JP7522348B2 - High Strength Bolts - Google Patents

Description

The present invention relates to high-strength bolts.

As is well known, in addition to various welding methods, bolted joints are commonly used to join steel materials used in various industrial fields. In such bolted joints, the pursuit of economy and technological advances have promoted the use of high tensile steel materials, and in the civil engineering and construction fields in particular, friction joints using high-strength bolts, which are highly reliable and easy to work with, are widely used.

As high-strength bolts used for friction joints, for example, a set of high-strength hexagonal bolts for friction joints, hexagonal nuts, and flat washers specified in the Japanese Industrial Standards JIS B1186, and a set of structural torsion-type high-strength bolts, hexagonal nuts, and flat washers specified in the Japanese Steel Construction Association Standard JSSII-09 are widely used. Under these circumstances, and particularly in recent years, with the increase in size of civil engineering and architectural structures, there is a strong demand for high-strength bolts with a tensile strength of 1200 N/ ^mm2 or more.

In light of the above circumstances, Patent Document 1 proposes a high-strength bolt with excellent delayed fracture resistance that exhibits the desired bolt tensile strength by setting the chemical composition of the steel so as to satisfy the relationship between the bolt tensile strength and tempering temperature, and the relationship between the bolt tensile strength and the carbon equivalent calculated from the chemical composition of the steel, and then performing quenching and tempering processes.

Patent No. 4142853

However, as a result of the above developments, high-strength bolts are increasingly being used in harsher environments. In these cases, there is a demand for improved hydrogen embrittlement resistance, among other bolt properties as described in Patent Document 1.

Therefore, the present invention aims to provide a high-strength bolt with improved hydrogen embrittlement properties.

［５］質量％で、Ｃ：０．３５～０．４５％、Ｓｉ：０．５０％未満、Ｍｎ：０．４０％超１．００％未満、Ｐ：０．０３０％未満、Ｓ：０．０３０％未満、Ｃｒ：０．０５～１．５０％未満、Ｍｏ：０．３０％超２．５０％未満、Ｖ：０．０５％超０．５０％未満、Ａｌ：０．００５％超０．０８０％未満、Ｎ：０．０１５０％未満を含有し、残部がＦｅおよび不可避的不純物からなる鋼材で形成され、引張強度が１３００ＭＰａ以上１８００ＭＰａ以下である、［１］から［４］のいずれか１項に記載の高強度ボルト。 [1] A high-strength bolt, in which, when a first portion of the valley bottom continuing with the pressure side flank and a second portion of the valley bottom continuing with the play side flank and being bordered from the first portion by a reference point where a normal to the valley bottom coincides with a line perpendicular to the axis in the cross-sectional shape including the axis are defined, the mean curvature κ ave of the cross-sectional shape in the range where a first angle that the normal makes with a line perpendicular to the axis in the first portion is 32° or less and in the range where a second angle that the normal makes with a line perpendicular to the axis in the second portion is 8° or less satisfies the relationship κ _ave H _< 5 with respect to the height H of the pointed crest of the thread.
[2] A high-strength bolt as described in [1], wherein the cross-sectional shape is formed by a _first and a second circular arc of radius r1 that are continuous with the pressure side flank and the play side flank, respectively, and a third circular arc of radius _r2 that is continuous with the first and second circular arcs, wherein _r1 < _r2 .
[3] A high-strength bolt as described in [1], whose cross-sectional shape is composed of a circular arc continuing from the play side flank and a higher-order curve that is continuing from the pressure side flank and the circular arc and includes a portion whose radius of curvature is greater than or equal to the radius of the circular arc.
[4] A high-strength bolt according to any one of [1] to [3], wherein in the first and second portions of the valley bottom, when the average hardness of the center including the axis is HVa, the average hardness of the outermost layer at a depth of 0.2 mm from the surface is HVs, the depth d (mm) at the position where the average hardness reaches the average hardness HVa at the center, and σ _ΔR (MPa) is the standard deviation of the axial residual stress at a position 0.5 mm deep from the surface, is 0.2≦d≦1.5 and the relationship of the following formulas (i) and (ii) is satisfied.

[5] The high-strength bolt according to any one of [1] to [4], which is formed of a steel material containing, by mass%, C: 0.35 to 0.45%, Si: less than 0.50%, Mn: more than 0.40% and less than 1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.05 to less than 1.50%, Mo: more than 0.30% and less than 2.50%, V: more than 0.05% and less than 0.50%, Al: more than 0.005% and less than 0.080%, N: less than 0.0150%, with the balance being Fe and unavoidable impurities, and has a tensile strength of 1300 MPa or more and 1800 MPa or less.

The above configuration allows the bolt to be shaped so that the residual stress at the root of the thread and the acting stress and plastic strain due to bolt tightening are smaller, making it difficult for hydrogen to enter even after the bolt is tightened, improving the hydrogen embrittlement resistance of the bolt.

