Seamless pipe and method for manufacturing the same

Abstract

The present invention provides a seamless pipe having excellent pressure resistance and a method for manufacturing the same. A seamless pipe is formed with a thin-walled portion in the circumferential direction of the pipe in the pipe axis direction. A line segment formed by connecting one end and the other end of the thin-walled portion along the pipe surface at the shortest distance in the formation direction of the thin-walled portion is inclined at an angle α of 5.0° or more with respect to the pipe axis direction. Preferably, the one end and the other end of the thin-walled portion are set in a region in the pipe selected from the shorter length among the length of 1.0 m in the pipe axis direction and 90% of the length in the pipe axis direction for one rotation of the thin-walled portion in the circumferential direction of the pipe.

Description

Technical Field

[0001] The present invention relates to a seamless pipe having excellent pressure resistance and a method for manufacturing the same. Background Art

[0002] For seamless pipes used for connections of pressure vessels and for oil well / gas well drilling, in addition to mechanical properties such as strength and toughness, and corrosion resistance for withstanding corrosive environments, pressure resistance for withstanding various temperature and pressure environments is also required. There are various forms of pressure and external force applied to seamless pipes, but for example, in pressure vessels and their piping, large internal pressure is often generated. In addition, when the content is at a high temperature or the external air temperature is high, the seamless pipe thermally expands, and thus axial compressive stress is generated between the connection parts at the pipe ends. On the contrary, when the content and the external air temperature are low, tensile stress is generated in the axial direction due to thermal contraction. That is, in addition to the internal pressure, axial stress is also generated.

[0003] In addition, for example, seamless pipes for oil wells / gas wells are inserted into the ground or the sea during exploitation. In this case, high external pressure is generated in the seamless pipe. The external pressure increases with the increase in depth, but at the same time the temperature also rises and the material softens, so plastic deformation caused by the external pressure is likely to occur. In addition, since seamless pipes for resource exploitation are connected in series from the ground, high tensile stress generated by their own weight is often applied together with the external pressure. Furthermore, the seamless pipe sometimes bends in the traveling direction during exploitation, and high compressive stress is sometimes generated on the inner side with a small bending radius of curvature, and high tensile stress is generated on the outer side with a large radius of curvature.

[0004] Since there is no seam in the circumferential direction of the seamless pipe, it is often used in such a severe pressure and stress environment. Here, the plastic deformation and failure of the seamless pipe due to pressure are called buckling. Buckling occurs when the generated internal pressure and external pressure are greater than the yield strength of the seamless pipe. In addition, if axial tensile stress and compressive stress are generated simultaneously with the external pressure and internal pressure, it is more likely to occur. Furthermore, a decrease in the roundness of the seamless pipe and an increase in wall thickness unevenness (non-uniformity of wall thickness) also easily cause buckling.

[0005] Buckling can be prevented by increasing the yield strength of the steel pipe material and ensuring excellent pressure resistance. Therefore, a seamless steel pipe having increased circumferential and axial yield strengths has been disclosed (Citation Document 1). In addition, since a reduction in the roundness and wall thickness unevenness of the seamless pipe is also effective in suppressing buckling, a method for manufacturing a seamless steel pipe with improved roundness and wall thickness unevenness has been disclosed (Citation Document 2).

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent No. 6596954

[0009] Patent Document 2: Japanese Patent No. 5831195 Summary of the Invention

[0010] In Cited Document 1 and Cited Document 2, in order to improve the pressure resistance performance, methods for increasing the strength and improving the dimensional accuracy of seamless steel pipes have been studied, but it cannot be said that they are sufficient as technologies for ensuring excellent pressure resistance performance.

[0011] The present invention has been completed in view of the above actual situation, and an object thereof is to provide a seamless pipe having excellent pressure resistance performance and a method for manufacturing the same.

[0012] In order to improve the pressure resistance performance, it is effective to increase the yield strength (yield stress) in the circumferential direction and axial direction of the seamless pipe so as to withstand pressures and external forces applied from various directions.

[0013] In addition, it is effective to make the roundness as good as possible in improving the pressure resistance performance. By optimizing the manufacturing conditions of sizing rolling and straightening rolling that can adjust the dimensions of the seamless pipe, the pressure resistance performance can be improved.

[0014] On the other hand, regarding the influence of wall thickness unevenness generated during the manufacture of the pipe on the pressure resistance performance, it is known that if the wall thickness unevenness becomes smaller, the pressure resistance performance improves, and in the case of buckling due to wall thickness unevenness, the thin wall portion caused by the wall thickness unevenness becomes the starting point of buckling.

[0015] In this regard, if a seamless pipe without wall thickness unevenness can be provided through various rolling processes with good productivity, products with high pressure resistance performance can be supplied. In its application, the degree of freedom in design can be increased, the range of utilization can be expanded, and safety can be improved.

[0016] Regarding this wall thickness unevenness, it is known that it is generated due to various factors such as uneven heating of the tube blank during hot rolling, tool wear, friction changes, and deviation from the equipment installation position.

[0017] Among the wall thickness unevenness generated due to such various factors, especially the wall thickness unevenness generated in the process of opening holes in the tube blank at the initial stage of hot rolling, for example, the wall thickness unevenness generated by the Ugine-Sejournet method, the Erhardt push bench method, and the Mannesmann method, is difficult to correct in the subsequent hot rolling downstream process and the subsequent cold rolling process, and remains in the product.

[0018] However, in order to obtain high pressure resistance performance, although the above yield strength and roundness can be adjusted by optimizing the added chemical components, sizing rolling conditions, etc., for wall thickness unevenness, a certain degree of generation is inevitable, and it is necessary to consider wall thickness unevenness in advance for product design.

[0019] Therefore, considering the reduction in pressure resistance performance caused by wall thickness unevenness, it is necessary to limit the size or use expensive additive elements while considering safety.

[0020] So far, various studies have been conducted to eliminate wall thickness unevenness. However, due to the complex and various causes of wall thickness unevenness, there is no technology that can stably reduce wall thickness unevenness, and it can only be managed by limiting the maximum amount of wall thickness unevenness.

[0021] The present inventors have conducted in-depth research in view of the above problems and found that by controlling the distribution of unavoidably generated wall thickness unevenness, the pressure resistance performance can be improved. That is, so far, research has mainly been conducted from the perspective of suppressing the generation of wall thickness unevenness. However, the present inventors believe that the generation of wall thickness unevenness is inevitable, and it has been found that by distributing the thin-wall portions generated by this wall thickness unevenness in a specific spiral shape or the like on the tube axis, the pressure resistance performance can be improved compared to a tube in which the thin-wall portions are distributed in a shape close to a straight line in the tube axis direction.

[0022] The gist of the present invention completed based on the above findings and further research is as follows.

[0023] [1] A seamless tube having a thin-wall portion in the circumferential direction formed in the tube axis direction,

[0024] A line segment formed by connecting one end and the other end of the above thin-wall portion along the tube surface at the shortest distance in the formation direction of the thin-wall portion is inclined at an angle α of 5.0° or more with respect to the tube axis direction.

[0025] [2] The seamless tube according to the above [1], wherein the one end and the other end of the thin-wall portion are set in a region of a tube selected from the shorter length of the length of 1.0 m in the tube axis direction and 90% of the length of the tube axis direction for one rotation of the thin-wall portion in the circumferential direction.

[0026] [3] The seamless tube according to the above [1] or [2], wherein the average outer diameter D ave [mm] and the angle α [°] satisfy the following formula (1):

[0027] D ave / α = 0.5 to 15.0 [mm / °] ··· Formula (1).

[0028] [4] The seamless pipe according to any one of [1] to [3] above, wherein the ratio of the compressive yield strength [MPa] in the pipe axis direction to the tensile yield strength [MPa] in the pipe axis direction is 0.85 or more.

[0029] [5] The seamless pipe according to any one of [1] to [4] above, wherein at least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the radius of curvature of the corner portion formed by the side surface of the fastening portion and the bottom surface of the thread groove is 0.2 mm or more.

[0030] [6] The seamless pipe according to [5] above, wherein at least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the fastening portion has a metal contact seal portion and a torque shoulder.

