JPS6364487B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing steel pipes with excellent collapse strength (strength against crushing due to external pressure), which is particularly important for oil country tubular products, and more specifically relates to a method for manufacturing steel pipes with excellent collapse strength (strength against crushing due to external pressure), which is particularly important for oil country tubular products. This invention relates to a method of processing steel pipes that can ensure the following properties. Recently, due to the tight oil and natural gas situation, oil wells and
Natural gas wells are becoming increasingly deep, and in addition, there are many cases of wet hydrogen sulfide being contained in the produced gas.Amid this trend, high requirements are being placed on oil country tubular goods in terms of corrosion resistance and collapse strength. is being served. However, the corrosion resistance and collapse strength of steel pipes are generally considered to be contradictory to each other.
In other words, to improve collapse strength, it is necessary to increase yield strength, but an increase in yield strength is usually accompanied by an increase in tensile strength, and this increase in tensile strength is directly linked to deterioration of corrosion resistance. be. Due to these circumstances, it is essentially difficult to achieve both corrosion resistance and collapse strength, and it is difficult to achieve both corrosion resistance and collapse strength using simple means such as adjusting the composition of pipe materials. It is simply impossible to deal with it. In order to meet these increasingly sophisticated demands, it can be said that a method of improving collapse strength independent of corrosion resistance is required. The following methods are currently known as this type of method. Perform diameter reduction processing on steel pipes. Straightener processing is omitted. Perform straightener processing at warm temperature. However, each of these methods has its own problems. First, diameter reduction processing increases only the yield strength in the circumferential direction of the pipe, which is directly involved in improving the collapse strength, to a limited extent, but this has a problem with the diameter reduction method itself of the steel pipe. In other words, as a diameter reduction means, it has been considered to use a number of segments divided in the circumferential direction.
This creates a slight difference in the way the segments touch in the circumferential direction of the pipe, and as a result, the rate of increase in yield strength varies at each part in the circumferential direction of the pipe, making it impossible to expect a stable and effective improvement in collapse strength. Next, the normal straightening process, which passes between the upper and lower chain-shaped rolls, generates compressive residual stress in the circumferential direction on the inner surface of the steel pipe and reduces the yield strength due to the housing effect, leading to a deterioration of the collapse strength. From this point of view, the straightener processing itself is omitted, but this does not require advanced pipe manufacturing technology from the viewpoint of maintaining the quality of the steel pipe, and it is especially important to avoid a decrease in the product value for small diameter pipes. is completely impossible. Another method is to perform straightener processing at a warm temperature to prevent the generation of compressive residual stress and decrease in yield strength of the steel pipe, but in order to suppress the generation of residual stress with this method, processing at a considerably high temperature is required. There is a need to do so and there is an economic problem. Furthermore, including the above, measures to prevent the occurrence of compressive residual stress in steel pipes are not proactive measures in the first place, and it cannot be denied that they alone have little effect. As described above, none of the conventionally known methods of increasing collapse strength independently of corrosion resistance is practically sufficient. Now, in order to overcome this situation, the present inventors have been conducting extensive research on effective measures to increase the collapse strength of steel pipes. found a useful relationship between the two. That is,
With the relationship shown in Figure 1, the residual stress (σ _R )
As is generally said, on the compression side (shown with a negative sign in the figure, the expression of compressive stress will follow this), it has a negative impact on the collapse strength, but on the tension side, the yield strength (σ When the stress value is less than 0.15 times _y ), it actually contributes to improving the collapse strength. This relationship is the strength level of the steel pipe,
It has also been confirmed that it is universal and holds true regardless of other conditions. The present inventors have focused on the relationship shown in Figure 1, and have repeatedly conducted experiments and studies on methods for stably obtaining the optimal tensile residual stress in the circumferential direction of the tube inner surface for collapse strength, and in particular on tube processing methods. As a result, we succeeded in completing the present invention shown below. In other words, the gist of the present invention is to apply parallel loads in opposing directions from opposite sides of the tube periphery to two points on the outer surface of the tube that are shifted by 40 to 90 degrees at the center angle of the tube in the same cross section of the tube. The present invention provides a method for manufacturing a high collapse strength steel pipe, which is characterized in that compression processing is applied in all directions of the pipe diameter or in each specific pipe diameter direction, and is carried out over the entire length of the pipe. By carrying out this method, the desired circumferential tensile residual stress can be applied to the inner surface of the tube,
Therefore, the collapse strength can be effectively improved by developing the tensile residual stress that is 0.15 times or less of the yield strength shown in FIG. When a parallel load P is simultaneously applied to four points G on the outer periphery of a tube in the same cross section as shown in FIG. 2 to compress the tube, the stress distribution in the tube cross section can be considered as follows. The position of point m on the inner surface of the tube is determined by the parallel loads P, P