1 is a diagram showing an entire high-strength bolt according to an embodiment of the present invention. FIG. FIG. 2 is an example of an enlarged cross-sectional view of a threaded portion of FIG. 1 . FIG. 2 is an example of an enlarged cross-sectional view of a threaded portion of FIG. 1 . FIG. 2 is an example of an enlarged cross-sectional view of a threaded portion of FIG. 1 . FIG. 2 is an example of an enlarged cross-sectional view of a threaded portion of FIG. 1 . FIG. 2 is an example of an enlarged cross-sectional view of a threaded portion of FIG. 1 . FIG. 13 is an example of an enlarged cross-sectional view of a threaded portion according to a reference example. FIG. 13 is an example of an enlarged cross-sectional view of a threaded portion according to a reference example. FIG. 1 is a diagram illustrating a schematic model of an FEM analysis performed to define conditions related to the shape of a high-strength bolt. 1 is a graph showing the maximum principal stress at the thread root surface layer portion calculated by analysis. 1 is a graph showing equivalent plastic strain (PEEQ) at each location of the thread root in the analysis. 1 is a graph showing the relationship between the peak value of equivalent plastic strain (PEEQ) at the thread root in the analysis and the average curvature in the section where the peak is observed. 1 is a graph for explaining the surface hardness of a thread root. FIG. 13 is a diagram for explaining residual stress parameters. FIG. 13 is a diagram for explaining residual stress parameters. FIG. 13 is a diagram showing an example of hardness distribution in the depth direction at the bottom of a thread valley. FIG. 13 is a diagram showing an example of hardness distribution in the depth direction at the bottom of a thread valley. FIG. 13 is a diagram showing an example of hardness distribution in the depth direction at the bottom of a thread valley. FIG. 13 is a diagram showing an example of hardness distribution in the depth direction at the bottom of a thread valley. FIG. 11 is a diagram showing an example of variation in residual stress in the circumferential direction at the thread root. FIG. 11 is a diagram showing an example of variation in residual stress in the circumferential direction at the thread root.

The preferred embodiment of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration will be assigned the same reference numerals to avoid duplicated explanations. Furthermore, unless otherwise specified, screw-related terms are defined in JIS B0101 "Screw Terminology."

(High strength bolt shape)
One of the causes of hydrogen embrittlement of bolts is that large tensile stress occurs at the thread root when the bolt is tightened, causing localized plastic deformation. Therefore, by shaping the thread root so that the residual stress and the acting stress and plastic strain due to tightening are reduced, it becomes more difficult for hydrogen to enter even after tightening, and the hydrogen embrittlement properties of the bolt can be improved.

1 is a diagram showing the entirety of a high-strength bolt according to an embodiment of the present invention, and FIGS. 2A to 2E are examples of enlarged cross-sectional views of the threaded portion of FIG. 1, and FIGS. 3A and 3B are examples of enlarged cross-sectional views of the threaded portion according to a reference example. The high-strength bolt 1 includes a head 2 and a shaft 3 integrally formed of steel, and a threaded portion 4 is formed on the shaft 3. FIG. 1 shows the axis X of the high-strength bolt 1, and FIGS. 2A to 2E, 3A, and 3B show cross-sectional shapes including the axis X of the high-strength bolt 1. In common with each example, a thread 5 and a thread groove 6 are formed on the threaded portion 4, and flanks 7A and 7B are formed on both sides of the thread groove 6. In the examples shown in each figure, the flank 7A is a pressure side flank, and the flank 7B is a play side flank. The valley bottom 8 connecting the flanks 7A and 7B on both sides of the thread groove 6 is defined as a first portion 8A continuing to the flank 7A (pressure side flank) and a second portion 8B continuing to the flank 7B (play side flank). The first portion 8A and the second portion 8B are bounded by a reference point 8P where the normal to the valley bottom 8 in the cross-sectional shape including the axis X coincides with the perpendicular line Xn to the axis X.

2A to 2E, when the first portion 8A and the second portion 8B are defined at the valley bottom 8 as described above, the average curvature κ ave in the cross-sectional shape including the axis X of the high-strength bolt 1 satisfies the relationship κ ave H < 5 with respect to the height H of the peak when the angle θ ₁ that the normal line makes with respect to the perpendicular line Xn to the axis X in the first portion 8A is in the range of 32° or less, and the angle θ ₂ that the normal line makes with respect to the perpendicular line Xn to the axis X in the second portion _8B is in the range of 8° or less. Here, the average curvature κ _ave is calculated by averaging the curvature in the cross-sectional shape including the axis X of the high-strength bolt 1 over the length along the cross-sectional _shape .

In the examples shown in Figures 2A to 2E, 3A and 3B, the thread angle α is 60°, and in the cross-sectional shape, the first portion 8A and the second portion 8B are continuous with the flanks 7A and 7B, respectively, so that the angles θ ₁ and θ ₂ in the first portion 8A and the second portion 8B are in the range of 0° to 60°. Schematically, θ ₁ = 32° and θ ₂ = 8° are shown in Figures 2A to 2E, and θ ₁ = θ ₂ = 13° are shown in Figure 2B (the angles are not necessarily exact because they are shown diagrammatically).

