JP6801727B2 - Mechanical screws and their manufacturing methods - Google Patents

Description

The present invention relates to a mechanical screw having a multi-threaded screw on the inner peripheral surface or the outer peripheral surface of the cylindrical body and a method for manufacturing the same. Mechanical screws function as mechanical joints for joining steel pipes, such as steel pipe piles, steel pipe sheet piles, and structural pillars, which are placed in the ground to form structures.
The present invention relates to a threaded mechanical joint having excellent toughness and a method for manufacturing the same, which suppresses brittle fracture from the thread bottom in the event of a large earthquake or the like.

Depending on the conditions of the construction site, steel pipe piles and long support piles for landslide prevention, for example, need to have a diameter of about 200 to 1500 mm and a length of several tens of meters. In such a case, it is necessary to sequentially join a plurality of short steel pipe piles during the pile driving at the construction site. For this joining, mechanical screws are used, which are attached to the respective ends of the steel pipe piles to be joined to form a mechanical joining joint.

When joining steel pipe piles for landslide prevention, long support piles, structural pillars, etc., the diameter of the mechanical screw used as the joint will exceed 1200 mm and the thickness will exceed 50 mm, depending on the construction site and assumed stress conditions. Sometimes. Further, the mechanical screw has a high strength of yield strength: 680 MPa or more and tensile strength: 780 MPa or more, and is required to have a large cross section and a thick wall.

As an example of such a mechanical joint, there is a "steel pipe pile joint" disclosed in Patent Document 1. A typical example of the joint of the steel pipe pile is shown in FIG. The joint joint 1 shown in FIG. 1 is a joint for joining steel pipe piles 2 and 3, and has a male side cylinder 10 having a male screw and fixed to the end face of one of the steel pipe piles 2 to be joined, and the other steel pipe pile. It is composed of a female side cylinder 20 having a female screw, which is fixed to the end face of 3. The screws formed on each cylinder are multi-threaded screws having three or more threads. By screwing the female side cylinder 20 into the male side cylinder 10, it becomes a joint for joining the steel pipe piles 2 and 3. At that time, the tip portion 21 of the male side cylinder 10 comes into contact with the inner peripheral seat 11 formed at the base end of the female side cylinder 20, and the tip inclined portion 12 of the female side cylinder 20 is the male side cylinder 10. It comes into contact with the base end inclined portion 22, and the male side cylinder 10 and the female side cylinder 20 are united without a gap.

The mechanical screw to be the male side cylinder 10 or the female side cylinder 20 is manufactured by, for example, the process shown in FIG. That is, a bloom made by continuous casting or a steel ingot by the ingot method is made into a large bar by hot rolling. After reheating this large rod, it is subjected to stationary forging, drilling, hole expansion, and ring forging to obtain a cylindrical mechanical screw material, which is then quenched and tempered. Next, by machining, the outer peripheral surface and the inner peripheral surface of the mechanical screw material are cut and threaded to form a predetermined mechanical screw. The mechanical screw shown in FIG. 2 is provided for the female side cylinder. The male side cylinder is threaded on the outer peripheral side of the mechanical screw material.

The cross-sectional shape of the mechanical screw is shown in FIG. 3 in comparison with the material shape after ring forging. Note that FIG. 3 shows the case of the mechanical screw which is the female side cylinder 20 in FIG. 1, and the typical dimensions are also shown. As shown in FIG. 3, after the outer peripheral surface and the inner peripheral surface of the material are evenly cut, in the case of the female side cylinder, the inner peripheral surface is threaded.
By the way, mechanical screws mainly cause stress concentration such as torsion and tensile stress on the screw bottom due to a huge earthquake or the like. On the other hand, the screw bottom after threading is generally the center of the radial thickness of the material for mechanical screws, as shown in FIG. 4 which is an enlarged view of the peripheral wall portion of the material of FIG. This also applies to the male side cylinder. Therefore, in order to suppress the destruction from the screw bottom due to a huge earthquake, it is necessary to provide a mechanical screw having high strength and excellent toughness at the center of the wall thickness of the material.

JP-A-2015-155622

However, in recent years, mechanical screws with a large cross section and thick wall with a diameter of more than 1200 mm and a thickness of more than 50 mm are caused by the presence of segregation at the center of the thickness and a decrease in the cooling rate during heat treatment. Therefore, it was extremely difficult to achieve both high strength and toughness.

Therefore, it is an object of the present invention to provide a method for achieving both high strength and toughness, particularly in the above-mentioned large-section and thick-walled mechanical screw. Another object of the present invention is to propose a method for producing a mechanical screw having high strength and high toughness.

