CN113423846B - Use method of high-strength electric welding steel pipe and high-strength electric welding steel pipe for foundation stabilization engineering - Google Patents

Abstract

The high-strength electric-resistance-welded steel pipe of the present invention has an outer diameter of 60.3mm to 318.5mm, a ratio of the wall thickness of the steel pipe to the outer diameter of the steel pipe of 0.02 to 0.06, and a tensile strength of 590N/mm ² As described above, when the steel pipe is cut at the center portion, the predetermined portion satisfies the specific numerical range.

Description

Use of high-strength electric-resistance welded steel pipes and high-strength electric-resistance welded steel pipes for foundation stabilization works method

technical field

The present invention relates to a high-strength electric-resistance-welded steel pipe used for perforating in soil and improving the foundation of slopes or ground in foundation stabilization engineering (including tunnel engineering or foundation stabilization engineering, etc.), and a high-strength electric-welded steel pipe for foundation stabilization engineering. Instructions.

This application claims priority based on Japanese Patent Application No. 2019-029437 for which it applied to Japan on February 21, 2019, the content of which is incorporated herein by reference.

Background technique

In recent years, in tunnel construction or ground stabilization construction for automobile roads, railways, etc., construction under harsh environments including construction requirements for lengthening and soft ground has been required. To achieve this purpose, ground improvement agents and lightweight and high-strength structural members are required, and high-strength steel pipes are attracting attention as lightweight and high-strength structural members.

As a method for producing a high-strength steel pipe, for example, Patent Document 1 and Patent Document 2 disclose a technique of increasing the tensile strength by heating to a high temperature after pipe-making, followed by rapid cooling. In addition, for example, Patent Document 3 discloses a technique for adjusting the chemical composition, yield strength, tensile strength, and yield ratio to specific ranges for electric-resistance-welded steel pipes for oil wells, which are one type of steel pipes buried in the ground. Heat treatment after tube making can improve tensile strength and toughness.

As described above, in the tunnel construction of the lengthening of the tunnel and the soft foundation, it is desirable to use the foundation improver, and to secure the heavy machinery and work space for injecting the improver. However, in recent years, in the construction of tunnels on expressways and high-speed railways, there have been increasing cases of construction in small spaces where heavy machinery is difficult to enter, such as in mountainous areas. In addition, the steel pipes for the above-mentioned purposes need to be processed after the pipe production in the steel pipe manufacturing plant, or by the intermediate operators or in the construction site of the construction site, and the female thread and the male thread are respectively formed on the two pipe ends in advance, or the connection function is used. The connecting member is joined to both pipe ends or one end of the steel pipe, and after being brought into the construction site, the excavation tool and the steel pipe or the steel pipe are connected and used at the construction site.

However, when heavy machinery cannot be used, the steel pipe must be carried in and connected by hand, and the physical load of the operator is very large. In particular, in recent years, with the aging of workers, it has become a problem to reduce the load of workers and ensure labor force, and high-strength and light-weight steel pipe components are required as a countermeasure.

Existing steel pipes for this purpose include, for example, standard STK400, tensile strength TS400 to 490 N/mm ² , outer diameter D = 114.3 mm, wall thickness t = 6.0 mm, length L = 3.0 to 3.5 m, and weight 48 to 56 kgf/ root. On the other hand, according to the Japan Labor Standards Law's guidelines for prevention of low back pain in the workplace, the weight of items handled manually by adult men should be approximately 40% or less of their body weight. As a standard example, when the weight of an adult male is 70kgf, the weight that can be handled by one person is 28kgf. Therefore, the existing steel pipe cannot be handled by a single operator, and from the viewpoints of difficulty in securing the operator and labor costs, a reduction in the weight of the steel pipe is required.

In many cases, the high-strength steel pipe for ground stabilization work of the present application is manufactured in a pipe factory with a length of about 10 m or more in view of production efficiency and price, and is cut to the above-mentioned predetermined length by an intermediate worker or the like. , carry out thread cutting, etc., and move it into the construction site for construction. When high-strength steel pipes for foundation stabilization work are driven into the foundation, the reason why threaded joints or high-precision fittings are used is that when the hard solid rock in the foundation becomes an obstacle when pressing into the foundation , in order not to bend from the joint part as a starting point and cause the embedding and press-fit to stop, the joint part needs to maintain the same strength as the base material part. This is due to the simple expansion of the pipe end of the steel pipe or the simple fixation of metal parts such as bolts. During the embedding, the obstacles may bend, fall off, or the metal parts collide and jam, so that the steel pipe cannot be pressed into the foundation. Ground stabilization works are hindered and are therefore not preferred. In addition, especially when the foundation for the tunnel is stable, since the steel pipe is pressed horizontally, slightly inclined, or laterally, it is difficult to ensure linearity in the above-mentioned direction at the construction site when joining by welding. It is also difficult to prepare such a welding apparatus for joining by welding.

Regarding thin-walled and high-strength enhancement for weight reduction of steel pipes, as shown in the above-mentioned Patent Document 3 and the like, various methods have been reported in the past. In addition, many steel pipes for this purpose do not rotate the steel pipe itself during or after foundation stabilization construction such as tunnel construction, so the roundness (roundness) of the central portion of the pipe is not required when burying the foundation. However, as described above, after the pipe is produced, the approximate center portion in the longitudinal direction of the steel pipe (hereinafter referred to as the center portion of the steel pipe, and before the steel pipe is cut, is located at a distance Le from the end of the steel pipe by the outer diameter of the steel pipe, which will be described later). The portion near the center of the steel pipe) is cut to the above-mentioned length L, so the steel pipe end portion of the steel pipe shipped from the steel pipe manufacturing plant and the steel pipe end portion generated by cutting the steel pipe at the center portion of the steel pipe afterward require the use of a rotary cutting device. Threading for joining the steel pipes to each other is performed at the ends of the steel pipes, and therefore high roundness is required at the ends of the steel pipes. Also, after cutting to the above-mentioned length L, a portion of the steel pipe ends may be fitted and joined via one or more jigs. In this case, the steel pipe ends are also required to have high roundness for stable joining.

As described above, high-strength steel pipes for ground stabilization works that require high roundness are produced by cold-working high-strength steel sheets. Therefore, the higher the strength and the greater the tensile strength, the greater the residual stress during processing. When the pipe is cut to the above-mentioned length L after pipe production, the residual stress is released at the steel pipe end of the cut portion, the deformation of the steel pipe ends becomes large, and the roundness tends to deteriorate.

Examples of the use of steel pipes or high-strength steel pipes with a length close to that of high-strength steel pipes for ground stabilization work include torsion beams, structural parts, and other automotive applications, and scaffolding parts at construction sites. Joining with other parts in automobile applications is mainly mechanical joining such as welding and bolting, and thread cutting affected by roundness is rarely used, so the subject of the present application is not obvious. The same is true for the scaffolding parts on the construction site, which are assembled by fastening of metal parts. In addition, as steel pipes with similar lengths, there are steel pipes for housing foundation piles and steel pipes for wire posts, but these are only connected by pins or simple metal fittings, and the subject of the present application is still not obvious.

In the case of thread cutting, for example, there is a steel pipe for oil country tubular goods. For a steel pipe with a length of about 10 m, the roundness has been ensured at a pipe manufacturing plant. The length of the direct link is used. However, in the final part of the excavation of oil wells up to several thousand meters, the intermediate workers sometimes perform thread cutting of several meters of short material for length adjustment. Problems related to shape change and roundness are not obvious. As for the high-strength steel pipe for foundation stabilization work, it is necessary to cut all the lengths shipped from the factory to the length at the time of use, for example, L=3.0 to 3.5m, and connect them one by one. Therefore, all the threads in the connection part are In the cutting part, roundness is very important, and joint problems may occur. In this way, it can be said that the deterioration of roundness at the time of cutting and the joining problems caused by thread cutting or fitting are problems specific to high-strength steel pipes for foundation stabilization work.

Here, as a conventional technique for improving the roundness of a steel pipe, generally known are drawing processing after pipe production, and forming steel processing in which a pipe end is pressed into a mold and subjected to warm processing. However, these may become a different process from a steel pipe manufacturing line, and the manufacturing cost may increase. The intermediate worker does not necessarily have a section steel processing facility, and as described above, the high-strength steel pipe may be brought into the construction site and then cut. In this case, it cannot be dealt with by another process, and it is necessary to ensure the roundness of the end of the steel pipe to be cut even for cutting by an intermediate worker or unplanned cutting at the construction site. In addition, the scope of tunnel engineering and foundation stabilization engineering is large, and a large number of steel pipes are used, so it is required to be as cheap as possible.

prior art literature

Patent Document 1: Japanese Patent Laid-Open No. 54-19415

Patent Document 2: Japanese Patent Application Laid-Open No. 6-93339

Patent Document 3: Japanese Patent No. 5131411

SUMMARY OF THE INVENTION

The problem to be solved by the invention

Therefore, the inventors provide a method of using a high-strength electric-resistance-welded steel pipe and a high-strength electric-resistance-welded steel pipe for ground stabilization work, which are lightweight and high-strength, and have high roundness at the end of the steel pipe produced by fresh cutting after pipe production.

means of solving problems

In order to solve the above problems and achieve the corresponding objects, the present invention adopts the following technical solutions.

(1) The high-strength electric-resistance welded steel pipe according to one aspect of the present invention contains, in mass % or mass ppm, C: 0.04-0.30%, Si: 0.01-2.00%, Mn: 0.50-3.00%, and P: 0.030% Below, S: 0.030% or less, Al: 0.005 to 0.700%, N: 100 ppm or less, Nb: 0 to 0.100%, V: 0 to 0.100%, Ti: 0 to 0.200%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, B: 0 to 50 ppm, Ca: 0 to 100 ppm, REM: 0 to 200 ppm, the balance consists of iron and impurities, and DCave is 60.3 mm or more and 318.5 mm or less, tCave/DCave is 0.02 or more and 0.06 or less, and tensile strength is 590 N/mm ² or more, and the following formula is satisfied when the central part of the steel pipe is cut.

