JP2006104549A - High strength bolt having excellent delayed fracture resistance and method for improving its delayed fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength bolt having satisfactory delayed fracture resistance and having a strength of ≥1,500 MPa, and to provide a method for improving its delayed fracture resistance. <P>SOLUTION: The high strength bolt having excellent delayed fracture resistance is composed of a steel having a composition comprising, by mass, 0.65 to 1.1% C, 0.05 to 2% Si, 0.2 to 2% Mn and 0.002 to 0.1% Al, and the balance Fe with inevitable impurities, and having a tensile strength of ≥1,500 MPa, and in which the compressive residual stress in the surface layer of the under head part in the bolt is 20 to 90% of the tensile strength in the steel. The method for improving its delayed fracture resistance is further provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bolt that is widely used in civil engineering / architecture, automobiles, various industrial machines, and the like, and in particular, a high-strength bolt having a strength of 1500 MPa or more and excellent delayed fracture resistance and improved delayed fracture resistance. Regarding the method.

There is a growing need for high-strength bolts to reduce the weight and performance of automobiles and various industrial machines, or to reduce construction costs for civil engineering and building structures. The high-strength bolt is manufactured by using a low alloy steel such as SCM435 or SCM440 defined in JIS G4105, and cold-forming into a predetermined shape, followed by quenching and tempering. However, when the tensile strength exceeds 1200 MPa, there is a problem that delayed fracture is likely to occur.
As a technique for improving delayed fracture resistance of high-strength steel, for example, Patent Document 1 is effective to reduce the P and S contents, and Patent Document 2 regulates the contents of Si and Mn. In addition, a method of bending or drawing in the tempering process after quenching is disclosed. Further, Patent Documents 3 to 6 disclose delayed fracture resistance improvement technology focusing on alloy elements and carbides precipitated during heat treatment. Furthermore, Patent Documents 7 and 8 disclose bolts in which pearlite steel is reinforced by wire drawing. Although these techniques improve the delayed fracture resistance of high-strength bolts to some extent, they have not yet led to a radical solution.
Japanese Patent Publication No. 5-59967 Japanese Patent Publication No. 5-41684 Japanese Patent Laid-Open No. 7-70695 JP-A-8-60291 Japanese Patent Laid-Open No. 11-236617 JP 2001-32044 A JP 54-101743 A JP 11-315348 A

  An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a high-strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more and a method for improving the delayed fracture resistance.

  The steel materials and structures that achieve both high strength and delayed fracture resistance are analyzed in detail. As a result, in order to improve the delayed fracture resistance of high-strength bolts, it is effective to use tempered martensite structure or drawn pearlite structure using high-C steel that has not been used in the past. Revealed. In addition, it has been clarified that the use of alloy carbide with precipitation strengthening and hydrogen trapping effect is extremely effective in improving the delayed fracture resistance of high-strength bolts. Furthermore, the steel material components and heat treatment conditions for maximizing the precipitation strengthening and hydrogen trap effect by alloy carbide were clarified. Based on these findings, the delayed fracture characteristics were analyzed in detail using real bolts of various strength levels. As a result, it was found that when the nut is completely tightened, if the bolt strength exceeds 1500 MPa, it is more likely to be delayed and broken at the lower part of the bolt neck than at the threaded part. Therefore, as a result of various investigations on means for improving the delayed fracture resistance at the bottom of the bolt neck, it is effective to apply compressive residual stress, and moreover, ultrasonic impact treatment than conventional shot peening is applied as a method of applying compressive residual stress. Has been found to be extremely effective in improving delayed fracture characteristics.