The inventors have described a method for obtaining high-strength and tough properties for improving seismic resistance in large-section and thick-walled mechanical screws having a diameter of more than 1200 mm and a thickness of more than 50 mm. We conducted diligent research. Specifically, the 0.2% proof stress is 680 MPa or more and the tensile strength is 780 MPa or more, and in order to suppress the fracture from the screw bottom at the time of a large earthquake, the radial direction of the above-mentioned cylindrical mechanical screw material. Various studies were repeated with the expectation that the 2 mm V notch Charpy absorption energy at 0 ° C. at the center of the thickness (hereinafter, also referred to as 1 / 2t portion) would be 47J or more.

That is, as shown in FIG. 3, the screw bottom of the mechanical screw corresponds to the 1 / 2t portion of the cylindrical mechanical screw material before machining, and this 1 / 2t portion corresponds to the bloom or the starting material. Since macrosegregation is likely to occur in the ingot and the cooling rate is slowed in the quenching process to roughen the structure, it is difficult to secure strength and toughness in the 1 / 2t portion. Therefore, it is important to improve the strength and toughness in this 1 / 2t portion.

As a result of diligent studies based on the above, in addition to regulating the component composition for each element, designing the component composition under predetermined constraints is especially important for the strength and toughness of the mechanical screw material at 1 / 2t. We have found that it is effective in improving the above, and have completed the present invention. The gist structure of the present invention is as follows.

1. 1. A mechanical screw having a multi-threaded screw on the inner peripheral surface or the outer peripheral surface of the cylindrical body, and the cylindrical body is
By mass%
C: 0.05-0.18%,
Si: 0.01-1.00%,
Mn: 0.6-2.0%,
P: 0.025% or less,
S: 0.015% or less,
Al: 0.001 to 0.050%,
Ti: 0.005 to 0.025%,
B: 0.0005 to 0.0030% and N: 0.0020 to 0.0060%
Including,
Cu: 0.8% or less,
Ni: 3.0% or less,
Cr: 1.5% or less,
Mo: 1.5% or less,
V: 0.3% or less and
Nb: Contains any two or more of 0.1% or less, the balance has the component composition of Fe and unavoidable impurities, the Di value shown in the following formula (1) is 140 to 280, and it is shown in the following formula (2). Mechanical threads with a Ceq value of 0.60 to 0.70%.
Di = 25.4 × DI0C × f ・ Si × f ・ Mn × f ・ Cu × f ・ Ni × f ・ Cr × f ・ Mo × f ・ V × 1.3… (1)
Here, DI0C = 0.3241 × √C (%)
f ・ Si ＝ 0.75Si (%) +1
f ・ Mn ＝ 3.33Mn (%) +1
f ・ Cu ＝ 0.35Cu (%) +1
f ・ Ni ＝ 0.36Ni (%) +1
f ・ Cr ＝ 2.16Cr (%) +1
f ・ Mo ＝ 3.00Mo (%) +1
f ・ V = 1.75V (%) + 1
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14… (2)
However, M (%) in the above formula (1) and M (M is an element symbol) in the above formula (2) mean the mass% of the element.

2. 2. The composition of the components is further increased by mass%.
Ca: 0.010% or less,
REM: 0.010% or less,
Mg: 0.010% or less and
Zr: The mechanical screw according to 1 above, which contains any one or more of 0.010% or less.

3. 3. The one or a cylindrical body having a second component composition, Ac ₃ + 30 ° C. to Ac After heating ₃ + 70 to a temperature range of ° C., cooling the temperature range of at least 800-400 ° C. at an average cooling rate of 2 to 20 ° C. / s A method for manufacturing a mechanical screw, which forms a multi-threaded screw on an inner peripheral surface or an outer peripheral surface of the cylindrical body after being subjected to a quenching treatment and then a tempering treatment.
Ac ₃ (° C) is as shown in the following equation.
Ac ₃ = 937.2-436.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo + 124.8V + 136.3Ti-19.1Nb + 198.4Al + 3315B + 42
Here, the element symbol indicates the content (mass%) of the element, and the element not contained is set to zero.

4. The method for manufacturing a mechanical screw according to 3 above, wherein the tempering treatment is performed by heating in a temperature range of 580 to 650 ° C. and then setting the average cooling rate in the temperature range of 550 to 400 ° C. to 1 ° C./s or more.