DCave×(－2/100)≤x≤DCave×(2/100) (1)

YN≤y≤YM (2)

x+K－3×SD≤y≤x+K+3×SD (3)

YM=MIN[{DEave×(2/100)}, {4×((tEave/3)－0.65)}] (4)

In the formula (4), the smaller of {DEave×(2/100)} and {4×((tEave/3)−0.65)} is defined as YM.

YN=MAX[{DEave×(-2/100)}, {-4×((tEave/3)-0.65)}] (5)

In the formula (5), the larger one of {DEave×(−2/100)} and {−4×((tEave/3)−0.65)} is defined as YN.

K={α+(β/I)+(γ×TS)}×DCave (6)

Standard deviation of the outer diameter of the central part of the steel pipe={p+(q/I)+(r×TS)}×DCave (8)

Where, x: longitudinal ovality (central part of steel pipe), y: longitudinal ovality (end of steel pipe), DCave: average outer diameter (mm) of the central part of steel pipe after pipe making and before cutting, tCave: after pipe making and Average wall thickness (mm) of the steel pipe at the center of the steel pipe before cutting, DEave: average outer diameter (mm) of the steel pipe end after pipe making and cutting, tEave: average of the steel pipe end after pipe making and cutting Wall thickness (mm), TS: tensile strength of the base metal part of the high-strength electric-resistance welded steel pipe (N/mm ² ), α, β, γ are constants,

α = -1.87×10 ^-3 (9)

β=1.35×10 ⁴ (10)

γ＝－6.65×10 ^-6 (11)

I is the secondary distance of the section at the central part of the steel pipe (mm ⁴ ),

I=π/64×{(DCave) ⁴ −(DCave−2×tCave) ⁴ } (12)

p, q, r are constants,

p=1.39×10 ^-3 (13)

q=4.17×10 ² (14)

r=6.05×10 ⁻⁷ (15).

(2) In the high-strength electric resistance welded steel pipe according to (1) above, the tensile strength may be 780 N/mm ² or more.

(3) The high-strength electric resistance welded steel pipe according to (1) or (2) above may further satisfy the following formula.

YN－K+3×SD≤x≤YM－K－3×SD (17)

(4) The high-strength electric resistance welded steel pipe according to (1) or (2) above may further satisfy the following formula.

DEave×(－2/100)－K+3×SD≤x≤DEave×(2/100)－K－3×SD (18)

(5) A method of using a high-strength electric-resistance-welded steel pipe for a ground stabilization project according to an aspect of the present invention, wherein the steel pipe center portion of the high-strength electric-resistance-welded steel pipe according to (1) or (2) is cut, and the resulting The end of the new steel pipe is threaded, and two or more high-strength electric-resistance welded steel pipes are connected with a threaded joint.

(6) A method of using a high-strength electric-resistance-welded steel pipe for ground stabilization work according to an aspect of the present invention, wherein one or both of the steel pipe ends of the high-strength electric-resistance-welded steel pipe according to (1) or (2) are connected to the steel pipe. The new steel pipe ends produced by cutting the central part are used by fitting the steel pipe ends to each other through one or more jigs, and connecting two or more high-strength electric resistance welded steel pipes.

effect of invention

According to the present invention, it is possible to provide a high-strength electric-resistance-welded steel pipe that is lightweight and high-strength, and has high roundness at the end of the steel pipe produced by fresh cutting after pipe production, and a high-strength electric-resistance-welded steel pipe for ground stabilization works. Thereby, the load of the coupling work of steel pipes can be reduced, and the efficiency of construction work can be achieved at low cost.

Description of drawings

Fig. 1 is used to show the basis of Le for determining the range of the central part of the steel pipe, which is the distance from the end of the steel pipe/the outer diameter of its position, the longitudinal ovality of the cross-section at the measurement position of the outer diameter, and the length of the steel pipe in the pipe making direction A graph showing the relationship between the longitudinal ovality differences at the 1/2 position. In addition, the steel pipe is 114.3 mm in outer diameter x 3.5 mm in thickness x 7400 mm in length.

2 is a graph showing the relationship between the tensile strength of the steel pipe and the longitudinal ovality (ΔDE) of the steel pipe end portion - the longitudinal ovality (ΔDC) of the steel pipe center portion. In addition, the steel pipe has an outer diameter of 114.3 mm and a wall thickness of 3.2 to 8.6 mm.

3 is a graph showing the relationship between the tensile strength of the steel pipe of each plate thickness and the longitudinal ovality (ΔDE) of the steel pipe end portion - the longitudinal ovality (ΔDC) of the steel pipe center portion. In addition, the outer diameter of the steel pipe was 114.3 mm.

FIG. 4 is a graph showing the relationship between the tensile strength of the steel pipe and the standard deviation of the average outer diameter of the center portion of the steel pipe. In addition, the steel pipe has an outer diameter of 114.3 mm and a wall thickness of 3.2 to 8.6 mm.

5 is a graph showing the relationship between the tensile strength of the steel pipe and the standard deviation of the average outer diameter of the center portion of the steel pipe for each plate thickness. In addition, the outer diameter of the steel pipe was 114.3 mm.

6 is a graph showing the relationship between the tensile strength of the steel pipe and the residual stress at the center of the steel pipe. In addition, the steel pipe has an outer diameter of 114.3 mm and a wall thickness of 3.2 to 8.6 mm.

7 is a diagram schematically showing a change in the average outer diameter of the steel pipe end portion and the state of the thread cross section when the steel pipe end portion is deformed by cutting. In addition, since it is a schematic representation, the ratio of an outer diameter and a wall thickness etc. are ignored and shown.

8 is a diagram showing the relationship between the longitudinal ovality ΔDC of the steel pipe center portion (before cutting) and the longitudinal ovality ΔDE of the steel pipe end portion (after cutting) when the steel pipe end portion is screwed.

Fig. 9 shows the longitudinal ovality ΔDC of the steel pipe center portion (before cutting) and the longitudinal ovality ΔDE of the steel pipe end portion (after cutting) when the steel pipe end portion is screwed, taking into account the variation in production A diagram of a more preferred relationship.

10 is a graph showing the longitudinal ovality ΔDC of the steel pipe center portion (before cutting) and the longitudinal ovality ΔDE of the steel pipe end portion (after cutting) when the area YY is larger than the area AA when the steel pipe end is threaded. Diagram of the relationship.

11 shows the longitudinal ovality ΔDC of the steel pipe center (before cutting) and the longitudinal ovality ΔDE of the steel pipe end (after cutting) when the area YY is larger than the area AA in the case where the steel pipe end is screwed , a graph of a more preferred relationship that takes into account manufacturing variations.

12 is a diagram showing the relationship between the longitudinal ovality ΔDC of the central part of the steel pipe (before cutting) and the longitudinal ovality ΔDE of the steel pipe end (after cutting) when the steel pipe ends are joined by fitting (the jig is chimeric case).

Fig. 13 shows the longitudinal ovality ΔDC of the central part (before cutting) of the steel pipe and the longitudinal ovality ΔDE of the steel pipe end (after cutting) when the steel pipe ends are joined by fitting, taking into account the variation in production A diagram of a more preferable relationship (in the case of fitting a jig).

FIG. 14 is a diagram showing an example of the outline of the equipment of the pipe making machine.

Detailed ways

The inventors measured the steel pipe cross-sectional dimension of the steel pipe center before and after cutting when the steel pipe center was cut to a predetermined length after pipe production, and investigated in detail changes in the steel pipe cross-sectional size due to the release of residual stress by cutting the steel pipe. As a result, it was successfully found that the cross-sectional size of the steel pipe after cutting was suitable for the cross-sectional size of the steel pipe before cutting for thread cutting or jig bonding, taking into account the dimensional change due to residual stress. In addition, the cross-sectional shape of the steel pipe before cutting is achieved by adjusting the position of the rolls of the respective roll stands in the pipe-making process, the welding process, and the straightening process. The manufacturing conditions will be explained in detail below. According to the specifications of the pipe manufacturing equipment, such as the number of rolls, the down force, the shape of the rolls and their arrangement, the conditions of each process are subtly different, so the range of the conditions cannot be specified. Dimensional measurement and roundness confirmation, and appropriate discovery and adjustment of each process condition suitable for the pipe manufacturing equipment can be implemented.

Most of the cutting of the steel pipe is sawing, but cutting by a lathe or the like is also possible.

In addition, in this specification, "high-strength electric resistance welded steel pipe" may be abbreviated as "steel pipe".

In addition, the numerical range represented using "-" in this specification means the range which includes the numerical value described before and after "-" as a lower limit and an upper limit.

Hereinafter, a high-strength electric resistance welded steel pipe according to an embodiment of the present invention will be described.

The high-strength electric-resistance welded steel pipe according to the present embodiment contains, in mass % or mass ppm, C: 0.04 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, P: 0.030% or less, and S: 0.030% Below, Al: 0.005 to 0.700%, N: 100 ppm or less, Nb: 0 to 0.100%, V: 0 to 0.100%, Ti: 0 to 0.200%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Cr : 0 to 1.000%, Mo: 0 to 1.000%, B: 0 to 50 ppm, Ca: 0 to 100 ppm, REM: 0 to 200 ppm, and the balance consists of iron and impurities.

The outer diameter (DCave described later) of the steel pipe is 60.3 mm or more and 318.5 mm or less. When the outer diameter of the steel pipe is 60.3 mm or more, the target steel pipe strength of the present invention can be easily obtained. If the outer diameter of the steel pipe is 318.5 mm or less, it will be easy to carry. The outer diameter of the steel pipe is preferably 113 mm or more and 116 mm or less. In addition, the outer diameter of a steel pipe is an average outer diameter.

