JP4535204B1 - Forged steel pipe with excellent flare workability - Google Patents

Abstract

A forged steel pipe having excellent flare workability is provided.
SOLUTION: The outer diameter is 21.7 mmφ to 114.3 mmφ, the wall thickness is 1.8 mm to 6.1 mm, the tensile strength (TS) is 290 N / mm ^{2 to} 500 N / mm ² , and the elongation is 23% to 90%. The pipe expansion ratio (A) when cracking occurred in the part was limited to forged steel pipes satisfying the following formula <1>. A = {H × tan (θ / 2) + (DR / 2)} / (DR / 2) ≧ −0.0004 × DR + 1.3 = B ... <1>, where A: cone expansion ratio, H: cone push-in Amount (difference between the pipe expansion start height and the height immediately after cracking after expansion), θ: full cone angle, DR: forged steel pipe outer diameter, B: specified value determined from steel pipe outer diameter [selection figure] Fig. 2

Description

  The present invention relates to a forged steel pipe excellent in flare workability in which cracks are hardly generated in a joint even when subjected to strong work such as flare work.

  When connecting steel pipes when laying water pipes or gas pipes, it is conventional to use a short steel pipe that has an inner diameter that is approximately the same as the outer diameter of the steel pipe. However, it took time and effort to connect the steel pipe with a short steel pipe, and it was necessary to prepare a short steel pipe separately and screw it to the internal diameter. It was costly.

Therefore, in recent years, a method of connecting steel pipes simply and efficiently has been started by performing so-called flare processing that expands the end of the steel pipe and making the flare part an integral flange with the steel pipe.
Since this flaring is a strong process that pulls out the end of the steel pipe, the ERW steel pipe is applied as having the ability to withstand this strong process, but since the ERW steel pipe is expensive, it is an inexpensive forged steel pipe. Is being applied.

Conventional forged welded pipes have low joint strength, and cracking is likely to occur at the joint when starting strong, so it has been said to have insufficient performance for applications such as flaring materials. It was.
As shown in FIG. 1, forged steel pipes are manufactured by forming a slit steel strip 2 with an edge forming machine 4, heating it with a heating furnace 5, and continuously forming the steel strip into a tubular shape with a forming forging machine 6. On the other hand, oxygen and / or air is blown to the abutting end (steel band width end) with a nozzle 7 and heated by oxidation heat, joined by forging, or further drawn and rolled into the forged steel joint 8. Finished. In addition, although not shown in figure, edge shaping | molding may be carried out after cutting the edge (width end part) of the slit steel strip.

  The manufactured forged steel pipes are prone to oxides remaining in the joints, and streaks are generated on the outer and inner surfaces of the joints, causing cracks in the joints during strong processing such as flaring. It was. The streaks on the outer surface side of the joint are the ones that occurred at the edge of the slit steel strip before heating and remained during forging. Further, the inner surface side muscles are raised near the abutting end portion at the time of forging to form a bead portion, and this valley is a muscle.

  Therefore, conventionally, as shown in Patent Literatures 1 to 3, the outer surface side muscle depth, the inner surface side bead height, the inner surface side muscle depth, the inclusions in the joint portion, and the like are specified in the joint portion. By restricting to the range, a forged steel pipe that has improved the strength of the joint has been provided.

JP 2007-152430 A Japanese Patent Laid-Open No. 10-263846 JP-A-4-270009

  This invention solves the said subject, and provides the forge-welded steel pipe which hardly breaks from a junction part, even if it performs strong processing like flare processing, The summary structure is as follows.

