JP4555749B2 - Method for improving delayed fracture resistance of high strength bolts - Google Patents

Description

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

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 method for improving delayed fracture resistance of a high-strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more.

The present inventors analyzed in detail about the steel material component and structure | tissue which satisfy | fills the high intensity | strength of a bolt, and delayed fracture resistance. As a result, in order to improve the delayed fracture resistance of high-strength bolts, it became clear that the use of alloy carbides with precipitation strengthening and hydrogen trapping effect is extremely effective. Furthermore, the steel material components and heat treatment conditions for maximizing the precipitation strengthening and hydrogen trap effect by alloy carbide were clarified. In addition, the delayed fracture characteristics were analyzed in detail using real bolts of various strength levels manufactured by quenching and tempering. As a result, it was revealed that when the nut was completely tightened, if the bolt strength exceeded 1500 MPa, it was more likely to break later than the threaded portion at the bolt neck. 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 results of the above studies, the conclusion was reached that a high-strength bolt with excellent delayed fracture resistance could be realized by optimally selecting the steel material composition, compressive residual stress, and compression residual stress application method by ultrasonic impact treatment. The present invention has been made.

The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.3-0.6%
Si: 0.05-2%
Mn: 0.1 to 2%,
Al: 0.002 to 0.1%,
Mo: 1 to 3%,
further,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1% of 1 type or 2 types or more, with the balance consisting of Fe and inevitable impurities,
And, after forming a bolt from a steel material having a component in which the mass% ratio of C satisfies the following formula (A) , heat treatment is performed under conditions of quenching temperature: 900 ° C. or higher, tempering temperature: 550 to 700 ° C., 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, and the ultrasonic vibrator is pressed to the lower part of the bolt neck. Under the condition of force: 10 to 1000 N, an ultrasonic vibration terminal is pressed against the neck lower part of the bolt to perform a striking treatment, and the compressive residual stress of the neck lower surface layer is 20 to 90% of the tensile strength of the steel material. A method for improving delayed fracture resistance of a high-strength bolt having a tensile strength of 1500 MPa or more.
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A)
(2) The steel material is further mass%,
Cr: 0.1 to 2%,
Ni: 0.05 to 1%,
Cu: 0.05 to 1%,
B: One type or two or more types of 0.0003 to 0.01% are contained, The method for improving delayed fracture resistance of a high-strength bolt according to claim 1.

According to the present invention, a method for greatly improving the delayed fracture resistance improvement method of a high strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more 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 0.3%, the required strength cannot be obtained, while if it exceeds 0.6%, the ductility is lowered. Limited to a range of 0.6%.
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. Therefore, the content is limited to a range of 0.05 to 2%.
Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for enhancing the hardenability for obtaining a martensite structure during the quenching treatment. On the other hand, even if added over 2%, an effect commensurate with the amount added cannot be obtained, so the content was limited to the range of 0.1 to 2%.

Al has the effect of preventing austenite grains from coarsening by forming AlN during deoxidation and heat treatment. Moreover, when adding B, it has the effect of fixing N and ensuring the solid solution B effective in improving hardenability and hydrogen embrittlement resistance. 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%.
Mo, V, Ti, and Nb all precipitate as fine alloy carbides during 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 1%, V is less than 0.05%, Ti is less than 0.01%, and Nb is less than 0.01%, the above effects cannot be sufficiently exhibited. Even if 1%, Ti is added to 1%, and Nb exceeds 1%, the above effect is saturated, so that Mo is 1 to 3%, V is 0.05 to 1%, Ti is 0.01 to 1% , Nb was limited to a range of 0.01 to 1%. Moreover, it is necessary to contain Mo and to include one or more of V, Ti, and Nb. This is because the precipitation strengthening ability and hydrogen trapping ability are weak only with Mo alloy carbides. By using a composite alloy carbide of V, Ti, and Nb, precipitation strengthening ability and hydrogen trapping ability are improved. Furthermore, it is important to add Mo, V, Ti, and Nb so that the mass% ratio with C satisfies the following formula (A).
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A)
If the mass% ratio with C is less than 0.2, the volume fraction of the alloy carbide to be precipitated is too low, the precipitation strengthening ability and the hydrogen trapping ability are not sufficient, and the excellent delayed fracture resistance intended in the present invention is excellent. Since it is difficult to realize a high strength bolt, the lower limit was limited to 0.2.

The above are the basic components of the steel that is the subject of the present invention. In the present invention, Cr: 0.1 to 2%, Ni: 0.05 to 1%, Cu: 0.05 to One or two or more of 1% and B: 0.0003 to 0.01% can be contained.
Cr is an effective element for improving the hardenability and increasing the softening resistance during the tempering treatment. However, if it is less than 0.1%, the effect cannot be sufficiently exerted, while adding over 2%. However, since the effect is saturated, it is limited to the range of 0.1 to 2%.
Ni is added to improve ductility which deteriorates with increasing strength and to improve the hardenability during heat treatment and increase the tensile strength, but less than 0.05% has little effect, On the other hand, even if it exceeds 1%, the effect of matching the added amount cannot be exhibited, so the content was limited to 0.05 to 1%.
Cu has the effect of improving the hardenability during the quenching process, but the effect is insufficient if it is less than 0.05%, and the effect is saturated even if added over 1%. , Limited to the range of 0.05 to 1%.
B has an effect of improving the delayed fracture resistance, and also has an effect of remarkably improving hardenability by segregating at the austenite grain boundary. However, when B is less than 0.0003%, the above effect is exhibited. Not exceeding 0.01%, the effect is saturated, so the content is limited to 0.0003 to 0.01%.
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.
The high-strength bolt of the present invention obtains a predetermined strength by quenching and tempering treatment, and is mainly composed of tempered martensite. As other structures, one or more of ferrite, bainite, and pearlite may be contained in an area ratio of 10% or less. The area ratio of ferrite, bainite, and pearlite can be measured by observing a field of view of 2 mm 2 or more with an optical microscope (500 times) at the center of the bolt.

