JPH093545A - Manufacturing method for large diameter welded steel pipe - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a simple method for increasing the crushing strength of UOE steel pipes used for submarine line pipes. [Structure] In a method of manufacturing a steel pipe in which a steel plate is processed into a tubular shape by U press and O press, welded, and expanded, a steel pipe after expansion is in a range of 150 ° C or higher and lower than 700 ° C (absolute temperature) x ( The heating index represented by 5 + log (time) is 1
A method for manufacturing a steel pipe, which comprises heating to a temperature of 300 or more and less than 4300. [Effect] It is possible to provide a UOE steel pipe in which the Bauschinger effect is reduced or eliminated by a simple heating device and the crushing strength is enhanced, without major equipment remodeling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a UOE steel pipe having a high crushing strength, which is used in a seabed pipeline.

[0002]

2. Description of the Related Art When laying a submarine line pipe, the first thing to be noted is to prevent the steel pipe from being crushed by seawater pressure. Therefore, when designing a submarine line pipe, it is necessary to design the steel plate strength, outer diameter, wall thickness, etc. so that crushing does not occur, but in reality it is not possible to accurately grasp such conditions. .

A UOE pipe manufacturing method is a typical method for manufacturing a thick, large-diameter welded steel pipe. This method comprises the steps of forming a steel plate into an annular shape by U-pressing and O-pressing, then seam-welding and expanding the pipe to produce a steel pipe. The last tube expansion is a step of expanding the tube diameter by a mechanical expander, and is a step necessary for adjusting the outer diameter and the shape. These plastic deformations are factors that promote crushing when an external pressure is applied, as described below.

The direction of plastic deformation in the UOE pipe manufacturing method will be described. By U pressing and O pressing, tensile deformation is given to the surface side of the steel sheet from the center of the thickness. In the subsequent pipe expanding step, all positions in the thickness direction are subjected to tensile deformation and shipped as they are. After all, the steel pipe is shipped in a state where it has undergone tensile plastic deformation in most of the plate thickness except the portion near the inner wall of the steel pipe, and is laid as a submarine line pipe. When laid on the seabed, steel pipes are successively welded circumferentially on the ship and are lowered to the seabed, so that the interior is exposed to atmospheric pressure and the exterior to water pressure. Therefore, on the seabed, the steel pipe is subject to compressive stress due to the pressure difference. That is, after the tensile deformation in the pipe expansion, when the cable is laid, the compressive stress opposite to that is applied.

Generally, when a compressive stress is applied after the tensile deformation, the proof stress against the compressive stress decreases. The same is true in the opposite case, that is, when first subjected to compressive deformation and then subjected to tensile stress, and the proof stress against tensile stress decreases. A phenomenon in which the proof stress in the direction opposite to the immediately preceding deformation direction is lowered when an alternating stress is applied is known as the Bauschinger effect.

Plastic deformation such as press working and pipe expansion is
It progresses with the movement and multiplication of a type of lattice defect called dislocation. Steel, which is easy for dislocation movement and multiplication, is soft and therefore has low yield strength. In general, steel is soft at the stage where plastic deformation is small and dislocation multiplication has just started, but the steel becomes harder as the plastic deformation progresses. When the plastic deformation progresses and becomes hard, there is a high-density array of dislocations in the steel that are entangled and hard to move.

The Bauschinger effect is caused by the formation of dislocation arrays that are difficult to move and proliferate only in a certain direction during the first plastic deformation. Such an array of dislocations will continue to harden as long as the deformation increases in the initial direction. However, when stress is applied in the opposite direction, and when deformation begins in that stress direction, some dislocations in the dislocation array initially created move and grow in that direction without large resistance. Therefore, plastic deformation in the opposite direction easily occurs. This means that the steel exhibits a low yield strength. When the compressive deformation proceeds as it is, hardening starts and the dislocations form an array of dislocations in the direction of the compression (in the direction opposite to the initial direction) similar to the direction of the initial tension. This explanation holds when the complicated phenomenon is extremely simplified.