(1) Outer diameter 21.7mmφ-114.3mmφ, Wall thickness 1.8mm-6.1mm, Tensile strength (TS) 290N / mm ² -500N / mm ² , Elongation 23% -90% The pipe expansion ratio (A) when cracking occurs satisfies the following formula <1>, the ratio of the joint thickness to the steel pipe thickness is 1.00 or more, and the ratio of the maximum bead thickness to the steel pipe thickness is A wrought forged steel pipe for flaring characterized by being 1.010 or more and the bead portion maximum wall thickness being larger than the joint wall thickness .
A = {H × tan (θ / 2) + (DR / 2)} / (DR / 2) ≧ −0.0004 × DR + 1.3 = B... <1>
However, A: Cone expansion ratio H: Cone push-in amount (difference between the expansion start height and the height immediately after cracking after expansion)
θ: full-width cone
DR: Forged steel pipe outer diameter B: Specified value determined from steel pipe outer diameter

  The forged welded steel pipe of the present invention has the effect of greatly improving the product yield of strong processing such as flaring because it hardly generates cracks even when subjected to strong processing such as flaring.

Schematic showing an example of manufacturing process of forged steel pipe Schematic showing an example of cone expansion test Schematic showing an example (part 1) of the cross-sectional shape around the joint of a forged steel pipe Schematic showing an example (part 2) of the cross-sectional shape around the joint of a forged steel pipe

The forged steel pipe has an outer diameter of 21.7 mmφ to 114.3 mmφ and a wall thickness of 1.8 mm to 6.1 mm. The material is a tensile strength (TS) of 290 N / mm ^{2 to} 500 N / mm ² and an elongation of 23. % To 90%. In addition, the tensile strength (TS) and elongation rate which are materials may be measured by any of JIS No. 11 test, JIS No. 12 test, JIS No. 5 test, JIS No. 1 test and the like.
When a forged steel pipe is subjected to strong processing such as flaring, it has been a problem because cracks have conventionally occurred in the joint. Therefore, it is extremely reliable to check whether the joint is good at the forged steel pipe manufacturing stage. It is necessary to secure.

  Therefore, a cone tube expansion test was adopted as a method for evaluating the strength of the joint. That is, since the flare process is a process of expanding the end of the steel pipe including the joint, an evaluation method for applying tension to the joint of the steel pipe is also required in the evaluation test. Therefore, evaluation was performed by a test in which tension was applied to the joint by cone expansion. In the cone expansion test, as shown in FIG. 2, the cone 9 as a tube expansion jig is fixed to the base and the forged steel pipe 8 is pushed in from the top to expand, or the forged steel pipe is fixed and the cone is pushed in from the top. As a result, the pipe is expanded, and tension is applied to the joint. Therefore, the tension applied to the welded steel pipe joint by flaring can be replaced by the cone expansion test, and crack evaluation is possible.

  Therefore, first, forged welded steel pipes that were subjected to flare processing and cracks did not occur, a raw pipe part adjacent to the flare processed part was collected, and a cone expansion test of the raw pipe was performed to observe cracks in the joint part. As a result, if the cone full angle (θ) is changed, if the full angle (θ) is small, increasing the cone push-in amount (H) will cause cracks in the joint, and if the full angle (θ) is large, the cone push-in amount Even if (H) was small, cracks occurred in the joint.

As a result of investigation by the present inventors, the crack of the joint due to the cone expansion occurs from the end of the forged steel pipe as in the case of flare processing, so the ratio of the circumferential length of the expanded pipe and the original pipe is large. As a result, it was found that the following formula <1> can be arranged using the specified value B determined from the pipe outer diameter regardless of the wall thickness of the forged steel pipe and the cone shape of the expanded pipe.
A = {H × tan (θ / 2) + (DR / 2)} / (DR / 2) ≧ −0.0004 × DR + 1.3 = B... <1>
However, A: Cone expansion ratio H: Cone push-in amount (difference between the expansion start height and the height immediately after cracking after expansion)
θ: full-width cone
DR: Forged steel pipe outer diameter B: Specified value determined from steel pipe outer diameter Therefore, if the expansion ratio (A) in the cone expansion test of the manufactured forged steel pipe enters the region that satisfies the above formula <1>, it is used for flare processing. It was found that no cracks occurred when