[0031] [7] A method for manufacturing a seamless pipe, which is a method for manufacturing the seamless pipe according to any one of [1] to [6] above, while rotating the pipe blank and advancing it in the pipe axis direction, the pipe blank is pierced by hot rolling,

[0032] As cold working of the pipe after the hot rolling, bending-unbending processing in the pipe circumferential direction is performed,

[0033] And, the pipe length LF [mm] after hot rolling, the pipe length LP [mm] after piercing rolling, and the advancement amount X [mm] of the pipe in the rolling direction when the pipe rotates one turn during piercing rolling satisfy the following formula (2):

[0034] (LF / LP)×X≤1100 [mm] ··· Formula (2)

[0035] According to the present invention, there is provided a seamless pipe having excellent pressure resistance and a method for manufacturing the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a diagram for explaining the distribution of the thin wall portions generated due to the wall thickness unevenness in the seamless pipes of the respective embodiments.

[0037] Figure 2 It is a diagram for explaining the bending-unbending processing in the pipe circumferential direction.

[0038] Figure 3 It is a longitudinal sectional view of the pipe axis direction (a sectional view parallel to the pipe axis direction) of the fastening portions of the external thread and the internal thread, Figure 3 (a) shows the case of a trapezoidal thread, Figure 3 (b) shows the case of a triangular thread.

[0039] Figure 4 It is a longitudinal sectional view of the pipe axis direction (a sectional view parallel to the pipe axis direction) of the threaded joint, Figure 4 (a) shows the case of an API threaded joint,Figure 4 (b) shows the case of a special threaded joint.

[0040] Figure 5 It is a schematic view near the head that is an extension of the pin. Figure 5 (a) is a cross-sectional view taken along the pipe axis direction parallel to the pipe axis of the pin and the coupling fastening part. Figure 5 (b) is the torque shoulder of the threaded front end of the pin observed from the front of the pin front end. Detailed implementation mode

[0041] <First implementation mode>

[0042] Hereinafter, the first implementation mode of the present invention will be described with reference to the drawings.

[0043] The seamless pipe of this implementation mode is formed with a thin-walled part in the circumferential direction of the pipe in the pipe axis direction. Along the pipe surface, a line segment formed by connecting one end and the other end of the thin-walled part at the shortest distance in the formation direction of the thin-walled part is inclined at an angle α of 5.0° or more with respect to the pipe axis direction (hereinafter also referred to as the inclination angle α or the wall thickness unevenness torsion angle α), and has excellent pressure resistance performance.

[0044] The thin-walled part of the seamless pipe of this implementation mode is formed to surround in the pipe axis direction, and for example, has a form like a spiral.

[0045] Here, the thin-walled part refers to the part with the smallest wall thickness in the circumferential direction of the pipe formed due to wall thickness unevenness (primary wall thickness unevenness) generated by using inclined rolling in piercing rolling and cold working. In addition, the above pipe surface can be either the inner surface or the outer surface of the pipe. In addition, one end and the other end of the above thin-walled part refer to one end and the other end of the thin-walled part in the area when an arbitrary measurement area is selected in the pipe axis direction.

[0046] The setting positions of the above one end and the other end are not particularly limited, but in order to further improve the measurement accuracy of the angle α, it is preferably selected from the length of 1.0 m in the pipe axis direction in (1) and 90% of the length in the pipe axis direction of one turn of the thin-walled part in the circumferential direction of the pipe in (2). The area in the pipe with the shorter length is used to set one end and the other end of the thin-walled part.

[0047] As the selection of the area in the above pipe, in the case of selecting the condition of (2), its length may not be 90% of the length in the pipe axis direction of one turn of the thin-walled part in the circumferential direction of the pipe. For example, it may be 40% or less than 40%.

[0048] Tilt angle α: 5.0° or more

[0049] Figure 1 It is a diagram for explaining the distribution of the thin-walled part generated due to wall thickness unevenness in the seamless pipe of this implementation mode.

[0050] Figure 1 The t min part can be measured by non-destructive investigation of the wall thickness distribution of the seamless pipe during production, and represents the thin wall part caused by the wall thickness unevenness due to piercing rolling. It should be noted that in the above non-destructive wall thickness distribution investigation, Fourier transform is performed after non-destructive inspection.

[0051] Figure 1 Among them, (a) and (b) are diagrams showing the distribution of the t min part on the pipe axis after cutting and unfolding the seamless pipe along the pipe axis direction. Figure 1 (a) is a comparative example (conventional example), Figure 1 (b) is a developed view of an embodiment of the present invention. α is the wall thickness unevenness torsion angle in the pipe axis direction of the line segment formed by connecting one end and the other end of the thin wall part along the pipe surface in the shortest distance in the formation direction of the thin wall part. Preferably, α can be the inclination angle of the line segment formed by connecting the wall thickness uneven part at one end and the wall thickness uneven part at the other end along the pipe surface in the shortest distance in the formation direction of the thin wall part relative to the pipe axis direction in a pipe selected from the shorter length among 90% (preferably 40%) of the length of 1.0 m in the pipe axis direction and the circumferential direction length of one turn of the thin wall part in the circumferential direction.

[0052] Here, the measurement of the inclination angle α is preferably performed at the center of the length direction of the measurement target area at the center of the length direction of the pipe.

[0053] The inventors used a plurality of seamless pipes that inevitably had wall thickness unevenness, had the same wall thickness unevenness amount, and different inclination angles α respectively, and studied the relationship between the inclination angle α and the pressure resistance performance. As a result, it was found that when the inclination angle α was 5.0° or more, the pressure resistance performance was significantly improved.

[0054] In addition, it was confirmed that this superiority was also exhibited in the case where tensile stress, compressive stress, and bending stress were simultaneously generated in addition to internal pressure and external pressure. From the viewpoint of further improving the pressure resistance performance, the above inclination angle α is preferably 15° or more, more preferably 25° or more. In addition, if the angle is too large, sometimes the inclined rolling efficiency decreases, so the above inclination angle α is preferably 80° or less, more preferably 60° or less.

[0055] Wall thickness non-uniformity

[0056] Wall thickness unevenness inevitably occurs in seamless pipes manufactured by rolling including hot and cold. The wall thickness unevenness amount [%] in the wall thickness distribution of the pipe during production by piercing rolling uses the wall thickness of the thickest part of the whole pipe: maximum wall thickness t max [mm], the wall thickness of the thinnest part: minimum wall thickness t min[mm] and the average value of the wall thickness distribution of the pipe: average wall thickness t ave [mm], which is represented by the following formula (3).

[0057] ((t max -t min ) / t ave ) × 100 [%] ··· Formula (3)

[0058] In the above formula (3), the average wall thickness t ave is obtained by measuring the wall thickness t at 32 points at intervals of 11.25° in the circumferential direction of the pipe starting from the thin-walled part at the central part in the length direction of the pipe and calculating their arithmetic mean. The thickness measurement can be carried out by non-destructive testing such as using ultrasonic waves.

[0059] In seamless pipes manufactured by rolling, although it varies depending on the product thickness, a wall thickness non-uniformity of about 2 - 15% is inevitably generated. Wall thickness non-uniformity is most likely to occur during piercing rolling. The causes of wall thickness non-uniformity at this time involve multiple aspects such as temperature non-uniformity of the billet before piercing, friction coefficient of the tool, backlash of the equipment, etc., so its generation is inevitable, and it is necessary to predict the product specifications with a certain degree of wall thickness non-uniformity. In addition, products with excessive wall thickness non-uniformity cannot meet the pressure resistance performance and are discarded. Therefore, it results in a reduction in productivity due to restrictions on manufacturing conditions, or restrictions on product shape and chemical composition in order to ensure pressure resistance performance. In this regard, in the pipe of the present embodiment, for the inevitably generated wall thickness non-uniformity, when the wall thickness non-uniformity is 2% or more, the effect can be confirmed without an upper limit. On the other hand, in the present embodiment, if there is too much wall thickness non-uniformity, it may sometimes have an adverse effect on characteristics other than pressure resistance performance, so the wall thickness non-uniformity is preferably managed to be 20% or less. More preferably, it is 12% or less, and further preferably, it is 10% or less.

[0060] It should be noted that, as described above, the wall thickness can be measured by various non-destructive inspections. For example, the wall thickness distribution of the entire length of the pipe can be measured by ultrasonic waves, and the above-mentioned maximum wall thickness t max , minimum wall thickness t min , and average wall thickness t ave can be output.