In the example shown in Fig. 2A, the cross-sectional shape of the entire valley bottom 8 including the first portion 8A and the second portion 8B is formed of a circular arc with a single radius of curvature. In this case, by determining the radius r of the circular arc to be r>H/5, κH<5 is satisfied for the curvature κ=1/r throughout the entire first portion 8A and the second portion 8B, and the condition κ _ave H<5 is also satisfied in the ranges of θ ₁ ≦32° and θ ₂ ≦8°. On the other hand, in the reference example shown in Fig. 3A, the radius r of the circular arc is equal to H/6, and in the reference example shown in Fig. 3B, the radius r of the circular arc is even smaller. In these cases, the above condition is not satisfied.

In the example shown in Fig. 2B, the cross-sectional shape of the valley bottom 8 is formed by combining two circular arcs with different radii of curvature. Specifically, in the range where the angle _θ1 exceeds 13° in the first portion 8A of the valley bottom 8 and the range where the angle _θ2 exceeds 13° in the second portion 8B, the cross-sectional shape is formed by an arc with a radius _r1 = H/6. On the other hand, in the range where the angle _θ1 is 13° or less in the first portion 8A and the range where the angle _θ2 is 13° or less in the second portion 8B, the cross-sectional shape is formed by an arc with a radius _r2 = 2H/3. With such a cross-sectional shape of the valley bottom 8, the height of the thread 5 is ensured while the above condition, i.e., the condition that κ _ave H < 5 in the range of _θ1 ≦ 32° in the first portion 8A and the range of _θ2 ≦ 8° in the second portion 8B, can be satisfied. As long as the conditions are satisfied, the radii _r1 and _r2 can be changed within the ranges of _r1 ≧ H/6, _r2 ≧ 2H/3 and _r1 < _r2 , and the connection point of the two arcs is not limited to a point where _θ1 , _θ2 = 13°.

In the example shown in Fig. 2C, the cross-sectional shape of the valley bottom 8 is constituted by a high-order curve in the first portion 8A, and is constituted by a circular arc in the second portion 8B. Here, the high-order curve constituting the cross-sectional shape of the first portion 8A includes a portion whose radius of curvature is equal to or greater than the radius r of the circular arc of the second portion 8B. Specifically, the cross-sectional shape of the first portion 8A is constituted by an elliptical curve with a minor axis b = H/6 and a major axis a = 1.8b, and the cross-sectional shape of the second portion 8B is constituted by a circular arc with a radius r = H/6. With such a cross-sectional shape of the valley bottom 8, the height of the thread 5 is ensured while the above condition, that is, the condition that κ _ave H < 5 is satisfied in the range of θ ₁ ≦ 32° in the first portion 8A and the range of θ ₂ ≦ 8° in the second portion 8B, can be satisfied. In the above condition, the range in which the relationship κ _ave H < 5 is satisfied is asymmetric between the first portion 8A side and the second portion 8B side, and is wider on the first portion 8A side. 2C, which allows the curvature of the cross-sectional shape to be smaller in the first portion 8A than in the second portion 8B, may be advantageous. As long as the conditions are satisfied, the major axis a, the minor axis b, and the radius r of the arc of the elliptical curve can be adjusted, and the high-order curve is not limited to an elliptical curve. The high-order curve and the arc may be connected at a point other than the reference point 8P.

In the example shown in Fig. 2D, the cross-sectional shape of the valley bottom 8 includes portions 81 and 82 formed of arcs of radius r, and a portion 83 located between the portions 81 and 82 and formed of a straight line. As shown in the figure, by making the central angle _β2 of the arc forming the portion 83 larger than the central angle _β1 of the arc forming the portion 81, it is possible to reduce the average curvature κ ave of the cross-sectional shape in the range of θ ₁ ≦32° on the flank 7A (pressure side flank) side and the range of θ ₂ ≦8° on the flank 7B (play side flank) side with respect to the reference point 8P. Specifically, since the curvature is 0 in the portion 82 formed of a straight line, it is possible to _reduce the average curvature κ _ave calculated by averaging the curvature over the length along the cross-sectional shape by including the portion 82 in the above range.

In the example shown in Fig. 2E, the cross-sectional shape of the root 8 includes portions 84, 85 formed by arcs of radius _r1 that are continuous with the flanks 7A, 7B, respectively, and a portion 86 formed by an arc of radius _r2 located between the portions 84, 85. Here, _r1 < _r2 . As a difference from the already-mentioned examples, in the example of Fig. 2B, the midpoint of the portion 86 formed by an arc of a relatively large radius _r2 coincides with the reference point 8P, but in the example of Fig. 2E, the midpoint 8Q of the portion 86 does not coincide with the reference point 8P and is located on the flank 7A (pressure side flank) side of the reference point 8P. This makes it possible to reduce the average curvature κ ave of the cross-sectional shape in the range of θ ₁ ≦ 32° on the flank 7A (pressure side flank) side and the range of θ ₂ ≦ 8° on the flank 7B (play side flank) side, based on the reference point _8P . Specifically, since the curvature is small in the portion 86 formed by an arc of a relatively large radius _r2 , the average curvature κ _ave can be reduced by increasing the proportion of the portion 86 in the above range.