  On the other hand, the reason why joints are easily cracked when performing strong processing such as flaring on forged steel pipes is that oxides, etc. are likely to remain in the joints during the manufacture of forged steel pipes. This was thought to be due to the formation of streaks. That is, it has been considered that the oxide remaining in the joint and the streaks on the inner and outer surfaces of the joint are likely to be the starting point of cracking. Therefore, based on the cause of the breakage of these joints, as described in Patent Documents 1 to 3, in the past, there is provided a forged steel pipe that reduces the inclusions in the joints and reduces the streaks on the outer surface or the inner surface. Has been made. However, if these are only appropriate, sufficient joint strength cannot be obtained, which is a problem.

Therefore, the inventors paid attention to the thickness of the joint portion 13 shown in FIG. When the conventional welded steel pipes with low joint strength are observed in detail, the joint thickness 14 is short, so the joints are resistant to strong tension acting in the circumferential direction of the steel pipe in strong processing such as flaring. It was found that the strength of the was insufficient.
When the strength of the joint portion is described in more detail, it is understood that the joint strength is lowered when the joint thickness 14 is shorter than the steel pipe thickness 15, and the joint strength is improved when the joint thickness 14 is longer. That is, in strong processing such as flaring, the steel pipe end and its periphery are expanded in the circumferential direction of the steel pipe while being expanded. At that time, excessive tension acts in the circumferential direction of the steel pipe at and around the end of the steel pipe. This tension tends to concentrate on the portion where the thickness of the steel pipe is thin. For this reason, when the thickness of the joint is thin, that is, when the thickness of the joint is shorter than the thickness of the steel pipe, stress is concentrated on the joint and it is easy to break.

In this onset bright, and longer than the thickness i.e. the joint thickness 14 steel pipe wall thickness 15 of the joint 13 shown in FIG. 4, i.e., 1.00 or more the ratio pipe wall thickness 15 of the joint thickness 14 and more and child, and can relax the stress concentration due to excessive tension in the large deformation such as flaring, is possible to prevent the cracking during flaring even when close to the a and B represented by formula <1> it can.

Further, the stress concentration that leads to the cracking of the joint acts not only on the interface of the joint but also on the periphery thereof. Therefore, attention is paid not only to the joint thickness 14 shown in FIGS. 3 and 4, but also to the bead portion 16 generated in the periphery thereof. The bead portion 16 is a portion where the abutting end portion swells at the time of joining. In the case of a forged steel pipe, the bead portion 16 tends to swell mainly on the inner surface side, but may swell slightly on the outer surface side. If the bulge of the bead part 16 is large, the joint thickness 14 is also likely to increase, the tension per unit area of the cross section is reduced, the stress concentration is relaxed, and the concentration of excessive tension at the joint part is mitigated. be able to. Therefore, by increasing the thickness of not only the joint portion but also the bead portion around the joint portion, stress concentration around the joint portion in strong processing such as flare processing can be further relaxed and cracking can be prevented. The present inventors have been quantitatively studied, the ratio of the maximum thickness 17 and the steel pipe wall thickness 15 of the bead portion shown in FIG. 4 at 1.010 or higher and the child, is A represented by formula <1> B It was understood that the occurrence of cracks during flare processing can be sufficiently prevented even when very close to.

Note that the steel pipe wall thickness 15 may be the average wall thickness in the circumferential direction of the forged steel pipe, may be the wall thickness on the opposite side of the joint, or a specific position where the wall thickness is substantially equal around the joint, for example, The thickness of a part separated from the joint by a distance corresponding to the thickness of the joint, or the thickness averaged in the range of 1/4 in the circumferential direction of the steel pipe across the joint may be used.
The specified value B expressed by the formula <1> can be used for quality inspection of forged steel pipes subjected to flare processing, and defective products can be eliminated to provide good products.

