JP4441434B2 - Manufacturing method of high-strength bolts with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a high-strength bolt excellent in delayed fracture resistance and a manufacturing method thereof widely used in civil engineering / architecture, automobiles, various industrial machines, and the like, and more particularly to delayed fracture resistance having a strength of 1400 MPa or more. The present invention relates to an excellent high-strength bolt and a manufacturing method thereof.

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 G 4105, cold forming into a predetermined shape, and then quenching and tempering. However, when the tensile strength exceeds 1200 MPa, there is a problem that delayed fracture is likely to occur.
As techniques for improving delayed fracture resistance of high strength steel, for example, Patent Documents 1 to 4 disclose inventions related to delayed fracture resistance improvement techniques that focus on alloy elements and carbides precipitated during heat treatment. Patent Documents 5 and 6 disclose inventions related to bolts made by strengthening pearlite steel by wire drawing. Patent Document 7 discloses an invention using shot peening as a method for producing a high-strength bolt having excellent delayed fracture resistance.

Japanese Patent Laid-Open No. 07-070695 Japanese Patent Laid-Open No. 08-060291 Japanese Patent Laid-Open No. 11-236617 JP 2001-32044 A JP 54-101743 A JP 11-315348 A JP 07-292434 A

However, each invention described in Patent Documents 1 to 6 has a problem that the delayed fracture resistance of the high-strength bolt is improved to some extent, but has not yet been fundamentally solved. Further, there is a limit to the improvement of the delayed fracture resistance according to the invention described in Patent Document 7.
Accordingly, an object of the present invention is to provide a high-strength bolt excellent in delayed fracture resistance and a method for manufacturing the same, which can advantageously solve the problems of the prior art as described above.

As a result of intensive studies on the above problems, the present inventors have found that it is extremely effective to apply a compressive residual stress near the surface while suppressing the roughness of the surface. This is because it is possible to suppress the occurrence of cracks due to the stress concentration in the surface recess and the accompanying hydrogen accumulation in the recess.
Further, it has been found that a bolt having excellent delayed fracture resistance and a tensile strength of 1400 MPa or more can be realized by combining optimum steel components. Furthermore, as a method of applying compressive residual stress while suppressing surface roughness, the inventors focused on laser irradiation treatment and found that a method of applying compressive residual stress near the surface by laser irradiation is extremely effective. Furthermore, it was concluded that a high-strength bolt with excellent delayed fracture resistance could be realized by optimally selecting the compressive residual stress, surface roughness and steel composition.

The present invention has been made for the first time based on the above knowledge, and the gist thereof is as follows.
(1) In mass%, C: 0.15-0.6%, Si: 0.05-2%, Mn: 0.1-2%, Al: 0.002-0.1%, Furthermore, Cr: 0.05-2%, Mo: 0.05-3%, V: 0.05-1%, Ti: 0.01-0.1%, Nb: 0.01-0.1% 1 part or more of the following, the balance is made of Fe and inevitable impurities, a predetermined forming process is performed, and a bolt whose tensile strength is adjusted to 1400 MPa or more by a predetermined heat treatment is immersed in a liquid, Alternatively, a liquid film is formed on the bolt surface, and a pulsed laser beam is condensed and irradiated on the bolt surface to generate a compressive residual stress of 20 to 95% of the tensile strength on the bolt surface. A manufacturing method of high strength bolts with excellent delayed fracture resistance.
(2) Further, it is characterized by containing one or more of Ni: 0.05-1%, Cu: 0.05-1%, B: 0.0003-0.01% by mass%. The manufacturing method of the high strength bolt excellent in the delayed fracture resistance as described in said (1).

( 3 ) The high strength bolt excellent in delayed fracture resistance according to ( 1 ) or (2) above, wherein the peak power density of the pulse laser beam on the bolt surface is 1 to 100 TW / m ^2. Manufacturing method.
( 4 ) The pulse laser beam is superimposed and irradiated spirally with respect to the bolt axis, and an average value of the number of irradiation times of the pulse laser beam at the same point on the bolt surface is 2 to 100 times. The manufacturing method of the high intensity | strength bolt excellent in the delayed fracture resistance of any one of said ( 1 ) -(3) .

  According to the present invention, by applying compressive residual stress to a high-strength bolt without roughening the surface, it is possible to provide a high-strength bolt excellent in delayed fracture resistance and a method therefor, and an industrially useful remarkable effect. Play.

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 0.15%, the required strength cannot be obtained. On the other hand, if it exceeds 0.6%, the ductility decreases. Limited to -0.6% range.