According to the present invention, it is possible to provide a mechanical screw having high strength and high toughness. Specifically, a thick mechanical screw having a 0.2% proof stress of 680 MPa or more, a tensile strength of 780 MPa or more, and a 1 / 2t portion of 0 ° C. Charpy absorption energy of 47 J or more can be provided at low cost. If steel pipe piles are connected using this mechanical screw, a strong joint can be realized, which can greatly contribute to the development of infrastructure in urban areas such as effective utilization of deep underground space.

It is a figure which shows the structure of the joint joint of a steel pipe pile. It is a figure which shows the manufacturing process of a mechanical screw. It is a figure which shows the cross-sectional shape of a mechanical screw and the material shape after ring forging. It is an enlarged view of the peripheral wall part of the material of FIG.

First, the reasons for limiting the content of each element in the component composition will be described. In addition,% display in a component means mass% unless otherwise specified.
C: 0.05-0.18%
C is an element useful for obtaining the strength required for a mechanical screw at a lower cost, and the content of C must be at least 0.05% in order to obtain the effect. On the other hand, if the content exceeds 0.18%, the toughness of the base metal and the welded portion is lowered, and the weldability is also deteriorated. Therefore, the upper limit was set to 0.18%. It is preferably in the range of 0.08 to 0.18%.

Si: 0.01-1.00%
Si is particularly useful as a solid solution strengthening element for increasing the strength of thick materials. In addition, a part also acts as a deoxidizing material. In order to obtain such an effect, a content of 0.01% or more is required. On the other hand, if the content exceeds 1.00%, the toughness of the weld heat-affected zone is reduced, so the content is in the range of 0.01 to 1.00%. Preferably, it is 0.05 to 0.60%.

Mn: 0.6-2.0%
Mn is contained to improve hardenability and increase the strength of the base metal, but if it is less than 0.6%, the effect is small. On the other hand, if the content exceeds 2.0%, it promotes macrosegregation of the material and reduces the toughness of the base material, so the range was set to 0.6 to 2.0%. Preferably, it is 0.8 to 1.8%.

P: 0.025% or less P is present in steel as an unavoidable impurity, but if it is present in excess of 0.025%, it should be 0.025% or less in order to promote grain boundary cracking, especially in the segregated portion. On the other hand, in order to make P less than 0.001% in the mass production process, the lower limit is preferably 0.001% because it causes a decrease in productivity and becomes very expensive. The preferred range is 0.003 to 0.020%.

S: 0.015% or less S is mixed in steel as an unavoidable impurity like P, but if it exceeds 0.015%, the toughness of the base metal and weld heat affected zone is reduced, so the upper limit is set to 0.015%. On the other hand, in order to make S less than 0.001% in the mass production process, the lower limit is preferably 0.001% because the product becomes very expensive due to a decrease in productivity. It is preferably in the range of 0.001 to 0.010%.

Al: 0.001 to 0.050%
Al is added as a deoxidizing material, but if it is less than 0.001%, its effect is small, so the content should be 0.001% or more. On the other hand, even if it is contained in excess of 0.050%, the effect is saturated, so the range is set to 0.001 to 0.050%. Preferably, it is 0.005 to 0.040%.

Ti: 0.005 to 0.025%
Ti is fixed as TiN and is contained in order to effectively function the improvement of hardenability by adding B. For that purpose, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.025%, the toughness of the base metal and the welded portion is lowered, so the content is set in the range of 0.005 to 0.025%. Preferably, it is 0.007 to 0.020%.

B: 0.0005 to 0.0030%
B is an element effective for improving hardenability, and is particularly effective for increasing the strength of the central portion of thick-walled materials. For that purpose, it is necessary to contain it at 0.0005% or more. On the other hand, when the B content exceeds 0.0030%, boride is formed and the toughness is lowered, so the range is set to 0.0005 to 0.0030%. Preferably, it is 0.0007 to 0.0025%.

N: 0.0020-0.0060%
N combines with Ti and Al to form a nitride and contributes to fine graining of the structure. To obtain these effects, 0.0020% or more is required. On the other hand, when the amount of N increases, N binds to B to form BN, which hinders hardenability. Therefore, the upper limit of the amount of N is set to 0.0060%. The preferred range is 0.0020 to 0.0055%.