  Moreover, in order to manufacture the forged welded steel pipe having excellent flare workability, the oxygen concentration in the air blown by the nozzle 7 in the forged welded pipe manufacturing process in FIG. 1 is preferably 22% by volume or more. As a result, the joint wall thickness increases and the ratio to the steel pipe wall thickness tends to be 1.00 or more, and the maximum bead thickness increases to the ratio of the steel pipe wall thickness to 1.010 or more. It is extremely effective for prevention.

  A forged steel pipe was manufactured in the manufacturing process shown in FIG. That is, the steel strip 2 is formed by the edge forming machine 4, heated to a temperature below the melting point by the heating furnace 5, and roll-formed by the forming forge machine 6, and air and / or oxygen at the abutting end by the nozzle 7. Immediately after that, the abutting ends were forged and joined, and then drawn and rolled to produce a forged steel pipe. The manufactured forged steel pipe was subjected to a cone expansion test with various cone expansion conditions changed to obtain a cone expansion ratio (A), and observed the occurrence of cracks in the joint in flare processing (push expansion ratio 1.3). . The results are shown in Table 1.

Table 1 also shows the size and material of the manufactured forged steel pipe. These forged steel pipes have an outer diameter of 21.7 mmφ to 114.3 mmφ, a wall thickness of 1.8 mm to 6.1 mm, and the material has a tensile strength (TS) of 290 N / mm ^{2 to} 500 N / mm ² and an elongation of 23. % To 90%. In this example, the steel pipe pushing speed into the cone was 5 mm / min. Note that the present invention is not limited to this, and any value may be selected from the range of the pushing speed of 0.001 mm / min to 1000 mm / min in a series of test ranges.

  In Table 1, Invention Examples Nos. 1 to 13 satisfy the formula <1> (the cone expansion ratio (A) is equal to or greater than a specified value (B) determined from the tube diameter). This is the best mode in which the ratio of the wall thickness 14 to the steel pipe wall thickness 15 is 1.00 or more, and the ratio of the bead portion maximum wall thickness 17 to the steel pipe wall thickness 15 is 1.010 or more. It was good without generating.

  On the other hand, Comparative Examples No. 14 to 17 did not satisfy the <1> formula, and cracks occurred in the joints during flare processing.

  The forged welded steel pipe of the present invention has a good joint strength, and even when subjected to strong processing such as flare processing, the joint does not crack, has a remarkably good performance, and is an inexpensive forged steel pipe. It can withstand severe demands, and its effects are extremely great.

DESCRIPTION OF SYMBOLS 1 Coiler 2 Slit steel strip 3 Looper 4 Edge forming machine 5 Heating furnace 6 Forming and forging machine 7 Nozzle 8 Forged steel pipe 9 Cone
13 Joint
14 Joint thickness
15 Steel pipe wall thickness
16 Bead section
17 Maximum wall thickness of bead
DR Forged steel pipe outer diameter H Cone push amount θ Cone full angle

Claims (1)

Outer diameter 21.7mmφ ~ 114.3mmφ, wall thickness 1.8mm ~ 6.1mm, tensile strength (TS) 290N / mm ² ~ 500N / mm ² , elongation 23% ~ 90% When the pipe expansion ratio (A) occurs, the following formula <1> is satisfied , the ratio of the joint wall thickness to the steel pipe wall thickness is 1.00 or more, and the ratio of the maximum bead thickness to the steel pipe wall thickness is 1.010 or more. A wrought forged steel pipe for flaring , wherein the maximum thickness of the bead portion is larger than the thickness of the joint portion .
A = {H × tan (θ / 2) + (DR / 2)} / (DR / 2) ≧ −0.0004 × DR + 1.3 = B... <1>
However, A: Cone expansion ratio H: Cone push-in amount (difference between the expansion start height and the height immediately after cracking after expansion)
θ: full-width cone
DR: Forged steel pipe outer diameter B: Specified value determined from steel pipe outer diameter

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