4 is a schematic diagram of a model of an FEM analysis performed to define conditions related to the shape of a high-strength bolt. The model is a solid element model of a solid body of revolution simulating a bolt and a nut as shown in the figure, and an analysis was performed using the FEM (finite element method) assuming a strength and load equivalent to a 1400 N/mm ² -class high-strength bolt.

In the analysis results described below with reference to Figures 5 to 7, Example A corresponds to the example in Figure 2A, Examples B-1 and B-2 correspond to the examples shown in Figure 2B, and the materials used in Examples B-1 and B-2 are different. Example C corresponds to the example in Figure 2C. Examples X-1 and X-2 correspond to the examples shown in Figure 3A, and the materials used in Examples X-1 and X-2 are different. Example Y corresponds to the example shown in Figure 3B.

Figure 5 is a graph showing the maximum principal stress at the surface layer of the thread root calculated by analysis. The graph shows the maximum principal stress (MPa) calculated in the range of 0 to 0.6 mm deep from the surface at the root (the position corresponding to the reference point 8P shown above) of the first thread groove (indicated by number 7 in Figure 4) of the bolt that engages with the thread of the nut. The peak value of the maximum principal stress is largest in Examples X-1 and X-2, which have a small radius of curvature of the cross-sectional shape of the thread root, and is smaller in Examples A, B-1, B-2, and C, which have a larger radius of curvature. Therefore, it is considered that increasing the radius of curvature of the cross-sectional shape of the thread root is effective in improving the stress state. In addition, in common to each example, the maximum principal stress has a peak between 0.1 mm and 0.5 mm deep from the surface of the thread root. Therefore, it is considered that improving the residual stress state in this depth region is effective in improving hydrogen embrittlement properties. This point is related to the consideration of materials described later.

FIG. 6 is a graph showing the equivalent plastic strain (PEEQ) at each portion of the thread root in the analysis. The graph shows the equivalent plastic strain (%) calculated for each portion of the root of the first thread groove (indicated by number 7 in FIG. 4) that engages with the thread of the nut in the analysis. The portion of the root is represented by the angles θ ₁ and θ ₂ that the normal to the cross-sectional shape described above makes with respect to the perpendicular to the axis. The angle θ ₁ is defined for the portion of the root that continues to the pressure side flank, and the angle θ ₂ is defined for the portion of the root that continues to the play side flank, and θ ₁ = θ ₂ = 0 is achieved at the point where the normal to the cross-sectional shape is perpendicular to the axis. In common with each example, the equivalent plastic strain has a peak in the range of 10° to 30° of the angle θ ₁ on the pressure side flank. In other words, the peak of the equivalent plastic strain appears biased toward the pressure side flank, not at the bottom of the root (position where θ ₁ = θ ₂ = 0).

On the other hand, among the examples in which the cross-sectional shape of the valley bottom is composed of an arc with a single radius of curvature, the equivalent plastic strain at the peak is largest in Example Y (Figure 3B), smaller in Example X-1 (Figure 3A), and even smaller in Example A (Figure 2A), which shows that the larger the radius of curvature of the cross-sectional shape, the smaller the equivalent plastic strain. In addition, in Example B-1 (Figure 2B), in which the cross-sectional shape of the valley bottom is composed of a combination of two arcs with different radii of curvature, a different value of equivalent plastic strain is calculated for each part with a similar radius of curvature than that of Examples X-1 and A, which have similar radii of curvature. This shows that the value of equivalent plastic strain at a certain position is not simply determined by the radius of curvature of the cross-sectional shape at that position, but is affected by the distribution of radii of curvature in nearby sections.

As a result, the graph shown in Figure 6 shows that the equivalent plastic strain is smallest in Example C (Figure 2C), where the cross-sectional shape of the valley bottom is composed of a high-order curve on the pressure flank and a circular arc on the play flank. When combined with the results of the other examples mentioned above, it is believed that in Example C, the radius of curvature of the cross-sectional shape of the valley bottom can be made larger on the pressure flank, where the peak of equivalent plastic strain appears, and therefore the equivalent plastic strain can be effectively reduced.

FIG. 7 is a graph showing the relationship between the peak value of the equivalent plastic strain (PEEQ) at the thread root in the analysis and the average curvature in the section where the peak is observed. As described above with reference to FIG. 6, the equivalent plastic strain has a peak at a position biased toward the pressure side flank of the thread root, specifically, at an angle θ ₁ on the pressure side flank side in the range of 10° to 30°. In this study, the average curvature κ _ave was calculated in an angle range of ±20° before and after the position where the peak appears, and the relationship between κ _ave H, which is obtained by multiplying the average curvature κ _ave by the height H of the peak, and the normalized equivalent plastic strain was graphed. The normalized equivalent plastic strain is the value obtained by normalizing the peak value PEEQ _max of the equivalent plastic strain by the equivalent plastic strain PEEQ _X at the same position in Example X-1 (FIG. 3A).

