidable impurities, Ac ₃ the steel having a structure mainly composed of martensite or bainite or pearlite martensite or tempered ~Ac
_A method for producing a high-strength bolt excellent in delayed fracture characteristics, wherein the bolt is heated to a temperature in the range of ₁ and thread-rolled, and then cooled at a cooling rate of 10 ° C./sec or more.

(11) C: 0.15 to 0.5% by weight
0%, Si: 0.05 to 2.0%, Mn: 0.2 to
Ac is a steel containing 2.0%, Al: 0.005 to 0.1%, the balance being Fe and unavoidable impurities and having a structure mainly composed of martensite or tempered martensite or bainite or pearlite. ₃ was heated to a temperature range of to Ac _1, after thread rolling,
Cool at a cooling rate of 10 ° C./sec or more, and then
A method for producing a high-strength bolt having excellent delayed fracture characteristics, characterized by tempering to a temperature range of 00 ° C. (12) By weight%, further Ti: 0.005 to 0.05
%, B: 0.0003-0.0050%, Cr: 0.
05-2.0%, Mo: 0.05-1.0%, N
i: 0.05-5.0%, Cu: 0.05-1.
0%, V: 0.05 to 0.5% Nb: 0.0
05 to 0.2%, Ta: 0.005 to 0.5%, W
: High-strength bolt excellent in delayed fracture characteristics according to any one of the above items (8) to (11), containing one or more of 0.05 to 0.5%. Manufacturing method.

[0019]

Next, an embodiment of the present invention will be described. (Structure form) First, the reason for limiting the structure form, which is the most important point for the improvement of the delayed fracture characteristic of the high strength bolt, which is the object of the present invention, will be described.

FIGS. 2 and 3 show the results of scanning electron microscope observation of the layered structure of martensite or tempered martensite and ferrite of the bolt defined in the present invention. 2 and 3 show the case of martensite.
It turns out that martensite and ferrite have a fine layered structure. In FIG. 3, it can be seen that a fine layered structure of martensite and ferrite oriented in the axial direction of the bolt is obtained. Controlling such a fine layered structure of martensite and ferrite or a fine layered structure of tempered martensite and ferrite by tempering the same makes it possible to improve delayed fracture characteristics.

FIG. 4 shows an example of analyzing the influence of the structure on the delayed fracture characteristics. By controlling to a fine layered structure of martensite or tempered martensite and ferrite, the amount of critical diffusible hydrogen becomes significantly higher than that of tempered martensite structure by conventional quenching and tempering. It can be seen that the critical diffusible hydrogen content is further improved by controlling to a fine layered structure of martensite or tempered martensite and ferrite oriented in the axial direction.

In order to improve the strength and the delayed fracture characteristics, the distance between the martensite or the tempered martensite is 10 μm in the above layered structure.
m is desirable, and a more desirable condition is 5 μm or less.

In the above layered structure, the range of the ferrite fraction is preferably from 10 to 70%. This is 10%
If it is less than 50%, the effect of improving the amount of critical diffusible hydrogen cannot be expected,
If it exceeds 70%, it is difficult to obtain a high strength of 1300 MPa or more. More desirable condition is 20-60%
It is.

The layered structure of ferrite and martensite or tempered martensite and the layered structure of ferrite and martensite or tempered martensite oriented in the axial direction of the bolt need to be realized in the entire cross section of the bolt. Not at least 50μ from the surface
If the layered structure is obtained in a region up to a depth of m or more, the critical diffusible hydrogen is improved. A more desirable condition is that the layered structure is obtained in a region from the surface to a depth of 300 μm or more.

(Steel Material Components) Next, the reasons for limiting the components of the steel which are the subject of the present invention will be described. C: C is an essential element for securing the strength of the bolt, but if it is less than 0.15%, the required strength cannot be obtained. On the other hand, if it exceeds 0.50%, the toughness is deteriorated and the delay resistance is increased. In order to degrade the fracture characteristics, the range was limited to the range of 0.15 to 0.50%.

Si: Si has an effect of increasing the strength by a solid solution hardening effect. If it is less than 0.05%, the above effect cannot be exerted. On the other hand, if it exceeds 2.0%, an effect commensurate with the added amount cannot be expected. Therefore, the content is limited to the range of 0.05 to 2.0%.

