JP2023073551A - Steel pipe and method for manufacturing the same - Google Patents

Abstract

To provide a steel pipe that has excellent flare processing property, and to provide a method for manufacturing the same.SOLUTION: In a steel pipe, a ratio H/W of a difference H between a bead height that is a maximum wall thickness of a thickened part formed in right and left parts of a butt joint part and a wall thickness of the butt joint part to a distance W between positions on an inner surface side of a steel pipe from which a right and left bead heights of the butt joint part are measured is 0.50 or less.SELECTED DRAWING: Figure 3(a)

Description

TECHNICAL FIELD The present invention relates to a steel pipe suitable for piping and a method for manufacturing the same.

Butt-welded steel pipes, which are inexpensive and have excellent workability, have been used for plumbing equipment such as water supply pipes. In recent years, as a method of joining such pipes through which fluids flow, a method of flaring the end of the pipe and using a flange has been widely used in order to save labor in construction work. Flaring is a process in which the end of a steel pipe is expanded and flared, and because it is processed by applying excessive tension in the circumferential direction of the steel pipe, it has excellent workability (hereinafter also referred to as "flare workability"). requested.

The butt-welded steel pipes are not melted at the ends after forming, but are joined in solid phase by collision.

For example, Patent Literatures 1 and 2 describe techniques for manufacturing a butt-welded steel pipe with improved flaring workability by defining the depth of the streaks in the abutting portion.

Japanese Patent No. 4577451 Japanese Patent No. 4077859

However, the butt welded steel pipes proposed in Patent Literatures 1 and 2 are not yet sufficient in flaring workability.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a steel pipe excellent in flaring workability and a method for producing the same in order to solve the above problems.

The inventors of the present invention conducted research on the flaring workability of steel pipes and found that the shape of the butt joint has an effect on the flaring workability.

In order to solve the above problems, the present inventors conducted extensive research on the optimum joint shape in the method of heating the steel plate and butting and forging the edges. As a result, it was found that the shape of the abutment part changes depending on the temperature and upset rate of the abutment part at the time of abutment, and the flaring workability also changes. In particular, the difference H between the bead height, which is the maximum thickness of the increased thickness formed on the left and right sides of the butt portion, and the thickness of the butt portion, and the inner surface of the steel pipe where the bead height on the left and right of the butt portion was measured It was found that a steel pipe having a large ratio H/W of the distance W between positions, that is, a steel pipe having a sharp groove at the abutting portion, cracks during flaring. They also found that by optimizing the temperature and upset rate at the butt joint, the adhesive strength and joint shape are improved, and high flaring workability can be achieved.

The present invention has been completed by further studies based on such findings, and the present invention employs the following means to solve the above problems.
[1] The difference H between the bead height, which is the maximum thickness of the thickened portions formed on the left and right sides of the abutment portion, and the thickness of the abutment portion, and the bead heights on the left and right sides of the abutment portion are measured. A steel pipe having a ratio H/W of 0.50 or less to a distance W between positions on the inner surface of the steel pipe.
[2] The edge portion of a steel plate is formed, the steel plate is heated in a heating furnace, and the heated steel plate is formed into a tubular shape by a forming forge welder on the exit side of the heating furnace, and is then pushed toward the abutment portion. When producing a steel pipe by butting and forging the edges while blowing air or oxygen-mixed air, the steel plate end temperature T (° C.) at the time of collision is set to 1400° C. or higher, and the upset rate at the time of collision is reduced. 0.0 to 3.0%, steel pipe manufacturing method.

According to the present invention, by setting the shape of the vicinity of the butt joint within a predetermined range, the steel pipe can be made to have better flaring workability than the conventional one. In addition, in the method of manufacturing steel pipes, by optimizing the steel plate edge temperature and upset rate at the time of butting, the bonding strength of the butting part is strengthened and the shape of the vicinity of the butting part is kept within a predetermined range. It is possible to manufacture a steel pipe with improved flaring workability compared to conventional steel pipes.

FIG. 1 is a schematic diagram of a steel pipe production line. FIG. 2 is a cross-sectional view of a steel pipe. FIG. 3(a) is an enlarged view of the abutting portion. FIG. 3(b) is a diagram showing the measurement positions of the bead height. FIG. 3(c) is a diagram showing the distance W between the bead height position and the position on the inner surface side of the steel pipe where the bead heights on the left and right sides of the abutting portion were measured. FIG. 4 is a schematic diagram of the rolls performing the bonding. FIG. 5(a) is a diagram showing a steel pipe before flaring. FIG. 5(b) is a diagram showing the steel pipe after flaring.

The steel pipe (butt welded steel pipe) of the present invention is manufactured by using a steel plate and abutting and forging the edges of the steel plate. The steel plate, which is the material, is preferably a steel strip, but cut plates such as thin plates and thick plates are also applicable. In the following, the case where the steel plate is a steel strip will be taken up.

First, FIG. 1 shows the process of manufacturing a steel pipe from a steel strip. A process of manufacturing a steel pipe from a steel strip will be described based on the steel pipe manufacturing line equipment with reference to FIG.

