JP2013139631A - High tensile bolt and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile bolt having a tensile strength of 1,7 00 MPa or more and excellent tensile deformation performance and delayed fracture resistance, and a method of manufacturing the same.SOLUTION: A steel containing, by mass, 0.35-0.70% C, 0.50-2.50% Si, 0.10-1.00% Mn, 0.30-3.00% Cr, 0.50-1.50% Mo, 0.001-0.1% Al and the balance Fe with inevitable impurities, is formed into a bolt shape in such a manner that a shape parameter S (=Ab/Ae, wherein, Ab is an effective cross-sectional area in a shaft part 2 and Ae is an effective cross-sectional area in a screw part) in the shaft part 2, becomes a value of 0.85 to <1.25, and after quenching the steel by austenization in the temperature range of 850-1,050°C, the steel is subjected to a tempering treatment in the temperature range of 500-650°C.

Description

  The present invention relates to a high-strength bolt used for a joint part of civil engineering, architecture, and the like and a method for manufacturing the same.

Currently, in the field of civil engineering and construction, F10T (JIS B 1186) and S10T (JSSII09) high-strength bolts (tensile strength of 1000 to 1100 MPa) are the mainstream, but the thickness of members such as high-rise buildings There is a demand for further increasing the strength of bolts for increasing the thickness and strength.
Recently, F14T ultra-high-strength bolts (tensile strength of 1400 MPa to 1600 MPa) capable of introducing axial force equivalent to about 1.5 times that of F10T have been commercialized, and the use record in the building field is increasing (for example, Non-patent document 1).

Furthermore, if an ultra-high-strength bolt exceeding 1700 MPa is put into practical use, it will be possible to further reduce the size of the joint and change the design of the steel structure. In other words, by increasing the strength of the bolts, it is possible to expect significant ripple effects such as resource saving, labor saving, energy saving and CO ₂ reduction.

However, in low alloy steels with a tensile strength exceeding 1200 MPa (addition amount of alloying elements other than carbon is 10% or less), delayed fracture is a serious problem, which greatly hinders the increase in strength of high strength bolts. Yes.
Delayed fracture is an abbreviation for time-delayed fracture, which is a breakdown that occurs when hydrogen is generated due to atmospheric corrosion and penetrates into the steel material and the steel material becomes brittle. Hydrogen that diffuses and accumulates in stress-concentrated portions in steel at room temperature, so-called diffusible hydrogen, is a cause of delayed fracture. Due to this delayed fracture, for about 30 years until the development of F14T ultra high strength bolts, the strength of high strength bolts for civil engineering and construction was flattened with high strength bolts of F10T and S10T.

In the above-mentioned F14T bolt, the delayed fracture resistance is improved by developing a steel material having a large allowable amount of diffusible hydrogen and developing a bolt shape that hardly causes delayed fracture (Patent Document 1, Non-Patent Document 1). Specifically, a steel material such as Mo and V carbonitrides that trap hydrogen in steel is finely dispersed in a matrix to increase the allowable hydrogen amount, and the bolt (1) screw shape (2) Improvement of the shape of the transition from the bolt shaft to the threaded portion, (3) Increase in the bolt head neck neck radius of curvature r, (4) High strength of conventional F10T and S10T with modified nut shape A unique shape different from the bolt is disclosed.
The allowable hydrogen amount indicates an allowable value for the amount of diffusible hydrogen that does not cause delayed fracture of the material under a certain load condition, and is an index for comparing delayed fracture resistance.

  On the other hand, in Patent Document 2, the steel for machine structural use of 1800 MPa class, which has excellent delayed fracture resistance by defining the tempering temperature from 500 ° C. to Ae1 along with the addition amount of C, Si, Mn, Cr, and Mo. Is disclosed that can be manufactured.

  However, when a high-strength bolt having the same screw shape as that of an existing F10T bolt is manufactured from the steel for machine structural use, delayed fracture starting from the screw portion of the bolt frequently occurs in the bolt air exposure test. There was also a point (nonpatent literature 2). Moreover, the ultrahigh strength steel material having a tensile strength of 1800 MPa has a problem that the notch toughness is low and the tensile deformability of the bolt is low.

  As shown in FIG. 1, the high-strength bolt is composed of a head portion 1, a shaft portion 2, and a screw portion 3, and the stress concentration is particularly large at the screw portion 3 and is plastic when tightening a bolt with a high axial force. Since strain increases, it is well known that delayed fracture occurs from the threaded portion 3 as a starting point.