Based on the above examination results, it is concluded that high strength bolts with excellent delayed fracture resistance can be realized by optimally selecting the steel material composition, structure, compressive residual stress and compressive residual stress application method by ultrasonic impact treatment. To achieve the present invention.
The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.65-1.1%, Si: 0.05-2%, Mn: 0.2-2%, Al: 0.002-0.1%, The balance is a high-strength bolt made of a steel material having Fe and inevitable impurities and having a tensile strength of 1500 MPa or more, and the compressive residual stress in the lower layer of the neck of the bolt is 20% of the tensile strength of the steel material. A high-strength bolt excellent in delayed fracture resistance, characterized by being -90%.
(2) Further, in terms of mass%, Cr: 0.1 to 2%, Mo: 0.05 to 3%, V: 0.05 to 1%, Ti: 0.002 to 0.5%, Nb: 0 The high-strength bolt excellent in delayed fracture resistance according to (1), comprising 0.002 to 0.5% of one kind or two or more kinds.
(3) The method for improving delayed fracture resistance of high-strength bolts according to (1) or (2), wherein the steel material is heated to 900 ° C. or higher, hot-formed with bolts, quenched, and 500 to After tempering at 700 ° C., the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or more, the vibration frequency of the ultrasonic vibrator: 10 to 60 kHz, the output of the ultrasonic wave: 500 to 5000 W, the ultrasonic vibration A method for improving the delayed fracture resistance of a high-strength bolt, characterized in that ultrasonic striking treatment is performed on the lower part of the bolt neck under the condition of the pressing force of the child on the lower part of the bolt neck: 10 to 1000N.
(4) The method for improving delayed fracture resistance of a high-strength bolt according to (1) or (2), wherein after quenching with the steel material, the quenching temperature is 900 ° C. or higher, and the tempering temperature is 500 to 700. Heat treatment is performed under the condition of ° C., and thereafter the hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the vibration frequency of the ultrasonic vibrator is 10 to 60 kHz, the output of the ultrasonic wave is 500 to 5000 W, the ultrasonic vibration A method for improving the delayed fracture resistance of a high-strength bolt, characterized in that ultrasonic striking treatment is performed on the lower part of the bolt neck under the condition of the pressing force of the child on the lower part of the bolt neck: 10 to 1000N.
(5) The method for improving delayed fracture resistance of high-strength bolts according to (1) or (2), wherein the steel material is cooled to a temperature range of 550 to 700 ° C. at 30 ° C./s or higher after hot rolling. Then, after maintaining in the temperature range for 30 to 300 s and performing pearlite transformation, wire drawing is performed in a range of true strain of 0.15 to 1.0, followed by cold bolt forming, and thereafter The hardness ratio of the ultrasonic vibrator: 1.2 or more, the vibration frequency of the ultrasonic vibrator: 10 to 60 kHz, the output of the ultrasonic wave: 500 to 5000 W, the pressing force of the ultrasonic vibrator on the bolt neck lower part: 10 to 1000 N A method for improving delayed fracture resistance of high-strength bolts, characterized in that ultrasonic hitting is performed on the lower part of the bolt neck under conditions.
(6) The method for improving delayed fracture resistance of a high-strength bolt according to (1) or (2), wherein the steel material is hot-rolled and then reheated to 900 ° C or higher, and 550 at 30 ° C / s or higher. After cooling to a temperature range of ˜700 ° C., maintaining in the temperature range for 30 to 300 s and performing pearlite transformation, wire drawing is performed in a range of true strain of 0.15 to 1.0, followed by cold bolt formation Thereafter, the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or more, the vibration frequency of the ultrasonic vibrator: 10 to 60 kHz, the output of the ultrasonic wave: 500 to 5000 W, to the lower part of the bolt neck of the ultrasonic vibrator A method for improving the delayed fracture resistance of high-strength bolts, characterized in that ultrasonic striking treatment is performed on the lower part of the bolt neck under the condition of a pressing force of 10 to 1000 N.
(7) After forming the bolt, after heating and cooling to a temperature range of 200 to 600 ° C., the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or more, the frequency of the ultrasonic vibrator: 10 to 60 kHz (5) or (6), characterized in that ultrasonic output processing is performed on the lower part of the bolt neck under the conditions of ultrasonic output: 500 to 5000 W and pressing force of the ultrasonic vibrator to the lower part of the bolt neck: 10 to 1000 N. The method for improving delayed fracture resistance of high-strength bolts as described in 1).
(8) After forming the bolt, after heating and cooling to a temperature range of 200 to 600 ° C. while applying a tension of 20 to 95% of the tensile strength to the bolt, the hardness ratio of the ultrasonic vibrator to the bolt : 1.2 or more, frequency of ultrasonic vibrator: 10 to 60 kHz, output of ultrasonic wave: 500 to 5000 W, pressing force of ultrasonic vibrator to lower part of bolt neck: 10 to 1000 N on lower part of bolt neck The method for improving delayed fracture resistance of high-strength bolts according to (5) or (6), wherein ultrasonic hitting is performed.