  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 an element effective for enhancing the hardenability for obtaining a martensite structure during the quenching process. However, if it is less than 0.1%, the above effect cannot be obtained, while it is added in excess of 2%. However, since an effect commensurate with the amount added cannot be obtained, the content is limited to a range of 0.1 to 2%.

  Al has the effect of preventing austenite grains from coarsening by forming AlN during deoxidation and heat treatment during steelmaking. 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%.

  Cr has the effect of improving hardenability. Furthermore, high temperature tempering is realized by temper softening resistance, and since it has corrosion resistance, it is an effective element for delayed fracture resistance. If it is less than 0.05%, the above effect is not exhibited, and if it exceeds 2%, the effect is saturated, so the content is limited to the range of 0.05 to 2%.

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. Furthermore, V, Ti, and Nb have the effect of grain refinement, and it can be said that this is also effective for delayed fracture resistance.
When Mo is less than 0.05%, 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 exerted, while Mo is 3%, Even if V is added in an amount of 1%, Ti is 0.1%, and Nb exceeds 0.1%, the above effect is saturated, so that Mo is 0.05 to 3%, V is 0.05 to 1%, Ti was limited to the range of 0.01 to 0.1%, and Nb was limited to the range of 0.01 to 0.1%. Further, it is necessary to contain one or more of Mo, V, Ti, and Nb. This is because, in a high-strength bolt exceeding 1400 MPa, a steel material component having excellent delayed fracture resistance as well as compressive residual stress is required.

  The above are the basic components of the steel that is the subject of the present invention. In the present invention, Ni: 0.05-1%, Cu: 0.05-1%, B: 0.0003- One or more of 0.01% can be contained.

  Ni is added to improve the toughness that deteriorates with increasing strength and to improve the hardenability during heat treatment to 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 producing carbonitrides of Al, V, Nb, and Ti. However, if it exceeds 0.015%, the ductility decreases, so 0.002 to 0 .015% is a preferable range.

  Next, the reason for limiting the compressive residual stress of the bolt will be described. When the compressive residual stress by laser irradiation 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, even if compressive residual stress exceeding 95% of the bolt strength is applied, the above effect is saturated, so the upper limit is limited to 95% of the bolt strength. From the viewpoint of improving delayed fracture resistance and the cost of laser irradiation, a preferable range of compressive residual stress of the bolt is 30 to 90% of the bolt tensile strength.

  Next, the reason for limiting the surface roughness will be described. If the 10-point average roughness is 10 μm or less, the delayed fracture that causes the surface recesses to be the starting point does not occur, so the upper limit of the 10-point average roughness is limited to 10 μm.

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 visual field of 2 mm ² or more at the center of the bolt with an optical microscope (500 times).

Next, a method for improving the delayed fracture resistance of the high-strength bolt of the present invention will be described.
FIG. 1 is a schematic configuration of a laser processing apparatus used in the present invention. The bolt 1 is immersed in the liquid 2, the laser beam 6 emitted from the laser beam oscillation device 3 is condensed using the lens 4, and the bolt surface is irradiated. The laser uses a pulsed laser to obtain a high peak power density. The reason for immersing in the liquid 2 is to suppress the expansion of the plasma generated from the laser beam irradiation part and increase the pressure of the plasma. The reaction force of the high-pressure plasma can give plastic deformation to the bolt surface and give compressive residual stress.

  The liquid 2 is an aqueous solution such as water or saline, an organic solvent such as alcohol, or a mixture thereof, and a liquid transparent to the wavelength of the laser is a preferable condition. Here, it is not always necessary to immerse the bolt in the liquid, and a liquid film may be formed on the surface of the bolt, and laser irradiation may be performed thereon. For example, the above-described state can be realized by spraying liquid onto the laser irradiation unit.

When the peak power density on the bolt surface of the laser beam is less than 1 TW / m ² , high compressive residual stress cannot be imparted to the bolt, while compression is performed even when laser treatment is performed at a peak power density exceeding 100 TW / m ^2. Since the effect of imparting residual stress is saturated, it is desirable to carry out in the range of 1 to 100 TW / m ² .

For delayed fracture, it is effective to selectively increase the axial compressive stress of the bolt. For this purpose, it is desirable to irradiate the irradiation spot of the pulse laser in a spiral manner with respect to the bolt. Here, as shown in FIG. 2, the spiral superimposed irradiation is formed continuously by scanning the irradiation spot of the pulse laser in the circumferential direction of the bolt surface and shifting the scanning region thus obtained in the axial direction. In this method, adjacent beam spots are irradiated so as to overlap each other. In order to increase the compressive stress in the axial direction of the bolt, it is desirable that the average value of the number of irradiation times of the pulse laser beam at the same point on the bolt surface is 2 to 100 times.
Further, the laser treatment is not necessarily applied to the entire surface of the bolt, and may be applied only to the base of the neck of the bolt or the vicinity of the valley of the screw portion, which is particularly problematic in delayed fracture because stress concentration occurs.