The ratio (tCave/DCave) of the thickness of the steel pipe (tCave described later) to the outer diameter of the steel pipe (DCave described later) is 0.02 or more and 0.06 or less. When the ratio (tCave/DCave) of the wall thickness of the steel pipe to the outer diameter of the steel pipe is 0.02 or more, the strength as the steel pipe can be easily achieved. If the ratio of the wall thickness of the steel pipe to the outer diameter of the steel pipe (tCave/DCave) is 0.06 or less, the weight reduction can be easily achieved.

The tensile strength of the steel pipe is 590 N/mm ² or more. When the tensile strength is 590 N/mm ² or more, the thickness can be reduced, and the weight can be easily handled manually. The tensile strength is preferably 780 N/mm ² or more. The tensile strength is preferably 1200 N/mm ² or less, and more preferably 1500 N/mm ² or less.

When the yield ratio of the steel pipe is 86% or more and 99% or less, the joint strength of the thread can be improved, which is preferable.

In addition, the tensile strength and the yield ratio of the steel pipe can be obtained by taking a full-thickness test piece along the pipe axis direction from the base material portion of the steel pipe after pipe production, and performing a tensile test in the pipe axis direction.

In the scope of the present specification and the claims, terms are defined as follows.

As for the outer diameter of the central part of the steel pipe, the welded part was placed at 12 o'clock on the clock, the position was set to 0°, the arbitrary outer diameter in the range of ±45° was set to D1, and the diameter orthogonal to D1 was set to be D3. The diameter of the position rotated 45° clockwise from D1 is set to D2, and the diameter of the position rotated 45° clockwise from D3 is set to D4.

The outer diameters of the central part of the steel pipe of D1, D2, D3, and D4 are respectively DC1, DC2, DC3, and DC4, and the average is taken as the average outer diameter of the central part of the steel pipe, and is called DCave. In addition, let the inner diameters of the central part of the steel pipe at the positions of D1, D2, D3, and D4 be dC1, dC2, dC3, and dC4, respectively, and the average of them is the average inner diameter of the central part of the steel pipe, which is called dCave. The wall thicknesses of the central portion of the steel pipe at the positions of D2, D3, and D4 were set to tC1, tC2, tC3, and tC4, and the average was taken as the average wall thickness of the central portion of the steel pipe, and referred to as tCave. Furthermore, the units of DC1, DC2, DC3, DC4, dC1, dC2, dC3, dC4, tC1, tC2, tC3, tC4, DCave, dCave, and tCave are all mm.

Next, regarding the outer diameter of the end of the steel pipe, similarly, the welded portion is placed at 12 o'clock on the timepiece, the position is set to 0°, the arbitrary outer diameter in the range of ±45° is set to D1, and the diameter is set to be orthogonal to D1. The diameter is set to D3. The diameter of the position rotated 45° clockwise from D1 is set to D2, and the diameter of the position rotated 45° clockwise from D3 is set to D4. The outer diameters of the steel pipe ends of D1, D2, D3, and D4 are taken as DE1, DE2, DE3, and DE4, and the average is taken as the average outer diameter of the steel pipe ends, and is referred to as DEave. In addition, let the inner diameters of the steel pipe ends at the positions of D1, D2, D3, and D4 be dE1, dE2, dE3, and dE4, and take the average as the average inner diameter of the steel pipe ends, which is called dEave. In addition, D1, D2, The wall thicknesses of the steel pipe ends at the positions of D3 and D4 were set to tE1, tE2, tE3, and tE4, and the average was taken as the average wall thickness of the steel pipe ends, and called tEave. In addition, the units of DE1, DE2, DE3, DE4, dE1, dE2, dE3, dE4, tE1, tE2, tE3, tE4, DEave, dEave, and tEave are all mm.

In addition, when cutting at the center of the steel pipe after pipe making, the position Le (mm) from the steel pipe end to the center in the longitudinal direction of the steel pipe by the outer diameter of the steel pipe is defined as the steel pipe end, which is larger than Le. The part separated to the center side of the steel pipe is called the center part of the steel pipe. The central portion of the steel pipe is a range in which residual stress generated during pipe manufacturing is released when the steel pipe is cut to deform the cross-sectional dimension of the steel pipe, an example of which is shown in FIG. 1 . The horizontal axis of Fig. 1 is "distance from the end of the steel pipe/outer diameter at the position". The vertical axis is "the difference between the longitudinal ovality of the cross-section at the outer diameter measurement position and the longitudinal ovality of the position of 1/2 of the length in the pipe-making direction". When the "distance from the end of the steel pipe/outer diameter at this position" on the horizontal axis is greater than 1.0, that is, from the cutting position of the end of the steel pipe to the position Le that is separated from the outer diameter of the steel pipe toward the center in the longitudinal direction of the steel pipe In the case of being separated to the center side of the steel pipe to a greater extent than that in the case of the center part of the steel pipe before cutting, "the longitudinal ovality of the cross-section at the outer diameter measurement position is the longitudinal ellipse at the position where the length in the longitudinal direction of the steel pipe is 1/2" "Difference in degree" is approximately 0, which means that the longitudinal ovality is the same with respect to the half position in the longitudinal direction of the steel pipe without deformation.

However, when the horizontal axis is 1.0 or less, that is, on the steel pipe end side from the cutting position of the steel pipe end to the longitudinal center of the steel pipe by the outer diameter of the steel pipe, the "outside diameter" The difference between the longitudinal ovality of the cross-section at the measurement position and the longitudinal ovality at the position of 1/2 of the length in the pipe making direction is biased to a negative value, and the closer to the end of the steel pipe, the more negative value side. This means that when the steel pipe is cut to form the steel pipe end, the residual stress is released, the deformation of the steel pipe end increases, and the roundness deteriorates.

Here, the longitudinal ovality (ΔDE) of the end portion of the steel pipe and the longitudinal ovality (ΔDC) of the central portion of the steel pipe will be described. D1-D3, which is the difference between D1 and D3 in the cross-section perpendicular to the longitudinal direction, is ΔD, and when the vertical ovality of the cross-section is defined as the vertical ovality of the cross-section, when the pipe cross-section is vertically long, D1>D3, so the vertical direction is D1>D3. The ovality>0, and when the tube cross section becomes horizontally long, the longitudinal ovality<0 because D1<D3. In the case of a true circle (perfect circle), since D1=D3, the longitudinal ellipticity=0. Therefore, the longitudinal ovality (ΔDE) at the end of the steel pipe and the longitudinal ovality (ΔDC) at the center of the steel pipe become:

The longitudinal ovality of the central part of the steel pipe ΔDC=DC1-DC3 (19),

The longitudinal ovality ΔDE=DE1-DE3 of the steel pipe end (20).

In addition, the cutting position of the steel pipe, that is, the end of the steel pipe, also includes the position that is cut to obtain the product during the production of the pipe, and the both ends of the steel pipe product at the time of shipping (delivery) after the pipe production. The end of the steel pipe formed by cutting at the construction site of the construction site. In addition, in Sample 1 and Sample 2 in FIG. 1 , the outer diameter is 114.3 mm×thickness 3.5 mm, TS=1000 N/mm ² , and the length L when newly cut to form the steel pipe end is 2000 mm to 5000 mm.

The inventors investigated the difference between the longitudinal ovality (ΔDE) at the end of the steel pipe and the longitudinal ovality (ΔDC) at the center of the steel pipe at various tensile strengths for the case of an outer diameter of 114.3 mm and a wall thickness of 3.2 to 8.6 mm. As a result, as shown in FIG. 2 , the relationship between the tensile strength and them (=slope) was clarified from the data of wall thickness = 3.2 to 3.5 mm, and the relationship was considered to be the same for each wall thickness, and the The relationship between the thicknesses was sorted out for each wall thickness, and it was found that, when the outer diameter was 114.3 mm, the relationship between FIG. 3 and the following formula (21) existed.

ΔDE=ΔDC+K (21)

Here, K is a constant obtained by the following formula (6).

K={α+(β/I)+(γ×TS)}×DCave (6)

Here, TS is the tensile strength (N/mm ² ) of the base metal portion of the steel pipe, α, β, and γ are constants, α=−1.87×10 ⁻³ , β=1.35×10 ⁴ , and γ=−6.65×10 ^{− 6} . I is the secondary distance (mm ⁴ ) of the cross-section of the central portion of the steel pipe, and is derived from the following formula (12).

I=π/64×{(DCave) ⁴ −(DCave−2×tCave) ⁴ } (12)

FIG. 3 shows an example of the calculation result of the formula (21) for each plate thickness.

The inventors investigated the standard deviation of the average outer diameter of the central portion of the steel pipe under various tensile strengths, as shown in FIG. As a result, as shown in FIG. 4 , the relationship between the tensile strength and them (=slope) was clarified from the data of wall thickness = 3.2 to 3.5 mm. The relationship between the thicknesses was sorted out for each wall thickness, and it was found that when the outer diameter was 114.3 mm, the relationship between FIG. 5 and the following formula (8) existed.

Standard deviation of the average outer diameter of the central part of the steel pipe={p+(q/I)+(r×TS)}×DCave (8)

Here, TS is the tensile strength (N/mm ² ) of the base metal portion of the steel pipe, p, q, and r are constants, p=1.39×10 ⁻³ , q=4.17×10 ² , and r=6.05×10 ⁻⁷ . I is the secondary distance (mm ⁴ ) of the cross-section of the central portion of the steel pipe, and is derived from the above-mentioned formula (12). An example of the calculation result of the formula (8) for each plate thickness is shown in FIG. 5 .

For the steel pipe for this purpose, when a plurality of steel pipes are connected and used, there are two methods of use. One is to directly use a rotary cutting device to thread the male thread and the female thread on both ends of the steel pipe to connect the steel pipe, and the other is to pass one or more clamps between the steel pipe and the steel pipe. A method used by fitting and connecting the ends of steel pipes.