As shown in the graph, in three examples (Example A, Example B-1, and Example C) in which the peak value of the equivalent plastic strain PEQ _max is lower than that of Example X-1 (FIG. 3A), the value of κ _ave H is less than 5. From this result, it can be said that by making κ _ave H<5, the equivalent plastic strain at the thread root can be effectively reduced. Furthermore, as explained with reference to FIG. 5, a similar relationship to that of the equivalent plastic strain is observed between the maximum principal stress state of the thread root surface layer and the curvature radius of the cross-sectional shape, so it is considered that by making κ _ave H<5, the maximum principal stress state can also be effectively improved. Furthermore, in Example C (FIG. 2C) in which the peak value of the equivalent plastic strain PEQ _max is lower, κ _ave H=2.60. Therefore, the shape of the thread root may be determined so that κ _ave H<3 is more preferably satisfied.

(High strength bolt material)
Next, the material of the high-strength bolt will be described. In order to suppress brittle fracture at the root of the thread when the bolt is tightened, it is desirable to reduce the surface hardness of the root of the thread. However, if the area where the surface hardness is low at the root of the thread is wide, the plastic strain at the root of the thread becomes too large when the screw is tightened, and brittle fracture due to hydrogen penetration is likely to occur. Therefore, the area where the surface hardness is low at the root of the thread must be limited. Conversely, if the surface hardness is too high compared to the hardness of the central portion, brittle fracture due to hydrogen penetration is likely to occur. Furthermore, in order to suppress brittle fracture at the root of the thread, it is also desirable to reduce the tensile residual stress at the root of the thread of the bolt before tightening. The axial residual stress at the root of the thread of the bolt occurs at the stage of forming the thread of the bolt by rolling. The axial residual stress at the root of the thread of the bolt varies in the circumferential direction of the thread, so the variation must be reduced. The stress acting on a bolt after it is tightened is the sum of the axial residual stress at the root of the bolt's thread and the axial stress generated by tightening. The greater this tensile stress, the more likely it is that brittle fracture due to hydrogen penetration will occur.

FIG. 8 is a graph for explaining the surface hardness of the thread valley root. The graph in FIG. 8 shows an example of HV hardness measured in the axial direction from the surface of the thread valley root (the position corresponding to the reference point 8P shown above). The HV hardness is measured at a load of 100 gf, starting from a depth of 0.2 mm from the surface in the thread valley root cross section, with a pitch of 0.1 mm up to a depth of 1.5 mm, and then at a pitch of 1 mm from a depth of 1.5 mm to the central depth including the bolt axis. In the same manner, the hardness distribution is measured at two or more points on the same thread valley root, and the average value at each measurement point is obtained. In the region where the surface hardness is reduced as described above, specifically the surface layer of the thread valley root, it is preferable that the hardness is reduced by 10 HV or more per mm of depth. However, if the hardness is reduced by 100 HV or more per mm of depth, the plastic strain when the bolt is tightened becomes large, making it easier for brittle fracture due to hydrogen penetration to occur, so it is preferable that the softening per mm of depth is 10 HV or more and less than 100 HV. This condition is expressed by the following formula (1), where HV _a is the average hardness of the center including the bolt axis, HV _S is the average hardness of the outermost layer located 0.2 mm deep from the surface, and d (mm) is the depth at which the average hardness reaches the average hardness HV _a at the center. Note that 0.2≦d≦1.5, and since brittle fracture due to hydrogen penetration is likely to occur if the surface hardness HV _S at the thread root becomes too low, it is desirable to satisfy 0<HV _a -HV _s ≦100. Furthermore, the average hardness HV _a at the center may be calculated by measuring the HV hardness from the center including the bolt axis to a position 1/4 of the bolt diameter at 10 or more points under a load of 100 gf.

9A and 9B are diagrams for explaining residual stress parameters. FIG. 9A is a schematic diagram of an enlarged thread root of a bolt, showing the measurement direction of the residual stress at the thread root. In this study, the following parameters were specified for the residual stress in the bolt axial direction at a depth of 0.5 mm from the surface of the thread root. Each parameter can be measured by X-ray diffraction at each point of one thread root rotated 0°, 90°, 180°, and 270° in the circumferential direction, and the measurement points of the thread root can be changed at three or more points on the same bolt. The graph in FIG. 9B shows a schematic example of a measurement of the residual stress at the thread root of one bolt, and illustrates how to obtain σ _Rmax, σ _Rmin , and σ _ΔR . Each parameter is expressed as positive on the tension side and negative on the compression side.
Maximum residual stress: σ _Rmax (MPa)
Minimum residual stress: σ _Rmin (MPa)
Standard deviation of residual stress in the bolt circumferential direction: σ _ΔR (MPa)

Of these, with regard to the standard deviation σ _ΔR , which indicates the variation in residual stress in the circumferential direction of the bolt, it is preferable that the following formula (2) be satisfied. Formula (2) specifies that the amount of softening in the surface layer of the thread root, i.e., the difference between the average hardness HV _a in the center of the bolt and the average hardness HV _S of the outermost surface layer at the thread root, and the standard deviation σ _ΔR are additively within a predetermined value. When the amount of softening in the surface layer is large, it is necessary to make the variation in residual stress smaller in order to prevent a decrease in hydrogen embrittlement resistance due to non-uniform deformation during bolt tightening.

