JP2019203579A - High tensile strength bolt and manufacturing method thereof - Google Patents

Abstract

To provide a high tensile strength bolt having a novel screw shape, and a low-cost manufacturing method thereof.SOLUTION: A bolt is composed of two components: a high-strength shaft part 1 and a low-strength soft steel nut 2, which eliminates the problem of lower neck fracture. High-frequency heating, flaw detection, hot rolling, and controlled cooling are directly connected using low-cost hot-rolled wire as a material, and a large pitch low-tilt angle trapezoidal rough threaded rod wire with a metallographic structure of fine pearlite and tensile strength of 1100 MPa or more is formed with high efficiency by isothermal transformation, and cut to form a bolt shaft. A gap is provided between the shaft 1 and the nut 2 so that pre- or post-paint or grout is press-fitted reliably along a thread surface. It is excellent in delayed fracture resistance.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high tensile strength bolt excellent in delayed fracture resistance.

  The strength of the so-called high-strength bolt is 1000 MPa or more, and the 1400 MPa class is also used in part. In order to obtain a predetermined strength, the metal structure is tempered martensite by forming and quenching and tempering the bolt. The weak point of the structure is that delayed fracture occurs at a strength of 1100 MPa or more (standard F11T). Delayed fracture is a phenomenon in which a steel material under tension develops local embrittlement due to H due to the progress of corrosion and breaks after several months to ten years of construction. In a corrosive environment, breakage is more accelerated.

  Various measures are taken in all processes to improve delayed fracture resistance. In steelmaking, de-H refining, addition of V and Mo to alleviate the harmfulness of H, reduction of coarse non-metallic inclusions that are the starting point of fracture, reduction of impurities such as P, S, etc. In the processing process, head cracks and neck flaws are prevented, and in the final heat treatment process, strict condition management is performed to obtain an appropriate metal structure, and the quality maintenance cost is considerable. .

  In many cases, the fracture starting point is the so-called neck near the boundary between the head and the shaft. The neck is an inflection point of plastic flow in forging, and material defects such as scratches and stretched non-metallic inclusions are opened, and new defects tend to be formed as stress concentration points. Also, the hot-rolled material has anisotropy from the beginning, and the method perpendicular to the rolling direction reduces the ductility. Under the neck, it is inconvenient because the tension during use also acts in the decreasing direction. This is an inevitable weakness in the current method of forming bolt heads by forging.

  Patent Document 1 discloses a method for improving delayed fracture resistance. According to this, a piano wire is used, a patenting treatment is performed to make a pearlite structure, and then a wire drawing process is performed, and then a bolt is finished by cutting and threading. It is well known that steel having a pearlite structure is more excellent in delayed fracture resistance than conventional quenched and tempered steel (hereinafter referred to as tempered steel), but it is described that it is further improved by the addition of cold working.

  The problem with this method is that various troubles occur because the work-hardened steel wire is forged and rolled. Both ends must be threaded. Heat generation during thread cutting is large. Tool wear is fast. Even if it can be applied to small diameter products, if the diameter is 15 mm or more, it is not practical to require special equipment for the patenting treatment of the steel bar as a raw material and the subsequent drawing.

  Patent Document 2 (referred to as Prior Example 1) shows another example of improving delayed fracture resistance. According to this, it is stated that the addition of V and Mo is extremely effective in suppressing the adverse effect of diffusible H that causes destruction. Addition of a large amount of expensive rare metal has a significant problem in material cost.

  Non-Patent Document 1 studies delayed fracture properties of PC (prestressed concrete) steel bars for piles and poles. It has been elucidated that shot peening of steel surfaces enhances delayed fracture resistance. High-strength steel under tension will eventually break with the progress of surface corrosion. In the case of PC steel bars, the steel bars are buried in close contact with the concrete to prevent corrosion and thus prevent delayed fracture. The treatment reduces the risk of delayed destruction if close contact is incomplete.

  Patent Document 3 discloses a method for producing a high-strength PC steel bar for bridges. According to this, a high-carbon low alloy steel is used as a material, and a tensile strength of 1350 MPa is obtained by induction into a structure composed mostly of dense pearlite by controlled cooling after hot rolling. Threaded bar steel with a diameter of 16 to 32 mm is also manufactured. Corrosion protection is an indispensable element in the construction of PC steel bars, so a grout mechanism is incorporated. Grout (milky alkaline covering material made of cement and water) for special tension / fixing members at both ends, consisting of nuts, etc., a special coupling nut that extends a steel rod to a predetermined length, and a rod body of several tens of meters ) Processing is done. As a result, delayed fracture does not occur and it has semi-permanent durability. If this technology can be diverted to bolts with simple work, the problem will be solved.