  According to the present invention, there is provided a high-strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more and a method for greatly improving the delayed fracture resistance by applying an ultrasonic impact treatment to the lower neck portion of the high-strength bolt. It is possible to provide a significant effect that is industrially useful.

The best mode for carrying out the present invention will be described below.
First, the reasons for limiting the components of the steel that is the subject of the present invention will be described.
C is an essential element for securing the strength of the bolt, but if it is less than 0.65%, it is difficult to obtain the required strength, while if it exceeds 1.1%, the bolt formability is deteriorated or delayed. Since the fracture characteristics tend to be lowered, the content is limited to a range of 0.65 to 1.1%.
Si has an effect of improving relaxation properties and increasing strength by a solid solution hardening effect. If the content is less than 0.05%, the above-described effect cannot be exhibited. On the other hand, if the content exceeds 2%, an effect commensurate with the amount of addition cannot be expected.
Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for improving hardenability. However, if it is less than 0.2%, the above effect cannot be obtained, while it exceeds 2%. Even if added, the effect commensurate with the amount added cannot be obtained, so the content was limited to 0.2 to 2%.
Al has the effect of preventing austenite grains from coarsening by forming AlN during deoxidation and heat treatment. If the content is less than 0.002%, the above effect is not exhibited. If the content exceeds 0.1%, the effect is saturated, so the content is limited to the range of 0.002 to 0.1%.

The above are the basic components of the steel that is the subject of the present invention. In the present invention, the steel further contains Cr: 0.1 to 2%, Mo: 0.05 to 3%, V: 0.05 to One or more of 1%, Ti: 0.002 to 0.5%, and Nb: 0.002 to 0.5% can be contained.
In the case of a pearlite structure, Cr has an effect of increasing the strength after the patenting process for strengthening the lamella refinement during the patenting process, and in the case of a martensite structure, there is an effect of increasing the temper softening resistance. If Cr is less than 0.1%, the effect cannot be sufficiently exhibited. On the other hand, even if added over 2%, the effect is saturated, so the content is limited to the range of 0.1 to 2%.
Mo, V, Ti, and Nb all precipitate as fine alloy carbides during patenting or tempering, and are extremely effective elements for increasing the strength of bolts. Further, since the alloy carbide has an effect of trapping hydrogen, it also has an action of improving delayed fracture resistance. When Mo is less than 0.05%, V is less than 0.05%, Ti is less than 0.002%, and Nb is less than 0.002%, the above effects cannot be sufficiently exerted, while Mo is 3%, Even if V is added to 1%, Ti is added to 0.5%, and Nb exceeds 0.5%, the above effect is saturated, so that Mo is 0.05 to 3%, V is 0.05 to 1%, Ti was limited to 0.002 to 0.5%, and Nb was limited to 0.002 to 0.5%.
P and S are not particularly limited, but from the viewpoint of improving the delayed fracture resistance of high-strength bolts, 0.015% or less is a preferable range. Further, N has an effect of refining austenite grains by forming carbonitrides of Al, V, Nb, and Ti. However, if it exceeds 0.015%, the ductility is lowered. 015% is a preferred range.

Next, the reason for limiting the compressive residual stress at the bottom of the bolt neck will be described. When the compressive residual stress by the ultrasonic impact treatment described below is less than 20% of the bolt strength, the effect of improving the delayed fracture resistance is small, so the lower limit of the compressive residual stress is limited to 20% of the bolt strength. On the other hand, since the above effect is saturated even when compressive residual stress exceeding 90% of the bolt strength is applied, the upper limit is limited to 90% of the bolt strength. From the viewpoint of improving delayed fracture resistance and the cost of ultrasonic impact treatment, a preferable range of compressive residual stress under the bolt neck is 30 to 70% of the bolt tensile strength. The residual stress of the present invention is measured by the X-ray method.
In the high-strength bolt of the present invention, when a predetermined strength is obtained by quenching / tempering treatment, the tempered martensite is a main structure. As other structures, one or more of ferrite, bainite and pearlite may be contained in an area ratio of 10% or less. In addition, when a predetermined strength is obtained in the wire drawing process and the bolt forming process after forming a pearlite structure by the patenting process, the processed pearlite structure is a main structure. As other structures, one or two of ferrite and bainite may be contained in an area ratio of 15% or less. The area ratio of these tissues can be measured by observing a visual field of 2 mm 2 or more with an optical microscope (500 times).

