JP4577451B1 - Forged steel pipe with good surface after flaring - Google Patents

Abstract

The present invention provides a forged steel pipe in which a joint depth of a steel pipe is remarkably shallow or flat at a joint contact portion after flaring.
[MEANS FOR SOLVING PROBLEMS] After flaring, characterized in that, in a finished steel pipe, the length 12 in the thickness direction of the joining portion of the steel pipe is set to 20 or more in the steel pipe thickness after being joined by forging or further rolling. This is a forged steel pipe with good surface.
[Selection] Figure 4

Description

  The present invention relates to a forged steel pipe that is manufactured by continuously forming by heating a slit steel strip while spraying oxygen or oxygen-mixed air or air to the end of the steel strip for diffusion welding and manufacturing. After being provided, the present invention relates to a forged steel pipe that has a remarkably shallow joint depth or is flat (that is, a forged steel pipe having a good surface after flaring).

  In Patent Literatures 1 to 3, by restricting the inner surface side of the steel pipe, the outer surface side, the inner surface side of the steel pipe, the inner surface side bead height, the inclusions of the joint portion, etc. to a specific range, It describes that the strength is improved to prevent cracking.

JP 2007-152430 A JP-A-10-263846 JP-A-4-270009

  In recent years, pipes that have a joint portion are increasing in number that flares the ends of steel pipes. Flare processing is a process of expanding the tube end of the steel pipe and protruding it, and is a severe process of applying an excessive tension in the circumferential direction of the steel pipe.

  Therefore, conventionally, an electric resistance welded steel pipe has been applied as having a performance that is difficult to break by this severe processing. However, since an electric resistance welded steel pipe is expensive, in recent years, an inexpensive forged steel pipe is being applied.

In addition, the forged steel pipe which is the object of the present invention has an outer diameter of 48.6 mmφ to 114.3 mmφ, a wall thickness of 1.8 mm to 6.1 mm, a tensile strength (TS) of 290 N / mm ^{2 to} 500 N / mm ² , and an elongation of 23. % To 90%.

  As shown in FIG. 1, forged steel pipes are manufactured by forming a slit steel strip 2 with an edge forming machine 5, heating it with a heating furnace 6, and continuously forming the steel strip into a tubular shape with a forming forging machine 7. On the other hand, oxygen or air or oxygen mixed air in which oxygen is mixed with air is blown to the end portion by a nozzle 8, the temperature is raised by oxidation heat, and forged and joined to finish the forged steel pipe 10. In some cases, the slit steel strip 2 is edge formed after the edge is cut by the edge cutting machine 4. In some cases, after heating in the heating furnace 6, the temperature of the steel strip end is raised by an induction heating device (not shown). Further, as shown in FIG. 2, after joining by forming and forging, there may be a case where the forged steel pipe 10 is finished by further drawing with a drawing mill 9.

  In the flared steel pipe, the end portion of the steel pipe and the flared portion are expanded, and the inner surface of the expanded steel tube becomes the contact portion of the joint portion. Therefore, if the flat state of the joint contact point is impaired, a gap is formed and the fluid passing through the steel pipe is liable to leak. Therefore, in the joint contact point, the degree of flatness is extremely important so that there is no gap. The

  However, the conventional welded steel pipe has deep streaks at the joint, so when flared, the streaks expand to form grooves, and both the depth and width increase to form large gaps. It was regarded as a problem because the flatness could not be maintained. In addition, it has been said that conventional forged steel pipes are susceptible to cracking due to their low joint strength when applied to flare processing.

  Therefore, conventionally, focusing on only the occurrence of cracks in the joint, as shown in Patent Literatures 1 to 3, the inner surface side of the steel pipe, the outer surface side, the inner surface side, and the inner surface side bead height. Forged steel pipes that improve the strength for preventing cracks in the joints by restricting the inclusions in the joints to a specific range have been provided. Although Patent Document 1 describes means for increasing the pressing pressure in order to increase the inner surface bead height, as will be described later, in the study by the present inventors, the bead height is increased only by the pressing pressure. The problem of the prior art was clarified.