  Patent Document 4 (referred to as Prior Example 2) proposes a product and a fixing method in which the above-mentioned threaded joint PC steel bar is developed on a high tensile strength bolt. The strength such as delayed fracture resistance and fracture under the neck is perfect from the results of PC steel bars. The manufacturing cost of the bolt does not include a rare metal and is a hot-rolled product, so it is much lower than the current high-strength bolt. For corrosion protection, the grout process of the PC method is used as a reference, and a clearance is provided between the bolt shaft and the nut to solve the problem by incorporating the grout process.

The problems of this method are listed. Since the screw knot is a screw thread missing twice in one round, and has only a substantial half round, there are two disadvantages.
1) The required coupling length of the nut is double that of a normal screw. Naturally, the axis is doubled.
2) In the grouting operation, it is relatively easy to press-fit from the end face of the nut, but due to the large gap on the rolling reduction side where the thread is intermittent, the flow may be short-circuited depending on the situation, and bubbles may remain between the screw faces, resulting in incomplete grout. It is to be.

  In terms of the market, high strength bolts have been solved with the long-lasting collaborative research and development results of academia and industry from a number of fatal accident cases. The current products in the oligopolistic market are coordinated. Although there are many problems in new entry, there is a certain demand for high-strength bolts and screw rods even if they are not “high strength”, and the name of the present invention is not “high strength”. It may be accepted overseas in the future.

Published Patent Publication 2002-337334 Published Patent Publication 2006-219718 Published Patent Publication No. 61-26730 Patent No. 4390157 Patent No. 4025851

CAMP-ISIJ Vol.8 (1995) -1507

As described above, the current high-strength bolts of 1100 MPa class or higher (previous example 1) are tempered steel and there is a risk of delayed fracture. As a countermeasure, the addition of a large amount of V and Mo can adversely affect diffusibility H. Although it is solved, there is a problem that the material cost is extremely high.
Bolts are constructed with a double nut method using a screw-joint PC steel rod as a shaft, and the method of corrosion protection with a one-touch grout from the nut end face (Prior Example 2) is a conversion of the excellent performance and results of PC steel to bolts. The above-mentioned delayed fracture problem and significant cost reduction are almost solved, but the required coupling length of the shaft and nut due to the screw joint (intermittent thread) is doubled and the grout short-circuits the screw missing part. There is a problem that it is difficult to cover the entire surface.
The present invention inherits both superiority (delayed fracture resistance and low cost) of the screw joint PC steel rod, while the problem of the bolt of the preceding example 2, that is, the problem of doubling the required coupling length caused by the screw joint, the bolt shaft and the nut The problem of incomplete grout is to be solved.

The following means are applied to solve the above problems.
1) Follow toughening with fine pearlite to ensure delayed fracture resistance.
2) Apply a reliable grout or paint for the same purpose (known).
3) Further improve the screw node shape with a track record and reliability as a PC steel bar for bridges (new).
4) Solve the difficulty of forming the improved screw shape (new).
5) Cost reduction by integrating high-speed thread forming (new), heat treatment, and removal of defective parts (new).

The first invention is to form a large pitch low-tilt angle trapezoidal rough screw of the following shape by hot rolling on the surface layer of hot rolled round steel bar or steel wire, and then control cooling to make the main metal structure fine pearlite, tensile strength A steel bar having a thickness of 1100 MPa or more is cut into a predetermined length to form a bolt shaft portion, and a nut is screwed to one end of the shaft portion to form a bolt. An adhesive is attached to the screwing surface between the shaft portion and both nuts. Or it is a high tensile-strength bolt characterized by having the following gap into which a paint or grout material can be pressed.
Thread height = (0.06-0.07) × rod diameter Thread pitch = (0.5-0.6) × rod diameter Ratio of thread bottom width C to thread hem width B C / B = 1.1 ~ 2.0
Thread slope = 40-50 ° vs. nut axial clearance = (0.3-0.5) mm
Clearance against nut radial direction = (0.2-0.4) mm