Next, the reason for limiting the method for improving delayed fracture resistance of the present invention will be described.
First, the reason for limitation when a predetermined bolt strength is obtained by quenching / tempering treatment will be described. When the bolt is formed hot, the steel material needs to be heated to 900 ° C. or higher. If it is less than 900 ° C., there are too many undissolved carbides and alloy carbides, and the precipitation strengthening ability is lowered. The upper limit is not particularly limited, but if it exceeds 1100 ° C., the austenite grains become coarse and delayed fracture resistance decreases, so the preferable upper limit of the heating temperature is 1100 ° C. In addition, after the hot bolt forming, a quenching process by water cooling or oil cooling is performed to obtain a martensite structure. In addition, when quenching is performed after cold bolting, if the heating temperature of the quenching process is less than 900 ° C., the amount of undissolved carbides and alloy carbides is too much and the precipitation strengthening ability decreases, so the quenching temperature Was limited to 900 ° C. Although the upper limit is not particularly limited, when the temperature exceeds 1100 ° C. as described above, the austenite grains become coarse and the delayed fracture resistance deteriorates, so the preferable upper limit of the heating temperature is 1100 ° C. Quenching is performed by water cooling or oil cooling to form a martensite structure. The tempering treatment temperature was limited to 500 to 700 ° C. in both hot bolt forming and cold bolt forming. If the tempering temperature is less than 500 ° C., the delayed fracture resistance of the bolt deteriorates, and the precipitation of alloy carbide is insufficient, so that it is not possible to expect precipitation strengthening and improved delayed fracture resistance due to the alloy carbide. On the other hand, when the tempering temperature exceeds 700 ° C., strength reduction and alloy carbide coarsening occur, and the precipitation strengthening ability and hydrogen trapping ability of the alloy carbide deteriorate. For the above reasons, the tempering temperature range was limited to 500 to 700 ° C.

Next, a method for improving delayed fracture resistance when a predetermined strength is obtained in a pearlite structure by wire drawing and a cold bolt forming process will be described. The method for improving delayed fracture resistance in this case is to rapidly cool the steel material after hot rolling, hold it in a temperature range where pearlite transformation is performed, increase the tensile strength, perform wire drawing, and perform cold bolt forming. is there. You may reheat after hot rolling. Moreover, you may heat after bolt shaping | molding, and when performing this heating, you may load tension | tensile_strength. Hereinafter, each limitation reason is demonstrated.
After re-heating after hot rolling or after hot rolling, it is necessary to rapidly cool to the temperature range for pearlite transformation. When the cooling rate is less than 30 ° C./s, pearlite transformation occurs during cooling, and the tensile strength after pearlite transformation decreases, so 30 ° C./s was set as the lower limit. A preferable cooling rate is 50 ° C./s or more. Although the upper limit of the cooling rate is not specified, it is technically difficult to exceed 500 ° C./s.

In the present invention, the temperature range for pearlite transformation is 550 to 700 ° C. This is because bainite is likely to be generated at a temperature lower than 550 ° C. after hot rolling or after hot rolling, and deteriorates the hydrogen embrittlement resistance of high-strength bolts, and Mo, V, Ti, In steel to which Nb is added, precipitation of alloy carbide is insufficient, so the lower limit was limited to 550 ° C. On the other hand, when the temperature exceeds 700 ° C., the tensile strength after pearlite transformation is reduced, so that it is difficult to realize the intended high-strength bolt, and the alloy carbides are coarsened, resulting in precipitation strengthening ability and hydrogen trapping ability. The upper limit was limited to 700 ° C.
It is necessary to hold in the temperature range of 550 to 700 ° C. until the pearlite transformation is completed. This is because bainite or martensite harmful to delayed fracture resistance is generated when cooling is started during the pearlite transformation. The holding time in the temperature range of 550 to 700 ° C. varies depending on the content of the chemical component, but is 30 to 300 s in the component range of the present invention.
In the present invention, after hot rolling or after reheating, pearlite transformation can be performed by dipping in a salt bath or lead bath at 550 to 700 ° C. Moreover, when reheating after hot rolling, if the reheating temperature is less than 900 ° C., solutionization is insufficient and undissolved carbide tends to remain, so the lower limit temperature is limited to 900 ° C. The upper limit is not particularly limited, but if it exceeds 1100 ° C., the austenite grains become coarse and ductility decreases after wire drawing. Therefore, the preferable upper limit of the reheating temperature is 1100 ° C.