  However, regarding the planar state of the joint contact portion after flaring described above, no correspondence has been taken as a forged steel pipe, and a technical solution has been highly desired.

  In view of the above situation, the present invention provides a forged steel pipe in which the joint depth of the steel pipe is remarkably shallow or flat at the joint contact location after flaring.

The present invention has been made in view of the above situation, and the gist of the present invention is as follows.
( 1 ) Forging welded and joined, and further subjected to drawing rolling, and in a finished steel pipe, joined by forging while blowing oxygen mixed air having an oxygen concentration of 22% by volume or more and 32% by volume or less to the end of the steel strip, performing reducing rolling is a junction thickness direction length of the steel pipe and the steel pipe wall thickness or, forge for you that flaring, characterized in that the bead height of the joint portion of the steel pipe inner surface is not less than 0.05mm Steel pipe.
( 2 ) The wrought forged steel pipe for flaring processing according to (1 ), wherein the joint depth of the inner surface of the steel pipe is 0.2 mm or less .

  The present invention provides a forged steel pipe having a remarkably shallow or flat steel pipe joint even when subjected to flaring, and having a good surface after processing. The forged steel pipe of the present invention is very useful for improving the efficiency and reliability of piping construction because the internal fluid is difficult to leak from the joint contact location when the flared steel pipe is succeeded. It is hard to break and enables processing with good efficiency and yield.

Schematic showing an example of manufacturing process of forged steel pipe (installation of edge cutting machine) Schematic showing an example of manufacturing process of forged steel pipe (drawing mill installation) Schematic cross-sectional view in the circumferential direction of the steel pipe near the joint in a conventional forged steel pipe Schematic diagram of the cross section in the circumferential direction of the steel pipe near the joint in the forged steel pipe

  When flaring a conventional forged steel pipe, the streaks of the joint portion on the inner surface side of the steel pipe are enlarged by processing, and the flatness of the joint contact portion is significantly impaired. The streaks of the joint portion on the inner surface side of the steel pipe are formed by forming a bead portion at the end portion on the inner surface side of the steel pipe when the forged steel pipe is joined, and this valley is a streak. In the case of a forged steel pipe, in a steel pipe that has been forged and joined and finished, the bead portion swells mainly on the inner surface side, but may swell slightly on the outer surface side. The reason why the inner side of the steel strip comes into contact with the inner end of the steel strip first is that the steel strip having a thickness is formed into a cylindrical shape when it is continuously formed by a forming and forging machine. This is because that part is deformed first. In addition, in a steel pipe that has been forged and joined and then drawn and rolled, the bead portion swells mainly on the inner surface side, but the outer surface side is not completely crushed into a perfect circle shape. There is a case where it gets excited.

  In the study by the present inventors, the conventional forged steel pipe has a deep line on the inner surface side of the finished steel pipe, so that the line expands in width and depth by flaring and becomes a groove, and the flatness of the joint contact point It was understood that it significantly inhibits. That is, in flare processing, the end portion of the steel pipe and the portion in the middle of flare processing are expanded in the circumferential direction while being expanded. At that time, excessive tension acts in the circumferential direction on the end portion of the steel pipe and the portion during the flaring process. This tension tends to concentrate on the portion where the thickness of the steel pipe is thin. In a conventional forged steel pipe, a streak is formed at the joint, and the length in the thickness direction of the joint is thinner than the thickness of the steel pipe. Therefore, tension is concentrated on the joint during flare processing, and the line is stretched in the circumferential direction to widen and widen into a groove. At the same time, the depth of the line at the joint also increases to make the groove deeper. It was a problem.

  Therefore, the present inventors have made the steel pipe end portion or flare by making the length in the thickness direction of the joint portion longer than the steel pipe wall thickness, that is, thicker than the steel pipe wall thickness when manufacturing the forged steel pipe. Concentration of excessive tension in flaring as in the past is less likely to occur at joints in the middle of processing, and as a result, the joint streaks are extended in the circumferential direction along with flare processing, Since the depth is reduced, the present inventors have found that the flatness of the surface after flare processing is remarkably improved and a good surface is obtained.