In the method of threading using a rotary cutting device, in order to ensure the threading accuracy and the threading function of the product during machining, a method of fitting the end of the steel pipe between the steel pipe and the steel pipe via one or more jigs In order to ensure the strength of the fitting surface, it is necessary to ensure the outer diameter tolerance of the steel pipe and high roundness at the pipe end. In terms of weight reduction by high strength, which is the object of the present invention, as shown in FIG. 6 in the case of an outer diameter of 114.3 mm and a wall thickness of 3.2 to 8.6 mm, the higher the strength, the higher the residual stress of the steel pipe. . Therefore, at the end of the steel pipe near the cutting position, the residual stress is released, the deformation force acts, and the thin wall is more easily deformed, and the change in the longitudinal ovality of the pipe end tends to increase, and ensuring the longitudinal ovality becomes a problem. . In addition, the measurement of residual stress was implemented by the Crampton method (for example, as described in Nippon Steel & Sumitomo Metal Technical Paper No. 397 (2013) p31).

Fig. 7 schematically shows that in the case where the steel pipe end is directly screwed, with respect to the design values of the screwing, that is, the outer diameter and the wall thickness are average values, and the cross-section is vertical (longitudinal ovality>0) Changes in the cross-section of the threading process. In addition, in FIG. 7, in order to demonstrate a principle, the ratio of the outer diameter and the wall thickness of an actual steel pipe is ignored and shown.

As shown in the cross section in the longitudinal direction of the threaded portion in Fig. 7, both the male thread and the female thread have residual portions that are not cut relative to the average thickness. The soundness of the thread shape and the reduction of residual material at the same time require the longitudinal ovality of the steel pipe end to be within a certain range. The residual portion is a portion represented by the following formulas (22) and (23).

Remaining part of male thread = (diameter of concave part of male thread min-inner diameter)/2 (22)

Here, inner diameter=outer diameter−2×thickness.

Remaining part of female thread = (outer diameter - diameter of concave part of female thread max)/2 (23)

Therefore, based on the above-mentioned new findings, the present inventors have discovered a method for clarifying the relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion when the tensile strength and size are different, that is, clearly cutting the steel pipe into predetermined The relationship between the longitudinal ovality before and after the length L is to adjust and control the longitudinal ovality of the central part of the pipe in the process of forming and setting the pipe within a certain range so that the central part of the steel pipe, that is, the central part of the steel pipe before cutting, is adjusted and controlled within a certain range. The longitudinal ovality of the cut steel pipe end is within a predetermined range, so that the cut steel pipe end has a high roundness.

In FIG. 8 , the case where the threading of the male thread and the female thread is directly performed on the end of the steel pipe by the cutting device will be described. In the relationship between the longitudinal ovality ΔDC of the central part of the steel pipe and the longitudinal ovality ΔDE of the end of the steel pipe, in order to reduce the residual material as much as possible in the thread cutting process, the weight of the steel pipe should be reduced, and the necessary thread function should be ensured. The shape of the steel pipe end, that is, the cut steel pipe end is such that ΔDC and ΔDE satisfy the area (hereinafter referred to as area XX) surrounded by the area AA and the area YY described below.

Here, in FIG. 8 , the area AA refers to the area required to secure the outer diameter tolerance, and is the area surrounded by the points A1, A2, A3, and A4 in FIG. The part satisfies the range of ±1% of the outer diameter tolerance (No. 1 tolerance) specified by JIS G3444 (2016) structural steel pipes. In addition, this outer diameter tolerance can be changed according to a specification. This range is a condition necessary to ensure the necessary circular shape when used as a structural steel pipe. If the range is not satisfied, the bending moment required for use as a structural steel pipe and the resulting bending resistance and resistance cannot be ensured. buckling. This range is a range necessary to secure the function as a structural pipe.

In FIG. 8 , the points A1 to A4 satisfy the following equations (24) to (31).

Point A1: x(A1)=DCave×(2/100) (24)

y(A1)=DEave×(2/100) (25)

Point A2: x(A2)=DCave×(2/100) (26)

y(A2)=DEave×(-2/100) (27)

Point A3: x(A3)=DCave×(-2/100) (28)

y(A3)=DEave×(-2/100) (29)

Point A4: x(A4)=DCave×(-2/100) (30)

y(A4)=DEave×(2/100) (31)

The above contents are arranged, and (x, y) satisfying the following formula (32) and formula (33) is the area AA.

DCave×(－2/100)≤x≤DCave×(2/100) (32)

DEave×(－2/100)≤y≤DEave×(2/100) (33)

The area YY is the range of the shape of the pipe end that should be ensured in order to reduce the residual material as much as possible in the thread cutting process, to reduce the weight of the steel pipe, and to ensure the necessary thread function. The inventors found the following formula (34) with respect to the average residual material schematically shown in FIG. 7 in order to secure the strength of the joint as a whole pipe when threading a high-strength thin-walled material.

Average residual material≥tEave/3 (34)

When the residual material is less than or equal to that, the joint strength required as the pipe body cannot be ensured, and the functions of the original application, such as breakage of the joint part during use, cannot be ensured.

On the other hand, as shown in FIG. 7 , in consideration of the fact that the actual outer diameter of the steel pipe deviates from the average outer diameter, the inventors started from the point of preventing local deformation of the threaded portion in the thread processing of the high-strength thin-walled material. , the following formula (35) was found about the residual material limit.

Limit residual material≥0.65mm (35)

If it is less than this value, there may be problems in manufacturing and use, such as an increase in manufacturing cost due to a defective product due to deformation of the thread portion during processing, and a failure to use the product due to deformation of the thread portion during use.

If the conditions required for the shape of the pipe end that should be ensured by reducing the residual material as much as possible in the thread cutting process to achieve the weight reduction of the steel pipe and ensuring the necessary thread function are obtained, as shown in the example of Fig. 7, in the longitudinally long In this case, the male thread side is the following formula (36).

Limit residual material = average residual material - (dE1 - dEave)/2≥0.65 (36)

In the electric resistance welded steel pipe, since the strip steel is used as the billet, the wall thickness is constant in terms of the average wall thickness, and the following formulas (37) and (38) are obtained.

dE1＝DE1－2×tEave(37)

dEave=DEave－2×tEave(38)

The following formula (39) is obtained by transforming the formula (36) from the formula (34), the formula (35), the formula (37) and the formula (38).

DE1－DEave≤2×{(tEave/3)－0.65} (39)

The female thread side is similarly represented by the following formula (40).

Limit residual material = average residual material - (DEave - DE3)/2≥0.65 (40)

By modifying the formula (34), the formula (40) becomes the following formula (41).

DEave－DE3≤2×{(tEave/3)－0.65} (41)

When both sides of the formula (39) and the formula (41) are satisfied, the following formula (42) is obtained.

ΔDE=DE1－DE3≤4×{(tEave/3)－0.65} (42)

When it is horizontally long, that is, when it is reversed vertically and horizontally in FIG. 7 , the male thread side is similarly represented by the following formula (43).

DEave－DE1≤2×{(tEave/3)－0.65} (43)

The female thread side is the following formula (44).

DE3－DEave≤2×{(tEave/3)－0.65} (44)

When both sides of the formula (43) and the formula (44) are satisfied, the following formula (45) is obtained.

DE3－DE1≤4×{(tEave/3)－0.65} (45)

The following formula (46) is obtained by rewriting the formula (45).

ΔDE=DE1－DE3≥－4×{(tEave/3)－0.65} (46)

Hereinafter, in Figs. 8 to 13 in which the x-axis is the longitudinal ovality ΔDC of the central portion of the steel pipe and the y-axis is the longitudinal ellipticity ΔDE of the steel pipe ends, the x-axis component of the point i in the figure is represented as x(i) , denoting the y-axis component as y(i).

In the expression of the formula described below, MAX(n, m) represents the larger value among n and m, and MIN(n, m) represents the smaller value among n and m. In addition, the conditions in FIGS. 8-9 and FIGS. 12-13 are TS=1000N/mm< ² >, a dimension is 114.3 mm of outer diameters, and a wall thickness is 3.5 mm. The conditions in FIGS. 10 and 11 are TS=1000 N/mm ² , the dimensions are 114.3 mm in outer diameter, and the wall thickness is 4.0 mm.

In FIG. 8 , if the line YH and the line YL that define the range of the line of the above-mentioned area YY are the following equations (47) and (48), the area YY is a region that satisfies both equations (47) and (48). In FIG. 8, the part surrounded by the line YH and the line YL is shown.

Line YH: y=4×{(tEave/3)−0.65} (47)

Line YL: y=-4×{(tEave/3)-0.65} (48)

In addition, YH and YL are the upper limit and lower limit of the range of ΔDE required to reduce the residual material as much as possible in the thread cutting process, to reduce the weight of the steel pipe, and to ensure the necessary thread function. It is represented by the formula, and (x, y) satisfying both the following formula (49) and formula (50) is the region YY.

-∞≤x≤∞ (49)

－4×{(tEave/3)－0.65}≤y≤4×{(tEave/3)－0.65} (50)

The area XX surrounded by the area AA and the area YY, that is, the area that can ensure the outer diameter tolerance for ensuring the function as a structural pipe, reduce the residual material as much as possible to reduce the weight of the steel pipe, and ensure the necessary thread function, It is an area|region enclosed by the point X1, the point X2, the point X3, and the point X4, and is represented by following formula (51) - formula (58).

Point X1: x(X1)=DCave×(2/100) (51)

y(X1)=YM (52)

Point X2: x(X2)=DCave×(2/100) (53)

y(X2)=YN (54)

Point X3: x(X3)=DCave×(-2/100) (55)

y(X3)=YN (56)

Point X4: x(X4)=DCave×(-2/100) (57)

y(X4)=YM (58)

Here, YN and YM are not shown in FIG. 8 , but are as follows. YN is the range as the lower limit of the y component when defining the range of the area XX, the y component y=DEave×(-2/100) of the area AA and the y component y=-4×(tEave/3)- of the area YY The larger of 0.65. YM is the range of the upper limit of the y component when the range of the area XX is defined, and the y component of the area AA is y=DEave×(2/100) and the y component of the area YY=4×(tEave/3)-0.65 The smaller value is Equation (4) and Equation (5).

