Method for inclined rolling of metal pipes, inclined rolling equipment for metal pipes, and method for manufacturing metal pipes.

Abstract

The objective is to provide a method for inclined rolling of metal pipes, inclined rolling equipment for metal pipes, and a method for manufacturing metal pipes. [Solution] The present invention relates to an inclined rolling method for metal pipes, in which a raw pipe is rotated in the circumferential direction and advanced in the axial direction of the pipe while being rolled to reduce its diameter, and includes a rolling step of obtaining a metal pipe by performing multiple passes of diameter reduction rolling on the raw pipe using two or more rolling rolls, wherein the roll gap of each pass is set to be less than or equal to the roll gap of the previous pass.

Description

Technical Field

[0001] The present invention relates to an inclined rolling method for reducing the diameter of a metal tube, an inclined rolling facility for a metal tube, and a method for manufacturing a metal tube.

Background Art

[0002] In fields where seamless metal tube products are used, especially in fields where particularly excellent corrosion resistance and high strength are required, in order to improve the corrosion resistance performance, duplex stainless steels (specifically equivalent to JIS G3459 SUS 329J1, 329J3L, 329J4L) and austenitic stainless steels (specifically equivalent to JIS G3459 SUS 301, 302, 304, 305, 309, 310, 312, 315, 316, 317, 836, 890, 321, 347) with a large amount of corrosion resistance improving elements such as Cr, Mo, Ni added, as well as seamless steel tubes of Ni-based alloys (specifically equivalent to JIS H4552 NW4400, NW6007, NW0276, NW6022, NW6002) are used.

[0003] These steel grades and alloys contain a large amount of alloying elements added to exhibit excellent corrosion resistance performance. Therefore, as the structure, it becomes a single-phase austenite phase or a multiphase structure containing a large amount of austenite phase. The austenite phase with a face-centered cubic lattice (fcc) structure often has a lower yield strength than the ferrite phase or martensite phase with a body-centered cubic lattice (bcc) structure in a service environment from low temperature to normal temperature. Therefore, when higher yield strength is required for a material containing the austenite phase, cold working is added to the material, and dislocation strengthening by the working is utilized to increase the yield strength.

[0004] For example, in high-strength and high-corrosion-resistant steel tubes used for oil well pipes, etc., cold working such as cold drawing and cold pilgering is frequently used, and high-strength and high-corrosion-resistant steel tubes with a yield strength of 125 ksi or more have been put into practical use (see Non-Patent Document 1).

[0005] The cold drawing method described in Non-Patent Document 1 is an effective technique not only for improving the strength of steel pipes in the longitudinal direction but also for uniformizing the wall thickness distribution in the longitudinal direction of steel pipes. However, it requires many processes before drawing, such as heat treatment to soften the steel pipe, pickling, chemical treatment to apply a lubricating film, and pipe end processing to create gripping parts for drawing. Furthermore, due to limitations on the pressure required for drawing and the need to prevent seizing of the tools, a wall thickness reduction of only about 20% can be achieved. Moreover, if the amount of wall thickness reduction is insufficient in a single drawing process, it is necessary to repeat the series of processes starting from the aforementioned heat treatment to soften the pipe. In addition, since the shape of the steel pipe after drawing is uniquely determined by the dimensions of the tool used for drawing, tool replacement is necessary when changing the size, making it unsuitable for small-batch, high-mix manufacturing. Furthermore, because many processes are required to perform drawing, there are problems with significant capital investment and energy consumption.

[0006] On the other hand, cold rolling eliminates the need for pre-treatment of steel pipes and allows for a high rate of wall reduction. However, the feed rate per pass is small, only a few tens of millimeters, resulting in poor production efficiency. In addition, the shape of the rolling rolls is complex, leading to a large burden on tool manufacturing (specifically, the labor and economic burden of manufacturing the rolling rolls).