  In Patent Document 3, the tensile strength is 1800 MPa by warm-forming the head 1 at a temperature higher than that of the screw part 3 and controlling the strength of the bolt to be gradually lowered from the shaft part 2 to the head part 1. A bolt that is excellent in ductility and delayed fracture characteristics, yet has excellent impact resistance, is disclosed. However, since the bolt forming is performed in the warm region, there is a problem in mass productivity of the bolt as compared with the conventional cold forming process.

The forefront of high strength technology in high strength bolt joints, 2008 Architectural Institute of Japan (Hiroshima), Structural Division (Steel Structure), Panel Discussion Materials, (2008), p. 1 Industrial Materials, Vol. 57, (2009), p. 34

JP 2002-276737 A JP 2003-073769 A JP 2011-058576 A

  The present invention solves the problems of the prior art as described above, can introduce an axial force equivalent to 1.7 times that of F10T bolts and S10T bolts, and has excellent delayed fracture resistance. We provide high strength bolt products.

  As a result of various studies in view of the above-described circumstances, the present inventors have increased the allowable hydrogen capacity and increased the delayed fracture resistance by reducing the stress concentration on the bolt screw portion 3 even with an ultrahigh steel material of 1700 MPa. If the bolt shaft 2 is optimized and the deformation of the bolt is carried out by the bolt shaft 2, the tensile deformation performance of the bolt product is improved and the high axial force is tightened. It has been found that the plastic strain and stress concentration in the force bolt thread portion 3 can be reduced.

  This invention is made | formed based on such knowledge, The place made into the summary is as follows.

The high-strength bolt of the present invention is, in mass%, C: 0.35 to 0.70%,
Si: 0.50 to 2.50%,
Mn: 0.10 to 1.00%,
Cr: 0.30 to 3.00%
Mo: 0.50 to 1.50%,
Al: 0.001 to 0.1%
The balance is made of Fe and inevitable impurities, and 90% by volume or more of the part from the bolt head 1 to the screw part 3 has an internal metal structure consisting of a tempered martensite structure, and the tensile strength of the above part Is 1700 MPa or more, and in the shaft part 2 of the high-strength bolt, the shaft part shape parameter S represented by the following formula (1) is a value of 0.85 or more and less than 1.25.
S = Ab / Ae (1)
(Ab in the formula represents the effective sectional area of the shaft portion, and Ae represents the effective sectional area of the threaded portion)

  Moreover, in the manufacturing method of the high strength bolt of the present invention, in the production of the high strength bolt, after forming the material into the bolt, the material is quenched after being subjected to the austenitizing treatment within a temperature range of 850 ° C. to 1050 ° C., It is characterized in that 90% by volume or more of the internal metal structure is made a martensite structure, and then tempered in a temperature range of 500 to 650 ° C.

According to the present invention, it is possible to introduce axial force more than 1.7 times that of conventional F10T, and provide high-strength bolts with excellent delayed fracture characteristics, especially tensile deformation performance by quenching and tempering after forming the material into bolts. Can contribute to the industry.
This is because (1) the ratio of the effective cross-sectional area (Ab) of the bolt shaft 2 and the effective cross-sectional area (Ae) of the screw part is adjusted, and the screw part 3 for high tension in the axial direction of the bolt. This is due to the fact that the bolt shaft part 2 yields and deforms in advance, thereby reducing the stress concentration on the screw part 3 and the occurrence of plastic strain, which are the origin of delayed fracture, and increasing the ductility of the bolt product. is there. In the present invention, the effective cross-sectional area (Ae) of the screw portion means an effective area defined in Appendix 1 of JIS B 1180. Moreover, the effective cross-sectional area (Ab) of the shaft part 2 means a cross-sectional area calculated by the minimum diameter of the shaft part.

It is the schematic of a volt | bolt. It is a figure which shows the shape and dimension (mm) of a notch test piece. It is a graph which shows the relationship between the stress concentration factor of a notch bottom, and notch tensile strength. It is a graph which shows the relationship between notch tensile strength and the amount of diffusible hydrogen. It is the schematic of a hydrogen cracking sensitivity test procedure. It is a figure which shows the shape of a volt | bolt. FIG. It is a load displacement relationship figure of a bolt product.