Mn: Mn is an element not only necessary for deoxidation and desulfurization but also effective for enhancing the hardenability for obtaining a martensitic structure. On the other hand, if the content exceeds 2.0%, segregation at the grain boundary during heating in the austenite region causes embrittlement of the grain boundary and deterioration of the delayed fracture resistance.
Limited to the range of ２．０2.0%.

Al: Al forms AlN during deoxidation and heat treatment, thereby preventing the austenite grains from being coarsened and also fixing N. Effect is not exhibited, and even if it exceeds 0.1%, the effect is saturated.
It was limited to the range of 5 to 0.1%.

The above are the basic components of the steel targeted by the present invention. In the present invention, the steel further contains Ti: 0.
005-0.05%, B: 0.0003-0.005
0%, Cr: 0.05 to 2.0%, Mo: 0.05
-1.0%, Ni: 0.05-5.0%, Cu:
0.05-1.0%, V: 0.05-0.5%
Nb: 0.005 to 0.1%, Ta: 0.005 to 0.
One or more of 5% and W: 0.05 to 0.5% can be contained.

Ti: Like Al, Ti forms TiN during deoxidation and heat treatment, thereby not only preventing the austenite grains from becoming coarse, but also fixing N. %, These effects are not exhibited, and even if it exceeds 0.05%, the effect is saturated. Therefore, the range is limited to the range of 0.005 to 0.05%.

B: B has the effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Further, B segregates at austenite grain boundaries, thereby significantly improving hardenability.
If it is less than 0.0003%, the above-mentioned effect is not exhibited,
Even if it exceeds 0050%, the effect is saturated.
Limited to 3-0.0050%.

Cr: Cr is an element effective for improving hardenability and increasing softening resistance during tempering, but if it is less than 0.05%, its effect cannot be fully exhibited.
On the other hand, if it exceeds 2.0%, the toughness and the cold workability deteriorate, so the content is limited to 0.05 to 2.0%.

Mo: Mo, like Cr, has a strong tempering softening resistance and is an effective element for increasing the tensile strength after heat treatment, but if it is less than 0.05%, its effect is small, while If the content exceeds 1.0%, the effect is saturated, and the cost is increased, so that the content is limited to 0.05 to 1.0%.

Ni: Ni is added in order to improve ductility, which deteriorates with increasing strength, and to improve the hardenability during heat treatment to increase the tensile strength.
If it is less than 0.05%, the effect is small. On the other hand, if it exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited.
Limited to 5.0% range.

Cu: Cu is an effective element for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, hot workability is deteriorated. Limited to 0.05-1.0%.

V: V has the effect of reducing the size of austenite grains by forming carbonitrides during the quenching treatment. However, if it is less than 0.05%, the above effect cannot be obtained. Since the effect saturates even if it exceeds 0.5%, it was limited to 0.05 to 0.5%.

Nb: Like V, Nb is also an effective element for forming carbonitrides to refine austenite grains, but if it is less than 0.005%, the above effect is insufficient. , 0.1%, the effect is saturated, so that the content is limited to 0.005 to 0.1%.

Ta: Ta also has an effect of refining austenite grains like Nb. However, if it is less than 0.005%, the above effect is not exerted. Is saturated, so the content is limited to 0.005 to 0.5%.

W: W is an element effective for improving the delayed fracture characteristics of high-strength bolts. However, if the content is less than 0.05%, the above-mentioned effect is not exhibited. However, the effect is saturated, so the range is limited to 0.05 to 0.5%.

Although P and S are not particularly limited,
From the viewpoint of improving delayed fracture characteristics,
A preferred range is 5% or less.

N is effective in reducing the size of austenite grains by forming nitrides of Ti, Al, V, and Nb, so that the preferred range is 0.003 to 0.015%. Further, with respect to N, the formation of nitrides of Al, V, Nb, and Ti has the effect of refining old austenite grains and increasing the yield strength.
-0.015% is a desirable range.