Furthermore, it contains any two or more selected from the group of Cu: 0.8% or less, Ni: 3.0% or less, Cr: 1.5% or less, Mo: 1.5% or less, V: 0.3% or less and Nb: 0.1% or less. There is a need to. Of these groups, if only one element is contained, it is difficult to balance the hardenability (Di value) and the strength after tempering (Ceq value) between the surface layer and the inside, and two or more types are included. It is necessary to contain it. Preferably, there are three or more.
Cu: 0.8% or less
Cu is mainly dissolved in steel and is effective for increasing strength without impairing toughness, but when added in excess of 0.8%, hot rolling, set-in forging, and drilling are performed. The upper limit was set to 0.8% because cracks occur during ring forging. Preferably, it is 0.05 to 0.6%.

Ni: 3.0% or less
Ni is an effective element that improves the strength and toughness of steel. However, it is also a very expensive element, and since it greatly reduces economic efficiency, the upper limit is set to 3.0%. Preferably, it is 0.05 to 2.0%.

Cr: 1.5% or less
Cr is an element that is effective in increasing strength by improving hardenability. However, since addition of more than 1.5% reduces weldability, the upper limit is set to 1.5%. Preferably, it is 0.05 to 1.3%.

Mo: 1.5% or less
Like Cr, Mo is an element that is effective in increasing strength by improving hardenability. However, since addition of more than 1.5% reduces weldability, the upper limit is set to 1.5%. Preferably, it is 0.03 to 1.2%.

V: 0.3% or less V is an element that forms VN to improve temper softening resistance and is effective in improving strength and toughness. However, addition of more than 0.3% is a very expensive element and causes a decrease in economy, so the upper limit is set to 0.3%. Preferably, it is 0.005 to 0.2%.

Nb: 0.1% or less
Like V, Nb is an element that forms Nb (C, N) to improve temper softening resistance and improve strength and toughness. However, addition of more than 0.1% is a very expensive element and causes a decrease in economy, so the upper limit is set to 0.1%. Preferably, it is 0.003 to 0.02%.

In the above-mentioned basic component composition, it is important that the Di value shown in the following formula (1) is 140 to 280 and the Ceq value shown in the following formula (2) is 0.60 to 0.70%.
Di = 25.4 × DI0C × f ・ Si × f ・ Mn × f ・ Cu × f ・ Ni × f ・ Cr × f ・ Mo × f ・ V × 1.3… (1)
Here, DI0C = 0.3241 × √C (%)
f ・ Si ＝ 0.75Si (%) +1
f ・ Mn ＝ 3.33Mn (%) +1
f ・ Cu ＝ 0.35Cu (%) +1
f ・ Ni ＝ 0.36Ni (%) +1
f ・ Cr ＝ 2.16Cr (%) +1
f ・ Mo ＝ 3.00Mo (%) +1
f ・ V = 1.75V (%) + 1
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14… (2)

The above Di value is an index showing the hardenability of the steel material, and can be obtained according to the above formula (1) depending on the alloying element to be added. When the Di value is less than 160, the material for mechanical screws (see FIG. 3) described above is hardened at the center of the thickness at the time of quenching, the strength of the center of the thickness is lowered, and the toughness is also lowered. On the other hand, when the Di value exceeds 280, the surface layer of the mechanical screw material becomes excessively hard, and the toughness of the surface is lowered. Therefore, the Di value is in the range of 140 to 280. It is preferably in the range of 150-250.

Next, the Ceq value defined by the above equation (2) is an index of strength and weldability, and if this Ceq value is less than 0.60%, it is necessary to lower the tempering temperature in order to obtain high strength. In particular, it reduces the toughness of the material for mechanical screws at the center of thickness. On the other hand, when the Ceq value exceeds 0.70%, it is necessary to raise the preheating temperature when welding the mechanical screw to, for example, a steel pipe, which lowers the weldability. Therefore, the Ceq value was set in the range of 0.60 to 0.70%. Preferably, it is 0.60 to 0.68%.
It contains the above basic components, and the rest is unavoidable impurities and Fe.

Further, if necessary, one or more of Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and Zr: 0.010% or less can be added to the above-mentioned basic components. By controlling the morphology of sulfides and oxides, these elements contribute to the improvement of the toughness of the weld heat-affected zone when welding with relatively high heat input is performed. For that purpose, it is preferable to add Ca at 0.0005% or more, and REM, Mg and Zr at 0.003% or more, respectively. However, even if any element is added in excess of 0.010%, the effect is saturated, so the upper limit is set to 0.010%. More preferably, Ca is 0.001 to 0.007% and REM, Mg and Zr are 0.003 to 0.008%, respectively.