More preferably, the maximum tensile residual stress σ _Rmax and the minimum tensile residual stress σ _Rmin satisfy the condition of the following formula (3).

The high-strength bolt according to the embodiment of the present invention as described above can be formed from a steel material containing, for example, by mass%, C: 0.35 to 0.45%, Si: less than 0.50%, Mn: more than 0.40% and less than 1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.05 to less than 1.50%, Mo: more than 0.30% and less than 2.50%, V: more than 0.05% and less than 0.50%, Al: more than 0.005% and less than 0.080%, N: less than 0.0150%, with the balance being Fe and unavoidable impurities. In addition, it is desirable for the tensile strength of the bolt to be 1300 MPa or more and 1800 MPa or less.

The reasons why the above ingredients are desirable are as follows.
C is an element that improves the strength of steel. In order to impart a predetermined strength to the bolt, the C content is 0.35% or more, and preferably 0.38% or more. On the other hand, if the C content exceeds 0.45%, the cold heading property during bolt manufacturing decreases. Preferably, the C content is 0.43% or less.
Silicon improves the strength, hardenability, and temper softening resistance of steel, but if it is contained in an amount of 0.50% or more, the cold heading property during bolt manufacturing decreases. Therefore, the amount of silicon is less than 0.50%, and preferably less than 0.10%.
Mn is a useful element for improving the strength and hardenability of steel. In this embodiment, in order to obtain this effect, the Mn content is set to more than 0.40%, and preferably to 0.45% or more. On the other hand, if the Mn content is 1.00% or more, brittle fracture is likely to occur, so the upper limit is set to less than 1.00%. Preferably, the upper limit of the Mn content is set to 0.80% or less.
Since P deteriorates toughness and delayed fracture resistance when its content is 0.030% or more, the upper limit is limited to less than 0.030%. Preferably, the upper limit of the P content is limited to 0.010% or less. From the viewpoint of production costs, the lower limit of the P content is set to 0.001% or more.
S is an element that forms MnS, and since cracks occur during cold heading and cold heading property deteriorates when the content is 0.030% or more, the S content is limited to less than 0.030%. Preferably, the upper limit of the S content is limited to 0.010% or less. The lower limit of the S content is set to 0.003% or more from the viewpoint of manufacturing costs.
Cr is a useful element for improving the strength, hardenability, and temper softening resistance of steel. In this embodiment, the Cr content is set to 0.05% or more in order to obtain the effect. Preferably, the Cr content is set to 0.10% or more, more preferably, 1.00% or more. On the other hand, if Cr is contained at 1.50% or more, the cold forgeability during bolt forming deteriorates due to an increase in hardness, so the upper limit of the Cr content is set to less than 1.50%. Preferably, the upper limit of the Cr content is set to 1.40% or less, more preferably, 1.30% or less.
Mo is a useful element for improving the strength, hardenability, and temper softening resistance of steel. In this embodiment, the Mo content must exceed 0.30% in order to obtain the effect. The Mo content is preferably 0.50% or more, more preferably 0.90% or more. On the other hand, if Mo is contained in an amount of 2.50% or more, the cold forgeability deteriorates due to an increase in hardness, so the upper limit of the Mo content is set to less than 2.50%. The upper limit of the Mo content is preferably 2.00% or less, more preferably 1.20% or less.
V is a useful element for improving temper softening resistance. In this embodiment, the V content must exceed 0.05% in order to obtain the effect. The V content is preferably 0.10% or more, and more preferably 0.30% or more. On the other hand, if 0.50% or more of V is contained, the cold heading property deteriorates due to an increase in hardness, so the upper limit of the V content is set to less than 0.50%. The upper limit of the V content is preferably 0.40% or less, and more preferably 0.35% or less.
Al is effective for deoxidizing steel, and is an element that forms _{fine Al2O3} and AlN, and is also effective for suppressing the coarsening of crystal grains. In this example, 0.005% or more of Al is added to obtain the effect. On the other hand, even if more than 0.080% of Al is added, the effect is saturated, so the upper limit of the Al content is set to 0.080% or less.
Since N generates VN and impairs the effect of improving softening resistance by adding V, the upper limit is set to 0.0150% or less, and preferably to 0.0080% or less. From the viewpoint of production costs, the lower limit of the N content is set to 0.0020% or more.
Using steel having the above composition, the head is formed by cold heading, the threads are formed by rolling, and the bolt is manufactured by hardening and tempering.