  In addition, as a result of further quantitative investigation, the present inventors have made the thickness of the joint portion in the thickness direction larger than the thickness as described above, and the depth of the streaks on the inner surface of the forged steel pipe is 0.2 mm or less. Thus, it was understood that the flatness of the joint portion can be further improved. That is, when the streak of the joint is deep like a conventional forged steel pipe, the length in the thickness direction of the joint is inevitably thinner than the thickness of the steel pipe. Therefore, excessive tension for expanding the steel pipe by flaring is concentrated on the joint, and the stripe further increases the depth to form a deep groove. On the other hand, if the streaks on the inner surface of the steel pipe are made as shallow as 0.2 mm or less, the length in the thickness direction of the joint can be almost the same as or thicker than the thickness of the steel pipe. The joint streaks are stretched in the circumferential direction and widened along with the peripheral parts, the depth of the streaks is reduced, the flatness is improved, and a better surface can be provided. .

  Furthermore, when the present inventors examined, the thickness direction length of a junction part was thickened more than thickness, and while making the muscle depth 0.2 mm or less, the bead height of the steel pipe inner surface was 0.05 mm or more Then, it discovered that the stripe | line | muscle of the junction part after flare processing became easy to become further flat. In other words, the bead generated during joining of the forged pipe tends to rise mainly to the inner surface side, and when the bead height is increased, the length in the thickness direction of the joined portion is likely to increase, which is almost the same as or thicker than the thickness of the steel pipe. As a result, the concentration of excessive tension in flare processing is less likely to occur at the joint, and the muscle is stretched in the circumferential direction, the depth is reduced, and the flatness of the joint portion is improved.

  Here, in order to manufacture a forged welded steel pipe having a good surface after flaring, the oxygen concentration of oxygen or oxygen-mixed air blown by the nozzle 8 in the forged welded pipe manufacturing process in FIG. 1 or FIG. Good. More specifically, the oxygen concentration in the vicinity of the end of the steel strip to be joined is preferably 22% by volume or more. As a result, the temperature at the joint and its vicinity increases from the circumferential direction of the steel pipe to reduce the strength (deformation resistance), the thickness at the joint and its vicinity increases to become greater than the thickness of the steel pipe, The depth is reduced and the thickness of the joint is increased, the bead height is increased and the thickness of the joint is further increased, and the flatness of the joint part after flaring is improved. .

  There is no problem if the oxygen concentration near the end of the steel strip is 32% by volume or less, but if it exceeds this, molten steel is generated in the joint and molten steel beads are generated and cannot be removed in the forged steel pipe production line. It is a big problem. Here, the vicinity of the steel strip end is a range from the end of the steel strip to the measurement probe diameter of the oximeter to about five times the diameter, and the oxygen concentration may be measured in advance before heating the steel strip.

  Regarding the bead height, when forging and joining, the bead part is formed by local pressure mainly on the inner surface side until the entire thickness direction of the steel pipe end part comes into contact. Even if only the pressing pressure of the part is increased, the bead does not rise above the conventional bead height. This is due to the following reason. In the conventional forged steel pipe manufacturing method, when the steel strip is heated and welded, the end portion of the steel strip and its vicinity are easily cooled, so that these temperatures are made substantially the same as the circumferential direction of the steel pipe, I was blowing air or oxygen. Therefore, the temperature at the end of the steel pipe immediately before joining and the temperature in the vicinity thereof are substantially the same as the temperature in the circumferential direction of the steel pipe. Become. Therefore, once the end of the steel strip comes into contact and the joint is formed to a length close to the wall thickness, even if only the pressing pressure is increased, the pressure in the circumferential direction of the steel pipe including the joint is increased. Since the steel pipe is uniformly dispersed, the thickness of the entire steel pipe is slightly increased, so that the pressure is not concentrated on the joint portion, and the bead does not rise above the conventional bead height.