Next, the production conditions of the present invention will be described.
That is, as shown in FIG. 2, blooms and ingots having the above-mentioned chemical composition are hot-rolled into large rods, and then stationary forging, drilling, hole expansion, and ring forging are performed for cylindrical mechanical screws. The basic form is to use the material (cylindrical body), then perform quenching and tempering, and finally manufacture by machining. Depending on the diameter and thickness of the mechanical screw, part of the hot working process can be omitted.

The above-mentioned manufacturing conditions may be in accordance with the general requirements for mechanical screws, but the quenching treatment after making a material for cylindrical mechanical screws is specified as follows from the viewpoint of ensuring high toughness. The temperature specified in the heat treatment shown below is based on the temperature in the 1 / 2t portion of the mechanical screw material.
[Quenching: Ac ₃ + 30 ℃ After heating to to Ac ₃ + 70 ° C. temperature range of the cooling at an average cooling rate of the temperature range of at least 800~400 ℃ 2~20 ℃ / s]
In order to improve the hardenability and toughness of the above mechanical screw material, the reheating temperature after hot working must be in the range of Ac ₃ + 30 ° C to Ac ₃ + 70 ° C. That is, if the temperature is lower than Ac ₃ + 30 ° C., a sufficient quenching structure (bainite or martensite) cannot be obtained and ferrite is partially formed, resulting in a decrease in strength. On the other hand, when Ac ₃ + 70 ° C. is exceeded, the toughness sharply decreases at the center of the thickness of the mechanical screw material. This is due to embrittlement after tempering due to the coarsening of the old γ grains. Therefore, in order to achieve both high strength and high toughness, it is necessary to heat in the temperature range of Ac ₃ + 30 ° C to Ac ₃ + 70 ° C.

Next, quenching is performed by immersing the material for mechanical screws heated in this temperature range in a refrigerant such as in water or oil. At that time, the average cooling rate in the temperature range of at least 800 to 400 ° C is set to 2 ° C to 20 ° C.
Let / s.
The above-mentioned quenching regulations are important for ensuring toughness. That is, when the average cooling rate is faster than 20 ° C./s, the toughness decreases due to excessive hardness. On the other hand, if the average cooling rate is less than 2 ° C./s, the toughness is also lowered because the structure is coarsened due to the precipitation of ferrite and the grain boundary embrittlement is caused by the lowering of the tempering temperature. Therefore, the average cooling rate at least 800 to 400 ° C. is 2 ° C. to 20 ° C./s.

In the quenching process, for example, a thermocouple is attached to 1 / 2t portion of a spare part (of the same plate thickness) of the material in advance, and a preliminary experiment for cooling is performed to obtain a desired cooling rate of water amount. It can be controlled by knowing what can be obtained by cooling or immersing in an oil tank with an oil concentration.

After quenching under the above conditions, tempering is performed according to the general procedure for mechanical screws. Further, in order to increase the strength and toughness, it is preferable to perform the tempering treatment under the following conditions.
[Tempering: After heating to a temperature range of 580 to 650 ° C, the average cooling rate in the temperature range of 550 to 400 ° C is 1 ° C / s or more]
First, the tempering temperature is 580 to 650 ° C. That is, when tempering is performed at a temperature lower than 580 ° C., P segregates at the old γ grain boundaries and causes tempering brittleness (grain boundary embrittlement), so that the toughness at the center of the thickness of the above mechanical screw material decreases. To do. On the other hand, if the tempering process is performed above 650 ° C, the target strength cannot be sufficiently satisfied. Therefore, the tempering temperature is preferably 580 to 650 ° C. More preferably, it is 580 to 630 ° C.
After heating to a temperature range of 580 to 650 ° C, set the average cooling rate in the temperature range of 550 to 400 ° C to 1 ° C / s or higher.

Further, if the average cooling rate of the tempering treatment in the temperature range of 550 to 400 ° C. after the heating is 1 ° C./s or more, the toughness can be further improved. When this average cooling rate is 1 ° C./s or more, there is no time for P to segregate at the old γ grain boundaries during the tempering cooling process, and grain boundary embrittlement can be suppressed. As a result, the toughness can be further improved as compared with the slow cooling (air cooling) material, and the toughness with 0 ° C. Charpy absorption energy of 100 J or more can be secured. Therefore, the average cooling rate is set to 1 ° C./s or more. In order to obtain this average cooling rate, it is appropriate to immerse the tempered mechanical screw material in a refrigerant such as in water or oil.