[0061] In addition, by performing frequency analysis on the wall thickness distribution of the entire length of the pipe using Fourier transform, only the wall thickness non-uniformity distribution (distribution of primary wall thickness non-uniformity) can be extracted, and the inclination angle (wall thickness non-uniformity torsion angle) α at the central part in the length direction of the pipe can be calculated. In the present invention, the inclination angle α measured in this way can be significantly controlled to control the pressure resistance performance.

[0062] Form of uneven wall thickness (D ave / α = 0.5 to 15.0)

[0063] In this embodiment, even when inevitable wall thickness unevenness occurs on the pipe, the pressure resistance performance can be improved through the control of the form of the wall thickness unevenness, and various restrictions related to the above-mentioned wall thickness unevenness can be reduced.

[0064] Specifically, by making Figure 1 the shown inclination angle α be 5.0° or more, for example, by adopting a form in which the thin-wall portions are spirally distributed in the pipe axis direction of the seamless pipe, compared with a form in which the inclination angle is less than 5.0° and the thin-wall portions are linearly distributed, the rigidity in the pipe axis direction can be improved, and the pressure resistance performance can be significantly improved.

[0065] In addition, if the inclination angle α is managed according to the outer diameter of the pipe, the effect is further improved. The inventors investigated various forms and as a result, confirmed that if the average outer diameter D ave [mm] of the seamless pipe is used to manage the inclination angle α [°] by the following formula (1), better pressure resistance performance can be obtained.

[0066] D ave / α = 0.5 to 15.0 [mm / °] ··· Formula (1)

[0067] If D ave / α is 0.5 mm / ° or more, good pressure resistance performance can be obtained, but if D ave / α is less than 0.5 mm / ° sometimes the efficiency of inclined rolling decreases. In addition, if D ave / α is 15.0 mm / ° or less, good pressure resistance performance can be obtained, but if D ave / α exceeds 15.0 mm / ° there is a possibility that the improvement effect of the pressure resistance performance decreases.

[0068] Thus, it is preferable that the inclination angle α is 5.0° or more and satisfies the above formula (1). More preferably, D ave / α is 3.0 mm / ° or more, further preferably 5.0 mm / ° or more, and still further preferably 6.0 mm / ° or more. In addition, more preferably, D ave / α is 12.0 mm / ° or less, further preferably 11.0 mm / ° or less, and still further preferably 8.0 mm / ° or less.

[0069] The above-mentioned average outer diameter D ave is obtained by measuring the outer diameters D at 9 points at 40° intervals in the circumferential direction of the pipe starting from the thin-wall portion and calculating the number average of these 9 points. In addition, the average outer diameter D ave is measured at a position between l / 5 and (4×l) / 5 in the pipe axis direction with respect to the pipe length l from the pipe end. In addition, it can be averaged by measuring at one or more positions within this range.

[0070] Compressive yield strength in the tube axis direction [MPa] / Tensile yield strength in the tube axis direction [MPa]: 0.85 or more

[0071] In addition, in the seamless pipe of the present embodiment, it is preferable that "compressive yield strength [MPa] in the pipe axis direction / tensile yield strength [MPa] in the pipe axis direction" is 0.85 or more. By having "compressive yield strength [MPa] in the pipe axis direction / tensile yield strength [MPa] in the pipe axis direction" be 0.85 or more, the pressure resistance performance is more excellent.

[0072] In addition, there is no need to particularly set an upper limit for "compressive yield strength [MPa] in the pipe axis direction / tensile yield strength [MPa] in the pipe axis direction". However, if the compressive yield strength in the pipe axis direction is too large relative to the tensile yield strength in the pipe axis direction, the orientation dependence on other mechanical properties such as toughness also becomes stronger. Therefore, it is preferably 1.15 or less.

[0073] It should be noted that the measurement of the compressive yield strength in the pipe axis direction can be measured by a cylindrical compression test. The cylindrical test piece for compression is collected from the center of the wall thickness parallel to the pipe axis direction. In addition, it is sufficient that the cylindrical outer diameter d [mm] and the cylindrical height h [mm] satisfy h / d ≤ 2.0. Specifically, a test piece is cut out from the center of the wall thickness of the pipe with a cylindrical outer diameter d = 5.0 mm and a cylindrical height h = 8.0 mm. The compression test is in the form of applying a load while clamping the test piece between flat plates at room temperature (25°C), and the compressive yield strength is calculated using the stress-strain curve obtained during compression. The stress-strain curve is obtained by performing 30% compression at a compression speed of 1.0 mm / min in a compression testing machine.

[0074] In addition, for the tensile yield strength in the pipe axis direction, according to JIS Z2241, first, as a test piece, a round bar tensile test piece with a parallel part diameter of 5.0 mm is cut out from the center of the wall thickness of the pipe parallel to the pipe axis direction. Then, at room temperature (25°C), a tensile test is performed at a crosshead speed of 1.0 mm / min until fracture. Using the stress-strain curve obtained thereby, the tensile yield strength is calculated.

[0075] Manufacturing method of seamless tube (control method of tilt angle α)

[0076] Hot rolling

[0077] A preferred method for manufacturing a seamless pipe for which the inclination angle α is 5.0° or more will be described. The inventors conducted in-depth research, and as a result, the inventors found that if an inclined rolling mill is used and its manufacturing conditions are controlled, a seamless pipe that satisfies the present embodiment can be obtained.

[0078] Specifically, there are several methods in piercing rolling for manufacturing seamless pipes, such as the Mannesmann method, the Mannesmann - Séjournet method, and the Erhardt plug mill method.

[0079] Among them, in the present embodiment, the Mannesmann method using inclined rolling is suitable, while the Mannesmann method and the Mannesmann plug mill method, which perforate and extrude by tools without rotating the tube blank, are not suitable.

[0080] Regarding the inevitable wall thickness non-uniformity described above, in the case of perforating by tools without rotating the tube blank, such as the Mannesmann method and the Mannesmann plug mill method, the thin-walled part is formed in a shape close to parallel to the tube axis direction, and the inclination angle α is close to 0° (less than 5.0°). In addition, in the perforation methods such as the Mannesmann method and the Mannesmann plug mill method, no consideration is given to controlling the torsional angle of the wall thickness non-uniformity, and its control is also impossible in terms of the device structure.

[0081] On the other hand, in the Mannesmann method, the tube blank is rotated for piercing rolling. In addition, in the Mannesmann method, torsional deformation of the tube occurs in the tube axis direction. Conventionally, in the case of piercing rolling by the Mannesmann method, various control methods for changing the rolling conditions are carried out to reduce the wall thickness non-uniformity. In addition, research has been conducted on suppressing excessive strain for the torsional deformation in the tube axis direction of the tube, but no active control of its distribution has been considered for the forward movement and torsional deformation in the tube axis direction per revolution of the blank. In this regard, in the present embodiment, the Mannesmann method is adopted to control the torsional deformation and rotation of the tube, control the inclination angle α, and manufacture a seamless tube with excellent pressure resistance.

[0082] In addition, in the method for manufacturing a seamless tube for controlling the wall thickness non-uniformity form of the present embodiment, in the piercing rolling using an inclined rolling mill based on the Mannesmann method, it is preferable to perform control satisfying the following formula (2) during rolling.

[0083] (LF / LP)×X≤1100[mm] ··· Formula (2)

[0084] In formula (2),

[0085] X [mm]: The forward movement of the tube in the rolling direction per revolution of the tube during piercing rolling

[0086] LF [mm]: The tube length after hot rolling

[0087] LP [mm]: The tube length after piercing rolling

[0088] Even when manufacturing tubes with the same average wall thickness and outer diameter, the forward movement X of the tube in the rolling direction can be variously changed by adjusting the inclination angle of the rolling rolls, the roll gap, and the protrusion amount of the plug on the rolled inner surface. Specifically, for example, if the inclination angle of the rolling rolls is increased, the component force in the forward direction increases, and the tube blank advances at a lower rotational speed. Therefore, the forward movement X of the tube in the rolling direction increases.

[0089] In addition, the average wall thickness of the tube is controlled by the balance between the roll gap and the protrusion amount of the plug. However, even when tubes with the same average wall thickness are obtained, the combinations are various, and the forward movement amount X in the rolling direction of the tube can be controlled by the combination. Specifically, for example, even when the roll gap is reduced without protruding the plug and when the roll gap is increased with the plug protruding, the same wall thickness can be obtained. However, when the plug protrudes, the forward movement amount X in the rolling direction can be further reduced by the resistance of the plug.