Here, steel material having the chemical composition shown in Table 1 was melted, rolled, subjected to softening heat treatment, and then cold forged to produce a bolt having a diameter of 22 mm. The bolts produced were six levels, from Example 1 to Example 6. In each example, the threaded portion was formed into a shape that satisfies the above condition, that is, the condition that κ _ave H < 5 in the range of θ ₁ ≦ 32° of the first portion and the range of θ ₂ ≦ 8° of the second portion, specifically, the shape described above with reference to FIG. 2B. For Example 1 and Example 2, the bolt shape was formed by cold forging, and then the threaded portion was formed by rolling. After forming the threaded portion, it was heated at 920°C for 2 hours using an atmosphere furnace in which the carbon potential (CP) can be controlled, and quenched. In order to control the Hv hardness of the surface layer of the thread valley root during quenching and heating, the CP was set to a value equivalent to 0.45% carbon content for the first hour and heated to a value equivalent to 0.25% carbon content for the remaining hour. After quenching, tempering was performed at 600 ° C. to obtain a tensile strength of 1300 MPa or more. In addition, for Example 3, in order to control the surface hardness of the thread valley root of the bolt, the CP was set to a value equivalent to 0.45% carbon content, heated at 920 ° C. for 1 hour, quenched, and then tempered at 600 ° C. In addition, for Example 4, the CP was set to a value equivalent to 0.25% carbon content, heated and quenched at 920 ° C. for 1 hour, and then tempered at 600 ° C. On the other hand, for Examples 5 and 6, in order to change the residual stress at the thread valley root of the bolt, heat treatment was performed under the same quenching and tempering conditions as Examples 1 and 2, and then thread rolling was performed at different processing speeds to prototype the bolt.

Figures 10A to 10D, 11A and 11B are diagrams showing the relationship between the hardness distribution in the depth direction at the thread root and the variation in residual stress in the bolt circumferential direction. Figures 10A, 10B, 10C and 10D show the hardness distribution in the depth direction at the thread root measured in Examples 1 to 4, respectively, and Figures 11A and 11B show the variation in axial residual stress measured at the thread root in Examples 1 and 5, respectively. Figures 10A to 10D show the average hardness (HV) measured at two locations on the cross section of the thread root of the bolt for a depth range of 0 to 11 mm from the surface of the thread root. Also, Figures 11A and 11B show the axial residual stress (MPa) at a position 0.5 mm deep from the thread root of the bolt for the 0°, 90°, 180° and 270° positions in the circumferential direction of the thread root.

Table 2 lists the HV hardness and residual stress data measured at the root of the thread for each prototype bolt. As shown in Table 2 and the graphs in Figures 10A and 10B, in Examples 1 and 2, the hardness is reduced within a range that satisfies formula (1) in the area 1.5 mm or less deep from the surface, and the surface hardness is reduced. On the other hand, as shown in Table 2 and Figure 10C, in Example 3, the hardness reduction is small in the area 1.5 mm or less deep from the surface, and the value of formula (1) is less than 10. Also, as shown in Table 2 and Figure 10D, in Example 4, the reduction in surface hardness is large, and the value of formula (1) exceeds 100.

As shown in Table 2 and Figure 11A, in Example 1, the value of formula (2) is 300 or less, and the variation in residual stress at the root of the thread valley is small. On the other hand, as shown in Table 2 and Figure 11B, in Example 5, the variation in residual stress at the root of the thread valley is large, and the value of formula (2) exceeds 300. As shown in Table 2, in Example 6, the value of formula (2) also exceeds 300.

The bolts prototyped as described above were each fastened to a fastening jig with a stress equivalent to plastic region tightening, and the jig was placed in a Ferris wheel-type accelerated corrosion tester. An accelerated corrosion test was performed in which the bolts were repeatedly immersed in a 3.5% NaCl solution for 10 minutes per hour and then dried. Hydrogen embrittlement was evaluated based on the presence or absence of fracture after 12 months. None of the bolts in Examples 1 to 6 fractured for 8 months, confirming that they had excellent hydrogen embrittlement resistance. The accelerated corrosion test was then continued for 12 months, and the results are shown in Table 2. Bolts that did not fracture even after 12 months are indicated by A, and bolts that fractured within 12 months are indicated by B. It can be seen that no fracture occurred for 12 months in Examples 1 and 2, and no fracture occurred for 8 months in Examples 3 to 6, but fracture occurred within 12 months. The bolts in Examples 1 to 6 all have improved hydrogen embrittlement resistance due to the threads being formed into the shapes described above, but in more severe corrosive environments, bolts that satisfy the conditions of Examples 1 and 2 exhibit better hydrogen embrittlement resistance.

The above describes in detail preferred embodiments of the present invention with reference to the attached drawings, but the present invention is not limited to these examples. It is clear that a person skilled in the art of the technical field to which the present invention pertains can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

1...high strength bolt, 2...head, 3...shank, 4...threaded portion, 5...thread thread, 6...thread groove, 7A...pressure side flank, 7B...play side flank, 8...root, 8A...first portion, 8B...second portion, 8P...reference point, X...axis, Xn...perpendicular to axis.

