JP2013139872A - High tensile bolt and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile bolt which has a tensile strength of ≥1,700 MPa, and is excellent in tensile deformation performance and a delayed fracture resistance characteristic, and a method for manufacturing the same.SOLUTION: Steel containing, by mass, C:0.35-0.70%, Si:0.50-2.50%, Mn:0.10-1.00%, Cr:0.30-3.00%, Mo:0.50-1.50%, Al:0.001-0.1%, and the residue comprising Fe and inevitable impurities is used as a material. The steel is formed in a bolt shape having a thread section shape at the thread section in which an angle of a flank face is set to 60°, the height of fundamental triangle of the thread section is set as H, and a transition point between the flank face and the bottom of the thread facing each other is set as H/2 from a bottom side of the fundamental triangle, and in which an arc-shaped curve is formed, the arc-shaped curve drawing a small abutting circle that abuts on the flank face at the transition point and that has a radius of curvature R of ≥H/3.5 and ≤H/2.5. The bolt-shaped steel is hardened by austenitizing treatment in a temperature range of 850-1,050°C, and then is subjected to tempering treatment in a 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 1400-1600 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 before the development of F14T super high-strength bolts, the increase in strength of civil engineering and construction high-strength bolts was flat with F10T and S10T high-strength bolts.

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, steel materials such as Mo and V carbonitrides that trap hydrogen in steel are finely dispersed in the base to increase the allowable hydrogen amount, and the bolt (1) new screw Development of the shape, (2) Improvement of the shape of the transition from the bolt shaft to the threaded portion, (3) Increase in the bolt head neck lower radius of curvature r, (4) Change the nut shape, the height of the conventional F10T and S10T A unique shape different from a force 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 head 1 is warm-formed at a temperature higher than that of the threaded portion, and the tensile strength is 1800 MPa level by controlling the strength of the bolt to be gradually lowered from the shaft 2 to the head 1. However, a bolt having excellent ductility and delayed fracture characteristics and 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 by this method.

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 is suitable for civil engineering and construction with excellent delayed fracture resistance. We provide high strength bolt products.

  As a result of various studies in view of the above circumstances, the inventors of the present invention can increase the hydrogen allowable amount of the material by reducing the stress concentration in the bolt screw portion 3 even if it is an ultrahigh strength steel material of 1700 MPa. It has been found that it is possible to improve the delayed fracture resistance, and for that purpose, the shape of the threaded portion 3 of the bolt should be optimized.

  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 A high-strength bolt having a length of 1700 MPa or more, and in the threaded portion 3 of the bolt, as shown in FIG. The shape of the bottom of the valley is set to H at the height of the pointed crest, and the transition point between the flank face of the opposing screw thread and the bottom of the valley is set to H / 2 from the bottom of the pointed crest mountain, and the flank surface at the transition point An arcuate curve is formed that draws a small contact circle having a radius of curvature R of contact of H / 3.5 or more and H / 2.5 or less.

  Moreover, in the manufacturing method of the high-strength bolt of the present invention, when the high-strength bolt is manufactured, the material is formed into a bolt, and then subjected to an austenitizing treatment within a temperature range of 850 ° C. to 1050 ° C. A tempering treatment is performed in a temperature range of 500 to 650 ° C. after 90% by volume or more of the metal structure is made a martensite structure.

  According to the present invention, it is possible to provide a high-strength bolt that can introduce an axial force 1.7 times or more that of conventional F10T and has excellent delayed fracture characteristics by quenching and tempering after forming the material into the bolt, Industrial contribution can be expected. Further, by adopting such a simple screw shape, 1) the mold can be easily manufactured and the manufacturing cost of the mold can be suppressed, 2) the durability of the mold can be improved, and 3) the molding load is high. Even if it is a high-strength bolt material, there is an effect that a screw shape with high dimensional accuracy can be formed.

It is the schematic of a volt | bolt. It is explanatory drawing which shows the positional relationship of a volt | bolt, a nut, and a clamping board, and the thread valley bottom shape and dimension (mm) of M22 volt | bolt. It is a graph which shows the relationship (a) of the screw position and maximum principal stress in the thread part 3 of M22 bolt, and the relationship (b) of the curvature radius of a screw bottom, and a stress concentration factor. 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 the schematic of a corrosion acceleration test procedure. It is a graph which shows the maximum, the minimum, and the average value of the breaking load in a JIS screw and an invention screw shape bolt.

  The high-strength bolt with a tensile strength of 1700 MPa or more in the present invention adopts the screw shape of the bolt of the present invention, so that the stress concentration factor of the thread portion 3 is a screw shape standardized with F10T and S10T bolts. By adopting this screw shape, it is possible to further increase the notch tensile strength of the threaded portion 3.

[Bolt thread shape]
In high-strength bolts, the angle of the opposed flank surfaces of the threads provided on the threaded portion 3 at an equal pitch is 60 °, and the shape of the valley bottom of the threaded portion is H. The transition point between the flank surface and the bottom of the valley is set to H / 2 from the bottom edge of the sharp edge, and the radius of curvature R abutting on the flank surface at each transition point is H / 3.5 or more, H / 2 It is necessary to form an arcuate curve that draws a contact circle of .5 or less. In addition, the tolerance and tolerance in the reference thread shape of the screw shall conform to JIS B 0209-1.

  As a result of the FEM analysis, when the radius of curvature R that abuts is smaller than H / 3.5, the stress coefficient at the threaded portion 9 rapidly increases as shown in FIG. On the other hand, if the radius of curvature R that abuts is larger than H / 2.5, there is a problem of screwing, so it is necessary to make it less than this. The radius of curvature R is preferably H / 2.8 or less, more preferably H / 3. The FEM analysis procedure is as follows.