If the steel material after pearlite transformation is subjected to wire drawing with a true strain exceeding 1.0, the delayed fracture resistance of high-strength bolts deteriorates. Therefore, in the present invention, the upper limit of the true strain of wire drawing is limited to 1.0. Further, if the true strain is less than 0.15, it becomes difficult to produce the high-strength bolt intended in the present invention, and the effect of improving the delayed fracture resistance is small, so the lower limit of the true strain is limited to 0.15. did. Here, the true strain of the wire drawing is a value represented by 2 × ln (wire diameter before wire drawing / wire diameter after wire drawing) (ln indicates a natural logarithm).
Heat treatment after wire drawing and bolt forming is performed for the purpose of improving delayed fracture resistance. When the heating temperature is less than 200 ° C., the above-described improvement effect is small. On the other hand, when the heating temperature exceeds 600 ° C., the strength is significantly reduced, so the heating temperature range is limited to 200 to 600 ° C. The heating time is not particularly limited because it varies depending on the method of the heating furnace, but 10 to 600 s is a preferable range in order to sufficiently exhibit the above effect.
Moreover, the delayed fracture resistance is further improved by performing the heat treatment while applying tension to the steel material after drawing or the bolt after forming. In this case, if the lower limit of the tension is less than 20% of the tensile strength, the effect of improving the delayed fracture resistance is small. On the other hand, the effect is saturated even if a tension exceeding 95% is applied. It was limited to the range.

FIG. 1 is a diagram illustrating an embodiment of a method for improving delayed fracture resistance of a high-strength bolt of the present invention.
In FIG. 1, 1 is a neck lower part, 2 shows an ultrasonic vibration terminal.
As shown in FIG. 1, the ultrasonic vibration terminal 2 is pressed against the lower neck portion 1 of the high-strength bolt that has been quenched and tempered, and subjected to ultrasonic striking treatment in the direction of the arrow in FIG. By applying a high compressive residual stress of 20 to 90% of the tensile strength of the steel material constituting the bolt to the surface layer of the lower neck portion 1, the delayed fracture resistance of the high strength bolt can be remarkably improved.
In the present invention, the bolt is molded as described above, and after necessary processing, the hardness ratio of the ultrasonic transducer to the bolt: 1.2 or more, the frequency of the ultrasonic transducer: 10 to 60 kHz, The ultrasonic striking process is performed on the lower part of the bolt neck under the conditions of output: 500 to 5000 W, pressing force of the ultrasonic vibrator to the lower part of the bolt neck: 10 to 1000 N.

The conditions for the ultrasonic hitting process will be described below.
If the hardness of the ultrasonic vibrator is less than 1.2 times the hardness of the bolt, it is difficult to efficiently apply the compressive residual stress to the lower part of the bolt neck by ultrasonic striking treatment. The hardness ratio was limited to 1.2 or more. The radius of curvature of the tip of the ultrasonic transducer is not particularly limited, but if it is larger than the radius of curvature of the bottom of the bolt neck (roundness of the bottom of the neck), compressive residual stress cannot be applied efficiently, so ultrasonic waves It is a preferable condition that the tip radius of the vibrator is equal to or less than the curvature radius of the lower neck. If the frequency of the ultrasonic vibrator is less than 10 kHz, the compressive residual stress cannot be applied efficiently, so the lower limit is limited to 10 kHz. On the other hand, since the effect of introducing the compressive residual stress is saturated even if the ultrasonic impact treatment is performed at a frequency exceeding 60 kHz, the upper limit of the frequency is limited to 60 kHz. A preferable range of the frequency is 20 to 40 kHz. If the output of the ultrasonic wave is less than 500 W, the ultrasonic striking treatment time for applying a predetermined compressive residual stress becomes long and not economical, so the lower limit is limited to 500 W. Even if the ultrasonic output exceeds 5000 W, the effect is saturated, so 5000 W was made the upper limit. If the pressing force of the ultrasonic vibrator to the lower part of the bolt neck is less than 10N, it is not economical because compressive residual stress cannot be efficiently applied, so the lower limit is limited to 10N. On the other hand, since the effect is saturated even if the pressing force exceeds 1000 N and the ultrasonic impact treatment is performed, the upper limit is limited to 1000 N.