The production method of (Production conditions) High-strength bolt of the present invention, either cooled at 10 ° C. / sec or more cooling rate after heating the steel of the component to a temperature range of Ac ₁ to Ac _3, or cooling After that, the steel tempered in the temperature range of 200 to 600 ° C., or the steel of the above component is thread-rolled in the temperature range of Ac ₁ to Ac ₃ and then cooled at a cooling rate of 10 ° C./sec or more, or 200 ° C. Tempering in a temperature range of up to 600 ° C.

The reasons for defining the manufacturing conditions will be described below. The present invention is to obtain a layered structure of ferrite and martensite by heating a steel material to a two-phase region of ferrite and austenite and then cooling, or by heating to a two-phase region and rolling after cooling. since the heating temperature is more than Ac ₁ below or Ac ₃ not obtained the layered tissue, the heating temperature range and the Ac ₁ or Ac ₃ or less. More desirable conditions are in the range of Ac ₁ + 10 ° C. to Ac ₃ -10 ° C.

Further, after heating / cooling or after heating / rolling /
In order to refine the layered structure of ferrite and martensite after cooling, it is desirable to adjust the structure of the steel material before heating to a structure mainly composed of martensite or tempered martensite, or bainite. A more desirable condition is that the ferrite fraction be less than 10% in these structures.

If the cooling rate after heating is less than 10 ° C./sec, a large amount of ferrite, pearlite and bainite are likely to be formed during cooling, so that the rate was set to 10 ° C./sec or more. More desirable conditions are 50 ° C./sec or more.

In the case of tempering after cooling, the tempering effect is small when the tempering temperature is lower than 200 ° C., and when it is higher than 600 ° C., it is difficult to obtain high strength of 1300 MPa or more. . A more desirable condition is 400 ° C. or higher.

[0047]

EXAMPLES Hereinafter, the effects of the present invention will be described more specifically with reference to examples. A test material having the chemical composition shown in Table 1 was heat-treated under various conditions to adjust the structure to martensite, tempered martensite, ferrite + pearlite, and bainite. Was rolled, then cooled at various speeds, and some materials were tempered after cooling.

Table 2 shows the results of evaluation of the mechanical properties, microstructure, and delayed fracture characteristics using the above samples.
The delayed fracture characteristics were evaluated based on the critical diffusible hydrogen amount described above, and the load stress was evaluated under the condition of 90% of the tensile strength.

Test No. 2 in Table 2 1 to 11 are examples of the present invention,
Others are comparative examples. As can be seen from the table, all of the examples of the present invention have a bolt tensile strength of 1300 MPa or more and have a layered structure of ferrite and martensite or tempered martensite. In these, the delayed fracture mode is intragranular cracking, the critical diffusible hydrogen content is higher than conventional bolts, and bolts having excellent delayed fracture characteristics are realized.

On the other hand, the comparative example No. 12 is a single-phase structure of tempered martensite because the temperature during heating is higher than Ac ₃ . Therefore, the delayed fracture mode is grain boundary cracking, the critical diffusible hydrogen content is low, and the delayed fracture characteristic is poor.

In Comparative Example No. In _No. 13, the temperature at the time of heating was lower than Ac1, and the predetermined strength was not obtained. No.
14 and 16 are examples in which the final structure is not fine because the structure before heating is F + P or P.

No. In No. 17, since the cooling rate was 10 ° C./sec or less, the final structure was F + P + M, and the delayed fracture characteristics were not improved.

Further, in Comparative Example No. 15, 18,
No. 19 is an example in which the chemical composition of steel is inappropriate. That is, No. In No. 15, because the C content is too low, a high-strength bolt of 1300 MPa or more intended in the present invention has not been realized. In addition, No. No. 18 has a too high C content. 19 is an example in which the Mn content was too high and the delayed fracture characteristics were all poor.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

As is clear from the above embodiments, the present invention reduces the layered structure of ferrite and martensite or tempered martensite to reduce the delayed fracture mode of bolts from intergranular cracks to intragranular cracks. Let me
It is possible to greatly improve the delayed fracture characteristics of high-strength bolts having a tensile strength of 1300 MPa or more, and at the same time, to optimally select the chemical composition of steel, heat treatment conditions, and the structure before heat treatment, thereby producing the steel. The industrial effects are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the relationship between the amount of diffusible hydrogen and delayed fracture time.