[0090] In the present embodiment, piercing rolling is performed in the hot rolling process. LP is the tube length after the piercing rolling.

[0091] In addition, after the piercing rolling, treatments such as thinning (hot thinning rolling) and sizing rolling are performed to finish the hot rolling. LF is the tube length after the hot rolling.

[0092] When performing hot rolling in the present embodiment, as LF, LP, and X, values estimated in advance based on the piercing rolling conditions and other hot rolling conditions are used. Then, by making "(LF / LP)×X" based on these values be 1100 mm or less, the inclination angle α of the finally obtained tube can be made 5.0° or more.

[0093] In addition, LF and X are measured after the piercing rolling, and LP is measured after the hot rolling. Further, together with the rolling conditions during the piercing rolling and the rolling conditions during other hot rolling, data is accumulated for manufacturing the next tube.

[0094] For the adjustment of the above X [mm], specifically, first, it can be predicted based on the rolling rolls and guide intervals that can adjust the outer diameter and outer peripheral length of the raw tube after the piercing rolling. In addition, the maximum circumferential speed of the rolling rolls can be obtained from the outer diameter and rotational speed of the rolling rolls. That is, the tube rotational speed [rotation / s] during the piercing rolling is obtained by (roll circumferential speed [mm / s] / outer peripheral length [mm])×rolling efficiency.

[0095] In addition, the forward speed [mm / s] of the tube during the piercing rolling is obtained by TAN (rolling roll inclination angle [°]×π / 180)×roll circumferential speed [mm / s]×rolling efficiency.

[0096] The rolling efficiency is about 0.4 to 0.8 according to the strength determination value of the rolling mill and the tube blank material.

[0097] In this way, since the information on the tube rotational speed, the forward speed of the tube, and the strength of the rolling mill and the tube blank material is obtained before the piercing rolling, the forward movement amount X in the rolling direction can be predicted before the rolling. For materials with unknown rolling efficiency, the rolling efficiency is predicted in the range of 0.5 to 0.7 in advance, and the actual piercing rolling time and rolling efficiency can be used in the next and subsequent operations.

[0098] The adjustment of the above-mentioned LF [mm] and LP [mm] can be predicted based on the volume of the tube blank before rolling and the target tube outer diameter and tube wall thickness after each rolling. That is, in plastic processing, the volume does not change before and after processing. Therefore, if the known initial volume before rolling is divided by the area of the cross-section obtained from the target tube outer diameter and tube wall thickness determined under the set conditions of the rolling mill, the length can be predicted. It should be noted that in thermoplastic processing, there is a slight volume change due to scale loss, thermal expansion, and thermal contraction, but LF and LP do not substantially change.

[0099] In addition, after various hot rolling processes, further thinning and outer diameter rolling can be carried out in cold rolling.

[0100] It should be noted that in the above-mentioned hot thinning rolling, for example, thinning rolling methods such as a stretcher, Assel mill, mandrel rolling, plug mill rolling, and hot Pilger rolling can be used. In addition, in sizing rolling, a sizing mill, reduction gear, straightening machine, etc. can be used. In cold processing, drawing, cold Pilger, and bending-unbending processing can be used.

[0101] Since LF and LP are the tube lengths after rolling, they can be easily measured, and the value of LF / LP represents the degree of stretching of the embedded thin-walled part such as a helix in the tube axis direction.

[0102] The smaller "(LF / LP)×X" is, the larger the inclination angle α is, and the higher the pressure resistance performance is. By making "(LF / LP)×X" 1100 mm or less and performing the bending-unbending processing described later, the inclination angle α can be made 5.0° or more. Therefore, "(LF / LP)×X" is preferably 1100 mm or less. In addition, "(LF / LP)×X" being 1000 mm or less becomes a more preferable range, and 800 mm or less becomes an even more preferable range. On the other hand, if "(LF / LP)×X" is too small, the forward movement amount X of the tube in the rolling direction per revolution in piercing rolling becomes small, so the productivity deteriorates. Therefore, "(LF / LP)×X" is preferably 100 mm or more.

[0103] Cold rolling

[0104] Bending and reverse bending processing in the circumferential direction of the tube

[0105] Especially in seamless tubes that require strength, cold rolling is often carried out after hot rolling.

[0106] Especially in tubes for oil wells, as cold rolling for obtaining strength, cold drawing rolling and cold Pilger rolling are standardized, and cold rolling is carried out by any of these methods. These methods are all processing methods in which a tool for inner surface rolling is inserted into the inner surface of the tube and stretched axially.

[0107] However, if tubes are manufactured by these processing methods, similar to piercing rolling in hot rolling, wall thickness non-uniformity is inevitably generated due to uneven friction coefficients, backlash of the equipment, etc. Moreover, in these processing methods, since the tube is processed by stretching it in the axial direction, in the final product, the distribution of wall thickness non-uniformity tends to be close to parallel to the tube axis direction, resulting in a reduction in pressure resistance performance.

[0108] Furthermore, if hot piercing rolling is combined with methods such as the Mannesmann - Séjournet process and the Mannesmann plug - mill process that result in a linear wall thickness non-uniformity distribution, as well as the above-mentioned processing methods, the wall thickness non-uniformity is promoted and the amount of wall thickness non-uniformity becomes larger, further reducing the pressure resistance performance. In addition, in these processing methods, since the tube is strengthened by stretching in the tube axis direction, it is known that the compressive yield strength in the tube axis direction decreases by 20 - 25% due to the Bauschinger effect. When in production, in addition to external pressure and internal pressure, the possibility of buckling increases in an environment where compressive stress in the tube axis direction is applied.

[0109] In contrast, the present inventors, while maintaining the form of wall thickness non-uniformity distribution with excellent pressure resistance performance obtained after hot rolling, in view of the above problems, have conducted in-depth research on cold rolling methods that do not reduce the compressive yield strength in the tube axis direction. As a result, the present inventors have come up with a cold working method by bending - unbending processing in the circumferential direction of the tube. If this bending - unbending processing is used, excellent pressure resistance performance can be obtained. Figure 2 This cold working method will be described.

[0110] Figure 2 FIG. is a diagram for explaining the bending - unbending processing in the circumferential direction of the tube. In the present embodiment, as described below, through the bending - unbending processing in the circumferential direction of the tube, while maintaining the distribution form of wall thickness non-uniformity, the high yield strength of the tube is achieved. This method is different from cold drawing rolling and cold Pilger rolling processing in which the strain caused by rolling is generated in the tube axis direction (tube axis long side direction). Figure 2As shown, strain is applied through flat bending of the tube (first flat bending) and then reverse bending when returning to a perfect circle again (second flat bending). In this method, without significantly changing the initial tube shape, the amount of strain is adjusted by repeating bending and reverse bending and changing the amount of bending. That is, compared with the conventional cold rolling method that utilizes tensile strain along the tube axis direction, the high strength of the tube caused by the processing hardening of using the cold working method of this embodiment utilizes bending deformation along the tube circumference direction. Since the control of this cold working method and the strain along the tube axis direction caused by this control are suppressed, in principle, the Bauschinger effect along the tube axis direction generated in the conventional cold rolling method does not occur. Furthermore, since rolling is not performed from the inner surface of the tube, but the tube is flattened by an external force from the outer surface of the tube, the wall thickness non-uniformity form formed by hot working is not affected. Therefore, while maintaining the distribution of wall thickness non-uniformity with excellent pressure resistance performance, the tensile yield strength and compressive yield strength in the tube axis direction can be simultaneously improved, and the pressure resistance performance in the environment of the combined action of these external forces can be significantly improved.

[0111] It should be noted that Figure 2 (a) and (b) are cross-sectional views when the tool contact parts are set at two places, Figure 2 (c) is a cross-sectional view when the tool contact parts are set at three places. Additionally, Figure 2 the thick arrows in [figures] are the directions of force application when flattening the tube. As Figure 2 shown, when performing the second flattening, the following methods can be adopted: moving the tool or staggering the position of the tool in such a way that the tool contacts the part where the first flattening has not been performed and rotating the tube ([ Figure 2 the slanted part in [figures] indicates the first flattened part.).