[0007] One example of a technology that solves these problems is Patent Document 1. The technology described in Patent Document 1 proposes a cold rolling method in which a metal pipe is passed through the roll gap of an inclined rolling mill having two or more rolling rolls arranged at an inclination with respect to the center line of the rolling path direction of the metal pipe (hereinafter sometimes referred to as the "pass line"). This eliminates the need for pre-treatment such as applying a surface coating to the pipe to be rolled before processing or processing the pipe ends, and enables improvement of the strength of the metal pipe by cold working with high processing efficiency, resulting in environmental protection and favorable effects in industry. Furthermore, by allowing the inner surface to deform freely, it prevents excessive surface pressure on the tool and allows for the addition of desired processing distortion without the occurrence of surface defects such as seizing that occur in cold drawing, making it suitable for high-mix low-volume production. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 6432614 [Non-patent literature]

[0009] [Non-Patent Document 1] The Iron and Steel Institute of Japan, "Current Status and Future of Steel Pipe Manufacturing Technology," The Iron and Steel Institute of Japan Publications, May 6, 1986, pp. 115-145. [Overview of the project] [Problems that the invention aims to solve]

[0010] As described above, the cold rolling method described in Patent Document 1 has various advantages. However, as a result of our investigation, we have found that, in the cold rolling method described in Patent Document 1, depending on the rolling conditions, the cross-sectional shape of the pipe may deform to an ellipse or the like after rolling, even if it is close to a perfect circle before rolling, resulting in a decrease in roundness. Furthermore, we have found that there is a method to make the cross-sectional shape of the pipe after rolling closer to a perfect circle.

[0011] Furthermore, there is a strong need to establish technologies that suppress the decrease in roundness of metal pipes after rolling, such as cold rolling. In addition, when providing technologies to improve the roundness of metal pipes after rolling, it is required that these technologies be applicable not only to seamless metal pipes but also to welded steel pipes and forge-welded steel pipes.

[0012] The present invention has been made in view of the above problems, and aims to provide a method for inclined rolling of metal tubes, inclined rolling equipment for metal tubes, and a method for manufacturing metal tubes that makes it possible to bring the cross-sectional shape of the metal tube after rolling closer to a perfect circle. [Means for solving the problem]

[0013] To solve the above problems, the inventors diligently studied a method for inclined rolling that improves the roundness of metal pipes and a method for manufacturing metal pipes, as well as inclined rolling equipment for metal pipes that would make this method feasible. As a result, they discovered that there are rolling conditions that allow for multiple passes in the inclined rolling process, which was previously done in a single pass, and that can suppress the deterioration of the roundness of the metal pipes after rolling.

[0014] Through further investigation, the inventors have completed an invention comprising the following gist. [1] A method for inclined rolling of a metal pipe, wherein the raw pipe is rotated in the circumferential direction and advanced in the axial direction of the pipe while being rolled to reduce its diameter, The process includes a rolling step to obtain a metal tube by performing multiple passes of diameter reduction rolling on the raw tube using two or more rolling rolls, A method for inclined rolling of a metal pipe, wherein in the rolling process, the roll gap of each pass is set to be less than or equal to the roll gap of the immediately preceding pass. [2] In the rolling process, The inclined rolling method for a metal pipe as described in [1] above, wherein the roll gap of the final pass is set to satisfy equation (1). 0 < (D - Gb) / (D - Gf) ≤ 0.60 …(1) Here, as shown in equation (1), D: Outer diameter of the raw tube before diameter reduction rolling [mm] Gf: Roll gap in the final pass [mm] Gb: Roll gap [mm] in the pass immediately preceding the final pass. That is the case. [3] A metal pipe inclined rolling machine that reduces the diameter of a raw pipe by rotating it in the circumferential direction and advancing it in the axial direction, Two or more rolling rolls are arranged at an inclination on the circumference centered on the pass line, The system includes control means for controlling the roll gap of the two or more rolling rolls, The control means, when performing multiple passes of diameter reduction rolling on the raw tube using the two or more rolling rolls, A metal pipe inclined rolling mill that sets the roll gap of each pass to be less than or equal to the roll gap of the previous pass. [4] The control means further sets the roll gap of the final pass so as to satisfy formula (1), and the inclined rolling equipment for metal pipes according to [3] above. 0 < (D - Gb) / (D - Gf) ≦ 0.60 …(1) Here, as shown in formula (1), D: Outer diameter of the raw pipe before diameter reduction rolling [mm], Gf: Roll gap in the final pass [mm], Gb: Roll gap in the pass immediately before the final pass [mm], [[ID=1x]] is. [5] A method for manufacturing a metal pipe, which has a pipe manufacturing process of subjecting a raw pipe to diameter reduction rolling using the inclined rolling method for metal pipes according to [1] or [2] above to obtain a metal pipe.