YN=MAX[{DEave×(-2/100)}, {-4×((tEave/3)-0.65)}] (5)

YM=MIN[{DEave×(2/100)}, {4×((tEave/3)－0.65)}] (4)

After finishing the above, (x, y) satisfying the following formula (59) and formula (60) at the same time is the region XX.

DCave×(－2/100)≤x≤DCave×(2/100) (59)

YN≤y≤YM (60)

Here, the inventors have discovered a method of clarifying the relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion as described above, and utilizing this relationship to control the longitudinal ovality of the steel pipe center portion within a certain range during pipe production As a result, the longitudinal ovality of the steel pipe end portion after the steel pipe is cut is ensured at a low level, and thread cutting can be performed. Hereinafter, this method and the area of the product obtained by this method are shown as area PP in FIG. 8 . The area PP is an area in which the above-mentioned area XX and the area WW described later overlap.

The area WW is ΔDC and ΔDE obtained when manufacturing is performed using the above-described relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion, and shows the range including the deviation. The area WW in FIG. 8 will be described. The longitudinal ovality of the steel pipe end portion and the steel pipe center portion has the relationship of Equation (61), which is indicated by the line WB in FIG. 8 .

y=x+K (61)

Here, y is ΔDE and x is ΔDC, and the above formula (21) is obtained by substituting them. In addition, K is a constant calculated|required by the said Formula (6).

As shown in FIG. 8 , in order to make ΔDE=0 from this formula, the longitudinal ovality x (=ΔDC) of the center portion of the steel pipe to be targeted during manufacture is expressed by the formula (62),

x(=ΔDC)=-K (62)

The point AIM in FIG. 8 , and if the forming and setting at the time of pipe making satisfy the formula (61), the longitudinal ovality of the pipe end can be easily reduced.

The range of ΔDC and ΔDE of the product manufactured by the relationship of the formula (61) is obtained by the standard deviation of the average outer diameter DCave of the central part of the steel pipe obtained by the above formula (8), and the deviation is taken into account, and the following lines WH and lines The area WW surrounded by WL. Here, WH represents the upper limit of ΔDE of the average +3σ, and WL represents the lower limit of the ΔDE of the average −3σ, and the following equations (63) and (64) are obtained.

Line WH: y=x+K+3×SD (63)

Line WL: y=x+K-3×SD (64)

Here, SD is the standard deviation of the longitudinal ovality, and since ΔD=D1-D3, the additivity based on the standard deviation can be represented by the following formula (7).

The standard deviation of the average outer diameter DCave of the central portion of the steel pipe is a number obtained from the above-mentioned formula (8). When represented by the formula, (x, y) satisfying the following formula (3) at the same time is the region WW.

x+K－3×SD≤y≤x+K+3×SD (3)

In the production where the longitudinal ovality of the steel pipe end after cutting the steel pipe is kept low using the relationship of the formula (61), the area PP in FIG. is the overlap of region XX with region WW. When represented by the formula, (x, y) satisfying the above-mentioned following formula (59), formula (60) and formula (3) at the same time is a region PP.

DCave×(－2/100)≤x≤DCave×(2/100) (59)

YN≤y≤YM (60)

x+K－3×SD≤y≤x+K+3×SD (3)

This is represented by coordinates in FIG. 8 , and is the inner area of the line connecting the point X1 , the point P1 , the point Z3 , the point X3 , the point P2 , the point Z1 , and the point X1 . Point P1: The intersection of the line passing through X1 and X2 and the line WL. Point P2: The intersection of the line passing through X4 and X3 and the line WH. Point Z1: The intersection of the line passing through X4 and X1 and the line WH. Point Z3: The intersection of the line passing through X3 and X2 with the line WL.

Within the range of x in the above-mentioned formula (59), when manufacturing is performed using the relationship of the above-mentioned formula (61) in the same manner, the region XX may not be satisfied due to variations in manufacturing. Therefore, in the thread cutting process, the range of ΔDC to be set and the ΔDE obtained at this time are shown as the area ZZ in FIG. 9 as a more preferable area in which the area XX can be stably secured in consideration of the variation in manufacture. . When represented by an equation, (x, y) satisfying both the following equation (65) and the above-mentioned equation (3) is the region ZZ.

YN－K+3×SD≤x≤YM－K－3×SD (65)

x+K－3×SD≤y≤x+K+3×SD (3)

This is represented by coordinates in FIG. 9 , and the region ZZ is a region that satisfies the region XX and is surrounded by a line connecting the following four points, namely, the point Z1 , the point Z2 , the point Z3 , and the point Z4 .

Point Z1: the intersection of the line passing through X4 and X1 and the line WH, and is represented by the following equations (66) and (67).

x(Z1)=y(X1)-K-3×SD=YM-K-3×SD (66)

y(Z1)=y(X1)=YM (67)

Point Z2: the intersection of x=x( Z1 ) and the line WL, and is represented by the following equations (68) and (69).

x(Z2)=x(Z1)=y(X1)-K-3×SD=YM-K-3×SD (68)

y(Z2)=x(Z1)+K－3×SD=YM－6×SD (69)

Point Z3: the intersection of the line passing through X3 and X2 and the line WL, and is represented by the following equations (70) and (71).

x(Z3)=y(X3)－K+3×SD=YN－K+3×SD (70)

y(Z3)=y(X3)=YN (71)

Point Z4: the intersection of x=x(Z3) and the line WH, and is represented by the following equations (72) and (73).

x(Z4)=x(Z3)=y(X3)－K+3×SD=YN－K+3×SD (72)

y(Z4)=x(Z3)+K+3×SD=YN+6×SD (73)

Next, when the wall thickness of the steel pipe increases, the area YY (the area required to reduce the residual material as much as possible during the thread cutting process, reduce the weight of the steel pipe, and ensure the necessary thread function) may become larger than the area AA (for The range required to secure the outer diameter tolerance), and the area PP in this case is shown in FIG. 10 .

In this case, the area XX where the area AA and the area YY overlap is the same as the area AA. When represented by a formula, (x, y) satisfying the above-mentioned following formula (32) and formula (33) at the same time is a region XX, and is represented by the following formula (32) and formula (33).

DCave×(－2/100)≤x≤DCave×(2/100) (32)

DEave×(－2/100)≤y≤DEave×(2/100) (33)

This is represented by coordinates in FIG. 10, and the area XX is the inner area of the line connecting the following four points, namely, the point X1, the point X2, the point X3, and the point X4, and is represented by the following equations (24) to (31) express.

Point X1 (=point A1): x(X1)=x(A1)=DCave×(2/100) (24)

y(X1)=y(A1)=DEave×(2/100) (25)

Point X2 (=point A2): x(X2)=x(A2)=DCave×(2/100) (26)

y(X2)=y(A2)=DEave×(-2/100) (27)

Point X3 (=point A3): x(X3)=x(A3)=DCave×(-2/100) (28)

y(X3)=y(A3)=DEave×(-2/100) (29)

Point X4 (=point A4): x(X4)=x(A4)DCave×(-2/100) (30)

y(X4)=y(A4)=DEave×(2/100) (31)

In FIG. 10 , regarding ΔDC and ΔDE obtained when manufacturing using the relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion, the region WW including the deviation indicating the range is the same as the above description, and satisfies the above-mentioned following (x, y) in the above formula (3) is the region WW, and is represented by the following formula (3).

x+K－3×SD≤y≤x+K+3×SD (3)

In FIG. 10, area PP is a portion where area XX and area WW overlap. If it is represented by the formula, (x, y) satisfying the following formula (32), formula (33) and formula (3) at the same time is the region PP.

DCave×(－2/100)≤x≤DCave×(2/100) (32)

DEave×(－2/100)≤y≤DEave×(2/100) (33)

x+K－3×SD≤y≤x+K+3×SD (3)

This is represented by coordinates in FIG. 10 , and is the inner area of the line connecting the point X1 , the point P1 , the point Z3 , the point X3 , the point P2 , the point Z1 , and the point X1 . in,

Point P1: The intersection of the line passing through X1 and X2 and the line WL.

Point P2: The intersection of the line passing through X4 and X3 and the line WH.

Point Z1: The intersection of the line passing through X4 and X1 and the line WH.

Point Z3: The intersection of the line passing through X3 and X2 with the line WL.

A region ZZ that can stably secure a preferable region of the region XX is shown in FIG. 11 in consideration of the manufacturing variation in this case. The idea is the same as above, but the y-component of the region XX is different,

y(X1)=y(X4)=DEave×(2/100) (25) and (31)

y(X2)=y(X3)=DEave×(-2/100) (27) and (29)

Therefore, if expressed by an equation, (x, y) satisfying both the following equation (74) and equation (3) is the region ZZ.

DEave×(－2/100)－K+3×SD≤x

≤DEave×(2/100)－K－3×SD (74)

x+K－3×SD≤y≤x+K+3×SD (3)

This is represented by coordinates in FIG. 11, and the region ZZ is the inner region of the line that satisfies the region XX and connects the following four points, namely, the point Z1, the point Z2, the point Z3, and the point Z4, and is represented by the following formula (75) ~ Formula (82) represents.