  A second invention is a method of manufacturing a shaft portion of a high tensile strength bolt described in the first invention, wherein a round rod having a predetermined component having a shaft diameter of 18 mm or more and 51 mm or less is caused to travel in the direction of the rod axis. The induction coil is passed through and rapidly heated so that the surface temperature reaches a reference value of 1050 ° C. or more at the exit of the coil, and the temperature is recorded. A thread is formed while rotating the rod around the axis through a machine, and then accelerated and cooled by passing through the center of a fixed centripetal spray provided in multiple stages, and then induced to a constant temperature transformation at a predetermined temperature. It is a manufacturing method of a high tensile strength bolt characterized by marking a deviation portion of the temperature from the reference value and subsequently removing the portion.

  A third invention is a method of manufacturing a shaft portion of a high strength bolt described in the first invention, wherein a steel wire having a predetermined diameter of 10 mm or more and 15 mm or less is linearly advanced in the axial direction, and a high frequency induction coil is provided. Through which the surface temperature reaches a reference value of 1050 ° C. or more at the outlet of the coil, and the temperature is recorded, and a roll die is immediately held while the center temperature is delayed, and the die rotates and revolves. Through a machine to form a screw of a predetermined shape, and then pass through the center of a centripetal spray rotating around a linear axis provided in multiple stages, accelerate cooling, induce a constant temperature transformation at a predetermined temperature, then the surface temperature of the said It is a method for manufacturing a high tensile strength bolt, characterized in that a portion that deviates from a reference value is marked and the portion is excluded later.

  The high tensile strength bolt of the present invention has excellent delayed fracture resistance because the metal structure is fine pearlite. It is even better if you add thread surface coating or shot peening. In the tempered martensite structure as in the preceding example 1, delayed fracture is a problem, and an expensive steel type containing V or Mo is indispensable as a countermeasure.

  The high tensile strength bolt of the present invention is composed of a high strength shaft part and a low strength mild steel nut, so there is no peculiar plastic working flow under the neck, eliminating the problem of delayed fracture resistance under the neck due to forging. Is done.

The high tensile strength bolt of the present invention has a continuous thread, and the required length of the joint between the shaft and the nut is halved as compared with the bolt having the intermittent thread joint as in the preceding example 2. In addition, since an appropriate screw gap is provided, the covering is surely advanced along the entire circumference of the screwing surface without short-circuiting the screw missing portion like the screw joint when the adhesive, paint or grout is press-fitted. The certainty of corrosion resistance and delayed fracture is improved.
Furthermore, it is not just a continuous thread, but a continuous and large pitch, low-inclined trapezoidal rough threaded section of a threaded PC steel bar with proven track record and reliability.

  In cost comparison, the current high-strength bolts are made of expensive Mo-V steel rods and have a long process of annealing, pickling, wire drawing, cutting, forging, rolling, and heat treatment, and are expensive. Although the cost is significantly reduced with the bolt obtained by subdividing the screw knot PC steel rod of Citation 2, there are other problems such as the intermittent screw described above. In the present invention, an economical steel rod similar to the latter is used as a material, and heating, rolling, and cooling are incorporated in one process, which is overwhelmingly more advantageous than Reference 1. Among them, the large pitch trapezoidal coarse screw leaps in the efficiency of rolling, and the direct connection with rapid heating assists the rolling. Surface defects are removed in the line, enhancing cost and reliability.

The shape of the large pitch low inclination angle trapezoidal rough thread high tensile strength bolt of this invention is shown. It is the schematic of the equipment which manufactures the rough threaded steel bar used as the axial part of the bolt of the present invention. A is a view of a fixed shaft screw rolling machine as viewed in the axial direction, and B is a view as seen from above. The fixed annular spray pipe which is a structural unit of a spray apparatus is shown. The figure which estimates the temperature of a core part in the rapid heating by a high frequency induction shows the surface cooling line immediately after a heating dimensionlessly, and B figure shows a vertical axis | shaft by logarithmic temperature. It is the schematic of the installation which manufactures the rough threaded steel bar used as the axial part of a bolt from a wire.