The steel pipe manufactured by the UOE pipe manufacturing method has a low crushing strength as a steel pipe due to the above Bauschinger effect when subjected to compressive stress. The compressive stress caused by bending due to the laying of the seabed pipeline causes local buckling due to superposition with the compressive stress due to water pressure. Can be transmitted and cause a serious accident. The UOE steel pipe having a low yield strength due to the Bauschinger effect is apt to cause such local buckling. Proposals for overcoming such weak points of UOE steel pipe have been made so far.
For example, (a) CKWTam & JGACroll: An Improveme
nt of the Propagation Buckle Performance of Subsea
Pipelines, Thin-Walled Structures, 4 (1986), P.423
There is a steel tube with spiral ribs proposed by.
This proposal is predicted to have a great computational effect. However, the cost of attaching the ribs is large and the handling is not easy, so the possibility of practical application is small.

Further, (b) the calculation is made that the crushing strength does not decrease when the expanding process of the UOE pipe making process is replaced with the reducing process to adjust the size and shape. .Kyriakides, E. Corona & FJFisher:
On the Effect of the UO-E Manufacturing Process o
n the Collapse Pressure of Long Tubes, J. of Engin
eering for Industry, 116 (1994), P.93). This method is certainly effective for external pressure, but for internal pressure there is a concern that the strength will be low due to the Bauschinger effect, and because this method requires the installation of a new large device for contraction, Not realistic from an economic point of view.

(C) In general, there is a report that the Bauschinger effect is reduced by processing in a heated state (W.
C. Leslie: The physical metallurgy of steel (Mcgra
w-hill intenational book company), p.159). However, this method is not economically feasible because the equipment for heating the steel sheet and the equipment for pressing the heated steel sheet must be modified.

(D) Proposal that the Bauschinger effect by the UOE pipe-making method can be reduced by changing the structure of the steel sheet as the material to the acicular-ferrite structure (WCLeslie: ibid., P.).
202) is also available. However, in order to obtain acicular-ferrite, it is necessary to add a large amount of alloying elements to the steel sheet, which increases the alloy cost and is not preferable.

[0012]

As described above, the conventional U
There is no example in which the OE pipe manufacturing method is used as it is and the reduction of the crushing strength is prevented without increasing the cost. Moreover, there is no example that suggests that the Bauschinger effect once generated is reduced or eliminated by the subsequent heating.

An object of the present invention is to provide a simple method for improving the crush strength of a steel pipe manufactured by using the existing UOE pipe manufacturing apparatus as it is.

[0014]

The inventor of the present invention has found that conventional UO
If the steel pipe manufactured by the E pipe manufacturing method is heated under appropriate conditions, the Bauschinger effect may be reduced or eliminated without impairing the original characteristics of the material, and the yield strength of the UOE steel pipe during compression may be improved. I guessed. That is, by appropriately heating, (a) the dislocation sequence itself, which is the source of the Bauschinger effect, is changed, or (b) the dislocation sequence is not significantly changed, and a new precipitate is formed on the dislocation having such a sequence. It is expected that the above-described effects can be obtained by expecting two effects of causing the dislocations to be pinned.

Therefore, steel pieces were cut out from a steel sheet which was melted and rolled in a laboratory and subjected to tensile deformation, and each steel piece was held at each temperature for various times and then allowed to cool, and a proof stress (0.2 % Yield strength) was evaluated. The tensile deformation before heating was in the range of 2.5% to 2.7%. The results are shown in FIG. The results in FIG. 1 show that the proof stress against the compressive stress after tensile deformation can be improved by selecting an appropriate heating temperature and heating time.

Curves A and B in the figure represent heating indices 1300 and 4300, respectively.

The gist of the present invention based on the above findings is as follows.