Point Z1: The intersection of the line passing through X4 and X1 and the line WH.

x(Z1)=y(X1)－K－3×SD=DEave×(2/100)－K－3×SD (75)

y(Z1)=y(X1)=DEave×(2/100) (76)

Point Z2: intersection of x=x(Z1) with line WL.

x(Z2)=x(Z1)=y(X1)-K-3×SD

＝DEave×(2/100)－K－3×SD (77)

y(Z2)=x(Z1)+K－3×SD=DEave×(2/100)－6×SD (78)

Point Z3: The intersection of the line passing through X3 and X2 with the line WL.

x(Z3)=y(X3)－K+3×SD=DEave×(-2/100)－K+3×SD (79)

y(Z3)=y(X3)=DEave×(-2/100) (80)

Point Z4: intersection of x=x(Z3) with line WH.

x(Z4)=x(Z3)=y(X3)－K+3×SD

=DEave×(-2/100)-K+3×SD (81)

y(Z4)=x(Z3)+K+3×SD=DEave×(-2/100)+6×SD (82)

Next, the case where the steel pipe and the steel pipe are fitted into the steel pipe end through one or more jigs and used in conjunction with each other will be described. In this case, the difference between the longitudinal ovality (ΔDE) of the end portion of the steel pipe and the longitudinal ovality (ΔDC) of the central portion of the steel pipe is used while ensuring the outer diameter tolerance for securing the function of the steel pipe. The area of the product obtained by the method of controlling the longitudinal ovality of the central part of the steel pipe within a certain range and ensuring that the longitudinal ovality of the end of the steel pipe after cutting the steel pipe is low is shown as the area in FIG. 12 . PP, a more preferred region is shown as region ZZ in FIG. 13 . In addition, in the case where the steel pipe and the steel pipe are connected and used by being fitted to the end of the steel pipe via one or more jigs, the area YY does not need to be considered.

Similar to thread cutting, if the range of the shape of the pipe end to be ensured when fitting is set as the region XX, the region XX is the same as the region AA, and if expressed by the formula, the following formula (32) and formula are satisfied at the same time. (x, y) of (33) is the area XX (= area AA).

DCave×(－2/100)≤x≤DCave×(2/100) (32)

DEave×(－2/100)≤y≤DEave×(2/100) (33)

This is represented by coordinates in FIG. 12 , and is an inner area of a line connecting the following points X1 , X2 , X3 , and X4 , and is represented by the following equations (24) to (31).

Point X1: x(X1)=DCave×(2/100) (24)

y(X1)=DEave×(2/100) (25)

Point X2: x(X2)=DCave×(2/100) (26)

y(X2)=DEave×(-2/100) (27)

Point X3: x(X3)=DCave×(-2/100) (28)

y(X3)=DEave×(-2/100) (29)

Point X4: x(X4)=DCave×(-2/100) (30)

y(X4)=DEave×(2/100) (31)

When the steel pipe and the steel pipe are connected and used by being fitted to the steel pipe end portion via one or more jigs, the relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion described above is obtained when manufacturing is performed. ΔDC and ΔDE, including regions WW in FIGS. 12 and 13 , which include deviations, are the same as those described above, and (x, y) satisfying the following formula (3) is the region WW.

x+K－3×SD≤y≤x+K+3×SD (3)

In FIG. 12 , as the relationship between the longitudinal ovality of the steel pipe central portion and the steel pipe end portion described above, the longitudinal ovality of the steel pipe central portion is controlled within a certain range during pipe manufacturing, and the steel pipe end portion after cutting the steel pipe is cut. The area PP of the area|region of the area|region of the product obtained by the method of ensuring the longitudinal ovality to be low is the part where the area|region XX and the area|region WW overlap. If it is represented by the formula, (x, y) satisfying the following formula (32), formula (33) and formula (3) at the same time is the region PP.

DCave×(－2/100)≤x≤DCave×(2/100) (32)

DEave×(－2/100)≤y≤DEave×(2/100) (33)

x+K－3×SD≤y≤x+K+3×SD (3)

This is represented by coordinates in FIG. 12 , and is the inner area of the line connecting the point X1 , the point P1 , the point Z3 , the point X3 , the point P2 , the point Z1 , and the point X1 . in,

Point P1: The intersection of the line passing through X1 and X2 and the line WL.

Point P2: The intersection of the line passing through X4 and X3 and the line WH.

Point Z1: The intersection of the line passing through X4 and X1 and the line WH.

Point Z3: The intersection of the line passing through X3 and X2 with the line WL.

Next, FIG. 13 shows the case where the steel pipe is fitted to the steel pipe end through one or more jigs and used in conjunction with the steel pipe. Considering the variation in production, a more preferable area ZZ of the area XX can be stably secured. . The idea is the same as above, but the y-component of the region XX is different,

y(X1)=y(X4)=DEave×(2/100) (25) and (31)

y(X2)=y(X3)=DEave×(-2/100) (27) and (29)

Therefore, if expressed by the formula, (x, y) satisfying both the following formula (74) and formula (3) is the region ZZ.

DEave×(－2/100)－K+3×SD≤x

≤DEave×(2/100)－K－3×SD (74)

x+K－3×SD≤y≤x+K+3×SD (3)

Represented by coordinates in FIG. 13 , the region ZZ is a region that satisfies the region XX and is surrounded by a line connecting the following four points, namely, the point Z1, the point Z2, the point Z3, and the point Z4, and is represented by the following equations (75) to ( 82) said.

Point Z1: The intersection of the line passing through X4 and X1 and the line WH.

x(Z1)=y(X1)－K－3×SD=DEave×(2/100)－K－3×SD (75)

y(Z1)=y(X1)=DEave×(2/100) (76)

Point Z2: intersection of x=x(Z1) with line WL.

x(Z2)=x(Z1)=y(X1)-K-3×SD

＝DEave×(2/100)－K－3×SD (77)

y(Z2)=x(Z1)+K－3×SD=DEave×(2/100)－6×SD (78)

Point Z3: The intersection of the line passing through X3 and X2 with the line WL.

x(Z3)=y(X3)－K+3×SD=DEave×(－2/100)－K+3×SD (79)

y(Z3)=y(X3)=DEave×(-2/100) (80)

Point Z4: intersection of x=x(Z3) with line WH.

x(Z4)=x(Z3)=y(X3)－K+3×SD

＝DEave×(-2/100)－K+3×SD (81)

y(Z4)=x(Z3)+K+3×SD=DEave×(-2/100)+6×SD (82)

Next, the manufacturing method of the high-strength electric resistance welded steel pipe of this embodiment is demonstrated.

The hot-rolled steel sheet used for the high-strength electric-resistance-welded steel pipe is produced by heating and hot-rolling the steel having the above-mentioned components, followed by controlled cooling and coiling.

The heating temperature of the steel is preferably 1150° C. or higher in order to make a carbide-forming element such as Nb dissolve in the steel as a solid solution. On the other hand, in order to obtain a fine-grained structure, it is preferably 1000°C to 1280°C. If the heating temperature is too high, the austenite grains become coarse, and as a result, the grain size of the ferrite becomes coarse, so it is preferably 1280° C. or lower.

In order not to generate ferrite during rolling, the finish rolling temperature of hot rolling is preferably 850°C or higher.

If the coiling temperature exceeds 300°C, sufficient strength may not be secured, so it is preferably 300°C or lower. More preferably, it is 150°C or lower.

Next, the obtained hot-rolled steel sheet was continuously formed into an open pipe by roll forming, and then the ends of the open pipe were butted against each other and electric-resistance welded to produce an electric-resistance-welded steel pipe. Seam heat treatment for heating and accelerated cooling of the electric welding part can be performed. Then, diameter reduction processing to reduce the outer diameter of the steel pipe by 0.5% to 4.0% can be performed with a sizing machine.

An example of the manufacturing process of the electric resistance welded steel pipe is shown in FIG. 14 . Electric-resistance-welded steel pipes are produced by cold working with multiple roll stands, including a forming step of bending the steel sheet to form a C-section, a welding step of electric-welding the pipe ends, a straightening step of reducing the diameter of the pipe to adjust the shape, and using a cutting machine. A cutting process for cutting a steel pipe to a desired level. The AA' section is the gantry position of the welding process, the BB' section is any gantry position with one or more straightening steps, and the CC' section is the roll at the final stage of the straightening step. The cross section of any position at a position farther from the cutting position than the position Le between the center position and the cut steel pipe end, the DD' cross section is the steel pipe end. In addition, the pipe width and pipe height in each section are set to Ah, Av, Bh, Bv, D1 (steel pipe center), D3 (pipe center), D1 (steel end), D3 (steel end), respectively. (mm). The tube width is the distance between the outer surfaces of the tube between 90° and 270°, and the tube height is the distance between the outer surfaces of the tube between 0° and 180° when the electric welding portion is set to the 0° position.

In order to manufacture such that ΔDC becomes an appropriate value, the upper, lower, and width rolls of the welding table are appropriately adjusted so that the tube width Ah and the tube height Av of the AA' cross-section become appropriate values, or the final stage of the leveling table is appropriately adjusted. The upper, lower and width rollers can be adjusted so that the tube width Bh and tube height Bv of the BB' section can be set to appropriate values. When the cold working at the time of straightening is minimized in consideration of the toughness and corrosion resistance of the steel pipe, it is preferable to manufacture in the former method. In addition, in the case of work hardening the steel pipe to further increase the strength, it is preferable to manufacture in the latter method. In addition, the manufacturing process of electric resistance welded steel pipe is not limited to the example of FIG. 14, since the number of objects, the number of stages, and the shape of a roll differ, the manufacturing conditions which satisfy the conditions of this invention were searched for each facility.

In the above description, in the method in which the steel pipe and the steel pipe are connected to the steel pipe end through one or more jigs, the fitting part also includes welding, bonding, or mechanical joining (for example, threading, using When the steel pipe and the jig are firmly joined, such as fitting of material elasticity, pin fixing, etc. Furthermore, "clamp" refers to a coupling or nipple, which does not directly cut threads on the steel pipe, but joins the coupling or nipple to the steel pipe by welding or mechanical joining.

In addition, the length of the high-strength electric resistance welded steel pipe according to the present invention is preferably 2000 mm to 5000 mm as described above, and more preferably 3000 mm to 3500 mm, which is generally used.

Next, the composition of the high-strength electric resistance welded steel pipe according to the present embodiment will be described.

Hereinafter, when each element is simply referred to as "content", it refers to the content in the steel pipe.

As described above, the steel pipe of the present embodiment contains, in mass % or mass ppm, C: 0.04 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, P: 0.030% or less, and S: 0.030% Below, Al: 0.005 to 0.700%, N: 100 ppm or less, Nb: 0 to 0.100%, V: 0 to 0.100%, Ti: 0 to 0.200%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Cr : 0 to 1.000%, Mo: 0 to 1.000%, B: 0 to 50 ppm, Ca: 0 to 100 ppm, REM: 0 to 200 ppm, and the balance is iron and impurities.