FIG. 1 shows the shape of the high tensile strength bolt of the present invention. The bolt includes a shaft portion 1 and a head nut 2 that is screwed to the shaft portion. The shaft portion 1 is obtained by simply cutting a large pitch low-inclined angle trapezoidal rough threaded steel bar having a predetermined diameter and shape with a metal structure of fine pearlite and a tensile strength of 1100 MPa or more into a predetermined length. It is as follows.
Screw height h = (0.06-0.07) × rod diameter D --- (1)
Screw pitch p = (0.5 to 0.6) × rod diameter D --- (2)
Ratio of screw bottom width C to thread skirt width B C / B = 1.1 to 2.0 −−− (3)
Thread inclination = 40-50 ° --- (4)
Nut axial clearance = (0.3 to 0.5) mm --- (5)
Clearance against nut radial direction = (0.2 to 0.4) mm −−− (6)

  The dimensional relationship described above follows the screw shape of a screw joint PC steel bar that has been proven and reliable for bridges, and has developed into a continuous screw that has never been manufactured. As a result of the change from the intermittent thread node to the continuous thread, the gap shape of the screwing surface changes. In screwing, the gap draws a helix, but in the former intermittent screw joint, the gap shorts the helix at the screw missing part. As a result, imperfections occur during the press-fitting of the anticorrosive grout in the gap. In the continuous trapezoidal screw, a spiral gap is formed in which the three surfaces of the thread, the screw bottom, and one inclined surface are parallel and adjacent to each other, and the reliability of grout and thread surface coating can be obtained.

The reason for “large pitch” is that 1) the quality stability of the threaded bar steel is followed, and 2) the forming speed (m / s) is efficient in rolling.
The reason for the “low inclination angle” is 1) to follow the results of the quality stability of the threaded bar steel in the same way as described above, and 2) to avoid stress concentration at the foot of the mountain that occurs in normal trapezoidal screws with a large inclination angle. The reason for the “trapezoidal shape” is to 1) follow the results of the quality stability of the threaded bar steel as described above, and 2) to avoid excessive cooling of the top of the metric screw when cooling after rolling. .
The reason for the “coarse thread” is to allow the dimensional accuracy of the hot-rolled steel bar, which is a raw material, although the thread is usually formed after the diameter is fixed by drawing. As a result, the gap between the nut and the nut increases, so that it is a screw with a so-called “backlash”, but it is used for grout and paint press-fitting.
The gap is preferably 0.2 mm or more so that it can be press-fitted, and 0.4 mm or less in terms of the strength of the screw. The axial gap dimension is approximately √2 times the gap.
The metal structure and tensile strength do not exceed conventional levels. Further, since high-strength bolts are often shot peened, it is effective to add the machining in the present invention. The tensile strength of the steel bar is specified as 1100 MPa or more, which increases the risk of delayed fracture.

FIG. 2 shows an outline of a production line (diameter of 19 mm or more) of a threaded bar steel of a shaft portion which is a main material of the high tensile strength bolt of the present invention.
A hot-rolled round bar 10 having a predetermined component and a predetermined bar diameter, which is a raw material, is sent from the supply base 12 to the processing path 11, and advances straight along the path 11 while being rotated by the transfer transfer device 13. The surface is rapidly heated to a predetermined temperature of 1050 ° C. or higher through a solenoid type high frequency coil 14. Due to the high frequency, the surface is preferentially heated. Although the central part is also transformed into austenite, a large temperature gradient is generated inside. The temperature measurement is recorded through the radiation type temperature sensor 5 in the immediate vicinity of the coil exit.

  At this time, although it is hardly known, if there is a surface flaw, the surface temperature varies slightly toward the high temperature side and is biased. The temperature sensor 15 functions as a flaw detector. In the present invention, this phenomenon is applied, and the temperature measurement signal is used for marking and exclusion of the part described later via a controller. If the temperature measurement position is delayed, soaking is progressed and the temperature abnormality is lost.

  A thread rolling machine 16 follows the coil 14 and immediately performs hot thread rolling with a temperature gradient. As shown in FIG. 3, in the screw rolling machine 16, the rotating shaft 33 of the two roll dies 32 is fixed, so that the workpiece 31 rotates around its own axis. On the surface of the die 32, a reverse thread of the shape of the product screw is provided with a plurality of turns with the height increasing in a slope, and the screw thread has a predetermined shape after being subjected to several rotations of one rotation twice. To grow up.

The material 31 to be processed passes through a spray device 17 provided after the rolling machine 16 and passes through a predetermined cooling process. As shown in FIG. 4, the spray device 17 is composed of a multi-stage annular spray tube 44 provided with a centripetal nozzle 42 on the inner side, and the cooling strength of each can be adjusted.
In the cooling process, the surface temperature is tracked by a temperature sensor 18 provided as appropriate, the spray strength is appropriately adjusted, and a predetermined constant temperature transformation is induced.