Claims (6)

When a first portion of the root is continuous with the pressure side flank in a cross-sectional shape including the axis, and a second portion of the root is continuous with the play side flank and is bounded by the first portion at a reference point where a normal line of the root coincides with a perpendicular line of the axis in the cross-sectional shape,
an average curvature κ ave of the cross-sectional shape in a range in which a first angle formed by the normal line with a perpendicular line to the axis in the first portion is 32° or less, and in a range in which a second angle _formed by the normal line with a perpendicular line to the axis in the second portion is 8° or less, satisfies a relationship of κ _ave H < 5 with respect to a height H of the thread crest ;
The cross-sectional shape is constituted by a first and a second circular arc of radius r1 continuing to the pressure side flank and the play side flank, respectively, and a third circular arc of radius r2 continuing to the first and _second circular arcs _,
A high strength bolt _, where r1 < r2 _. When a first portion of the root is continuous with the pressure side flank in a cross-sectional shape including the axis, and a second portion of the root is continuous with the play side flank and is bounded by the first portion at a reference point where a normal line of the root coincides with a perpendicular line of the axis in the cross-sectional shape,
an average curvature κ ave of the cross-sectional shape in a range in which a first angle formed by the normal line with a perpendicular line to the axis in the first portion is 32° or less, and in a range in which a second angle _formed by the normal line with a perpendicular line to the axis in the second portion is 8° or less, satisfies a relationship of κ _ave H < 5 with respect to a height H of the thread crest ;
A high-strength bolt, the cross-sectional shape of which is composed of a circular arc continuing from the play side flank, and a higher-order curve continuing from the pressure side flank and the circular arc and including a portion whose radius of curvature is equal to or greater than the radius of the circular arc . 3. A high-strength bolt according to claim 1 or 2, wherein, in the first and second portions of the valley bottom, when an average hardness of a center portion including the axis is HVa, an average hardness of a surface layer portion at a depth of 0.2 mm from the surface is HVs, a depth d (mm) at a position where the average hardness reaches the average hardness HVa of the center portion, and a standard deviation of residual stress in the axial direction at a position 0.5 mm deep from the surface is σ _ΔR (MPa), 0.2≦d≦1.5 and the relationship of the following formulas (i) and ( ii ) is satisfied.

When a first portion of the root is continuous with the pressure side flank in a cross-sectional shape including the axis, and a second portion of the root is continuous with the play side flank and is bounded by the first portion at a reference point where a normal line of the root coincides with a perpendicular line of the axis in the cross-sectional shape,
an average curvature κ ave of the cross-sectional shape in a range in which a first angle formed by the normal line with a perpendicular to the axis in the first portion is 32° or less, and in a range in which a second angle formed by the normal line with a perpendicular to the axis in the second portion is 8° or less, satisfies a relationship _of κ _ave H < 5 with respect to a height H of the thread crest ;
A high-strength bolt in which, in the first and second portions of the valley bottom, an average hardness of a center portion including the axis is HVa, an average hardness of a surface layer portion at a depth of 0.2 mm from the surface is HVs, a depth d (mm) at a position where the average hardness reaches the average hardness HVa of the center portion, and a standard deviation of residual stress in the axial direction at a position 0.5 mm deep from the surface is σ _ΔR (MPa), where 0.2≦d≦1.5 is satisfied and the relationship of the following formulas (i) and (ii) is satisfied .

A high-strength bolt according to any one of claims 1 to 4, which is formed from a steel material containing, by mass%, C: 0.35 to 0.45%, Si: less than 0.50%, Mn: more than 0.40% and less than 1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.05 to less than 1.50%, Mo: more than 0.30% and less than 2.50%, V: more than 0.05% and less than 0.50%, Al: more than 0.005% and less than 0.080%, N: less than 0.0150%, with the balance being Fe and unavoidable impurities, and has a tensile strength of 1300 MPa or more and 1800 MPa or less. When a first portion of the root is continuous with the pressure side flank in a cross-sectional shape including the axis, and a second portion of the root is continuous with the play side flank and is bounded by the first portion at a reference point where a normal line of the root coincides with a perpendicular line of the axis in the cross-sectional shape,
an average curvature κ ave of the cross-sectional shape in a range in which a first angle formed by the normal line with a perpendicular line to the axis in the first portion is 32° or less, and in a range in which a second angle _formed by the normal line with a perpendicular line to the axis in the second portion is 8° or less, satisfies a relationship of κ _ave H < 5 with respect to a height H of the thread crest ;
A high-strength bolt having a tensile strength of 1,300 MPa or more and 1,800 MPa or less, which is formed from a steel material containing, by mass%, C: 0.35 to 0.45%, Si: less than 0.50%, Mn: more than 0.40% and less than 1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.05 to less than 1.50%, Mo: more than 0.30% and less than 2.50%, V: more than 0.05% and less than 0.50%, Al: more than 0.005% and less than 0.080%, N: less than 0.0150%, and the balance being Fe and unavoidable impurities .

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