In a method for producing a steel pipe in which a steel plate is processed into a tubular shape by U press and O press and welded, and then expanded, the expanded steel pipe is expressed in the range of 150 ° C. to less than 700 ° C. at the following temperature and time. The value of the equation (heating index) is 1
A method for producing a steel pipe having excellent crush resistance, which comprises heating to 300 or more and less than 4300.

Heating index = T × (5 + logt) where T (absolute temperature K) = 273 + heating temperature (° C.) (heating temperature is 150 ° C. or higher and lower than 700 ° C.) t: 150 ° C. or higher heating time (h) log: Common logarithm

[0020]

When the steel pipe is heated to 150 ° C. or higher, it takes a long time to precipitate on dislocations or rearrange rearrangement at a temperature lower than this temperature, and in the actual production where efficiency is important, the bauschinger is used.
This is because the effect cannot be reduced or eliminated until there is practically no problem. The reason why the temperature is lower than 700 ° C. is to prevent deterioration of the material.

The reason why the heating index is set to 1300 or higher is that the proof stress can be improved when the heating index is set to 430 or higher.
The reason why it is less than 0 is that if it is more than that, tempering proceeds too much and the steel is softened. The reason why the heating index is introduced is that the effect of time and temperature can be expressed by one index because the target improvement in yield strength can be obtained if the temperature is high and the temperature is short and the temperature is long. In addition, the reason why 5 is adopted as the constant and 20 which is often used in the tempering index is not used is that the effect of the fluctuation of the heating temperature sensitively affects the Bauschinger effect even in a short heating time. This is because the fact is represented by a heating index. When 20 is used, the effect of the temperature change is less likely to appear in the change of the heating index as the effect of the heating time unless heating is performed for a long time, whereas when 5 is adopted, the effect of the temperature change is changed even if the heating is performed for a short time. This is because it can be expressed as, and can be associated with the actual Bauschinger effect.

Accelerated cooling steel sheets are often used as materials for UOE steel pipes. The accelerated cooling steel plate refers to a thick steel plate which is controlled at a constant accelerated cooling rate by a predetermined cooling device having a controlled water content after the controlled rolling of the thick steel plate. Since it is possible to suppress the amount of ferrite and improve the strength, it is an effective manufacturing method for improving the weldability. Niobium (Nb) is usually added to these accelerated cooled steel sheets in order to take advantage of strength increase and controlled rolling. Since the solid solution concentration of Nb in the accelerated cooling steel sheet is higher than that in the case where it is not subjected to accelerated cooling, the amount of carbonitrides deposited during heating according to the present invention increases. At this time, the amount of precipitation on dislocations also increases, so
It is possible to more strongly pin the dislocation activation under low stress in the opposite direction. Therefore, U to which the method of the present invention is applied
The steel plate for the OE steel pipe is Nb manufactured by the accelerated cooling method.
Additive steel sheet is preferred.

There is no particular restriction on the method of heating the steel pipe. A particularly important point is that the temperature is not necessarily uniform over the entire length of the steel pipe. If the above conditions of temperature and time are satisfied, the Bauschinger effect can be reduced or eliminated to the extent that there is practically no problem regardless of whether the steel pipe part is isothermal or non-isothermal. Therefore, it is also possible to heat the steel pipe while rotating it in a spiral shape with one gas burner, and to heat several pipes or dozens of burners arranged in a strip for the purpose of improving efficiency. You can also Alternatively, the same effect can be obtained by passing the gas burner arranged in a ring shape at high speed without rotating. Needless to say, the objective can also be achieved by performing uniform overall heating in a heat treatment furnace, ignoring efficiency and equipment costs.

Although there are no particular restrictions on the conditions for cooling to a temperature range lower than 150 ° C. after heating, it is preferable to allow it to cool. This is because the solute nitrogen in the steel material forms an atmosphere around the dislocations during the cooling, and the effect of stopping the dislocations is obtained.

The calculation of the heating index when the isothermal condition is not maintained is performed by the approximation method described below.