Hereinafter, each element, content, and impurities will be described.

<C: 0.04 to 0.30%>

C (carbon) is an element effective for improving the strength of the steel pipe.

The content of C in the steel pipe of the present invention is 0.04% or more. Thereby, the strength of the hot-rolled steel sheet can be secured, and as a result, the strength of the steel pipe can be secured.

On the other hand, when the content of C is too large, the strength of the steel pipe becomes too high, and the toughness deteriorates. Therefore, the upper limit of the content of C is 0.30%. The upper limit of the content of C is preferably 0.25%, and more preferably 0.20%.

<Si: 0.01 to 2.00%>

Si (silicon) is effective as a deoxidizer.

However, when the content of Si is too large, the low-temperature toughness is impaired, and further the electric weldability is impaired. Therefore, the upper limit of the content of Si is 2.00%. The content of Si is preferably 1.20% or less, and more preferably 0.60% or less.

On the other hand, in order to obtain the effect as a deoxidizer more effectively, the content of Si is 0.01% or more. In addition, the content of Si is preferably 0.10% or more, and more preferably 0.20% or more, from the viewpoint of further increasing the strength of the steel pipe by solid solution strengthening.

<Mn: 0.50 to 3.00%>

Mn (manganese) is an element that increases the strength of steel by improving the hardenability of steel.

From the viewpoint of securing high strength, the content of Mn (manganese) in the steel pipe of the present invention is 0.50% or more. The content of Mn is preferably 0.80% or more.

However, if the content of Mn is too large, the formation of martensite is promoted, and the toughness is deteriorated. Therefore, the upper limit of the content of Mn is 3.00%. In order to obtain higher toughness, the upper limit is preferably 2.00%.

<P: 0.030% or less>

P (phosphorus) is an impurity.

Since the toughness is improved by reducing the content of P, the upper limit of the content of P is 0.030%. The content of P is preferably 0.020% or less.

The lower the content of P, the better, so the lower limit of the content of P is not particularly limited. However, the content of P is usually 0.001% or more from the viewpoint of the balance between properties and cost.

<S: 0.030% or less>

S (sulfur) is an impurity.

By reducing the content of S, MnS elongated by hot rolling can be reduced and toughness can be improved, so the upper limit of the content of S is 0.030%. The content of S is preferably 0.020% or less, and more preferably 0.010% or less.

The lower the content of S, the better, so the lower limit of the content of S is not particularly limited. However, the content of S is usually 0.001% or more from the viewpoint of the balance between properties and cost.

<Al: 0.005 to 0.700%>

Al (aluminum) is an element effective as a deoxidizer.

However, if the content of Al is too large, inclusions increase and ductility and toughness are impaired. Therefore, the upper limit of the content of Al is 0.700%.

On the other hand, in order to obtain the effect as a deoxidizer more effectively, the content of Al is 0.005% or more. In order to reduce inclusions and obtain higher ductility and toughness, the upper limit is preferably 0.100% or less.

<N: 100ppm or less>

N (nitrogen) is an element inevitably present in steel.

However, when the content of N is too large, inclusions such as AlN increase excessively, and there is a possibility that defects such as surface damage and deterioration of toughness may occur. Therefore, the upper limit of the N content is 100 ppm. The content of N is preferably 80 ppm or less, particularly preferably 60 ppm or less.

On the other hand, the lower limit of the content of N is not particularly limited, but the content of N is preferably 10 ppm or more in consideration of the cost and economy of de-N (denitrogenation).

<Nb: 0 to 0.100%>

Nb (niobium) is an element that lowers the recrystallization temperature, suppresses recrystallization of austenite during hot rolling, and contributes to the refinement of the structure.

However, when the content of Nb is too large, the toughness is deteriorated by coarse precipitates. Therefore, the upper limit of the content of Nb is 0.100%. The content of Nb is preferably 0.06% or less, and more preferably 0.05% or less.

On the other hand, the content of Nb is preferably 0.010% or more, and particularly preferably 0.020% or more, from the viewpoint of obtaining the effect of reducing the structure more reliably.

<V: 0 to 0.100%>

V (vanadium) is an element that forms carbides and nitrides and improves the strength of steel by precipitation strengthening.

However, if the content of V is too large, the carbides and nitrides will become coarse, and the toughness may be deteriorated. Therefore, the content of V is 0 to 0.100%. The content of V is more preferably 0.060% or less.

On the other hand, from the viewpoint of further increasing the strength of the steel pipe, the content of V is preferably 0.010% or more.

<Ti: 0 to 0.200%>

Ti (titanium) is an element that forms fine nitrides (TiN), suppresses the coarsening of austenite grains during heating of the slab, and contributes to the refinement of the structure.

However, if the content of Ti is too large, the coarsening of TiN or precipitation hardening by TiC may occur, and the toughness may be deteriorated. Therefore, the content of Ti is 0 to 0.200%. The content of Ti is more preferably 0.100% or less, and particularly preferably 0.050% or less.

On the other hand, the content of Ti is preferably 0.010% or more, and more preferably 0.015% or more, from the viewpoint of further improving the toughness through the refinement of the structure.

<Ni: 0 to 1.000%>

Ni (nickel) is an element that increases the strength of steel by improving the hardenability of steel. In addition, Ni is also an element which contributes to the improvement of toughness.

However, since Ni is an expensive element, the content of Ni is 0 to 1.000% from the viewpoint of economical efficiency. The content of Ni is more preferably 0.500% or less.

On the other hand, from the viewpoint of further improving toughness, the content of Ni is preferably 0.100% or more.

<Cu: 0 to 1.000%>

Cu (copper) is an element that increases the strength of steel by improving the hardenability of steel. In addition, Cu is also an element which contributes to solid solution strengthening.

However, if the content of Cu is too large, the surface properties of the steel pipe may be impaired. Therefore, the content of Cu is 0 to 1.000%. The content of Cu is more preferably 0.500% or less.

On the other hand, the content of Cu is preferably 0.100% or more.

Furthermore, when the steel pipe contains Cu, it is preferable to contain Ni at the same time from the viewpoint of preventing deterioration of the surface properties.

<Cr: 0 to 1.000%>

Cr (chromium) is an element effective for improving strength.

However, if the content of Cr is too large, the weldability may be deteriorated, so the content of Cr is 0 to 1.000% or less. The content of Cr is more preferably 0.500% or less.

On the other hand, from the viewpoint of further improving the strength of the steel pipe, the content of Cr is preferably 0.100% or more.

<Mo: 0 to 1.000%>

Mo (molybdenum) is an element which contributes to the high strength of steel.

However, Mo is an expensive element, so the content of Mo is 0 to 1.000% from the viewpoint of economy. The content of Mo is more preferably 0.500% or less, and particularly preferably 0.300% or less.

On the other hand, the content of Mo is preferably 0.050% or more.

<B: 0 to 50ppm>

B (boron) is an element which remarkably improves the hardenability of steel and contributes to the enhancement of the strength of steel when contained in a trace amount.

However, even if B is contained in an amount exceeding 50 ppm, the hardenability cannot be further improved, and there is a possibility that a precipitate may be formed and the toughness may be deteriorated, so the upper limit of the content of B is 50 ppm. On the other hand, B may be mixed from raw material impurities. In order to sufficiently obtain the effect of hardenability, the content of B is preferably 3 ppm or more.

<Ca: 0 to 100ppm>

Ca (calcium) is an element that controls the form of sulfide-based inclusions, improves low-temperature toughness, and further refines oxides in the welded portion, thereby improving the toughness of the welded portion.

However, if the content of Ca is too large, oxides and sulfides may become larger, which may adversely affect toughness. Therefore, the content of Ca is 0 to 100 ppm.

On the other hand, the content of Ca is preferably 10 ppm or more.

＜REM: 0～200ppm＞

In this specification, "REM" refers to rare earth elements, which are composed of Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (Samarium), Eu (Europium), Gd (Gadolinium), Tb (Terbium), Dy (Dysprosium), Ho (Holmium), Er (Erbium), Tm (Thulium), Yb (Ytterbium) and Lu (Li) The general term for 17 elements.

In addition, "REM: 0-200 ppm" means that at least one of the above-mentioned 17 elements is contained, and the total content of these 17 elements is 200 ppm or less.

REM is an element that controls the form of sulfide-based inclusions, improves low-temperature toughness, and further refines oxides in the welded portion to improve the toughness of the welded portion.

However, if the content of REM is too large, oxides and sulfides may become larger, and the toughness may be adversely affected. Therefore, the content of REM is 0 to 200 ppm.

On the other hand, the content of REM is preferably 10 ppm or more.

＜Impurities＞

In the present invention, an impurity means a component contained in a raw material or a component mixed in a production process, and is not a component intentionally contained in steel.

Specific examples of impurities include O (oxygen), Sb (antimony), Sn (tin), W (tungsten), Co (cobalt), As (arsenic), Mg (magnesium), Pb (lead), Bi (bismuth), H (hydrogen).

Among them, the content of O is preferably controlled to be 0.004% or less.

The usage method of the high-strength electric resistance welded steel pipe for foundation stabilization works of this invention is demonstrated.

In the method of using a high-strength electric-resistance welded steel pipe for a foundation stabilization project of the present invention, a new steel pipe end portion formed by cutting the central portion of the high-strength electric-resistance welded steel pipe is threaded, and two or more high-strength electric-resistance welded steel pipes are welded with a threaded joint. Steel pipe connection is used.

Further, in the method of using the high-strength electric-resistance-welded steel pipe for a foundation stabilization project of the present invention, the steel pipe is used for a new steel-pipe end portion formed by cutting one or both of the steel pipe ends of the high-strength electric-resistance-welded steel pipe at the center of the steel pipe. The ends are fitted to each other via one or more jigs, and two or more high-strength electric resistance welded steel pipes are connected and used.