In this analysis, paying attention to the threaded portion of the bolt and nut, an axial object analysis is performed on the portion including the bolt, nut and fastening plate as shown in FIG. The finite element model is discretized by a three-node linear triangular element at the bolt and nut portion and a four-node linear quadrilateral element at the fastening plate portion. The element size of the thread and valley of male and female threads is 0.05 mm, and the element size of other parts is basically 1 mm. Moreover, about the meshing part of a nut and a thread part, a node is shared as what does not generate | occur | produce a slip. The boundary conditions in this analysis are as shown in FIG. 2, and the lower surface of the clamping plate is completely fixed, and an evenly distributed surface load is applied to the lower surface of the shaft portion.
Thereby, the stress concentration factor of the screw part 3 can be reduced as compared with the JIS bolt while maintaining the machining accuracy of the screw part 3 in the same manner as the JIS bolt, and the delayed fracture resistance can be improved.

  The reason for this is that, as shown in FIG. 3, the FEM analysis results of the maximum principal stress (a) at each screw position in the thread portion 3 and the stress concentration factor (b) at the maximum principal stress position (screw position 9) are shown. By optimizing the shape parameters of the threaded portion 3, the stress concentration factor of the threaded portion is made smaller than the stress concentration factor of the thread shape defined by existing F10T bolts and commercially available F14T bolts (Patent Document 1), especially delayed fracture This is because the notch tensile strength of the threaded portion is increased by defining a screw shape that lowers the maximum principal stress of the nut hooking portion (screw position 9) that has the highest probability of becoming the starting point of the threaded portion.

〔Chemical composition〕
The chemical component of the high-strength bolt of the present invention is, by 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 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 reducing the austenitizing temperature and making the austenite finer, and also effective in suppressing the coarsening of the cementite by solid solution in the hardenability and cementite.
If the amount is less than 0.10% by mass, the desired effect cannot be obtained. More preferably, 0.20 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 it is less than 90% of the base metal structure, a 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 in order 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 steels A and B can obtain a tensile strength of 1700 MPa or more 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 smaller than the lower limit, a tensile strength of 1700 MPa or more could not be obtained by tempering treatment at 500 ° C. or higher. However, even in 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. 4 shows the shape and dimensions of a tensile test piece with a notch that simulates the shape of the screw bottom.
Table 3 shows the relationship between the stress concentration factor (kt) of the notch specimen and the curvature radius r of the notch bottom.

  For example, FIG. 5 shows the relationship between the stress concentration factor and the notch tensile strength of notched specimens of invention steels A and 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. 6 shows the results of the hydrogen split sensitivity test of Invention Steel A, Comparative Steels D and E for the notch test. The test procedure 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. The procedure for the corrosion acceleration test is shown in FIG.
When the allowable amount of diffusible hydrogen is compared with the same load, for example, at a load of 1400 MPa corresponding to 0.8 times the yield strength of the invention steel A, as compared with the comparative steels D and E as indicated by arrows in the figure. Invented steel A has a higher hydrogen tolerance. 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.
In particular, when the stress concentration factor is 2 or less, a diffusible hydrogen amount of 1 mass ppm or more can be allowed. On the other hand, the maximum value of hydrogen entering the air exposure test of this material is about 0.8 ppm by mass from the corrosion acceleration test shown in FIG. As shown in FIG. 3 (b), it can be proved that the M22 bolt has an invention bolt with a stress concentration factor of 2 or less at the screw position 9 and the allowable amount of diffusible hydrogen exceeds the amount of diffusible hydrogen penetration, and delayed fracture hardly occurs. .

  Table 4 summarizes the maximum principal stress at the main screw positions in the threaded portion 3 of the M22 bolt made of the inventive steel B. The maximum principal stress at the most important screw position 9 is lower than that of a JIS screw or a commercially available F14T screw. The screw shape of each bolt is as shown in FIG.

  About the bar material of 22 mm in diameter of the invention steel B, the steel material was subjected to spheroidizing annealing treatment, and then a conventional thread-shaped torcia-type bolt was produced with various shaft shapes. 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.

A tensile test was performed on a high-strength bolt having a cut JIS screw shape and an inventive screw shape. FIG. 9 shows the maximum, minimum and average values of the bolt breaking load of the JIS screw shape and the invention screw shape.
The bolt having the invention screw shape shows a breaking load of 1940 MPa, which is 100 MPa or more lower than the bolt having the JIS screw shape. Further, no screw unscrewing occurred in the JIS screw shape and the invention screw shape.

If the high-strength bolt of the present invention is used, compared to the conventional high-strength bolt of 1000 to 1100 MPa class, (1) the joint is made compact and lightweight, and (2) the bolt joint of the high-strength and thin steel plate is achieved. It becomes possible, and the freedom degree of design increases. 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 made of Fe and inevitable impurities, and 90% by volume or more of the internal metal structure in the part from the bolt head to the screw part is a tempered martensite structure, and the tensile of the part The strength is 1700 MPa or more, the screw shape of the bolt is provided at equal pitches on the screw portion, the angle of the opposed flank of the screw thread is 60 °, and the shape of the valley bottom of the screw portion is the height of the point Is set to H / 2, and the transition point between the flank face and the valley bottom of the opposing screw thread is set to H / 2 from the edge of the sharp ridge, and the radius of curvature R contacting the flank face at the transition point is H / 3. A high-strength bolt characterized by forming an arcuate curve that draws a contact small circle of 5 or more and H / 2.5 or less.   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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