[0112] As Figure 2 shown, by intermittently or continuously applying circumferential bending and reverse bending along the tube circumference that flattens the tube over the entire circumference of the tube, strain caused by bending is applied near the maximum value of the curvature of the tube, and strain caused by reverse bending is applied to the minimum value of the curvature of the tube. As a result, the strain caused by the bending and reverse bending deformation required to increase the strength of the tube (dislocation strengthening) is accumulated. Additionally, in the case of using this processing form, different from the processing form of compressing the wall thickness and outer diameter of the tube, it is characterized in that since a large amount of power is not required and it is a deformation caused by flattening, processing can be performed while keeping the shape change before and after processing to a minimum.

[0113] Regarding as Figure 2The flat tool shape for the tube shown can use rollers. If the tube is rotated flat between two or more rollers arranged in the circumferential direction of the tube, the strain caused by repeated bending-unbending deformation can be easily applied. Further, if the rotation axis of the roller is inclined by 90° or less with respect to the rotation axis of the tube, the tube advances in the direction of the tube rotation axis while being flattened, so that the continuity of processing can be easily achieved. In addition, in the processing continuously performed using this roller, for example, if the interval between the rollers is appropriately changed so that the flattening amount changes with respect to the advancement of the tube, the curvature (flattening amount) of the tube in the first and second times can be easily changed. Therefore, by changing the interval between the rollers to change the movement path of the neutral line, the strain in the wall thickness direction can be made uniform. Similarly, the same effect can be obtained by changing the flattening amount by changing the roller diameter instead of the roller interval. In addition, they can also be combined. Although it becomes complicated in terms of equipment, if the number of rollers is three or more, the jumping of the tube during processing can be suppressed and stable processing can be performed.

[0114] By hot piercing rolling to satisfy the formula: (LF / LP)×X≤1100, and further by cold working through bending-unbending processing as described above, the inclination angle α can be 5.0° or more, and excellent pressure resistance performance can be obtained in various pressure and stress environments.

[0115] The seamless tube of this embodiment is independent of the chemical composition and strength level of the tube, and compared with a tube (α: a tube with an angle less than 5.0°) in which the distribution of the thin-walled part caused by inevitably generated wall thickness unevenness is nearly parallel to the tube axis direction, the effect of improving the pressure resistance performance can be obtained.

[0116] According to the manufacturing method of the seamless tube of this embodiment, regardless of the chemical composition and strength of the tube blank material, for the wall thickness uneven distribution of the manufactured seamless tube, α can be 5.0° or more. The seamless tube of this embodiment can be, for example, a carbon steel tube, various stainless steel tubes, or non-ferrous metals.

[0117] In addition, when the seamless pipe of the present embodiment is applied to a steel pipe for oil wells that is subjected to high external pressure and axial stress of the pipe in a severe corrosion environment, acid-resistant steel pipes can also be used. The acid-resistant steel pipes have a composition containing, by mass%, C: 0.20 to 0.35%, Mn: 0.1 to 1.2%, Cr: 0.3 to 2.0%, Mo: 0.1 to 1.5%, and the balance being composed of Fe and inevitable impurities such as S or P, Al, or O. They have a martensitic structure, high strength, and excellent acid resistance. In addition, duplex stainless steels (UNS S32205, S31260, S32750, S32760) with excellent corrosion resistance and high strength through the above-mentioned bending-unbending processing can also be used. As the duplex stainless steel, it can be a steel containing, by mass%, Cr: 11.5 to 35.0% and Mo: 0.5 to 6.0% and having a ferritic and austenitic structure. In addition, from the viewpoint of improving corrosion resistance, as the composition of the duplex stainless steel, in addition to the above composition, it is preferably contains, by mass%, C: 0.08% or less, Si: 1.0% or less, Mn: 10.0% or less, Ni: 15.0% or less, N: less than 0.400%, and the balance being composed of Fe and inevitable impurities such as S or P, Al, or O. From the viewpoint of improving corrosion resistance, as the composition of the duplex stainless steel, it is further preferably contains one or two selected from W: 6.0% or less and Cu: 4.0% or less. In addition, from the viewpoint of improving strength, as the composition of the duplex stainless steel, it is preferably further contains one or more selected from Ti: 0.30% or less, Al: 0.30% or less, V: 1.0% or less, and Nb: 1.0% or less. In addition, from the viewpoint of improving workability during hot forming and corrosion resistance in an acidic atmosphere, as the composition of the duplex stainless steel, it is preferably further contains one or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, REM: 0.010% or less, and Ag: 0.30% or less.

[0118] In addition, when the seamless pipe of the present embodiment is exposed to a more severe corrosion environment during oil well exploitation, Ni-based alloys (UNS N06600, N08800) can also be used. As the Ni-based alloy, it preferably has a composition containing Cr: 11.5 to 35.0% by mass, Ni: 23.0 to 60.0% by mass, Mo: 0.5 to 17.0% by mass, and the balance is composed of inevitable impurities such as Fe and S or P, Al or O, and has an austenite phase structure. In addition, from the viewpoint of improving corrosion resistance, the Ni-based alloy preferably contains C: 0.05% or less, Si: 1.0% or less, Mn: 5.0% or less, and N: less than 0.400% by mass. In addition, from the viewpoint of improving corrosion resistance, the Ni-based alloy preferably contains one or two selected from W: 5.5% or less and Cu: 4.0% or less by mass. In addition, from the viewpoint of improving strength, the Ni-based alloy preferably contains one or more selected from Ti: 1.5% or less, Al: 0.30% or less, V: 1.0% or less, and Nb: 1.0% or less by mass. In addition, from the viewpoints of improving workability during hot forming and improving corrosion resistance in an acidic atmosphere, the Ni-based alloy preferably contains one or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.20% or less.

[0119] <Second Embodiment>

[0120] Next, a second embodiment of the present invention will be described.

[0121] The seamless pipe of the present embodiment is characterized in that, compared with the seamless pipe of the first embodiment, at least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the radius of curvature of the corner formed by the side surface of the fastening portion and the bottom surface of the thread groove is 0.2 mm or more. Since the other configurations and functions of the seamless pipe of the present embodiment are the same as those of the seamless pipe of the first embodiment, only the fastening portions of the external thread and the internal thread will be described below. It should be noted that the seamless pipe with excellent pressure resistance of the present embodiment can be used for a threaded joint directly connected (integral type) to other pipes or a threaded joint connected by a coupling (T&C type).

[0122] Here, first, the thread provided at the pipe end portion will be described.

[0123] From the viewpoints of preventing fire and repeated plugging and unplugging, pipes for oil wells, gas wells, or geothermal wells sometimes use fastening by threads instead of welding in the connection between pipes.

[0124] Seamless pipes used in oil wells, gas wells, and hot water extraction applications exposed to high external pressure are required to have a high tensile yield strength in the pipe axis direction. In addition, the connection part of the pipe is required to have a high compressive yield strength in the pipe axis direction.

[0125] The threaded joint consists of a pin with an external thread and a box (coupling) with an internal thread. As threaded joints, standard threaded joints specified in API (American Petroleum Institute) standards and high-performance special threaded joints called special threaded joints that have not only a threaded part but also a metal contact seal part and a torque shoulder can be cited.

[0126] In order to achieve firm fastening of the threads, the threads can be designed to generate contact surface pressure in the diameter direction. For example, tapered threads are used.

[0127] With the contact surface pressure in the diameter direction, the pin (external thread side) undergoes diameter reduction deformation and is stretched in the pipe axis direction, and the box (internal thread side) undergoes diameter expansion deformation and contracts in the pipe axis direction. Therefore, contact surface pressure is generated on the side surfaces at both ends of the thread.

[0128] Therefore, a compressive stress in the pipe axis direction corresponding to the fastening force is generated in the thread. Therefore, in seamless pipes used in oil wells, gas wells, and hot water extraction applications exposed to high external pressure, in addition to high pressure resistance performance, a compressive yield strength in the pipe axis direction that can withstand compressive stress is usually also required. Especially in special threaded joints, since a large compressive stress in the pipe axis direction is generated at the torque shoulder, it is preferably to have a high compressive yield strength in the pipe axis direction.