The application of compressive residual stress by ultrasonic impact treatment is superior in delayed fracture resistance to the application of compressive residual stress by shot peening. The reason is
1) The compressive residual stress by ultrasonic hitting treatment is higher than that of shot peening 2) The compressive residual stress by ultrasonic hitting treatment is applied to the inside of the steel material rather than shot peening 3) The site of ultrasonic hitting treatment is plastically deformed 4) The delayed fracture resistance is improved. 4) It is estimated that the surface roughness due to the ultrasonic hitting process is smaller than that of shot peening.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Using steel materials having chemical components shown in Table 1, M6 hexagon bolts having the shape shown in FIG. 1 were formed cold or hot. When the bolt was formed hot, a quenching treatment was performed immediately after the bolt formation, followed by a tempering treatment. When the bolt was formed cold, quenching / tempering treatment was performed thereafter. As for the microstructure, tempered martensite was 95 to 100% in area ratio, and the balance was one or more of ferrite, bainite and pearlite. After the quenching and tempering treatment, the bolt was subjected to ultrasonic hitting treatment. Table 2 shows the heat treatment conditions of the bolts and the ultrasonic hitting conditions. Table 2 also shows the tensile strength of the bolt and the residual stress at the bottom of the neck. In the delayed fracture test, 100 bolts manufactured under the same conditions were each subjected to an atmospheric exposure test, and the fracture rate (%) of delayed fracture was evaluated. The bolt tightening load in the air exposure test was 90% of the bolt breaking load, and the air exposure period was evaluated for 2 years. Table 2 also shows the fracture ratio (%) in the air exposure test. Test Nos. 1 to 19 in Table 2 are examples of the present invention, and Test Nos. 20 to 39 are comparative examples. As can be seen from the table, in all of the examples of the present invention, the tensile strength of the bolt is 1500 MPa or more and a high compressive residual stress is introduced to the lower part of the bolt neck. As a result, all fracture rates of delayed fracture are 0%, and a high-strength bolt excellent in delayed fracture resistance is realized.

On the other hand, No. 21, 25, 27, 29, 33, and 35, which are comparative examples, are all examples of bolts that are still quenched and tempered. This is an example in which the fracture ratio of delayed fracture is high because the residual stress of the lower neck is not a high compressive residual stress.
Nos. 22, 24, 26, 28, 34, 36, 37, and 39, which are comparative examples, are all examples in which the conditions of the ultrasonic hitting process are inappropriate. That is, No. 22 has a low hardness ratio of the ultrasonic vibrator to the bolt, and No. 24 has a low frequency of the ultrasonic vibrator. Since No. 28 is too low in ultrasonic output because the pressing force is too low, No. 34 is too low in the vibration frequency and pressing force of No. 34, and No. 36 is an ultrasonic vibration against the bolt. Since the hardness ratio and pressing force of the child are too low, No. 37 is too low in the vibration frequency and ultrasonic output of the ultrasonic vibrator. Since the output is too low, none of them is in a compressive residual stress state in which the residual stress under the bolt neck is high. As a result, the fracture rate of delayed fracture was high in the exposure test, and this was an example in which delayed fracture could not be prevented.

Nos. 23, 30, and 38, which are comparative examples, are examples in which the residual stress at the bottom of the neck is changed to a compressive residual stress by a conventional shot peening process. In shot peening, it is difficult to efficiently apply high compressive residual stress to the lower neck, so this is an example of delayed fracture in an atmospheric exposure test.
Comparative examples Nos. 20 and 31 are examples in which the heat treatment conditions of the tempering treatment are inappropriate. No. 20 is an example in which delayed fracture occurred in the delayed fracture test because the tempering temperature was too low. No. 31 is an example in which the intended strength of the bolt could not be increased because the tempering temperature was too high.
No. 32, which is a comparative example, is an example in which delayed fracture resistance deteriorates because the C content of the bolt is too high, and delayed fracture occurs in the delayed fracture test.