Example

After heating the steel slab of the composition shown in the table of Examples to 1050°C or higher, rough rolling is performed at the recrystallization temperature or higher, and then finishing rolling is performed at Ar3°C or higher and 950°C or lower with a cumulative reduction of 65% or higher. The steel plate cooled from Ar3°C or higher is cold-formed into a hollow state in a pipe-making facility having a forming process, a welding process, and a straightening process, and then resistance welding is performed to produce a tensile strength of 590 N/mm ² The above high-strength steel pipe is subjected to "joining after thread cutting" or "fitting via a jig" to a new steel pipe end which is cut at the center of the steel pipe after pipe production.

In addition, the tensile strength is obtained by taking a full-thickness test piece from the base material portion of the heat-treated steel pipe in the pipe axis direction, and performing a tensile test in the pipe axis direction.

In the table of Examples, the conditions and results of Examples and Comparative Examples under each condition are shown. In each table, "G" of each area indicates that each area can be satisfied, and "NG" of each area indicates that each area cannot be satisfied.

table 3

Table 4 x = WW value of ΔDC

Table 7

Table 8 x = WW value of ΔDC

If the area AA cannot be satisfied, the necessary outer diameter tolerance cannot be secured at the ends of the steel pipe and the center of the steel pipe, which can be determined by measuring the outer diameter of the steel pipe. In this case, the required circular shape cannot be ensured when used as a structural pipe, so the required bending moment or bending resistance cannot be ensured, deformation and bending may occur during use, and the required function as a structural pipe cannot be satisfied.

If the area YY is not satisfied, the necessary residual material cannot be secured as the thread, and the thread may be deformed during threading, and the connection failure may occur during use, etc., and the function of the thread cannot be secured. This can be determined by dimensional measurement or visual inspection with a thread gauge or the like. In addition, since the necessary residual material cannot be secured as the pipe body, the strength of the joint cannot be secured, and deformation and breakage such as bending of the joint portion may occur during use, and the function for the original application cannot be secured. This can be determined by visual inspection.

When the area XX cannot be satisfied, that is, when one or both of the area AA and the area YY cannot be satisfied, there will be a problem that cannot be satisfied, respectively.

If the region WW cannot be satisfied, the work result will deviate from the relationship between the longitudinal ovality of the steel pipe center portion and the steel pipe end portion obtained in the present invention, so that accurate forming cannot be performed. This means that due to local defects in the shape of the product, equipment abnormalities, etc., the manufacturing cannot be carried out correctly, and a certain quality cannot be obtained in the manufacturing batch, so it cannot be used as a product. This can be determined by visual inspection of the product, inspection of the equipment. In addition, if the region WW cannot be satisfied, the shape of the steel pipe required for threading cannot be formed because it cannot be accurately formed, so that deformation during threading may occur, and connection failure during use may occur, and it cannot be ensured. function of the thread. In addition, since the outer diameter tolerance cannot be guaranteed to be a constant value in a manufacturing lot, these cannot be satisfied. In order to perform threading without deformation and to secure the outer diameter tolerance, securing the area WW is a prerequisite.

As the evaluation of the steel pipes in the examples in Table 1, the threading conditions and the securing of the steel pipe outer diameter tolerance are shown. Ensuring the outer diameter tolerance of the steel pipe means that both the steel pipe end and the center of the steel pipe satisfy the outer diameter tolerance.

In the threading state, good threading can be performed when the condition for accurate forming and ensuring a certain quality as a steel pipe product, namely the area WW, and the condition for securing the necessary residual material as the thread, namely the area YY, are satisfied. Regarding the assurance of the tolerance of the outer diameter of the steel pipe, the outer diameter tolerance of the steel pipe can be ensured when the conditions for correct molding and ensuring a certain quality as a steel pipe product, that is, the area WW, and the conditions for ensuring the outer diameter tolerance, that is, the area AA are satisfied at the same time. .

When the area PP cannot be satisfied, that is, when either one or both of the area XX and the area WW cannot be satisfied, there will be a problem that the area cannot be satisfied, respectively. Only if the area XX is not satisfied, the necessary residual material cannot be secured as the thread, and the thread may be deformed during threading, and the connection failure may occur during use, etc., and the function of the thread cannot be secured.

Only if the area WW is not satisfied, the shape of the steel pipe required for threading cannot be formed because it cannot be properly formed, so it may deform during threading, and may cause poor connection during use, etc., and the thread cannot be secured. function. At the same time, since the outer diameter tolerance cannot be guaranteed to be a certain value in the manufacturing lot, the outer diameter tolerance cannot be satisfied either. If both the area XX and the area WW are not satisfied, the residual material required for threading during threading cannot be secured, and deformation during threading may occur. In addition, since the desired shape of the steel pipe cannot be formed, deformation may occur during threading, and due to both reasons, connection failure occurs during use, etc., and the function of the thread cannot be ensured. Similarly, since the outer diameter tolerance cannot be guaranteed to be a constant value in a manufacturing lot, the outer diameter tolerance cannot be satisfied either.

Zone ZZ is the scope of a better embodiment, even if it deviates from zone ZZ, as long as it is within the scope of zones XX and WW, it is an embodiment.

In addition, No. 2 and No. 31 of the comparative example of Table 1 are demonstrated. In these comparative examples, the outer diameter tolerance of the steel pipe was "bad", but the threading condition was "good". This is the case where the outer diameter tolerance is not satisfied at the center of the steel pipe, but is satisfied at the end of the steel pipe. In these embodiments, since the regions WW and YY are satisfied, thread cutting can be performed. However, since the outer diameter tolerance of the central part of the steel pipe was not satisfied, the outer diameter tolerance was "defective", and the necessary function as a structural pipe was not satisfied, so it could not be used as a product and a comparative example.

The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to these examples. Those who know the common knowledge in the technical field to which the present invention pertains can obviously think of various modifications or amendments within the scope of the technical idea described in the claims, and these should of course be understood as belonging to the technical scope of the present invention.

industry availability

According to the present invention, it is possible to provide a high-strength electric-resistance-welded steel pipe with high roundness and a high-strength electric-resistance-welded steel pipe for ground stabilization work, which is lightweight and high-strength, and has a high roundness at the end of the steel pipe produced by fresh cutting after pipe production. Thereby, the industrial applicability is large.

Claims (6)

1. a high-strength electric-welded steel pipe, characterized in that, In mass % or mass ppm, containing C: 0.04 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.700%, N: 100ppm or less, Nb: 0 to 0.100%, V: 0 to 0.100%, Ti: 0 to 0.200%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, B: 0～50ppm, Ca: 0～100ppm and REM: 0～200ppm, The balance consists of iron and impurities, When DCave is 60.3 mm or more and 318.5 mm or less, tCave/DCave is 0.02 or more and 0.06 or less, and tensile strength is 590 N/mm ² or more, when the central part of the steel pipe is cut, the following formula is satisfied: DCave×(-2/100)≤x≤DCave×(2/100) (1), YN≤y≤YM (2), x+K－3×SD≤y≤x+K+3×SD (3), YM=MIN[{DEave×(2/100)}, {4×((tEave/3)－0.65)}] (4), Equation (4) takes the smaller of {DEave×(2/100)} and {4×((tEave/3)−0.65)} as YM, YN=MAX[{DEave×(-2/100)}, {-4×((tEave/3)-0.65)}] (5), In formula (5), the larger one of {DEave×(−2/100)} and {−4×((tEave/3)−0.65)} is used as YN, K={α+(β/I)+(γ×TS)}×DCave (6),

Standard deviation of the outer diameter of the central part of the steel pipe={p+(q/I)+(r×TS)}×DCave(8), Where, x: the longitudinal ovality of the central part of the steel pipe, y: the longitudinal ovality of the end of the steel pipe, DCave: the average outer diameter of the central part of the steel pipe after pipe making and before cutting, in mm, tCave: after pipe making and cutting The average wall thickness of the steel pipe at the center of the front steel pipe, in mm, DEave: the average outer diameter of the steel pipe end after pipe making and cutting, in mm, tEave: after pipe making and cutting the steel pipe end Average wall thickness, in mm, TS: tensile strength of the base metal part of the high-strength electric-resistance welded steel pipe, in N/mm ² , α, β, γ are constants, α=-1.87× ^10-3 (9), β=1.35×10 ⁴ (10), γ=-6.65× ^10-6 (11), I is the secondary distance of the section at the central part of the steel pipe, the unit is mm ⁴ , I=π/64×{(DCave) ⁴ −(DCave−2×tCave) ⁴ } (12), p, q, r are constants, p=1.39× ^10-3 (13), q=4.17×10 ² (14), r=6.05×10 ⁻⁷ (15). 2. The high-strength electric-welded steel pipe according to claim 1, characterized in that, The tensile strength is 780 N/mm ² or more. 3. The high-strength electric-resistance welded steel pipe according to claim 1 or 2, characterized in that, It also satisfies the following formula: YN-K+3×SD≤x≤YM-K-3×SD (17). 4. The high-strength electric-resistance welded steel pipe according to claim 1 or 2, characterized in that, It also satisfies the following formula: DEave×(－2/100)－K+3×SD≤x ≤DEave×(2/100)－K－3×SD (18). 5. A method of using high-strength electric-welded steel pipe for foundation stabilization engineering, characterized in that, The high-strength electric-resistance-welded steel pipe according to claim 1 or 2 is cut at the center of the steel pipe, threaded at the end of the resulting new steel pipe, and used by connecting two or more high-strength electric-resistance-welded steel pipes with a threaded joint. 6. A method of using high-strength electric-welded steel pipe for foundation stabilization engineering, characterized in that, One or both of the steel pipe ends of the high-strength electric-resistance welded steel pipe according to claim 1 or 2 and a new steel pipe end produced by cutting at the center of the steel pipe, and the steel pipe ends are connected to each other via one or more jigs Fitting is used by connecting two or more high-strength electric resistance welded steel pipes.

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