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, hexagonal bolts of M10 were formed. Thereafter, heat treatment was performed at a quenching temperature of 950 ° C. and a tempering temperature of 400 ° C. to 650 ° C. 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 quenching / tempering treatment, laser irradiation was performed using the apparatus shown in FIG. Water was used as the liquid, and a second harmonic (wavelength: 532 nm) of an Nd: YAG laser having good water permeability was used as the laser beam. The time width of the pulse laser beam was 10 ns. The laser beam was condensed with a convex lens having a focal length of 100 mm. The spot shape on the test piece was circular, and the spot diameter of the irradiation mark was 0.3 mm. In the process, the laser beam irradiation spot was superimposed in a spiral by moving the bolt 1 while continuously rotating. The laser treatment was performed on the entire surface of the bolt, and the average number of times of irradiation with the pulse laser beam at the same point on the bolt surface was 3.5.
Table 2 shows the laser irradiation conditions and ten-point average roughness of the bolt.

  Table 2 also shows the tensile strength and residual stress of the bolt. The residual stress is measured by the X-ray method. In the delayed fracture test, 30 bolts manufactured under the same conditions were each subjected to an atmospheric exposure test and evaluated by the fracture rate (%) of delayed fracture. 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 No. in Table 2 1 to 16 are examples of the present invention. 17-23 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 1400 MPa or more, high compressive residual stress is introduced into the bolt, and the ten-point average roughness is 10 μm or less. 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, the comparative example No. 17 and 18 are examples where the chemical composition of steel is inappropriate. That is, no. No. 17 is an example in which the target strength was not obtained because the strength after heat treatment was low because C was too low. Moreover, No. which is a comparative example. No. 18 is an example in which delayed fracture occurred in an atmospheric exposure test because it does not contain one or more of Cr, Mo, V, Ti, and Nb.
No. which is a comparative example. Nos. 19 and 21 are examples in which bolts are manufactured only by conventional quenching and tempering processes. Since no compressive residual stress was applied to the surface layer of the bolt, delayed fracture occurred in the atmospheric exposure test.
No. which is a comparative example. No. 20 is an example in which the laser irradiation conditions are inappropriate. That is, no. No. 20 is an example in which sufficient compressive residual stress was not obtained on the surface layer of the bolt because the peak power density of laser irradiation was low. 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.
No. which is a comparative example. 22 and 23 are examples in which the residual stress of the bolt is changed to a compressive residual stress by a conventional shot peening process. This is an example of delayed fracture in the atmospheric exposure test because the surface is rough in shot peening.

It is a figure which illustrates embodiment of the delayed fracture-proof characteristic improvement method of the high strength bolt of this invention. It is a figure which shows the irradiation method of a laser beam.

Explanation of symbols

1 bolt 2 liquid 3 laser beam oscillator 4 condenser lens 5 mirror 6 laser beam 7 laser beam irradiation spot

Claims (4)

% By mass
C: 0.15-0.6%,
Si: 0.05-2%
Mn: 0.1 to 2%,
Al: 0.002 to 0.1%
In addition,
Cr: 0.05-2%
Mo: 0.05-3%,
V: 0.05 to 1%
Ti: 0.01 to 0.1%,
Nb: 0.01 to 0.1%
1 part or more of the following, the balance is made of Fe and inevitable impurities, a predetermined forming process is performed, and a bolt whose tensile strength is adjusted to 1400 MPa or more by a predetermined heat treatment is immersed in a liquid, Alternatively, a liquid film is formed on the bolt surface, and a pulsed laser beam is condensed and irradiated on the bolt surface to generate a compressive residual stress of 20 to 95% of the tensile strength on the bolt surface. A manufacturing method of high strength bolts with excellent delayed fracture resistance. Furthermore, in mass%,
Ni: 0.05 to 1%,
Cu: 0.05 to 1%,
B: 0.0003 to 0.01%
The manufacturing method of the high strength bolt excellent in the delayed fracture resistance of Claim 1 characterized by including 1 type, or 2 or more types of these. Characterized in that said peak power density is 1~100TW / m ² in the bolt surface of the pulsed laser beam, the delayed method for producing a high strength bolt fracture characteristics according to claim 1 or 2. The pulse laser beam is superimposed and irradiated spirally with respect to a bolt axis, and an average value of the number of times of irradiation of the pulse laser beam at the same point on the surface of the bolt is 2 to 100 times. Item 4. A method for producing a high-strength bolt excellent in delayed fracture resistance according to any one of Items 1 to 3 .

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