JPS5952690B2 - Manufacturing method for large-diameter heat-treated steel pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing heat-treated steel pipes, and particularly a method for manufacturing large-diameter heat-treated steel pipes in which a large-diameter welded steel pipe is expanded and then continuously quenched and tempered by induction heating while being transported. Regarding.

For natural gas or oil pipelines or oil well pipes,
A large diameter welded steel pipe is used which has excellent tensile strength and toughness as well as dimensional accuracy.

Such tubes are used, for example, in the so-called UOE system or in the C
It is usually manufactured using a pipe manufacturing method such as the FE method (continuous roll forming method).

All of these methods involve forming a steel plate into a tube using a die, seam welding the edges of the plate, and then expanding the tube radially to obtain high dimensional accuracy and optimal mechanical properties.

In recent years, as pipelines have become longer distances, the steel pipes used there have become increasingly larger in diameter in order to reduce the cost of transporting natural gas, etc. Steel pipes with excellent resistance to hydrogen-induced cracking are required, and in recent years, thick-walled, large-diameter steel pipes have also been required for marine structures and the like.

In order to meet these demands, there is an increasing need to perform heat treatment such as quenching and tempering on large diameter steel pipes after welding.

However, normally, when such large-diameter steel pipes are continuously heated and cooled during transport, the outer diameter of the pipe ends becomes larger than that of the pipe center part (the middle part of the pipe other than the pipe ends). Therefore, it becomes difficult to keep the outer diameters of both the tube end portion and the tube center portion within a certain standard value.

In other words, if the steel pipe is heated and cooled continuously during transport,
As the tube moves, the cooled portion rapidly shrinks due to thermal contraction.

Therefore, compressive thermal stress occurs in the circumferential direction in the uncooled portion that is continuous with this portion, and since it is in a high temperature state, permanent compression deformation easily occurs.

However, the above-mentioned compressive thermal stress does not occur in the end portion of the tube where there is no cooled portion, particularly in the front end portion of the tube in the transport direction, and permanent compressive deformation is unlikely to occur.

As a result, the outer diameter of the end of the tube in the final product is larger than that of the center of the tube.

The object of the present invention is to eliminate such problems and provide a method for manufacturing heat-treated steel pipes that can make the outer diameter uniform over the entire length direction. The tube expansion step is characterized in that the tube expansion rate near the tube end is made smaller in advance than the tube expansion rate at the tube center (portion other than the tube end).

Next, the present invention will be explained in more detail with reference to the drawings.

FIG. 1 schematically shows the steps from expansion to quenching of a large-diameter heat-treated steel pipe.

The tube to be treated 1 formed into a tubular shape and seam welded is mechanically expanded by about 1.0 to 1.5% of the tube diameter by an expander 2, and then transferred to a quenching step 3.

In the hardening step 3, the tube 1 to be treated is passed through an induction heating coil 4 and heated to a maximum temperature of 950° C., and then water-cooled from the inside and outside of the tube with a water cooling nozzle 7 to shrink and harden the tube.

In the tempering step (not shown), the material is heated to below the transformation point using a similar induction heating coil and cooled using an air injection nozzle or the like to stabilize the structure during quenching and provide toughness.

As the pipe to be treated 1 progresses, the middle part B of the pipe excluding the pipe end part A passes through the induction heating coil 4 in the quenching process 3, and when it is just before water cooling, the previous part (the pipe end part) Since it is cooled and in a contracted state, compressive thermal stress acts on this portion B in the circumferential direction, which easily causes permanent compression deformation.

However, when the tube end portion A is just before water cooling, there is no already cooled portion connected ahead of it in the direction of travel.
Such thermal stress is not applied from the front of the tube end, and the outer diameter of the tube end portion A inevitably increases in the end.

Such a phenomenon occurs not only in the quenching process but also in the tempering process when the diameter rapidly contracts as in the case of water cooling.

FIG. 2 shows variations in the outer diameter of large-diameter steel pipes heat-treated in this manner in the longitudinal direction of the pipes.

The outer diameter of the tube is largest at the tube end (front end), and the portion with poor outer diameter continues up to about 1 m from the tube end.

The portion where the outer diameter increases in this way is up to approximately 1 m from the end of the tube, regardless of the overall length of the tube to be treated.

Therefore, in the present invention, at the stage of the tube expansion process before starting heat treatment, the tube is expanded in advance by an amount corresponding to the amount of permanent compressive deformation caused by thermal stress, from both ends of the tube to be treated to the portion where the outer diameter defect occurs. The expanded outer diameter of the portion up to about 1 m from the front end of the tube is made smaller than that of the center of the tube, and in this state heat treatment for quenching and tempering is performed.

For a steel pipe whose outer diameter is small only at the tube end, the difference in outer diameter is canceled out by the heat treatment described above, resulting in a heat-treated steel pipe having a uniform outer diameter as a whole.

Pipe expansion methods include methods using water pressure or mechanical means.

As a mechanical means, segments 9 (Fig. 1) arranged in a cylindrical shape with a length of about 1 m are inserted into the pipe from both ends of the pipe, and are pushed apart by a wedge action using a hydraulic device (not shown) via a rod 10. Expand the tube.

When using such a mechanical expander,
In the present invention, by adjusting the stroke length of the rod, the expansion rate near the front end of the tube is approximately 0.65% smaller than that at the center of the tube (the part after 1 m from the front end of the tube). The tube expansion rate is gradually changed.

FIG. 3 shows the outer diameter distribution of a steel pipe after being heat-treated by the method of the present invention, in the same longitudinal direction as in FIG. 2.

As can be seen from this figure, the outer diameter of the end portion of the tube is not large, and the entire length is close to the nominal outer diameter of the tube.

In the above embodiment, the steel pipe is moved and heat treated sequentially along the length of the pipe, but the steel pipe is fixed and the heat treatment equipment is moved along the pipe axis to perform quenching and tempering. Of course, this is a good thing.

The tube expanding means is also not limited to mechanical means as in the embodiments.

[Brief explanation of the drawing]

Fig. 1 is a diagram schematically showing the process from expansion to quenching heat treatment of a large-diameter heat-treated steel pipe, and Fig. 2 is a diagram showing the outer diameter distribution of a heat-treated steel pipe manufactured by a conventional method in the pipe longitudinal direction. FIG. 3 is a diagram similar to FIG. 2, showing the outer diameter distribution of a heat-treated steel pipe manufactured by the method of the present invention. 1...Pipe to be treated, 2...Mechanical expander, 3...Quenching process, 4...
・Induction heating coil.

Claims (1)

[Claims] 1. In a method for manufacturing a heat-treated steel pipe in which a steel pipe is heat-treated by quenching and tempering along the axial direction, before the heat treatment, the front end of the steel pipe is expanded at a smaller expansion ratio than other parts, and then, A method for manufacturing a heat-treated steel pipe, which comprises performing heat treatment.