  The high-strength bolt having a tensile strength of 1700 MPa or more in the present invention is (1) the ratio of the effective cross-sectional area (Ab) of the shaft portion 2 of the bolt to the effective cross-sectional area (Ae) of the screw portion, that is, the S value is 0.85 As described above, as a value less than 1.25, with respect to high tension in the axial direction of the bolt, the threaded portion 3 that becomes a point of occurrence of delayed fracture due to yield deformation of the bolt shaft portion 2 ahead of the threaded portion 3. This can be achieved by reducing the stress concentration and plastic strain and increasing the ductility of the bolt product.

[Bolt shaft shape]
In the case of existing F10T and S10T bolts, the S value in the equation (1) is a value from 1.25 to 1.28, and the cross-sectional area (Ab) of the bolt neck lower shaft portion is the effective cross-sectional area of the screw portion ( Since it is larger than Ae), if tension is applied to the bolt, the bolt breaks at the threaded portion.
In the case of the bolt of the present invention, the bolt shaft portion has such a shape that the S value is less than 1.25. In such a bolt, the stress concentration on the bolt screw portion 3 can be reduced by setting the S value to less than 1.25. More preferably, by setting the S value to 1.05 or less, the shaft portion breaks at the end, so that the deformability is remarkably improved as compared with the bolt where the thread portion breaks. On the other hand, if the S value becomes too small, the tension that can be introduced into the bolt also decreases, so the S value is desirably 0.85 or more.

〔Chemical composition〕
The high-strength bolt of the present invention is, in mass%, C: 0.35 to 0.70%,
Si: 0.50 to 2.50%,
Mn: 0.10 to 1.00%,
Cr: 0.30 to 3.00%
Mo: 0.50 to 1.50%,
Al: 0.001 to 0.1%
The balance is composed of Fe and inevitable impurities.
The reason for limiting the chemical component of the bolt of the present invention will be described below.

C:
C forms carbide particles and is the most effective component for increasing the strength. However, if it exceeds 0.70% by mass, the toughness is deteriorated, so the content is set to 0.70% by mass. In order to sufficiently expect an increase in strength, 0.35% by mass or more, preferably 0.40% by mass or more, more preferably 0.50% by mass is contained.

Si:
Si is an element effective for deoxidizing and dissolving in ferrite to increase the strength of the steel and finely disperse cementite.
Accordingly, the content added to the steel as a deoxidizing material and remaining in the steel is set to 0.50% by mass or more. The upper limit is not particularly limited for increasing the strength, but considering the cold forgeability and workability of the steel, it is 2.5% by mass or less, preferably 2.0% by mass or less, more preferably 1.0%. It is preferable to set it as the mass%.

Mn:
Mn is an element effective in lowering the austenitizing temperature and effective in refining austenite, and also in hardenability and solid solution in cementite to suppress cementite coarsening.
If the amount is less than 0.10% by mass, the desired effect cannot be obtained. More preferably, 0.2 mass% or more is contained. The upper limit is not particularly limited for increasing the strength, but considering the toughness of the steel material to be obtained, it is preferably 1.00% by mass or less.

Cr:
Cr is an element effective for improving the hardenability, and is also an element having a strong effect of delaying the growth of cementite by dissolving in cementite. In addition, it is also one of the important elements in the present invention that, by adding a relatively large amount, forms a high Cr carbide that is more thermally stable than cementite and improves corrosion resistance.
Therefore, it is necessary to contain at least 0.30% by mass or more. Preferably it is 0.80 mass% or more, More preferably, 1.00 mass% or more is contained. However, if too much Cr is added, many coarse carbides remain undissolved during the quenching process, and the mechanical properties are deteriorated. Therefore, the upper limit was made 3.00 mass% or less.

Mo:
Mo is an element effective for increasing the strength of steel in the present invention, and not only improves the hardenability of the steel but also solidifies a small amount in cementite to make the cementite thermally stable. In particular, secondary hardening occurs and steel is strengthened by nucleating alloy carbides newly on dislocations in the matrix phase completely separately from cementite. Moreover, the formed alloy carbide is effective for atomization and also as a hydrogen trap site.
Therefore, preferably 0.50% by mass or more, more preferably 1.00% by mass or more is contained. However, it is an expensive element and excessive addition forms coarse undissolved carbides or intermetallic compounds to produce toughness. Therefore, the upper limit of the addition amount was set to 1.50% by mass.
Since W and V also show the same effect as Mo, a part of Mo can be replaced with these elements.

  Al is an element that is effective for deoxidizing and forming elements such as O and N, oxides, nitrides and the like to refine the base structure. However, since excessive addition reduces toughness, it is preferably 0.1% by mass or less. More preferably it is 0.04 mass% or less.