[0129] In this regard, the seamless pipe of the present embodiment has a fastening part with an external thread or an internal thread at at least one of the pipe ends on both sides, and further has the structure and function of the seamless pipe of the above first embodiment.

[0130] As described above, when fastening the threaded fastening part, tensile and compressive stresses in the pipe axis direction are generated due to the bending deformation after fastening.

[0131] In the present embodiment, in order to improve the strength, bending and reverse bending processing is performed by cold working. Therefore, the compressive yield strength in the pipe axis direction relative to the tensile yield strength in the pipe axis direction (compressive yield strength in the pipe axis direction [MPa] / tensile yield strength in the pipe axis direction [MPa]) can be 0.85 or more. In addition to high pressure resistance performance, excellent threaded joint performance can also be obtained.

[0132] Figure 3 It is a cross-sectional view in the pipe axis direction (a cross-sectional view parallel to the pipe axis direction) of the fastening part of the external thread and the internal thread, and is a schematic diagram showing the position of the curvature radius of the corner R in the fastening part of the thread. Figure 3 (a) is a schematic diagram in the case of trapezoidal threads, Figure 3(b) is a schematic diagram when it is a triangular thread.

[0133] When fastening the seamless pipe of this embodiment with a thread, it is preferable that at least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the radius of curvature of the corner formed by the side surface of the fastening portion and the bottom surface of the thread groove is 0.2 mm or more.

[0134] That is, if the bending-unbending process is used in this embodiment, regardless of the type of thread, by fastening, the external thread and the internal thread are brought into contact with each other, and the radius of curvature of the corner R formed by the side surface where pressure is generated by fastening and the bottom surface of the thread groove is 0.2 mm or more, thereby improving the fatigue characteristics of the fastening portion of the thread. It should be noted that for the side surface, the thread inclined surface on the side closer to the pipe end in the external thread (pin) is called the inserted tooth side, and the thread inclined surface on the side farther from the pipe end is called the load-bearing tooth side. In the internal thread (box), the thread inclined surface opposite to the inserted tooth side of the pin is called the inserted tooth side, and the thread inclined surface opposite to the load-bearing tooth side of the pin is called the load-bearing tooth side.

[0135] It should be noted that the thread processing method can use any method such as a cutting-based method, rolling that transfers the thread shape by plastic processing, etc. Cutting is preferred because better dimensional accuracy can be obtained and the surface layers of the inner and outer surfaces of the pipe are not easily deformed.

[0136] Figure 4 is a cross-sectional view in the pipe axis direction of the threaded joint (a cross-sectional view parallel to the pipe axis direction), Figure 4 (a) is a cross-sectional view when the threaded joint is an API threaded joint, Figure 4 (b) is a cross-sectional view when the threaded joint is a special threaded joint. In a threaded joint composed only of threads like an API threaded joint, when the threads are fastened, the maximum surface pressure is generated at both ends of the threads. The threads on the front end side of the pin contact the inserted tooth side, and the threads on the rear end side of the pin contact the load-bearing tooth side. In the case of a special threaded joint, the reaction force generated by the torque shoulder also needs to be considered. When the threads are fastened, the maximum surface pressure is generated on the load-bearing tooth sides at both ends of the threads. In the case of a cold working method that is not the bending-unbending process, due to the influence of the Bauschinger effect in the pipe axis direction, the compressive yield strength in the pipe axis direction is lower than the tensile yield strength in the pipe axis direction, and compressive stress is generated at the stress concentration part. Moreover, due to the low compressive yield strength, minute deformation occurs. If a thread-based fastening method is adopted, the fatigue life of the thread is reduced. In contrast, by using the bending-unbending process of this embodiment and making the radius of curvature of the corner R 0.2 mm or more, the fatigue characteristics of the threads in the seamless pipe are improved, and good pressure resistance performance is obtained.

[0137] Increasing the radius of curvature of the corner R to more than 0.2 mm is effective in further alleviating stress concentration. However, a large corner R may deprive the freedom of thread design, limit the size of the pipe that can be thread - machined, or make design impossible. In addition, if the corner R is increased, since the area of the side surfaces of the external and internal threads in contact is reduced, a decrease in sealing performance and fastening force may occur. Therefore, the corner R is more preferably in the range of 0.2 - 3.0 mm. Alternatively, it is appropriate to define the area of the side surface reduced due to the size of the corner R in relation to the thread height. For the radius of curvature of the above - mentioned corner R, a radial length (length in the diameter direction starting from the pipe - axis center side) less than 20% of the thread height can be set as the radius of curvature occupied by the corner R, and the radius of curvature of the corner R is designed to be 0.2 mm or more.

[0138] Figure 4 (b) The special thread joint shown not only has threads but also a metal - contact seal part and a torque shoulder. In the special thread joint, the sealing performance of the pipe is ensured by fastening with the metal - contact seal part ( Figure 4 (Seal in (b)). On the other hand, the torque shoulder ( Figure 4 (Shoulder in (b)) acts as a stopper during fastening and plays an important role in ensuring a stable fastening position, but high compressive stress is generated during fastening. If the torque shoulder is deformed due to the high compressive stress, the sealing performance is impaired. In addition, due to the deformation towards the inner - diameter side, the inner diameter is constricted. Therefore, in order not to deform the torque shoulder, it is necessary to increase the wall thickness to improve the compressive strength, and a thin - walled pipe shape cannot be designed. In addition, material waste occurs due to the remaining wall thickness.

[0139] Furthermore, usually when fastening threads, the fastening - torque value (the value of the torque during thread fastening) is confirmed. Also, the torque value for sealing (if it exceeds a certain reference during fastening, it becomes a torque value indicating the sealing state, so it refers to the torque value during fastening) and the torque value at which the torque shoulder as the upper limit does not deform (if the torque value exceeds a certain reference and becomes large, the thread tip deforms, so the torque value not exceeding this reference) are managed. That is, fastening is managed within the range from the torque value for sealing to the torque value at which the torque shoulder does not deform.

[0140] At this time, when the axial - compression yield strength of the pipe is small, in order to suppress the deformation of the torque shoulder, the upper limit of the torque value becomes smaller. Therefore, the management range of the torque value becomes narrower, and stable fastening cannot be performed.

[0141] Figure 5 is a schematic view near the head which is an extension part of the pin, Figure 5 (a) is a cut - away cross - sectional view parallel to the pipe axis of the pin and the coupling fastening part, Figure 5(b) is the torque shoulder when observing the threaded front end of the pin from the front of the pin front end.

[0142] In the present embodiment, if a tube having a high compressive yield strength in the tube axis direction is obtained by bending-back cold working, deformation of the torque shoulder can be suppressed while maintaining high pressure resistance performance. In order to suppress deformation of the torque shoulder and perform fastening stably, it is only necessary to ensure that Figure 5 the cross-sectional area of the front end thickness (the part that bears the front end of the external thread on the coupling side, (Ds1 - Ds0) / 2) of the torque shoulder of the external thread (pin) shown in is 25% or more with respect to the cross-sectional area of the original tube. If the front end thickness of the torque shoulder of the external thread is increased, the head rigidity becomes too high and galling is likely to occur during fastening. Therefore, the preferred range is 25 to 60%.

[0143] In addition, by designing the head in a manner that further improves the compressive strength resistance of the torque shoulder, high torque performance can be further achieved (the torque value without deformation becomes higher and a higher fastening torque can be applied), and thus it is preferred. In order to achieve high torque performance, as shown in Figure 5 (a), it is preferred that when the sealing point position from the tube end is set as x, the ratio x / L of x to the head length L with respect to the threadless part at the pin front end is 0.01 to 0.1. By setting the sealing point position near the shoulder, the actual cross-sectional area of the shoulder (shoulder cross-sectional area: π / 4×(Ds1 2 -Ds0 2 )) increases, and high torque performance can be obtained (refer to Figure 5 (b)). At this time, if the head length L is too long, the head rigidity decreases and it cannot withstand a high compressive force. Therefore, the head length L is preferably 0.5 inches or less. On the other hand, if the head length L is too short, there is no room for arranging the sealing part. Therefore, it is preferably 0.2 inches or more.

[0144] It should be noted that in Figure 5 :

[0145] δ: represents the sealing interference amount, which is defined as the maximum value of the overlapping margin when overlapping the attached drawings

[0146] Ds1: the outer diameter of the shoulder contact area

[0147] Ds0: the inner diameter of the shoulder contact area.