A steel strip 2 discharged from a coil 1 is passed through a looper 3 , and an edge forming device 4 forms an edge portion (not shown) of the steel strip 2 . Although not particularly limited in the present invention, a method of shaping the edge portion with an upset roll and a method of shaving the edge portion by cutting are conceivable for forming the edge portion. After that, the entire width of the steel strip 2 is heated in the heating furnace 5, the width of the steel strip is measured by the strip width measuring machine 6, and the edges are abutted and forged while being continuously formed into a tubular shape by the forming and forging machine 7. Immediately before the edge portion is abutted and forged, air or oxygen-mixed air is blown from the oxygen-mixed air spray nozzle 8 to raise the temperature and join by forge welding. A steel pipe (butt-welded steel pipe) 10 is manufactured by rolling to an outer diameter. A steel pipe (butt welded steel pipe) 10 is formed with an abutting portion 11 . In this equipment line, the steel pipe (butt-welded steel pipe) 10 is finished by forge-welding and joining the butting portions 11 and then performing the reducing rolling, but the steel pipe (butt-welding steel pipe) 10 may be finished without performing the reducing-rolling. .

In the steel pipe of the present invention, the distance W between the positions on the inner surface side of the steel pipe where the maximum wall thickness (hereinafter referred to as "bead height") of the thickened portions formed on the left and right of the abutting portion on the inner surface of the steel pipe (hereinafter referred to as "bead height") is measured, and the bead The butt-welded steel pipe has a ratio H/W of 0.50 or less to the difference H between the height and the thickness of the butt portion (hereinafter referred to as "the thickness of the bead portion"), and is excellent in flaring workability. Next, a method for measuring each value will be described with reference to FIGS.

Steel pipe wall thickness t
As shown in FIG. 2, the wall thickness t of the steel pipe (butt welded steel pipe) 10 is measured at 90°, 180°, and 270° in the circumferential direction when the position of the abutting portion 11 is 0°. 3 (indicated by 13 in FIG. 2) are measured with a single-ball micrometer or the like, and the average value of the three points is taken as the steel pipe wall thickness t. This steel pipe wall thickness t is a value used for later calculation of the upset rate.

Bead Portion Thickness As shown in FIG.

Bead height Bead height 15 is obtained by first drawing a tangent line to the abutting portion 11 as shown in FIG. In the range 14, as shown in FIG. 3(b), points are created at intervals of 0.1 mm. From each point, a line perpendicular to the tangent line is drawn down the outer surface of the steel pipe to create a point that intersects the outer diameter 12 of the steel pipe. Draw a tangent line to the outer surface of the steel pipe at the point of intersection, and measure the wall thickness in the direction perpendicular to the tangent line. A measurement method using a point micrometer, a microscope, or the like can be used for measurement, but is not particularly limited. After that, the maximum value among the measured wall thicknesses is taken as the bead height 15 .

Distance W between the positions on the inner surface of the steel pipe where the bead heights on the left and right of the butt part are measured
As shown in FIG. 3(c), the distance 18 between the positions on the inner surface of the steel pipe where the bead heights 15 on the left and right sides of the abutting portion 11 are measured is measured, and the value is defined as W. As shown in FIG. In FIG. 3(c), since the maximum thickness of the thickened portion on the left side of the abutment portion is larger than the maximum thickness of the thickened portion on the right side, the maximum thickness of the thickened portion on the left side of the abutment portion is defined as the bead height 15. do.

Difference H between bead height and thickness of abutment
The thickness at the abutment portion 11 is measured, and the difference from the bead height 15 is taken to be the difference H between the bead height 15 and the abutment portion thickness 16 .

Next, the regulations are described.

H/W is 0.50 or less Even if the groove depth of the abutting portion 11 is the same, the degree of stress concentration varies depending on the width of the groove, and the effect of degrading the flaring workability varies. The steeper the groove, the larger the value of H/W. As this value increases, stress concentrates in the groove, cracking occurs during flaring, and a flared portion of desired dimensions cannot be obtained. As a result of investigating the relationship between H/W and flaring processability, it was found that H/W should be 0.50 or less in order to obtain desired flaring processability. Therefore, in the steel pipe of the present invention, it is essential to set H/W to 0.50 or less. A more preferable range is H/W of 0.30 or less. Also, the lower limit of H/W is 0.0%.

Steel plate end temperature T (°C) at collision 1400°C or higher If the temperature is low, the surface scales will not melt and an excessive amount of upset will be required to remove them. Scale is melted by heating the steel plate edge temperature at the time of collision to 1400°C or higher, and is removed by the upset or the like specified in the present invention. It is mandatory to do the above. On the other hand, if the ends of the steel plate are heated excessively, not only will the energy required for heating become enormous, but the ends of the steel plate will be extremely softened, resulting in excessive thickening even at an upset rate of 0.0 to 3.0%. and H/W exceeds 0.50. In the present invention, if the temperature is 1480° C. or lower, the H/W can be easily reduced within the range of 0.0 to 3.0% upset. For this reason, it is preferable that the steel plate edge temperature is 1480° C. or less. As for the air or oxygen mixed gas, conventionally used ones may be used, and conventional methods for spraying them may be used.