If it is defined by the central angle α taken from the line y in the pipe radial direction passing through the center between the points of application G and G, then this α is from 0 to The moment M ₁ regarding point m when it is within the range of the central angle θ is given by the following equation (1). M ₁ /PD=1/π{(π−2θ)sinθ−2cosθ}=C<
0...(1) Here, D: Outer diameter of the pipe C: Constant Moreover, the above moment _M2 in the range of α=θ to π/2 is obtained by the following equation. M ₂ /PD=1/π{(π−2θ)sinθ−2cosθ} +sinα−sinθ …(2) According to equations (1) and (2) above, specifically, the moment shown in Figure 3 is You can know the distribution state. The figure illustrates the case where θ = π/6, but the bending moment is a negative value at point A on the inner surface of the tube where the reference line y passes, so the stress on the inner surface of the tube has a tensile value at that position. On the other hand, at point B, which is shifted by 90 degrees from point A, the bending moment takes a positive value and the stress on the inner surface of the tube becomes compressive. In this case, if the absolute value of compressive stress (σ _B ) near point B exceeds the tensile stress (σ _A ) at point A, that is, if the condition |σ _B |>σ _A is satisfied, then near point B It is possible to cause the inner surface of the tube to undergo compressive yielding. Therefore, by applying this compressive yielding to the entire circumference of the inner surface of the tube, it is possible to generate residual stress in the tensile direction on the inner surface of the tube. Here, satisfying the above |σ _B |>σ _A means that
The moment _M2 at a point on the inner surface of the tube near point B is
This means that the following formula M ₂ −(−M ₁ )>0 (3) is satisfied. The example of θ = π/6 shown in Fig. 3 is a case that satisfies this equation (3), but here, as an index representing the range around point B on the inner surface of the tube that satisfies equation (3),
Introducing the central angle β taken with reference to the line X in the tube radial direction passing through point B, this β can be found as follows. That is, by substituting equations (1) and (2) into equation (3) above, equation (4) below can be obtained. 2/π {(π−2θ) sinθ−2cosθ} +sinα−sinθ＞0 sinα＞(4θ/π−1) sinθ+4/πcosθ...(4) In other words, this is the central angle θ indicating the point of action of the load P.
This represents the relationship between α and α such that the relationship (3) above holds true. On the other hand, if the limit α that satisfies the condition (3) is now expressed as α ₁ , then between α ₁ and β, β=π/2−α ₁ …(5) Equation (5) holds true. Two. By combining both equations (4) and (5) and eliminating α, the relationship between θ and β can be derived.
FIG. 4 is a diagram of this relationship, and it is understood from the figure that β>0 only in the range of θ20°. This shows that if the load application point G is set at a position where θ is 20° or more, compressive yield on the inner surface of the tube can be obtained without fail. Therefore, based on the method of the present invention, the compression process using the parallel load is performed in all directions of the tube diameter or in each specific tube diameter direction, so that the inner surface of the tube is subjected to compression yielding over the entire circumference, This makes it possible to apply tensile residual stress in the circumferential direction of the inner surface of the tube. In this case, the magnitude of the circumferential tensile residual stress applied to the inner surface of the tube can be controlled to a desired level by adjusting the above θ and applied load P, and therefore the optimal level for collapse strength can be achieved. The above tensile residual stress can be obtained. The method of the present invention is to move the loaded state according to the present invention as shown in Fig. 2 in the circumferential direction of the pipe while maintaining its shape, and to exactly shift it to the initial axially symmetrical position (180° inverted position). , compression processing is performed in all directions of the pipe diameter, or the same load state is made to appear in each pipe diameter direction D ₁ , D ₂ . . . at a predetermined angle γ interval over the entire circumference of the pipe as shown in FIG. It is something.
In this latter case, γ is determined in the relationship shown in Figure 4 above, taking into account the range of β that is determined by θ, and is set within a range that ensures compressive yield on the inner surface of the tube throughout the entire circumference. That's fine. In the present invention, θ is 20° or more (2θ: 40° or more)
The reason for this is obvious from the above explanation of the action, but the reason why this θ is limited to 45° or less (2θ: 90° or less) is because at θ exceeding 45°, it is difficult to apply parallel loads P and P. This is because it is difficult in practice and the generation of stress in the X-ray direction is difficult to avoid, taking into account practicality. Note that in the method of the present invention, the central angles θ and θ facing each other in FIG.
It is sufficient as long as the condition of 45° is satisfied. The method of the invention can be specifically implemented in a wide variety of ways. Here are a few examples.
I'll list it. A pair of machining jigs 2 having a V-block cross-sectional shape as shown in FIG. 6 are used facing each other. By applying both edges 3, 3 of the V-shaped recess of the jig 2 to the pipe 1 and applying pressure from behind, a parallel load P, P is applied to the pipe as shown in Fig. 2.
It is a load. This compression processing operation is performed within the same cross section of the pipe at a predetermined angle γ _every other pipe diameter direction.
Execute every D ₁ , D ₂ …. This γ ₁ is γ shown in Fig. 5, and is determined so that compressive yielding of the inner surface of the tube over the entire circumference is achieved from the range β of the inner surface of the tube where compression yields in one processing as shown in Fig. 4. I decide. Of course, this processing may be performed continuously in the circumferential direction, and in this case, as shown in FIG.