  Subsequently, a rotary drawing device 18 is provided, which is driven in synchronism with the rolling machine 16 and travels on the path at the same speed and at the same rotation speed even after the rear end of the workpiece has left the rolling machine 16.

  The paint is sprayed on the temperature abnormal part detected by the temperature sensor 15 by the marker 19 provided subsequently. The abnormal material 22 marked as the normal material 23 is transferred in a different direction by the carry-out table 21.

The overall process has been outlined above, but the main points of the important process will be described.
Regarding the phenomenon that surface scratches affect the surface temperature in high frequency heating, there are cases where only the skin is heated (eg, skin quenching), but when heating the entire cross section, a normal temperature sensor is used to determine the exact temperature. Measure after soaking away from the coil outlet. Therefore, the above phenomenon is missed. As a process in which surface scratches increase the surface temperature, the induced current that circulates in the circular cross section is concentrated on the surface due to the skin effect, but scratches on the outermost layer increase the resistance value of the outermost layer and raise the temperature. Further, it is presumed that the resistance increase due to the temperature rise overlaps.
There may be no scratch in the part. Detailed investigation reveals that the surface temperature varies finely when the decarburized layer is large. In any case, it is effective for eliminating defective portions.

One of the reasons why the surface temperature is specified to be 1050 ° C. or higher is to make the temperature at which the central portion reaches sufficient austenitization (about 900 ° C.).
A method for accurately estimating the temperature behavior of the central portion when the central portion of the material to be treated is transformed into austenite by induction heating will be described below.
A round bar is heated in a coil, and the progress of temperature rise during energization and the surface temperature during air cooling after turning off are traced. FIG. 5 shows an example, and FIG. 5A shows a cooling line (both temperature and time are dimensionless). The temperature drops rapidly after turning off, but then the cooling rate gradually decreases and becomes a smooth curve. The rapid drop in surface temperature is due to heat conduction into the interior. When soaking is progressing and the heat output from the surface layer is only air cooling, it becomes a slow air cooling line. The transition time is a soaking state, indicating the maximum temperature in the center.

In order to detect the transition time, the temperature axis in FIG. The air-cooled part rides on a straight line, but the gradient is different in the process of soaking immediately after turning off, and the inflection point can be easily grasped (arrow in the figure). This is because in the soaking state, the cooling equation is as follows and is described by a logarithmic function.
logeθ = −k · t −−−−− (7)
k = 4α / cρD
θ; temperature k; time constant t; time α; heat transfer coefficient c; specific heat ρ; density D;

The thread rolling will be described. The screw shape formed as shown in FIG. 1 follows the current screw node PC steel rod, and becomes a continuous trapezoidal screw including the weight reduction of the screw portion disclosed in Patent Document 5. Unlike an accurate metric screw that is formed only at the end of a round bar in the cold, the features are the same as the screw height h, but the slope (α) is loose, and the top of the screw thread (width b), the bottom of the screw (width C) ) Is flat and long, and therefore the screw pitch p (= mountain hem width B + valley bottom width C) becomes abnormally large and becomes 8 to 9 times that of the metric screw. In the case of continuous rolling along the axial direction, a large screw pitch means that the rolling speed (m / s) increases proportionally apart from the difficulty of rolling. The screw shape was already described [Equations (1) to (6)].
When the rod diameter D is given, the pitch p, the ridge width B, and the valley bottom width C are determined, and the effective thread width of the internal thread (= the thread width of the intermediate thread height) and the external thread are calculated geometrically. The thread thickness ratio q is determined. If the yield strength ratio of the two materials is set to the reciprocal of the ratio q, both of them eliminate the waste of necessary material dimensions. The optimum value of B may be determined from the nut material strength.

In the case of forming the trapezoidal screw by plastic working, it becomes easy to form an intermittent screw node in rolling. When rolling down from the leader oval to the thread nodal section, the material flows into the caliber thread groove simply by increasing the rolling reduction.
In the rolling of metric threads, the groove width (between mountains) and the groove depth (becomes peak height) of the dies are almost equal and small, so that the material is easily raised and the thread formation is not difficult.
In the trapezoidal screw, the groove width is large, so that the flow from the pressed surface (thread bottom) to the groove is delayed. The problem arises that the rolling dies must be lengthened to increase the repetitive machining. If the diameter is large, the die becomes excessive and screw formation becomes more difficult. In order to solve the problem, the present invention can be devised as follows.

