JP3161285B2 - Manufacturing method of large diameter welded steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a UOE steel pipe having a high crushing strength used for a submarine pipeline or the like.

[0002]

2. Description of the Prior Art When laying a submarine line pipe, the first thing to be noted is to prevent the steel pipe from being collapsed by seawater pressure. Therefore, when designing a submarine line pipe, the strength, outer diameter, wall thickness, etc. of the steel sheet must be designed so as not to cause crushing, but such conditions are not accurately understood. .

[0003] A typical method of manufacturing a thick-walled large-diameter welded steel pipe is a UOE pipe manufacturing method. This method comprises the steps of forming a steel pipe into an annular shape by a U-press and an O-press, performing seam welding, expanding the steel sheet, and producing a steel pipe. The last expansion is a step of expanding the diameter of the pipe by a mechanical expander, and is a step necessary for adjusting the outer diameter and the shape. As described below, these plastic deformations are factors that promote crushing when an external pressure is applied.

[0004] The direction of plastic deformation in the UOE pipe manufacturing method is described below. In both cases, the U-press and the O-press apply a tensile deformation to the surface side from the center of the thickness of the steel sheet. In the subsequent pipe expansion process, all positions in the thickness direction undergo tensile deformation and are shipped as they are. Eventually, the steel pipe is shipped in a state where it has undergone tensile plastic deformation at most of its thickness except for the part near the inner wall of the steel pipe, and is laid as a submarine line pipe. When laid on the sea floor, the steel pipes are lowered onto the sea floor while being sequentially welded on the ship, so the inside is subjected to atmospheric pressure and the outside is subjected to water pressure. For this reason, steel pipes are subjected to compressive stress due to the pressure difference on the seabed. In other words, after the tube is subjected to a tensile deformation, the tube is subjected to a compressive stress that is opposite to the compressive stress when the tube is laid.

In general, when a compressive stress is applied after a tensile deformation, the proof stress against the compressive stress decreases. The reverse is also true, that is, the case where compression deformation is first applied and then tensile stress is applied, and the proof stress against tensile stress is reduced. The phenomenon in which the proof stress in the direction opposite to the immediately preceding deformation direction is reduced when an alternating stress is applied is known as the Bauschinger effect.

[0006] Plastic deformation such as press working and pipe expansion is as follows.
It proceeds with the movement and proliferation of a kind of lattice defect called dislocation. Steel that is easy to move and propagate dislocations is soft and therefore has low yield strength. Generally, steel is soft at the stage when plastic deformation is small and dislocation multiplication has just started, but the steel becomes harder as plastic deformation progresses. When plastic deformation progresses and hardens, high density dislocations that are entangled and hard to move are formed in the steel.

[0007] The Bauschinger effect is caused by the formation of an array of dislocations which, during the first plastic deformation, are difficult to move and grow only in a certain direction. Such an arrangement of dislocations will continue to harden as long as the deformation increases in the first direction. However, when a stress is applied in the opposite direction and deformation starts in the direction of the stress, some of the dislocations in the array of dislocations initially formed move and proliferate in that direction without great resistance. Therefore, plastic deformation in the opposite direction easily occurs. This means that the steel has a low yield strength. As the compression deformation proceeds, the hardening starts, and the dislocations form an array of dislocations in the direction of compression (in the direction opposite to the initial direction) similar to the direction of the initial tension. Such an explanation holds when a complicated phenomenon is greatly simplified.

When a steel pipe manufactured by the UOE pipe manufacturing method is subjected to a compressive stress, the crushing strength of the steel pipe is low due to the Bauschinger effect described above. Compressive stress due to bending, etc. associated with the construction of submarine pipelines overlaps with compressive stress caused by water pressure, causing local buckling.Depending on conditions, this local buckling may cause the already installed part to buckle over a long distance. Can spread and lead to serious accidents. UOE steel pipes whose proof strength has been reduced by the Bauschinger effect are liable to cause such local buckling. There have been proposals to overcome such weaknesses of UOE steel pipes.
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 spiral-ribbed steel pipe proposed by A.
This proposal is expected to have significant computational benefits. However, the cost of attaching the ribs is large and handling is not easy, so that the possibility of practical use is small.