[0148] The sealing performance indicating airtightness is also important as a characteristic of the thread. It is preferably to satisfy a compression rate of 85% or more shown in the sealing test of ISO13679:2019, and high strength can be achieved by the bending-unbending cold working of the present invention. To achieve high sealing performance, it is preferable that the head length L of the threadless part at the front end of the pin is 0.3 inches or more, and when the sealing point position from the pipe end is set as x, the ratio x / L to the head length L is 0.2 to 0.5. However, if the head length is extended more than necessary, cutting takes time, the head rigidity decreases, and the performance becomes unstable. Therefore, the head length L is preferably 1.0 inch or less.

[0149] Examples

[0150] [Example 1]

[0151] Hereinafter, the present invention will be described based on examples.

[0152] The pressure resistance performance of various materials was evaluated. First, Table 1 shows the specifications of the test materials.

[0153] [Table 1]

[0154]

[0155] For this material, hot piercing rolling (the Mannesmann method, the Mannesmann plug mill method, or the Mannesmann method) shown in Table 2 was performed to manufacture seamless pipes with an average outer diameter of Φ45 to 460 mm. In hot rolling, the billet was heated in a heating furnace at 1300 °C, and then each hot piercing rolling and subsequent thinning and sizing rolling were performed, and then the product shape was formed by air cooling. For Material A, the pipe after air cooling was directly used. For Materials B, C, and D, heat treatment was performed by heating the pipe after air cooling to 1000 to 1150 °C and performing water quenching. For the pipe subjected to hot piercing rolling by the Mannesmann method, regarding the pipe length LF [mm] after hot rolling, the pipe length LP [mm] after piercing rolling, and the advancement amount X [mm] in the rolling direction when the pipe rotates one turn during piercing rolling, "(LF / LP) × X" was controlled to the values shown in Table 2.

[0156] In addition, various cold rolling operations were performed on a part of them (refer to cold drawing (cold drawing process), bending-unbending (bending-unbending process in the circumferential direction of the pipe) in Table 2).

[0157] For cold drawing, a 15% reduction was given when finishing to the product size. For the bending-unbending process, the pipe was introduced and bent-unbent by rotating rolls having a rotation axis inclined 2 to 5° with respect to the pipe axis direction and arranged at 120° intervals in the circumferential direction of the pipe. It should be noted that the bending-unbending was applied to the pipe with rolls whose roll interval was reduced by 5 to 15% with respect to the initial outer diameter of the pipe.

[0158] For the manufactured seamless pipe, the yield strength characteristics in the pipe axis direction (pipe axis direction tensile yield strength (pipe axis tensile yield strength), pipe axis direction compressive yield strength (pipe axis compressive yield strength)) and the inclination angle α are measured.

[0159] The inclination angle α is the inclination angle of the line segment formed by connecting one end and the other end of the thin-walled part along the pipe surface at the shortest distance in the formation direction of the thin-walled part with respect to the pipe axis direction. In addition, for the set positions of the above one end and the other end, the area in the pipe selected from the shorter length among (1) the length of 1.0 m in the pipe axis direction and (2) 90% of the length in the pipe axis direction when the thin-walled part rotates one circle in the pipe circumferential direction is used to set one end and the other end of the thin-walled part. For the inclination angle α measured in each pipe, it was confirmed that the same value was obtained even when one end and the other end of the thin-walled part were set in the area of the pipe selected at 40% of the length in the pipe axis direction when the thin-walled part rotates one circle in the pipe circumferential direction.

[0160] In addition, the measurement of the inclination angle α is carried out with the center in the length direction of the measurement target area at the center of the length direction of the central part of the pipe in the length direction.

[0161] The above inclination angle α is measured by using ultrasonic wall thickness distribution in the state of the product length, and is calculated based on the maximum wall thickness t max [mm], minimum wall thickness t min [mm], average wall thickness t ave [mm] and the wall thickness non-uniform distribution during piercing rolling obtained by using the Fourier transform of this distribution. The average wall thickness t ave is obtained by measuring the wall thickness t at 32 points at intervals of 11.25° in the pipe circumferential direction starting from the thin-walled part at the center of the length direction of the central part of the pipe in the length direction and calculating their number average.

[0162] For the pipe axis direction compressive yield strength and pipe axis direction tensile yield strength, a round bar tensile test piece and a cylindrical compressive test piece with an outer diameter (diameter) of 5.0 mm are cut out from the wall thickness center part of the end of the pipe used in the pressure test, and the tests are carried out at compression and tensile speeds of 1.0 mm / min respectively. The stress-strain curves are measured in the normal temperature tensile and compression tests. The pipe axis direction tensile yield strength and pipe axis direction compressive yield strength are calculated from this stress-strain curve.

[0163] Specifically, first, the compressive yield strength in the tube axis direction is measured by a cylindrical compression test. The cylindrical test piece for compression is collected from the center of the wall thickness parallel to the tube axis direction. The test piece is cut out from the center of the wall thickness of the tube in the form of a cylinder with an outer diameter d = 5.0 mm and a cylinder height h = 8.0 mm. The compression test is carried out in the form of applying a load with the test piece clamped between flat plates at room temperature (25 °C), and the compressive yield strength is calculated using the stress-strain curve obtained during compression. The stress-strain curve is obtained by performing 30% compression at a compression speed (= crosshead speed) of 1.0 mm / min in a compression testing machine.

[0164] In addition, for the tensile yield strength in the tube axis direction, according to JIS Z2241, first, as a test piece, a round bar tensile test piece with a parallel part diameter of 5.0 mm is cut out from the center of the wall thickness of the tube parallel to the tube axis direction. Then, a tensile test is carried out at a crosshead speed of 1.0 mm / min at room temperature (25 °C) until fracture. Using the stress-strain curve obtained therefrom, the tensile yield strength is calculated.

[0165] The evaluation is carried out by giving various inclination angles α to seamless tubes that are consistent with the wall thickness unevenness, t min , outer diameter, and tensile yield strength in the tube axis direction of the test material, respectively performing a pressure resistance test, and making a relative evaluation when the comparison example with an inclination angle α less than 5.0° is set to 100.

[0166] The above-mentioned pressure resistance test is carried out by closing the tube ends of the obtained tube, inserting it into a shell with an inner diameter larger than the tube outer diameter and sealing it, and applying a water pressure inside and outside the tube. For the pressure resistance test conditions a and b, the external pressure or internal pressure is increased from 0 MPa to 150 MPa at a rate of 1 MPa each, and the pressure at the point where the water pressure change is observed due to the crushing of the tube is taken as the crushing strength (pressure resistance performance). For the application methods of the external pressure and internal pressure in c and d, they are the same as a and b in sequence, but are carried out while applying a constant bending moment to the tube. The bending moment is applied such that the axial tensile stress on the outer surface of the tube is constantly 80% of the axial tensile yield strength of the tube obtained through the tensile test. The determination of the crushing strength (pressure resistance performance) is the same as in conditions a and b, which is the pressure at the point where the water pressure change is confirmed.

[0167] It should be noted that the average outer diameter D ave is obtained by measuring the outer diameter D at 9 points at intervals of 40° in the circumferential direction of the tube starting from the thin wall part and calculating the arithmetic mean of these 9 points.

[0168] In addition, for the wall thickness distribution generated during piercing rolling, the wall thickness unevenness is expressed by the following formula (3) using the maximum wall thickness t max [mm], the minimum wall thickness t min [mm], and the average wall thickness t ave [mm].

[0169] Wall thickness non-uniformity = ((t max - t min ) / t ave ) × 100 [%] ··· Equation (3)

[0170] From the results in Table 2, it can be seen that the pressure resistance performance of the examples of the present invention is excellent. Furthermore, the strength characteristics in the pipe axis direction of the pipes subjected to bending-back bending in cold working are excellent, and good pressure resistance performance is also exhibited in the pressure resistance test where axial compressive stress is generated.