  P and S are elements that should be removed as much as possible in order to reduce the grain boundary strength, and are each preferably 0.01% by mass or less.

  In addition, it is permissible for elements other than those described above to be contained in various elements as long as the effects of the present invention are not reduced.

[Refining treatment of bolt products]
In order to obtain a bolt tensile strength of 1700 MPa or more, it is necessary to perform quenching and tempering treatment after forming the material into bolts.
The reasons for limiting the tempering treatment conditions for bolt products in the present invention will be described below. Here, the bolt tensile strength is defined as the tensile strength at normal temperature of a tensile test piece machined from a bolt product into a shape and dimensions specified in JIS Z 2201.

  From the above chemical components, the austenitizing temperature of the bolt product needs to be 850 ° C. or higher. However, if the austenitizing temperature is too high, the base crystal grain structure becomes coarse and the toughness of the bolt decreases. Therefore, the upper limit of the austenitizing temperature is set to 1050 ° C. or lower. The austenitizing temperature is preferably 1000 ° C. or lower, more preferably 950 ° C. or lower. The austenitizing time varies depending on the shape and size of the bolt product, and the conditions are not particularly limited. However, it is desirable that the austenitizing time is within a range of 10 to 90 minutes after the bolt product reaches the austenitizing temperature.

  The quenching conditions after austenitization vary depending on the shape and size of the bolt product, so the conditions are not particularly limited. However, (1) no quench cracking occurs at the bottom of the bolt screw by quenching, and (2) the as-quenched bolt product It is indispensable that 90 volume% or more of the base metal structure to comprise comprises a martensite structure. The reason is that when the base metal structure is less than 90% by volume, the tensile strength of 1700 MPa or more cannot be obtained by the subsequent tempering treatment.

  In the as-quenched structure, it is necessary to perform a tempering treatment at 500 ° C. or higher to ensure the toughness and delayed fracture resistance of the bolt with a tensile strength of 1700 MPa or more. As the tempering treatment is performed at a higher temperature, the toughness is improved, but the tensile strength is lowered. Therefore, the upper limit of the tempering temperature was set to 650 ° C. In particular, in order to effectively utilize the effect of the Mo addition, the tempering temperature is preferably 530 ° C. or higher, more preferably 550 ° C. or higher. The tempering time varies depending on the shape and size of the bolt product, and the conditions are not particularly limited. However, it is desirable that the tempering time is within a range of 30 to 90 minutes after the bolt product reaches the tempering temperature.

Examples of the present invention will be described below.
Table 1 shows the chemical composition of the test steel in this example, and Table 2 shows the heat treatment conditions and the tensile deformation characteristics at room temperature.
In Table 1, steel materials A, B and C are steel materials having chemical components within the scope of the present invention, and D and E are steel materials outside the scope of the invention components. The comparative steel D steel corresponds to JIS-SCM440 steel which is also used as F10T high-strength bolt steel.

Inventive steel A and inventive steel B can obtain a tensile strength of 1700 MPa or higher at a tempering temperature of 500 ° C. or higher. Comparative steel D has Si and Mo contents, and comparative steel E has Mo contents. Since the temper softening resistance is less than the lower limit of the invention, a tensile strength of 1700 MPa or more could not be obtained by tempering treatment at 500 ° C. or more. However, even with the invention steel A, when the tempered martensite volume fraction was less than 90%, it was impossible to obtain a tensile strength of 1700 MPa or more.
The tensile test piece used was a JIS No. 4 or No. 14 A round bar test piece defined in JIS Z 2201, and the tensile test piece method conformed to JIS Z 2241.

FIG. 2 shows the shape and dimensions of a tensile test piece with a notch simulating the shape of the screw bottom.
Table 3 shows the relationship between the radius of curvature r of the notch bottom and the stress concentration factor.

  For example, FIG. 3 shows the relationship between the stress concentration factor and the notch tensile strength of the notched specimens of Invention Steel A, Invention Steel B, and Comparative Steel D. Here, a universal type tensile test was used, and a tensile test was performed until the test piece was broken at a crosshead speed of 0.5 mm / min. Compared with the comparative steel D, the invention steels A and B have higher stress concentration factor dependency of the notch tensile strength, and higher notch tensile strength can be obtained by lowering the stress concentration factor.