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, after forming the bolt, heat treatment is performed under the conditions of quenching temperature: 900 ° C. or higher and tempering temperature: 550 to 700 ° C., and then the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or higher. The ultrasonic vibrator has a vibration frequency of 10 to 60 kHz, an ultrasonic output of 500 to 5000 W, and a pressing force of the ultrasonic vibrator to the lower neck of the bolt: 10 to 1000 N. A striking process is performed by pressing the vibration terminal.

Below, the reason for limitation of the manufacturing method of this invention is demonstrated.
When the heating temperature of the quenching treatment is less than 900 ° C., there are too many undissolved carbides and alloy carbides and the precipitation strengthening ability is lowered, so the lower limit of the quenching 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 delayed fracture resistance decreases, so the preferable upper limit of the heating temperature is 1100 ° C. In addition, quenching performs water cooling or oil cooling to make a martensite structure. In the tempering treatment, when the tempering temperature is lower than 550 ° C., the precipitation of alloy carbide is insufficient, and therefore precipitation strengthening and delayed fracture resistance cannot be expected to be improved by the alloy carbide. On the other hand, when the tempering temperature exceeds 700 ° C., the alloy carbide coarsens, and the precipitation strengthening ability and hydrogen trapping ability of the alloy carbide are reduced. For the above reasons, the tempering temperature range was limited to 550 to 700 ° C.

Next, the conditions of the ultrasonic impact treatment after the quenching / tempering treatment will be described.
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, M10 hexagon bolts having the shape shown in FIG. 1 were formed. Thereafter, quenching and tempering were performed. As for the microstructure, tempered martensite was 95-100% in area ratio in all cases, 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 25 in Table 2 are examples of the present invention, and Test Nos. 26 to 47 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.-26-32 which is a comparative example are all examples in which the chemical composition of steel is inappropriate. That is, No. 26 is an example of low strength after heat treatment because it does not contain Mo. Nos. 27, 28, and 32 are examples in which the high strength bolt of 1500 MPa or more which was the target could not be realized because the content of Mo was low but the content was low. Further, Nos. 29 and 31, which are comparative examples, are examples in which delayed fracture occurred in an atmospheric exposure test because none of them contained one or more of V, Ti, and Nb. Further, No. 30 is an example in which the intended strength of the bolt could not be increased because the C content was too low.
Nos. 33 to 36 as comparative examples are examples in which the heat treatment conditions for quenching and tempering treatment are inappropriate. Nos. 33 and 34 are examples in which the quenching temperature is too low, so that a large amount of undissolved carbide and alloy carbide exist and the strength after tempering is low. Nos. 35 and 36 are examples in which the tempering temperature is inappropriate, No. 35 is too high in tempering temperature and the strength is lowered, and No. 36 is too low in tempering temperature so that no alloy carbide precipitates. This is an example in which the intended strength was not obtained.

Nos. 37, 39, and 41, which are comparative examples, are 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. 38, 40, 42, 45 to 47, which are comparative examples, are examples in which the conditions of the ultrasonic hitting process are inappropriate. That is, since No. 38 has a low hardness ratio of the ultrasonic vibrator to the bolt, No. 40 and 47 both have the low frequency of the ultrasonic vibrator. Since the pressing force of the vibrator on the lower part of the bolt neck is too low, and No. 45 is too low in ultrasonic output, none of them is in a compressive residual stress state where the residual stress at the lower part of 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. 43 and 44, which are comparative examples, are examples in which the residual stress at the bottom of the neck 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 the atmospheric exposure test.

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 (2)

% By mass
C: 0.3-0.6%
Si: 0.05-2%
Mn: 0.1 to 2%,
Al: 0.002 to 0.1%,
Mo: 1 to 3%,
further,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1% of 1 type or 2 types or more, with the balance consisting of Fe and inevitable impurities,
And, after forming a bolt from a steel material having a component in which the mass% ratio of C satisfies the following formula (A) , heat treatment is performed under conditions of quenching temperature: 900 ° C. or higher, tempering temperature: 550 to 700 ° C., 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, and the ultrasonic vibrator is pressed to the lower part of the bolt neck. Under the condition of force: 10 to 1000 N, an ultrasonic vibration terminal is pressed against the neck lower part of the bolt to perform a striking treatment, and the compressive residual stress of the neck lower surface layer is 20 to 90% of the tensile strength of the steel material. A method for improving delayed fracture resistance of a high-strength bolt having a tensile strength of 1500 MPa or more.
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A) The steel material is further in mass%,
Cr: 0.1 to 2%,
Ni: 0.05 to 1%,
Cu: 0.05 to 1%,
B: One type or two or more types of 0.0003 to 0.01% are contained, The method for improving delayed fracture resistance of a high-strength bolt according to claim 1.

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