When the heating progress is plotted with time on the horizontal axis and temperature on the vertical axis, when a mountain shape is obtained, when the temperature rises, 1
The point where the temperature reaches 50 ° C and the maximum temperature point are connected by a straight line, and it is assumed that 150 ° C has elapsed until the intermediate temperature on the line (the value of 150 ° C and the maximum temperature divided by 2) has passed. Up to the intermediate temperature, the approximate calculation is performed.
When lowering the temperature, connect the maximum temperature to 150 ° C with a straight line, the maximum temperature elapses until the time when the temperature reaches the intermediate temperature on the straight line, and then 1
It is substituted in the formula assuming that the intermediate temperature is reached up to 50 ° C.
In short, a two-step staircase approximation is performed for the time integration of the heating index. As a result of such an approximate calculation, the heating index is 13
It may be 00 or more and less than 4300. Even when there are a plurality of peaks, similar two-step staircase approximation is performed, and as a result of addition, the value may fall within the above range.

[0027]

[Example] Outer diameter of 508 m according to the crushing test method shown in FIG.
m and wall thickness 28.6 mm API standard X65 UO
The crush strength of the E steel pipe was evaluated. One of the test steel pipes is UO
After making a pipe by the E pipe making method, the pipe diameter is at least 0.8% and the maximum is 1.5%.
Expanded between. The steel pipe was heated by a linear heating method in which the steel pipe was heated by one fixed gas burner while being fed in the longitudinal direction while being rotated. The surface temperature 10 seconds after passing the burner is displayed by a non-contact thermometer, and the time at which the temperature is 283 ° C at the lowest part and 625 ° C at the highest part and 250 ° C at the highest part is about 55 minutes. Yes, the heating conditions within the scope of the present invention are satisfied. One of the other test steel pipes was as manufactured by the usual UOE pipe manufacturing method.

In the crush test of FIG. 2, the test steel pipe 1 is placed in a large-diameter and high-strength sheath pipe 2, and both ends of the sheath pipe and the outer periphery of the test steel pipe 1 are hermetically welded with a sealing plate 3. Water pressure was applied until the test steel pipe 1 was crushed. Water pressure is generated by increment of water volume. The test results are shown in FIG. The crushing pressure (strength) of the linearly heated steel pipe according to the present invention is 55 MPa, while that of the unheated UOE steel pipe is 46 MP.
a. The linearly heated steel pipe has a pressure (strength) up to about 20% higher until it is crushed.

[0029]

According to the method of the present invention, by heating a UOE steel pipe to a predetermined temperature range with a simple device, the Bauschinger effect is reduced or eliminated, and a UOE steel pipe excellent in crush resistance is obtained. Is possible.

[Brief description of drawings]

FIG. 1 is a drawing showing that the proof stress at the time of compression after applying tensile deformation changes depending on the heating temperature and the heating time. The vertical axis represents heating temperature and the horizontal axis represents holding time. The value in each circle in the figure represents the proof stress (MPa) against the compressive stress.

FIG. 2 is a drawing showing a crushing test method for steel pipes.

FIG. 3 is a drawing showing the relationship between the water amount increment and the pressure of a steel pipe that was heated after pipe expansion (example) and a steel pipe that was not heated (comparative example).

[Explanation of symbols]

 A: Curve showing the heating index 1300 B: Curve showing the heating index 4300 1: Test steel pipe 2: Sheath pipe 3: Sealing plate 4: Water introduction pipe

Claims (1)

[Claims] 1. A method for producing a steel pipe in which a steel plate is processed into a tubular shape by U-press and O-press and welded, and then expanded. In the method, the expanded steel pipe is in the range of 150 ° C. to less than 700 ° C. at the following temperature and time. The value of the expression (heating index) represented is 1
A method for producing a steel pipe having excellent crush resistance, which comprises heating to 300 or more and less than 4300. Heating index = T x (5 + logt) where T (absolute temperature K) = 273 + heating temperature (° C) (heating temperature is 150 ° C or higher and lower than 700 ° C) t: heating time at 150 ° C or higher (h) log: Common logarithm