As mentioned above, by strictly optimizing the chemical composition and quenching conditions, the 0.2% proof stress is 680MPa or more, the tensile strength is 780MPa or more, and the 0 ° C Charpy absorption energy of 1 / 2t part is 47J or more. Mechanical screws can be obtained. Further, by optimizing the tempering conditions, the 0 ° C. Charpy absorption energy can be 100 J or more.

Steels having the component compositions shown in Table 1 are melted and hot-rolled, stationary forged, drilled, drilled, and ring-forged according to those shown in FIG. 2 to obtain a cylindrical mechanical screw material. , The quenching treatment and the tempering treatment were carried out under the conditions shown in Table 2.

Tensile test pieces and Charpy impact test pieces (2 mm V notch) were collected from the surface of the mechanical screw material thus obtained at a depth of 10 mm and at the center of the thickness (1 / 2t part), and their mechanical properties were examined. The results are shown in Table 2.
In the mechanical screw material in which the quenching and tempering conditions were met within the chemical composition range of the steel of the present invention, both the surface 10 mm and 1 / 2t portions had high strength and sufficiently high toughness. On the other hand, when the range of the component composition deviates from the provisions of the present invention, the strength and toughness cannot satisfy the target. Moreover, even if the chemical composition was within the scope of the invention, the strength and toughness were low when the quenching conditions were deviated.

1 Joined joint 2, 3 Steel pipe pile 10 Male side cylinder 11 Inner peripheral seat 12 Tip inclined part 20 Female side cylinder 21 Tip part 22 Base end inclined part

Claims (4)

A mechanical screw having a multi-threaded screw on the inner peripheral surface or the outer peripheral surface of the cylindrical body, and the cylindrical body is
By mass%
C: 0.05-0.18%,
Si: 0.01-1.00%,
Mn: 0.6-2.0%,
P: 0.025% or less,
S: 0.015% or less,
Al: 0.001 to 0.050%,
Ti: 0.005 to 0.025%,
Ni: 0.05-3.0%,
B: 0.0005 to 0.0030% and N: 0.0020 to 0.0060%
Including,
Cu: 0.8% or less,
Cr: 1.5% or less,
Mo: 1.5% or less,
V: 0.3% or less and
Nb: Contains any two or more of 0.1% or less, the balance has the component composition of Fe and unavoidable impurities, the Di value shown in the following formula (1) is 140 to 280, and it is shown in the following formula (2). The Ceq value is 0.60 to 0.70%, the 0.2% proof stress of the 1 / 2t part is 680MPa or more, the tensile strength of the 1 / 2t part is 780MPa or more, and the 0 ° C Charpy absorption energy of the 1 / 2t part is 47J or more. Mechanical screw.
Di = 25.4 × DI0C × f ・ Si × f ・ Mn × f ・ Cu × f ・ Ni × f ・ Cr × f ・ Mo × f ・ V × 1.3… (1)
Here, DI0C = 0.3241 × √C (%)
f ・ Si ＝ 0.75Si (%) +1
f ・ Mn ＝ 3.33Mn (%) +1
f ・ Cu ＝ 0.35Cu (%) +1
f ・ Ni ＝ 0.36Ni (%) +1
f ・ Cr ＝ 2.16Cr (%) +1
f ・ Mo ＝ 3.00Mo (%) +1
f ・ V = 1.75V (%) + 1
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14… (2)
However, M (%) in the above formula (1) and M (M is an element symbol) in the above formula (2) mean the mass% of the element. The composition of the components is further increased by mass%.
Ca: 0.010% or less,
REM: 0.010% or less,
Mg: 0.010% or less and
Zr: The mechanical screw according to claim 1, which comprises any one or more of 0.010% or less. A cylindrical body having a component composition according to claim 1 or 2, after heating to a temperature range of _{Ac 3 + 30 ℃ ~Ac 3 +} 70 ℃, at an average cooling rate of the temperature range of at least 800~400 ℃ 2~20 ℃ / s Upon cooling, the quenching treatment performed, then after the tempering process to form a multiple thread on an inner peripheral surface or an outer peripheral surface of the cylindrical body, 1 / 2t part 0.2% proof stress than 680MPa of, 1 / 2t part A method for manufacturing a mechanical screw, which has a tensile strength of 780 MPa or more and a 1 / 2t portion of 0 ° C. Charpy absorption energy of 47 J or more. The method for manufacturing a mechanical screw according to claim 3, wherein the tempering treatment is performed by heating the machine in a temperature range of 580 ° C to 650 ° C and then setting the average cooling rate in the temperature range of 550 to 400 ° C to 1 ° C / s or more.

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