  Apply high-frequency induction rapid heating to promote rolling. Since it is heated rapidly to about 1050 ° C. or higher, preferably 1100 ° C. or higher, the surface layer is considerably softened and the lower layer is somewhat hard, and therefore the surface layer tends to flow. Furthermore, compressive stress is generated in the surface layer due to the temperature gradient. Under these two conditions, the initial reduction during rolling can be set large (required die length is shortened), the rise of the meat is quick, and the formation of a wide trapezoidal screw is promoted. This is the reason why the induction coil and the rolling machine are closely arranged.

The cooling will be described. The material to be processed rotates by rolling, and the centripetal nozzle is fixed, so the spray 42 is blown around the screw surface so as to circulate (spiral). In principle, the material to be processed is tangential and axial. Both are evenly cooled. Since the material to be treated is suspended by a rolling machine on the upstream side and by a rotary drawing device on the downstream side, there is no contact object that disturbs the cooling conditions. Rolling rotation and fixed spray are a convenient combination.
The spray cooling capacity (indicated by heat transfer coefficient) is approximately proportional to the water density (g / s · m 2). If it is set too large, control accuracy becomes a problem. It is easy to use a medium collar. In addition, the air / water mixed spray (commonly called mist spray) is more stable than various sprays at various points (eg, less clogging). From experience, it is desirable that the heat transfer coefficient is 600 (kcal / hm2 ° C.) or less.

  In the cooling of the present invention, fine pearlite is obtained by inducing the isothermal transformation. For this purpose, the cooling is usually first adjusted to the desired transformation temperature and then maintained so as to maintain the temperature. When the cooling is not strong, it is regarded as soaking in the cross section, and the cooling rate decreases in inverse proportion to the rod diameter. However, when the cooling becomes strong, the surface center difference occurs and the difference increases as the rod diameter increases. As a result, the core part deviates from the isothermal transformation. Addition of hardenability strengthening alloys alleviates the problem. For isothermal transformation, it is necessary to set production conditions incorporating three factors: rod diameter, cooling strength, and hardenability. The relationship between isothermal transformation temperature, pearlite lamellar pitch and strength is well known.

Consider the core cooling delay. The delay can be estimated from the Biot number Bi, which is a dimensionless number. In the case of one-dimensional steady heat transfer from solid to fluid, the transfer of heat on the solid surface is equal,
λ (θa−θb) / d = α (θb−θc) −−−− (8)
λ: Solid thermal conductivity θa: Solid heat source temperature θb: Boundary temperature θc: Fluid temperature d: Solid thickness α: Heat transfer coefficient From the above equation, the following equation is derived.
α × d / λ = (θa−θb) / (θb−θc) −−−− (9)
The left side is defined as the Biot number Bi. When Bi = 1
θb = (θa + θc) / 2 −−− (10)
The surface temperature is an intermediate value between the heat source refrigerants. If the Bi number is larger than 1, the surface is lower than the intermediate value, and if it is about 0.1, the surface may be regarded as soaking in the solid.

In the case of steel bar, the standard is the two-dimensional steady heat transfer of the cylinder. When the surface area and mass are geometrically corrected assuming that the center is a heat source, equations (8) and (10) are as follows.
λ (θa−θb) / d = 4α (θb−θc) −−− (11)
θb = (θa + θc) / 5 −−− (12)
From the above equation, the boundary temperature is distributed to the solid side 1 fluid side 4 with respect to the total temperature difference. Specifically, when a steel bar of 920 ° C. is cooled under a condition of Bi number = 1 by spraying at 20 ° C., cooling of the center starts when the surface temperature decreases from 920 ° C. by 180 ° C. to 740 ° C. Cooling proceeds with a temperature difference between the surface and the refrigerant of 1: 4 with respect to the temperature difference between the surface and the surface.
The average cooling rate of the rod is inversely proportional to the rod diameter and proportional to the heat transfer coefficient, but the delay to the center surface also depends on the rod diameter and the heat transfer coefficient, so that the center portion is delayed twice. It is.