[0171]

[0172] [Table 3]

[0173]

[0174] [Example 2]

[0175] Next, the fastening part of the threaded joint was provided on a part of the pipes for evaluation. A trapezoidal thread was formed at the end of the pipe by machining (refer to Figure 3 (a)), and after fastening two pipes with the thread, a fatigue test of the thread was carried out in which the two pipe ends were rotated in a state of eccentricity of 3 to 10% according to the tensile yield strength in the pipe axis direction. It should be noted that for the thread, the corner R as the stress concentration part was changed as shown in Table 4, and the number of rotations until the fatigue crack at the stress concentration part and the fracture of the thread due to the progress of the fatigue crack were compared. It should be noted that in order to show that the thread characteristics are more excellent and the effect of the bending-back bending as a preferred manufacturing method, the evaluation of the number of rotations is expressed by the ratio when the number of rotations in other cold working methods is set to 1 for the same steel type and size, and the pipe with a ratio greater than 1.00 is judged to be more excellent, and the effect of extending the fatigue life is evaluated.

[0176] As shown in Table 4, for Materials A and B which are examples of the present invention, threaded joints composed of pins (pipe dimensions) with an outer diameter of Φ88.9 mm and a wall thickness of t6.5 mm and their corresponding couplings, and threaded joints composed of pins with an outer diameter of Φ244.5 mm and a wall thickness of t13.8 mm and their corresponding couplings were prepared. The types of threaded joints prepared were joints composed only of threads and special threaded joints composed of threads, metal contact sealing parts and shoulders, and the above-mentioned fatigue tests were carried out. Table 4 shows the radii of curvature of the corner R on the load-bearing flank and the inserted flank of the thread root of the pin and the radii of curvature of the corner R on the load-bearing flank and the inserted flank of the thread root of the coupling.

[0177]

[0178] From the results in Table 4, it can be seen that the fatigue characteristics of the seamless pipes of the present invention are excellent.

[0179] Next, in the special threaded joint, the design of the torque shoulder is evaluated. As shown in Table 5, in a threaded joint (special threaded joint) composed of a pin with an outer diameter of Φ88.9 mm and a wall thickness of t6.5 mm and its corresponding coupling, a tightening test (Yield torque evaluation test) is carried out based on ISO13679:2019.

[0180] [Table 5]

[0181]

[0182] It can be seen that if the cross-sectional area of the shoulder is 25% or less of the cross-sectional area of the unprocessed part of the pin (the cross-sectional area of the original pipe) (if the cross-sectional area ratio of the shoulder is 0.25 or less), Yield occurs at a tightening torque (Yield torque) of 3000 N·m. Here, Yield refers to the state where the threaded joint undergoes a sufficiently large plastic deformation and cannot guarantee the performance of the joint. Since the Yield torque is high, the usable torque range is expanded, and it can be said that the threaded joint is easy to use. In the results shown in Table 5, it cannot be said that there is a sufficient Yield torque at 3000 N·m, and it can be said that the threaded joint has high performance at 4000 N·m.

[0183] From this point, it can be known that in the steel of the present invention, even if the cross-sectional area of the shoulder is 20% of the cross-sectional area of the unprocessed part of the pin, the Yield torque is 4000 N·m or more, sufficient high torque can be ensured, and tightening can be achieved.

[0184] For this value, the conventional duplex stainless steel with low compressive strength requires more than 25%, so it can be confirmed that the cross-sectional area of the shoulder of the duplex stainless steel of the present invention is 20% or more of the cross-sectional area of the unprocessed part of the pin and the superiority of ensuring the same torque.

[0185] In addition, as the second high-performance threaded joint, a threaded joint with high sealing performance that passes the sealing test of ISO13679:2019 can be cited. Therefore, as shown in Table 6, in a threaded joint (special threaded joint) composed of a pin with an outer diameter of Φ88.9 mm and a wall thickness of t6.5 mm and its corresponding coupling, and a threaded joint (special threaded joint) composed of a pin with an outer diameter of Φ244.5 mm and a wall thickness of t13.8 mm and its corresponding coupling, a sealing test is carried out based on ISO13679:2019.

[0186] [Table 6]

[0187]

[0188] First of all, as described above, from the results of Table 5, it can be seen that by applying the seamless pipe of the present invention, a threaded joint that can be tightened even with a lower shoulder cross-sectional area can be realized.

[0189] In addition, as can be seen from the results in Table 6, the seamless pipe of the present invention having excellent compressive yield strength in the pipe axis direction passes with a sealing compression rate of 85% or more and has excellent thread characteristics.

[0190] This feature can increase the degree of freedom in the design of threaded joints and enable the following two types of high-performance threaded joints.

[0191] First, as the first high-performance threaded joint, a high-torque threaded joint that can ensure sealing performance even when a high tightening torque is applied can be cited. By using a stainless steel seamless pipe with high compressive strength like the present invention for the threaded joint, high torque performance can be obtained. In addition, by optimizing the design of the threaded joint, further high torque can be achieved. Specifically, the head length of the non-threaded portion at the pin tip is 0.2 inches to 0.5 inches, and when the position of the sealing point from the pipe end is set as x, the ratio x / L to the head length L is designed to be 0.01 to 0.1.

[0192] In addition, according to the results of the sealing test, in order to achieve a highly airtight metal contact sealing portion, it is preferable that the head length L of the non-threaded portion at the pin tip is 0.3 inches to 1.0 inches, and when the position of the sealing point from the pipe end is set as x, the ratio x / L to the head length L is 0.2 to 0.5. As described above, if the head length is extended and the sealing point is moved away from the pipe end, there is a high possibility that the cross-sectional area of the shoulder becomes small and becomes a cross-sectional area where the Yield problem occurs in conventional materials, making it impossible to design. This problem becomes significant in the case of thin walls, and it is impossible to achieve a wall thickness of 6.5 mm or less.

[0193] In the seamless pipe of the present invention, since the compressive strength is high, if the cross-sectional area of the shoulder can be ensured to be 20%, the problem of Yield can be avoided, and the design of ensuring the cross-sectional area of the shoulder and high sealing performance can be achieved at the same time. As shown in Table 6, it was confirmed that the threads of Pipe Nos. 28 and 30 with a pipe axis direction compressive yield strength / pipe axis direction tensile yield strength of 0.85 or more passed the sealing test with a compression rate of 85% or more under the test load of ISO13679:2019. Specifically, it was confirmed that if the threads of Pipe Nos. 28 and 30 with a pipe axis direction compressive yield strength / pipe axis direction tensile yield strength of 1.0 or more were used, they passed the sealing test with a compression rate of 100%.

Claims

1. A seamless pipe having a thin-walled portion in the circumferential direction of the pipe formed in the pipe axis direction. A line segment formed by connecting one end and the other end of the thin-walled portion along the pipe surface at the shortest distance in the forming direction of the thin-walled portion is inclined at an angle α of 15° to 39.0° with respect to the pipe axis direction. Moreover, the ratio of the compressive yield strength [MPa] in the pipe axis direction to the tensile yield strength [MPa] in the pipe axis direction is 0.85 or more.

2. The seamless pipe according to claim 1, wherein, The one end and the other end of the thin-walled portion are set in a region in the pipe selected as the shorter length from the length of 1.0 m in the pipe axis direction and 90% of the length in the pipe axis direction for one rotation of the thin-walled portion in the circumferential direction of the pipe.

3. The seamless pipe according to claim 1 or 2, wherein Average outside diameter D ave [mm] and the said angle α [°] satisfy the following formula (1): D ave / α = 0.5 to 15.0 [mm / °] ··· Equation (1).

4. The seamless pipe according to any one of claims 1 to 3, wherein, At least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the radius of curvature of the corner portion formed by the side surface of the fastening portion and the bottom surface of the thread groove is 0.2 mm or more.

5. The seamless pipe according to claim 4, wherein, At least one of the pipe end portions on both sides has a fastening portion with an external thread or an internal thread, and the fastening portion has a metal contact seal portion and a torque shoulder.

6. A method for manufacturing a seamless pipe, which is a method for manufacturing the seamless pipe according to any one of claims 1 to 5. While rotating the pipe blank and advancing it in the pipe axis direction, the pipe blank is pierced by hot rolling. As cold working of the pipe after hot rolling, bending-unbending processing in the circumferential direction of the pipe is performed. Moreover, the hot-rolled pipe length LF [mm], the pierced-rolled pipe length LP [mm], and the amount of advance X [mm] of the pipe in the rolling direction when the pipe rotates one turn during pierced rolling satisfy the following formula (2): (LF / LP)×X ≤ 1100 [mm] ··· Formula (2).

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