Advantages of the Invention

[0015] According to the present invention, it is possible to suppress a decrease in the roundness of the cross-sectional shape of the metal pipe after rolling.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a schematic diagram for explaining an embodiment of the inclined rolling equipment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of the inclined rolling equipment shown in FIG. 1 viewed from the side. [Figure 3] FIG. 3 is a schematic diagram of the inclined rolling equipment shown in FIG. 1 viewed from above. [Figure 4] FIG. 4 is a schematic diagram for explaining the face angle of the rolling roll in the inclined rolling equipment of the present invention. [Figure 5] FIG. 5 is a schematic diagram for explaining an example of the cross-sectional shape of the metal pipe after rolling.

Embodiments for Carrying Out the Invention

[0017] The present invention will be described with reference to the respective drawings. Note that the following description shows preferred embodiments of the present invention, and the present invention is not limited to this embodiment.

[0018] [Inclined rolling equipment] The inclined rolling mill of the present invention will be described with reference to Figures 1 to 4.

[0019] Figure 1 shows one embodiment of the inclined rolling equipment of the present invention, and is a front view of the inclined rolling equipment from the rolling exit side. As an example, Figure 1 shows the state in which a pipe to be rolled is being rolled by an inclined rolling equipment having two rolling rolls (i.e., a two-roll type inclined rolling equipment). Figure 2 is a schematic diagram of the inclined rolling equipment and pipe to be rolled shown in Figure 1, viewed from the side, and is a cross-sectional view taken along line AA in Figure 1. Figure 3 is a schematic diagram of the inclined rolling equipment and pipe to be rolled shown in Figure 1, viewed from above, and is a view taken along arrow BB in Figure 2. Figure 4 is a diagram illustrating the face angle of the rolling rolls used in the inclined rolling equipment of the present invention. For ease of understanding, the lower of the two rolling rolls 3 is omitted from Figure 3, and Figures 2 to 4 show cross-sectional views of the raw pipe 1 and the pipe to be rolled 2.

[0020] The inclined rolling equipment of the present invention is one type of rolling equipment used in the manufacturing process of metal pipes. Below, as an example of "rolling equipment," a cold rolling equipment for performing cold rolling on a material to be rolled will be described. For example, an inclined rolling equipment having two or more rolling rolls in which the rotation axis of the rolling rolls is inclined with respect to the center line of the rolling path direction of the raw pipe (pass line, i.e., the pipe axis direction) can be mentioned. As an example of "rolling," the case of performing cold rolling will be described, but the present invention is applicable not only to cold rolling but also to hot rolling and warm rolling. In this invention, "metal pipe" refers to seamless steel pipes, welded steel pipes, forge-welded steel pipes, and UOE pipes.

[0021] The inclined rolling mill 10 of the present invention is a facility having two or more rolling rolls 3 arranged at an inclination in the circumferential direction of the raw pipe 1 and the pipe to be rolled 2 (i.e., on a circle centered on the pass line 6). This inclined rolling mill 10 has control means 4 for controlling the roll gap of at least the rolling rolls 3.

[0022] As shown in Figure 1, etc., a raw pipe 1 is supplied to the roll gap of the inclined rolling mill 10 from the inlet side of the inclined rolling mill (i.e., the rolling inlet side). Then, the raw pipe 1 is passed through the rolling direction (direction of travel) while being pressed between two or more rolling rolls 3, thereby subjecting the pipe material to inclined rolling (hereinafter sometimes simply referred to as "rolling") and producing a metal pipe with a reduced outer diameter to the desired size. This raw pipe includes seamless steel pipes, welded steel pipes, forge-welded steel pipes, and UOE pipes before diameter reduction rolling. In the example of a two-roll type inclined rolling mill used for cold rolling, as shown in Figures 1 to 4, the raw pipe 1 is passed through the direction of travel while being compressed between a pair of rolling rolls 3. In this example of a cold rolling mill, the metal pipe obtained after the rolling process is called a cold-rolled pipe.

[0023] First, let me explain the rolling mill roll 3.

[0024] Figures 2 and 3 illustrate the intersection angle γ and inclination angle β of the rolling roll 3. Figure 2 is a side view of the rolling roll 3, and Figure 3 is a top view of the rolling roll 3.