(B) It has been evaluated by calculation that the crushing strength does not decrease when the dimensions and shape are adjusted by replacing the expanding step of the UOE pipe forming step with the contracting step (S). .Kyriakides, E.Corona & FJFisher:
On the Effect of the UO-EManufacturing Process o
n the Collapse Pressure of Long Tubes, J. of Engin
eering for Industry, 116 (1994), p. 93). Although this method is certainly effective for external pressure, there is a concern that the strength will be reduced due to the Bauschinger effect for internal pressure, and this method requires the installation of a new large-scale device for contraction. Not realistic from an economic point of view.

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

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

[0012]

As described above, the conventional U
There is no example in which the OE pipe manufacturing method is used as it is to prevent a decrease in the crushing strength without increasing the cost. Moreover, there is no example suggesting 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 crushing strength of a steel pipe manufactured using the existing UOE pipe manufacturing apparatus as it is.

[0014]

The present inventor has proposed a 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 properties of the material, and the proof stress of the UOE steel pipe during compression may be improved. I guessed. That is, by appropriate heating, (a) the arrangement of the dislocation itself, which is the source of the Bauschinger effect, or (b) the arrangement of the dislocation is not largely changed, and a precipitate is newly generated on the dislocation of such an arrangement. The two effects of dislocation pinning can be expected, and it is considered that the above effects can be obtained.

Therefore, a steel slab was cut out from a smelted and rolled steel plate in the laboratory and subjected to tensile deformation, and each steel slab was kept at each temperature for various times, allowed to cool, and subjected to a proof stress (0.2%) by a compression test. % Proof stress) was evaluated. 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 if an appropriate heating temperature and heating time are selected, the proof stress against the compressive stress after tensile deformation can be improved.

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

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

[0018] In the method for producing a steel pipe to be expanded after forming and welding a steel sheet into a tubular shape by a U press and an O press, the expanded steel pipe is expressed in the range of 150 ° C or more and 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, wherein the heating is performed so as to be 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 more and less than 700 ° C.) t: heating time of 150 ° C. or more (h) log: Common logarithm

[0020]

The purpose of heating a steel pipe to 150 ° C. or more is that, at temperatures lower than this, it takes a long time to precipitate on dislocations or rearrange the dislocations.
This is because the effect cannot be reduced or eliminated until practically no problem occurs. The reason why the temperature is set to less than 700 ° C. is to prevent deterioration of the material.

The reason why the heating index is set to 1300 or more is that if the heating index is higher than 1300, the yield strength is improved.
The reason why it is set to less than 0 is that if it is more than 0, the tempering proceeds too much and the steel is softened. The reason why the heating index is introduced is that if the temperature is high for a short time, and if the temperature is low for a long time, the target yield strength can be improved, so that the effect of time and temperature can be expressed by one index. In addition, the reason why the constant 5 is adopted here and the variable 20 frequently 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 with a short heating time. This is because the fact is represented by a heating index. In the case of using 20, the effect of the temperature change is hard to appear in the change of the heating index as the effect of the heating time unless the heating is performed for a long time. This is because it can be correlated with the actual Bauschinger effect.

As a material for the UOE steel pipe, an accelerated cooling steel sheet is often used. The accelerated cooling steel sheet refers to a steel sheet which is cooled at a constant accelerated cooling rate by a predetermined cooling device in which the amount of water is controlled after the steel sheet is controlled and rolled. Since the strength can be improved by suppressing the amount of ferrite, this is an effective production method for improving weldability. Normally, niobium (Nb) is added to these accelerated cooling steel sheets in order to utilize the effects of increasing the strength and controlling rolling. Since the Nb solid solution concentration of the accelerated cooling steel sheet is higher than that of the steel sheet not subjected to accelerated cooling, the amount of carbonitride precipitated increases during heating according to the present invention. At this time, the amount of precipitation to dislocations also increases,
Dislocation activation with low stress in the opposite direction can be more strongly pinned. Therefore, U to which the method of the present invention is applied
The steel plate for OE steel pipe is made of Nb manufactured by the accelerated cooling method.
Addition steel sheets are preferred.