FIG. 2 is a diagram showing a fine layered structure of ferrite and martensite.

FIG. 3 is a view showing a fine layered structure of ferrite and martensite oriented in the axial direction of the bolt.

FIG. 4 is a diagram showing the relationship between the average interval between martensite and the critical diffusible hydrogen amount in the layered structure of martensite and ferrite.

Claims (12)

[Claims] 1. A high-strength bolt excellent in delayed fracture characteristics, wherein a region from a surface layer to a depth of at least 50 μm is composed of a layered structure of martensite and ferrite, and has a tensile strength of 1300 MPa or more. 2. A high strength with excellent delayed fracture characteristics, characterized in that a region from a surface layer to a depth of at least 50 μm is composed of a layered structure of tempered martensite and ferrite and has a tensile strength of 1300 MPa or more. bolt. 3. A region from the surface layer to a depth of at least 50 μm is composed of a layered structure of martensite and ferrite oriented in the axial direction of the bolt, and further has a tensile strength of 13 μm.
A high-strength bolt excellent in delayed fracture characteristics, characterized in that it is not less than 00 MPa. 4. A lag comprising a region from the surface layer to a depth of at least 50 μm comprising a layered structure of tempered martensite and ferrite oriented in the longitudinal direction of the bolt, and further having a tensile strength of 1300 MPa or more. High strength bolt with excellent breaking characteristics. 5. In a layered structure of martensite or tempered martensite and ferrite, the distance between martensite or tempered martensite is 10 or more.
The high-strength bolt according to any one of claims 1 to 4, wherein the high-strength bolt has excellent delayed fracture characteristics. 6. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0% Mn: 0.2 to 2.0%, Al: 0.005 to 0.1 The high-strength bolt excellent in delayed fracture characteristics according to any one of claims 1 to 5, wherein the high-strength bolt contains% and a balance of Fe and unavoidable impurities. 7. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0% Ni: 0.05 to 5.0%, Cu: 0.05 to 1.0%, V: 0.05 to 0.5%, Nb: 0.005 to 0.2%, Ta: 0.005 6. 0.5%, W: 0.05 to 0.5%, one or more of the following: excellent in delayed fracture characteristics according to any one of claims 1 to 5, High strength bolt 8. In weight%, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, the balance being Fe and unavoidable impurities,
After heating a bolt having a structure mainly composed of martensite, tempered martensite, bainite, or pearlite to a temperature range of Ac ₃ to Ac ₁ , 10 ° C.
A method for producing a high-strength bolt having excellent delayed fracture characteristics, characterized in that the bolt is cooled at a cooling rate of at least / sec. 9. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, the balance being Fe and unavoidable impurities,
After heating a bolt having a structure mainly composed of martensite, tempered martensite, bainite, or pearlite to a temperature range of Ac ₃ to Ac ₁ , 10 ° C.
A method for producing a high-strength bolt excellent in delayed fracture characteristics, characterized in that the bolt is cooled at a cooling rate of not less than / sec and then tempered to a temperature range of 200 to 600. 10. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, the balance being Fe and unavoidable impurities,
After heating a steel having a structure mainly composed of martensite, tempered martensite, bainite, or pearlite to a temperature range of Ac ₃ to Ac ₁ and thread rolling, the steel is cooled at a cooling rate of 10 ° C./sec or more. A method for producing a high-strength bolt excellent in delayed fracture characteristics, characterized by cooling. 11. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, the balance being Fe and unavoidable impurities,
After heating a steel having a structure mainly composed of martensite, tempered martensite, bainite, or pearlite to a temperature range of Ac ₃ to Ac ₁ and thread rolling, the steel is cooled at a cooling rate of 10 ° C./sec or more. Cool, then 200
A method for producing a high-strength bolt having excellent delayed fracture characteristics, characterized by tempering to a temperature range of up to 600 ° C. 12. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0% Ni: 0.05 to 5.0%, Cu: 0.05 to 1.0%, V: 0.05 to 0.5%, Nb: 0.005 to 0.2%, Ta: 0.005 12. The delayed fracture characteristic according to claim 8, wherein a steel containing one or more of W to 0.05% and W: 0.05 to 0.5% is used. Method for manufacturing high strength bolts with excellent quality.