  Next, an example in which a pearlite structure is formed and bolted by wire drawing and cold will be described. Using steel materials having chemical components shown in Table 1, immediately after hot rolling under the conditions shown in Table 3, it is immersed in a salt bath at 550 to 700 ° C. or reheated after hot rolling, and 550 to 700 ° C. Was immersed in a lead bath and pearlite transformed. The cooling rate from the hot rolling temperature or the reheating temperature was 50 ° C./s or more, and the immersion time in the salt bath and lead bath was 30 to 300 s. Further, the steel material after the pearlite transformation was subjected to a true strain drawing process shown in Table 3, and then cold-formed into a M6 size bolt shown in FIG. After bolt formation, heat treatment and ultrasonic impact treatment were performed under the conditions shown in Table 3. For the delayed fracture test of the bolt, the method implemented in Table 2 was used. Test Nos. 1 to 20 in Table 3 are examples of the present invention, and Nos. 21 to 41 are comparative examples. As can be seen from the table, in all of the examples of the present invention, the tensile strength of the bolt is 1500 MPa or more and a high compressive residual stress is introduced to the lower part of the bolt neck. As a result, all fracture rates of delayed fracture are 0%, and a high-strength bolt excellent in delayed fracture resistance is realized.

On the other hand, No. 21, 23, 28, 30, 31, 35, 39, and 40 in Table 3, which are comparative examples, are all examples of bolts as manufactured. This is an example in which the fracture ratio of delayed fracture is high because the residual stress of the lower neck is not a high compressive residual stress.
Nos. 22, 24 to 26, 29, 36, and 38 in Table 3, which are comparative examples, are all examples in which the conditions of the ultrasonic impact treatment are inappropriate. That is, No. 22 has a low hardness ratio of the ultrasonic vibrator to the bolt, and No. 24 has a low vibration frequency of the ultrasonic vibrator. Since No. 26 is too low in ultrasonic power because the pressing force is too low, No. 29 is too low in hardness ratio and ultrasonic output of the ultrasonic vibrator to the bolt, No. 36 is low in the bolt. Since the hardness ratio and the pressing force of the ultrasonic vibrator are too low, No. 38 is too low in the vibration frequency and ultrasonic output of the ultrasonic vibrator. It is not in a state. As a result, the fracture rate of delayed fracture was high in the exposure test, and this was an example in which delayed fracture could not be prevented.
Nos. 27, 37, and 41 in Table 3, which are comparative examples, are examples in which the residual stress in the lower neck portion is changed to compressive residual stress by the conventional shot peening process. In shot peening, it is difficult to efficiently apply high compressive residual stress to the lower neck, so this is an example of delayed fracture in an atmospheric exposure test.
Nos. 32 and 33 in Table 3 as comparative examples are examples in which the wire drawing distortion is inappropriate. No. 32 is an example in which the intended strength of the bolt could not be increased because the wire drawing distortion was too low. No. 33 is an example in which the delayed fracture resistance deteriorated because the wire drawing distortion was too high, and delayed fracture occurred in the delayed fracture test.
No. 34 of Table 3 which is a comparative example is an example in which the chemical composition of the steel material is inappropriate. In other words, since the C content is too high, proeutectoid cementite is generated during the pearlite transformation treatment, and as a result, disconnection occurs in the wire drawing process.

It is a figure which illustrates embodiment of the delayed fracture-resistance improvement method of the high strength bolt of this invention.