Estimate under what conditions Bi number = 1. Rod diameter D = 51 mm, heat transfer coefficient α = 400
(Kcal / m2h ℃), thermal conductivity λ = 20 (kcal / mh ℃)
Bi = 400 × 0.05 / 20 = 1
The α value is a fairly strong mist spray but is in a practical range.
Comparing and examining TTT diagrams describing the transformation of steel so that the center part assumes a predetermined isothermal transformation assuming the cooling delay described above, and assuming an appropriate value and increasing the hardenability Don't be. Specifically, Mn, Cr, Mo (a trace amount), V (a trace amount), etc. are increased or newly added.
When the rod diameter is 20 mm, a sufficient cooling rate can be obtained even if the α value is lowered, and the Bi number is small, so that the cooling delay may be ignored. In this case, it is wise to reduce the alloy amount by strengthening the α value to some extent.

Regarding the induction of isothermal transformation, if the Bi number is 0.5 or more (α value is default, rod diameter is large), sufficiently consider the cooling delay at the center, and induce the surface temperature to be slightly lower than the desired transformation temperature and promptly. The cooling is stopped, and after waiting for some temperature increase, the desired temperature is maintained thereafter.
If the Bi number is 0.3 or less, it is assumed that the cross section is uniform, and the isothermal transformation is induced.

In order to ensure delayed fracture resistance, shot peening is applied to the surface of the steel bar. This treatment is a safety measure to prevent delayed destruction even when there is a local deficiency in the grout operation. The scale on the surface is removed to show a grayish white satin finish. The grout adhesion in this surface property is greater than that of hot-rolled steel with scale adhesion. To implement, in the production line of FIG. 2, a projector is inserted (not shown) after the controlled cooling and before marking. Note that the steel bar temperature during processing is 500 ° C or less. Above, the effect decreases due to softening.
It is also preferable to perform anticorrosion coating immediately after shot processing. This can be done easily at low cost. Corrosion protection improves delayed fracture resistance. Since there is a sufficient gap on the thread surface, there is no problem in fitting.

  Ordinary mild steel is used for the material of the nut. The first reason is that if a high-strength nut is used, the dimensional accuracy of the threaded joint of the steel bar is not precise, so there is a risk that the contact pressure of the screw thread at the fitting portion varies depending on the location and is partially unreasonable. This is because in mild steel, the thread is appropriately shear-deformed and stress is easily dispersed on the fitting surface. The second reason is to reduce the cost because two are used.

When the bolt diameter is 15 mm or less, a wire can be used as the material. FIG. 6 shows a line for processing the wire. The main points are described below.
1) A wire is inserted from the wire coil 61 into the processing path 62 and straightened and straightened by the pinch roll 63.
2) Heating and temperature measurement are the same as in the case of a round bar, but the rolling machine 66 requires a revolving / rotating structure for the roll die. The wire does not rotate.
3) The spray cooling device 67 is similarly multistage, but it must be of a rotary type around the path core. The structure can be achieved by attaching a nozzle to the inner tube of the swivel joint.
4) After rolling and heat treatment, it may be a straight bar or a coil 72. In the latter case, the coil diameter is 200 times or more of the wire diameter. Since the surface stress is less than the yield stress, no bending defects remain.

Consider production efficiency and cost. Bolts obtained by subdividing the screw joint PC steel bar of the preceding example 2 are not recognized in the market, so they are excluded and compared with the current high-strength bolt of the preceding example 1.
The current high-strength bolts (F13T or higher) are made of expensive Mo-V steel rods and have a long process of annealing, pickling, wire drawing, forging, rolling, and heat treatment, and are expensive.
In the present invention, heating, rolling, and cooling are incorporated in one process using an economical steel type hot-rolled bar wire similar to that in the preceding example 2, which is overwhelmingly advantageous over the reference 1.
The diameter of the hot-rolled bar wire is somewhat inaccurate, but there is no problem in the formation of a rough thread by hot rolling. Drawing or the like is omitted.

In the series of steps of the present invention, the rate-determining method is the induction heating capacity (t / h) and the rotational speed (rpm) of the rolling machine. The heating capacity is determined and designed in advance according to the production plan. The efficiency P (t / h) of the rolling machine is as follows in the case of continuous rolling in the axial direction.
P = πN × p × D ² × ρ / 4 (13)
N: Rotational speed p; Pitch D; Rod diameter ρ; Density The rod diameter D is assumed to be 18 to 51 mm by expanding the screw node rod having a diameter of 23 to 36 mm. The pitch p is 8 to 9 times that of a cold rolled metric thread. Therefore, in principle, the efficiency leap is easy. Next, it is hot working, and the amount of rolling reduction and the area of rolling reduction are extremely small compared to rolling, so the rolling load and power are lighter than that of a rolling mill. There is no particular difficulty in equipment cost and design for the capacity enhancement (several t / h) of the rolling machine.