Upset rate 0.0-3.0%
By increasing the upset ratio at the time of collision, the ends of the steel plate are strongly bonded and the flaring workability is improved. On the other hand, however, if the upset rate is increased, the ends of the steel sheet are excessively deformed, and lip-shaped grooves are generated on the inner diameter side. In addition, steep grooves are formed on the outer surface by buckling of the steel plate. If such steep grooves on the inner and outer surfaces exist in the steel pipe, stress concentrates during flaring, and cracks start. Also, it is conceivable that reducing the upset rate improves the shape of the abutment portion. On the other hand, however, if the upset rate is negative, the ends of the steel sheets cannot be joined, and pipemaking cannot be performed.

In the present invention, it is essential that the upset rate is 0.0% to 3.0% in order to suppress the formation of steep grooves during pipemaking. A preferred range is 0.5 to 2.5%. The method of calculating the upset rate is to measure the strip width L of the steel strip (steel strip) on the delivery side of the heating furnace, and use this L and the thickness t of the steel strip to be used, as shown in FIG. In the roll 19 and the forge lower roll 20, the roll bottom radius R indicated by 21 and the inter-roll gap Rg indicated by 22 are calculated using the following formula (1).
X=100×(L-2π(Rt/2)-2Rg)/L (1)
X: Upset rate (%)
L: Width of steel strip (mm)
t: thickness of steel strip (mm)
R: roll bottom radius (mm)
Rg: Gap between rolls (mm)

A butt-welded steel pipe was manufactured according to the manufacturing process shown in FIG. That is, it is discharged from the coil, passes through the looper, forms the edge, then heats the entire width of the steel strip in a heating furnace, continuously forms it into a tubular shape with a forming and forge welding machine, and immediately before the edge portion is collided and forged. Mixed air was blown from a nozzle, the temperature was raised, the pipes were forged and joined, and the pipes were further reduced to a desired outer diameter by a diameter reducing roll to produce a butt welded steel pipe. At this time, in the invention examples, the temperature T (° C.) of the steel plate edge portion during collision was set to 1400° C. or higher, and the upset rate was set to 0.0 to 3.0%. On the other hand, a butt-welded steel pipe manufactured under conditions that do not satisfy any of these conditions was used as a comparative example.

The outer diameter of the manufactured butt-welded steel pipe was measured, and then flared using a flare processing machine. Fig. 5(a) shows a butt-welded steel pipe before flaring, and Fig. 5(b) shows a butt-welded steel pipe after flaring. Flaring was performed until the outer diameter after flaring was 1.7 times the outer diameter before flaring. The outer diameter 23 after flaring was measured with a vernier caliper at a portion including the abutting portion after flaring as shown in FIG. 5(b). Table 1 shows the results.

Steel pipe No. 1, which is an invention example of the present invention. As can be seen from Table 1, the shapes of the abutting portions of samples 1 to 10 are within an appropriate range. As a result, cracking (separation of the forged portions of the flange portion of the flare tube) did not occur during flaring, and flaring workability was good (◯). On the other hand, steel pipe No. of the comparative example. In all of Nos. 11 to 14, cracking of the joint portion was visually confirmed before flaring to the desired outer diameter, and the flaring workability was poor (x).

1 coil 2 steel plate (steel strip)
3 Looper 4 Edge Forming Device 5 Heating Furnace 6 Band Width Measuring Machine 7 Forming Forge Welding Machine 8 Oxygen Mixed Air Blowing Nozzle 9 Reducing Mill 10 Steel Pipe (Butt Welded Steel Pipe)
11 Colliding portion 12 Outer diameter of steel pipe 13 Steel pipe wall thickness 14 Bead height measurement range 15 Bead height 16 Wall thickness of abutting portion 17 Maximum wall thickness of thickened portion on right side of abutting portion Distance W between positions on the inner surface of the steel pipe where the bead height is measured
19 forge upper roll 20 forge lower roll 21 roll bottom radius R
22 Gap between rolls Rg
23 Outer diameter after flaring

Claims (2)

The inner surface of the steel pipe where the difference H between the bead height, which is the maximum wall thickness of the thickened portions formed on the left and right sides of the butting portion, and the wall thickness of the said butting portion, and the bead height on the left and right sides of the said butting portion are measured. A steel pipe having a ratio H/W to a distance W between side positions of 0.50 or less. The edge portion of a steel plate is formed, the steel plate is heated in a heating furnace, and the heated steel plate is formed into a tubular shape by a forming forge welder on the exit side of the heating furnace, while air or oxygen is introduced toward the abutting portion. When producing a steel pipe by colliding and forge-welding the edge portion while blowing mixed air, the temperature T (° C.) of the steel plate end portion during colliding is set to 1400° C. or more, and the upset rate during colliding is 0.0. ~3.0%, steel pipe manufacturing method.

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