  Here, the bead height on the inner surface of the steel pipe is the difference between the length from the center of the thickness of the steel pipe to the maximum thickness of the inner surface bead portion and the length that is 1/2 the thickness of the steel pipe. Also, the steel pipe wall thickness may be the average wall thickness in the circumferential direction of the forged steel pipe, or the wall thickness of the portion located on the opposite side of the steel pipe joint. The thickness may be a position, for example, a thickness at a position separated from the joint by a distance corresponding to the length in the joint thickness direction, or a thickness averaged in a range of 1/4 in the steel pipe circumferential direction across the joint.

  Hereinafter, the present invention will be described more specifically with reference to examples.

A forged steel pipe was manufactured in the manufacturing process shown in FIG. That is, the steel strip 2 is formed with an edge forming machine 5, heated to a temperature below the melting point with a heating furnace 6, roll-formed with a forming forging machine 7, and oxygen or air with a nozzle 8 at the end of the steel strip, Alternatively, a forged steel pipe was manufactured by blowing oxygen mixed air and forging and joining. Some steel pipes were subjected to drawing rolling after edge cutting of the steel strip 2 and / or forging and joining. The manufactured welded steel pipe has an outer diameter of 48.6 mmφ to 114.3 mmφ, a wall thickness of 1.8 mm to 6.1 mm, a tensile strength (TS) of 290 N / mm ^{2 to} 500 N / mm ² , and an elongation of 23% to 90%. . Flare processing was performed under the conditions of 1.3 to 1.4 in terms of expansion ratio (average value in the circumferential direction of the outermost diameter of the flare after flaring / outer diameter of the forged steel pipe before flaring).

  After flaring these forged steel pipes, together with the ratio of the steel pipe wall thickness direction length and steel pipe wall thickness at the joints of the forged steel pipes manufactured above, the depth of the inner surface of the steel pipe, and the bead heights on the inner and outer surface sides of the steel pipe Table 1 shows the muscle depth.

No. which is an example of the present invention. 1 , no. 3, no. 5, no. No. 6 shows that the depth of the joint after flaring is significantly lower than the depth of the forged steel pipe before processing, and the flatness of the steel pipe joint contact portion can be improved. On the other hand, the comparative example No. 7 to No. Up to 10, it was found that the reinforcement depth of the joint after flaring increased more than the reinforcement depth of the forged steel pipe before machining, and the flatness of the steel pipe joint contact portion could not be improved.

  Forged welded steel pipe of the present invention, even if it is subjected to flaring processing, when the steel pipe joint has a remarkably shallow or flat surface, the surface after flaring is good, and the flared steel pipe is succeeded The internal fluid is difficult to leak from the joint part, which is very useful for improving the efficiency and reliability of piping construction, and enables processing with high efficiency and yield, and the effect is extremely large. .

DESCRIPTION OF SYMBOLS 1 Coiler 2 Slit steel strip 3 Looper 4 Edge cutting machine 5 Edge forming machine 6 Heating furnace 7 Forming and forging machine 8 Nozzle 9 Drawing mill 10 Forging steel pipe 11 Joining part 12 Thickness direction length of joining part 13 Reinforcement 14 Reinforcement depth 15 on the inner surface of the steel pipe 15 Bead portion 16 on the inner surface side Length 17 from the thickness center to the maximum height of the inner surface bead portion 1/2 of the wall thickness
18 Inner side bead height 19 Thickness center 20 Thickness (steel pipe thickness)
21 Streaks on the outer surface side of the steel pipe 22 Beads on the outer surface side

Claims (2)

Forged and joined, and further drawn and rolled, in the finished steel pipe, oxygen-mixed air having an oxygen concentration of 22% by volume to 32% by volume is blown and joined to the end of the steel strip, and further drawn and rolled. go and is the junction thickness direction length of the steel pipe and the steel pipe wall thickness or, forge welded steel pipe for flaring you, characterized in that the bead height of the joint portion of the steel pipe inner surface was not smaller than 0.05 mm. The wrought forged steel pipe for flaring according to claim 1, wherein the depth of the joint at the inner surface of the steel pipe is 0.2 mm or less.

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