  Specific conditions when the product of the present invention is manufactured by the manufacturing method of the present invention will be summarized. Table 1 shows the steel types suitable for the product. The composition range is almost the same as that of the current screw-node PC steel bar. The larger the rod diameter, the greater the alloy composition. If it is 26 mm or less, strengthening of hardenability by Mo and V is unnecessary.

  Table 2 summarizes the properties and processing conditions of the current high-strength bolts of the first example, the subdivided bolts of the screw-node PC steel rod of the first example, and the large pitch low-inclined angle base rough screw bolt of the present invention. There is no particular difficulty in implementation.

  It can replace the existing high strength bolt. Especially suitable for tall bolts.

DESCRIPTION OF SYMBOLS 1; Shaft part 2; Head 10; Round bar 11; Processing path 12; Supply stand 13; Time transfer feeder 14; High-frequency coil 15; Temperature sensor 16; Rolling machine 17; Spray cooling device 18; Marker 20; Temperature sensor 21; Unloading table 22; Abnormal material 23; Normal material 31; Processing material 32; Rolling die 33; Die rotating shaft 41; Processing material 42; Spray 43; Spray nozzle 44; Coil 62; Processing path 63; Pinch roll 64; High frequency coil 65; Temperature sensor 66; Rolling machine 67; Spraying device 68; Pinch roll 69; Temperature sensor 70; Marker 71;

Claims (3)

A hot-rolled round bar or steel wire surface layer is formed by hot rolling to form a large pitch low-tilt angle trapezoidal rough screw with the following shape, and then controlled cooling to make the main metal structure fine pearlite and tensile strength of 1100 MPa or more. A steel bar is cut into a predetermined length to form a bolt shaft portion, and a nut is screwed to one end of the shaft portion to form a bolt. An adhesive, paint, or grout material is formed on the screwed surface between the shaft portion and both nuts. A high tensile strength bolt characterized by having the following gaps that can be press-fitted.
Thread height = (0.06-0.07) × rod diameter Thread pitch = (0.5-0.6) × rod diameter Ratio of thread bottom width C to thread hem width B C / B = 1.1 ~ 2.0
Thread slope = 40-50 ° vs. nut axial clearance = (0.3-0.5) mm
Clearance against nut radial direction = (0.2-0.4) mm   A method of manufacturing a shaft portion of a high tensile strength bolt according to claim 1, wherein a round rod of a predetermined component having a shaft diameter of 18 mm or more and 51 mm or less is caused to travel in the rod axis direction, and the high-frequency induction coil is penetrated. The coil is rapidly heated so that the surface temperature becomes a reference value of 1050 ° C. or more at the exit of the coil, and the temperature is recorded, and the center temperature is kept delayed and immediately passed through a screw rolling machine with a rotating shaft fixed with a roll die. A thread is formed while rotating the rod around the axis, and then it is accelerated and cooled by passing through the center of a fixed centric spray provided in multiple stages, and then induced to a constant temperature transformation at a predetermined temperature, and then the reference value of the surface temperature. A method for manufacturing a high tensile strength bolt, characterized in that a portion of the bolt is marked to be removed later.   A method of manufacturing a shaft portion of a high tensile strength bolt according to claim 1, wherein a steel wire having a predetermined component with a shaft diameter of 10 mm or more and 15 mm or less is linearly advanced in the axial direction, and the high-frequency induction coil is passed through the coil. The surface temperature is rapidly heated so that the surface temperature becomes a reference value of 1050 ° C. or more at the outlet portion, and the temperature is recorded and passed through a thread rolling machine in which a roll die is immediately held with the center temperature being delayed and the die rotates and revolves. Forming a screw having a shape, then passing through the center of a centric spray rotating about a line axis provided in multiple stages, accelerating cooling, inducing a constant temperature transformation at a predetermined temperature, and then the deviation of the surface temperature from the reference value A method of manufacturing a high tensile strength bolt, characterized by marking a part and removing the part later.

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