For example, FIG. 4 shows the results of the hydrogen split sensitivity test of Invention Steel A, Comparative Steel D, and Comparative Steel E for the notch test. The procedure of this test is shown in FIG. The amount of hydrogen in the steel was measured by thermal desorption analysis, and the amount of hydrogen released up to 300 ° C. when the test piece was heated at 100 ° C./hour was defined as the amount of diffusible hydrogen.
When the allowable amount of diffusible hydrogen is compared with the same load load, for example, at a load load of 1400 MPa, the hydrogen allowable amount is higher in the inventive steel A than in the comparative steel D and the comparative steel E as indicated by arrows in the figure. In invention steel A, the allowable hydrogen amount can be further increased by reducing the stress concentration factor at the notch bottom, that is, by reducing the stress concentration on the threaded portion.

  About the bar material of 22 mm in diameter of the invention steel B, after carrying out the spheroidizing annealing process to the steel material, the torcia type | mold bolt was produced with various shaft shape. The bolt head was cold produced by an existing bolt former. The bolt molded body was austenitized at 940 ° C. for 1 hour, quenched, and tempered at 540 ° C. for 1 hour. The tensile strength of the JIS No. 4 test piece cut out from the bolt product was 1815 MPa.

  FIG. 6 shows the shape of the bolt with the neck shape changed. The screw shape is the same as that of the existing F10T or S10T bolt.

  FIG. 7 shows a schematic diagram of the applied force. The applied force was a compressive force applied by an Amsler universal testing machine, and a tensile force was applied to the specimen through a jig. A displacement meter was attached to the four sides of the steel plate, and the relative displacement of the upper and lower steel plates was measured until the specimen broke. The test is performed in advance by performing a calibration test, tightening by tension control, and initial tension corresponding to 0.75 times the yield strength of the threaded portion of the bolt product (= 0.75 × yield strength × Ae) was introduced.

By this test, the relationship between the displacement of the bolt and the load was investigated (FIG. 8). The right end of the load displacement curve in the figure is the breaking point. From the load-displacement relationship, in the specimens with S values of 1.25 and 1.05, the thread part breaks at the end. However, when the S value is 1.05, the maximum displacement is improved as compared with the bolt of 1.25, which indicates that the stress concentration on the screw portion 3 of the bolt is alleviated.
Further, in the specimen having an S value of 1.00 to 0.85, the shaft portion was broken at the end, and in addition to the stress concentration on the screw portion 3 being relaxed, the proof stress was maintained and the deformation capability was also improved. You can see that it ’s big. It can be seen that the inventive bolts exhibit excellent energy absorption capabilities. When the S value is 0.8, the maximum load is reduced to 430 kN.

  Table 4 shows the relationship between the tensile deformation characteristics of the bolt and the S value (= Ab / Ae).

  The S value of existing F10T bolts is 1.25 to 1.28. By making the S value less than 1.25 of the present invention and within the range of 0.85 or more, the deformability of the bolt can be increased without significantly reducing the tensile strength of the bolt product.

If the high-strength bolt of the present invention is used, compared with the conventional high-strength bolt of 1000 to 1100 MPa class, (1) the joint portion is made compact and lightweight, and (2) the bolt strength is higher than that of the thick steel plate. This increases the degree of freedom in design. As a result, it is possible to develop a new civil engineering building structure with resource saving, labor saving, energy saving and CO ₂ reduction in mind.

1 Head 2 Shaft 3 Screw

Claims (2)

In mass% C: 0.35 to 0.70%,
Si: 0.50 to 2.50%,
Mn: 0.10 to 1.00%,
Cr: 0.30 to 3.00%
Mo: 0.50 to 1.50%,
Al: 0.001 to 0.1%
The balance is a high-strength bolt composed of Fe and inevitable impurities, and 90% by volume or more of the internal metal structure in the portion from the bolt head to the screw portion is a tempered martensite structure, A high-strength bolt having a tensile strength of 1700 MPa or more and a shaft shape parameter S represented by the following formula (1) having a value of 0.85 or more and less than 1.25.
S = Ab / Ae (1)
(Ab in the formula represents the effective sectional area of the shaft portion, and Ae represents the effective sectional area of the threaded portion)   It is a manufacturing method of the high strength bolt of Claim 1, Comprising: After shape | molding a raw material into a volt | bolt, it hardened after performing the austenitizing process within the temperature range of 850 degreeC-1050 degreeC, and 90 volume of an internal metal structure. % Or more is made into a martensite structure, and then a tempering treatment is performed in a temperature range of 500 ° C. to 650 ° C.

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