It is recommended to allow smooth relative movement in the circumferential direction with respect to the tube 1 while maintaining the pressurized state. Needless to say, either the tube or the jig may be rotated during processing. In this method, a jig is used that is long enough to process the entire length of the pipe, or a jig with a short length is used to machine the pipe sequentially from one end of the pipe to the other in unit length increments. By adding processing, the entire length of the pipe can be processed. As shown in FIG. 8, a required number of parallel fixed coaxial pair rolls 5, 5 are arranged in tandem, and the roll axis R of each stage is set in a plane perpendicular to the pipe 1 passing between the rolls. The tube 1 is fed between the opposing rolls 5, 5 and allowed to pass through. This method compresses the entire length and circumference of the pipe.
It is characterized by the fact that it is carried out all at once by moving the tube in the axial direction, and can be said to be a highly efficient method suitable for continuous tube manufacturing lines. The number of roll stages and the inclination angle of the roll axis R of each stage are determined based on β in FIG. 4, using the same concept as γ ₁ described above. In this method,
It is rational that each roll is actively driven and used for traveling the pipe 1. However, from the pure point of view of processing, there is no particular difference whether the drive is driven or not.Therefore, even if a drive means for the tube is provided separately and the rolls are simply rotated as the tube travels, there is no difference. No problem. Incidentally, as a conventional method for processing pipes, there has been a method using a chain roll for the purpose of straightening the pipe, but in this method, as shown in Fig. 9, the pipe 1 is subjected to concentrated loads at opposing positions on the outer circumferential surface. P and P are applied, and tensile stress is generated at point A on the inner surface of the tube where the load is applied, and at the same time, compressive stress is generated at point B, which is 90 degrees off from point A at the central angle. However, in this case, the absolute value of the tensile stress at point A is always greater than the compressive stress at point B, regardless of the conditions. In other words, it is completely impossible to obtain a mechanical form that satisfies the aforementioned |σ _B |>σ _A. Next, the effects of implementing the present invention will be described. Outer diameter 177.8mm, with chemical components shown in Table 1.
Steel pipe with wall thickness of 18.54 mm and length of 500 mm (yield strength: 66.2
Kgf/mm ² ) was processed according to the present invention.

[Table] Processing was carried out by using the pair of jigs shown in Fig. 7 as shown in Fig. 6, and by continuously processing in the circumferential direction while rotating the pipe as described above. . The jig used had a length that exceeded the total length of the target pipe, 500 mm, and therefore the entire length of the pipe could be processed in one process. 2θ in Figure 2 is
It was 60 degrees. FIG. 10 is a plot of the load applied during the above-mentioned machining (load value applied per unit length) and the residual stress value in the circumferential direction of the tube inner surface caused by the machining. As is clear from the figure, in the method of the present invention, a circumferential tensile residual stress can be applied to the inner surface of the tube, and by adjusting the applied load P, the tensile residual stress value can be set to 0.15, which is preferable for collapse strength. It can be managed within the range of σ _y (indicated by I in the figure). As a comparative example, we will briefly describe the case of straightener machining using the chain-shaped rolls shown in Fig. 9. As shown in Fig. 11, the amount of crushing, in other words, the applied load P (see Fig. 9) should be controlled. Even if it is set, residual stress in the compressive direction always occurs in the circumferential direction of the inner surface of the tube, and residual stress in tension cannot be obtained. As is clear from the above description, the method of the present invention is capable of generating circumferential tensile residual stress on the inner circumferential surface of a steel pipe that contributes to improving collapse strength. It can be said that this method is effective when applied to the production of oil country tubular goods.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the effect of residual stress in the circumferential direction on the tube inner surface on the collapse strength of the tube, Fig. 2 is a schematic diagram explaining compression processing based on the present invention, and Fig. 3 is a diagram showing the influence of the inner surface of the tube during compression processing. A graph showing an example of moment distribution, FIG. 4 is a diagram showing the relationship between θ and β in the method of the present invention, FIG. 5 is an explanatory diagram showing the procedure for carrying out compression processing based on the present invention, and FIG. FIG. 7 is a front view showing a preferred example of a jig used for the above compression process; FIG. 8 is a front view showing another example of a specific means for performing the above compression process; FIG. 9 is a diagram for explaining the mechanical relationship during conventional straightener processing, and FIG. 10 is a diagram showing the residual stress in the circumferential direction of the tube inner surface when the method of the present invention is applied, corresponding to the load applied. 11 shows the relationship between the amount of crushing and the residual stress in the circumferential direction of the inner surface of the tube after processing when conventional straightener processing is carried out. In the figure, 1: pipe, 2: processing jig, 4: roll,
5: Coaxial fixed type pair roll.

Claims (1)

[Claims] 1. In the same cross-section of a steel pipe, parallel loads in opposite directions from opposite sides of the pipe periphery are applied to the outer surface of the pipe, which is shifted by 40 to 90 degrees at the center angle of the pipe.
1. A method for manufacturing a high collapse strength steel pipe, characterized in that compression processing that applies a load to a point is performed in all directions of the pipe diameter or in each specific pipe diameter direction, and this is carried out over the entire length of the pipe.