Explanation of symbols

1 Lower neck 2 Ultrasonic vibration terminal

Claims (8)

% By mass
C: 0.65-1.1%
Si: 0.05-2%
Mn: 0.2-2%
Al: 0.002 to 0.1%
A balance of Fe and inevitable impurities, and a high-strength bolt composed of a steel material having a tensile strength of 1500 MPa or more, wherein the compressive residual stress of the lower neck layer of the bolt is a tensile strength of the steel material. A high-strength bolt excellent in delayed fracture resistance, characterized by being 20 to 90% of strength. Furthermore, in mass%,
Cr: 0.1 to 2%,
Mo: 0.05-3%,
V: 0.05 to 1%
Ti: 0.002 to 0.5%,
Nb: 0.002 to 0.5%
The high-strength bolt excellent in delayed fracture resistance according to claim 1, comprising one or more of the following.   The method for improving delayed fracture resistance of a high-strength bolt according to claim 1 or 2, wherein the steel material is heated to 900 ° C or higher and hot-formed with a bolt, and then subjected to a quenching treatment and 500 to 700 ° C. After the tempering process, the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or more, the vibration frequency of the ultrasonic vibrator: 10 to 60 kHz, the output of the ultrasonic wave: 500 to 5000 W, the bolt of the ultrasonic vibrator A method of improving delayed fracture resistance of a high-strength bolt, characterized in that an ultrasonic impact treatment is performed on the lower part of the bolt neck under a condition of pressing force on the lower part of the neck: 10 to 1000N.   The method for improving delayed fracture resistance of a high-strength bolt according to claim 1 or 2, wherein after the bolt is formed using the steel material, the quenching temperature is 900 ° C or higher and the tempering temperature is 500 to 700 ° C. Then, the hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the vibration frequency of the ultrasonic vibrator is 10 to 60 kHz, the output of the ultrasonic wave is 500 to 5000 W, the bolt of the ultrasonic vibrator A method of improving delayed fracture resistance of a high-strength bolt, characterized in that an ultrasonic impact treatment is performed on the lower part of the bolt neck under a condition of pressing force on the lower part of the neck: 10 to 1000N. The method for improving delayed fracture resistance of a high-strength bolt according to claim 1 or 2, wherein the steel material is hot-rolled and then cooled to a temperature range of 550 to 700 ° C at 30 ° C / s or more, After maintaining in the temperature range for 30 to 300 s and performing pearlite transformation, wire drawing is performed in the range of true strain of 0.15 to 1.0, followed by cold bolt forming, and then an ultrasonic transducer for the bolt Hardness ratio: 1.2 or higher, ultrasonic vibrator frequency: 10-60 kHz, ultrasonic output: 500-5000 W, force of ultrasonic vibrator to the bottom of bolt neck: 10 to 1000 N A method for improving delayed fracture resistance of high-strength bolts, characterized in that ultrasonic hitting is performed on the lower neck.   The method for improving delayed fracture resistance of high-strength bolts according to claim 1 or 2, wherein the steel material is hot-rolled and then reheated to 900 ° C or higher and 550 to 700 ° C at 30 ° C / s or higher. After cooling to the above temperature range and maintaining the temperature range for 30 to 300 s to transform pearlite, the wire is drawn in the range of true strain of 0.15 to 1.0, then cold-formed with bolts, and then The hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the vibration frequency of the ultrasonic vibrator is 10 to 60 kHz, the output of the ultrasonic wave is 500 to 5000 W, and the pressing force of the ultrasonic vibrator to the lower part of the bolt neck : A method for improving delayed fracture resistance of high-strength bolts, characterized in that an ultrasonic impact treatment is performed on the lower part of the bolt neck under conditions of 10 to 1000 N.   After forming the bolt, it is heated and cooled to a temperature range of 200 to 600 ° C., then the hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the frequency of the ultrasonic vibrator is 10 to 60 kHz, and the ultrasonic wave The ultrasonic impact treatment is performed on the lower part of the bolt neck under the condition of the output of 500 to 5000 W and the pressing force of the ultrasonic vibrator on the lower part of the bolt neck: 10 to 1000 N. 8. To improve delayed fracture resistance of high-strength bolts. After forming the bolt, after heating and cooling to a temperature range of 200 to 600 ° C. while applying a tension of 20 to 95% of the tensile strength to the bolt, the hardness ratio of the ultrasonic vibrator to the bolt: 1. 2 or more, ultrasonic vibrator frequency: 10 to 60 kHz, ultrasonic output: 500 to 5000 W, ultrasonic vibrator pressing force on bolt neck lower part: ultrasonic impact on bolt neck lower part under conditions of 10 to 1000 N The method for improving delayed fracture resistance of high-strength bolts according to claim 5 or 6, wherein the treatment is performed.

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