There are no particular restrictions on the method of heating the steel pipe. It is particularly important 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 substantially no problem irrespective of whether it is isothermal or non-isothermal and whether the steel pipe portion is uniform or non-uniform. Therefore, it is possible to heat a steel pipe while rotating it in a spiral with one gas burner, and to heat several or several tens of burners in a strip shape for the purpose of improving efficiency. Can also. Alternatively, the same effect can be obtained by passing the gas burner arranged in a ring at a high speed without rotating the gas burner. Of course, if efficiency and equipment costs are neglected, it is needless to say that the object can be achieved by performing uniform overall heating in the heat treatment furnace.

After heating, the condition for cooling to a temperature lower than 150 ° C. is not particularly limited. This is because the solid solution nitrogen in the steel material forms an atmosphere around the dislocation during cooling, and an effect of stopping the dislocation can be obtained.

The calculation of the heating index when the isothermal holding is not performed is performed by the following approximation method.

The progress of heating is plotted with time on the horizontal axis and temperature on the vertical axis.
The point at which 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 up to an intermediate temperature (a value obtained by dividing the sum of 150 ° C. and the maximum temperature by 2) on the straight line. Approximate calculation is performed assuming that the time has elapsed at the intermediate temperature.
At the time of cooling, the maximum temperature and 150 ° C are connected by a straight line, and the maximum temperature elapses until the intermediate temperature is reached on the straight line.
It is substituted in the equation assuming that the intermediate temperature passes up to 50 ° C.
In short, two steps are approximated for the time integration of the heating index. As a result of such an approximate calculation, the heating index is 13
What is necessary is just to be 00 or more and less than 4300. Even when a plurality of peaks are formed, similar two-step staircase approximation is performed, and as a result of the addition, the value may be within the above range.

[0027]

EXAMPLE An outer diameter of 508 m was obtained by the crush test shown in FIG.
U of API standard X65 material of m and wall thickness 28.6mm
The crushing strength of the E steel pipe was evaluated. One of the test steel tubes is UO
After making pipe by E pipe making method, minimum 0.8%, maximum 1.5% in pipe diameter
Expanded between. The steel pipe was heated by a linear heating method in which it was heated by one fixed gas burner while being fed in the longitudinal direction while rotating. The surface temperature after 10 seconds after the burner is indicated by a non-contact thermometer, and the time during which the temperature was at least 283 ° C at the lowest temperature, 625 ° C at the highest temperature, and at least 250 ° C at the highest temperature was about 55 minutes. Yes, it satisfies the heating conditions within the scope of the present invention. One of the other test steel pipes has been manufactured by a usual UOE pipe manufacturing method.

In the crush test shown in 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 increasing water volume. The test results are shown in FIG. The crushing pressure (strength) of the steel pipe linearly heated according to the present invention is 55 MPa, while that of the unheated UOE steel pipe is 46 MPa.
a. The linearly heated steel pipe has about 20% higher pressure (strength) until crushing.

[0029]

According to the method of the present invention, the UOE steel pipe is heated to a predetermined temperature range by a simple device to reduce or eliminate the Bauschinger effect and to provide a UOE steel pipe excellent in crush resistance. Is possible.

[Brief description of the drawings]

FIG. 1 is a drawing showing that the proof stress at the time of compression after applying tensile deformation changes with heating temperature and heating time. The vertical axis is the heating temperature, and the horizontal axis is the 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 method for crushing a steel pipe.

FIG. 3 is a drawing showing the relationship between the water amount increment and the pressure for each of a steel pipe heated after pipe expansion (Example) and a steel pipe not heated (Comparative Example).

[Explanation of symbols]

 A: Curve representing heating index 1300 B: Curve representing heating index 4300 1: Steel tube for test 2: Sheath tube 3: Sealing plate 4: Water inlet tube

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/00-9/44 C21D 9/50 B21C 37/08

Claims (1)

(57) [Claims] 1. A method for producing a steel pipe which is formed by processing a steel sheet into a tube by a U-press and an O-press and welding, and then expanding the expanded steel pipe in a range of 150 ° C. or more and 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, wherein the heating is performed so as to be 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 more and less than 700 ° C.) t: heating time at 150 ° C. or more (h) log: Common logarithm