[0025] As shown in Figures 2 and 3, the rolling roll 3 is a roll that rolls the raw pipe 1 supplied to the inclined rolling equipment 10. The rolling roll 3 has a rolling section 3a, and the raw pipe is subjected to inclined rolling in this rolling section 3a. The shape of the rolling roll 3 is not particularly limited. Examples of the shape of the rolling roll 3 include a barrel-shaped roll, a rolling roll with a shape other than a barrel-shaped roll in which the diameter is changed linearly or continuously in the longitudinal direction of the roll to control the contact position with the raw pipe 1, and a conical roll. Figures 1 to 4 show an example in which a barrel-shaped roll is used as the rolling roll 3.

[0026] In this invention, the rolling roll 3 is positioned at an inclination such that its rotation axis 7 has an inclination angle β with respect to the pass line 6. The inclination angle β refers to the angle (in degrees) between the straight line in the direction of the pipe axis (pass line 6) and the rotation axis 7 of the rolling roll 3 when the rolling roll 3 is viewed from above (i.e., in a direction perpendicular to the pipe axis direction and in the direction in which the rolling load is applied to the raw pipe 1), as shown in Figure 3.

[0027] In this way, by arranging two or more rolling rolls 3 at an inclination, the rolling rolls 3, which rotate around their rotation axis 7, utilize the frictional force generated by the contact between the rolling rolls 3 and the raw tube 1 to pull the raw tube 1 supplied to the roll gap in the rolling direction (i.e., the rolling path direction). Here, the end of the raw tube being pulled in is considered the tip. Therefore, the raw tube 1 is rolled in a helical shape while being rotated by the rolling rolls 3. That is, the raw tube 1 is rolled to reduce its diameter while rotating in the circumferential direction and advancing in the axial direction of the tube. Such a rolling configuration can be achieved by making the roll gap of at least two rolling rolls 3 of the inclined rolling mill smaller than the outer diameter of the raw pipe 1, and by inclining each of the rolling rolls 3 as described above (see Figure 1).

[0028] From the viewpoint of ensuring that the raw pipe 1 and the rolled pipe 2 advance stably in the direction of travel, it is preferable to set the inclination angle β in the range of 0.5 to 40.0°.

[0029] In addition to having an inclination angle β, the rolling roll 3 may also be positioned with a crossing angle γ on the rolling exit side. The crossing angle γ refers to the angle (in degrees) between the pass line 6 and the rotation axis 7 of the rolling roll 3 when the rolling roll 3 is viewed from the side (i.e., in a direction perpendicular to the pipe axis direction and perpendicular to the direction in which the rolling load is applied to the raw pipe 1), as shown in Figure 2. The crossing angle γ is not specifically defined. From the viewpoint of preventing deterioration of the roundness of the cross-sectional shape by minimizing the change in the peripheral speed of the roll in the pipe axis direction and stabilizing the progress of the raw pipe, it is preferable to set the crossing angle γ in the range of 0 to 45.0°. Roundness will be discussed later, so the explanation is omitted here.

[0030] As shown in Figure 1, in the case of an inclined rolling mill having a pair of rolling rolls 3, the inclination angle β and intersection angle γ formed by each rolling roll may be in opposite directions relative to the pipe axis direction (pass line 6). Although not shown in the figures, even when there are three or more rolling rolls 3, the direction in which the inclination angle β and intersection angle γ are formed may be changed for each rolling roll.

[0031] Referring to Figure 4, the face angles (specifically, the entry side angle M and the exit side angle N) in the rolling roll 3 will be explained.

[0032] The entry side angle M of the rolling roll 3 refers to the angle (in degrees) formed between the side of the rolling roll 3 that is on the rolling entry side relative to the direction of pipe travel (i.e., the tapered side of the rolling roll 3 where the cross-sectional shape gradually decreases toward the rolling entry side) and a straight line 6a parallel to the pipe axis direction (pass line 6), as shown in Figure 4.

[0033] Furthermore, the exit side angle N of the rolling roll 3 refers to the angle (in degrees) formed between the side of the rolling roll 3 that is on the exit side relative to the direction of travel of the pipe (i.e., the tapered side of the rolling roll 3 where the cross-sectional shape gradually decreases toward the exit side) and a straight line 6a parallel to the direction of the pipe axis, as shown in Figure 4. Note that "when the rolling roll 3 is viewed from the side" above means when viewed in a direction perpendicular to the direction of the pipe axis and also perpendicular to the direction in which the rolling load is applied to the raw pipe 1.

[0034] The entry angle M and exit angle N are not specifically defined. For example, when the shape of the rolling roll 3 is a barrel-shaped roll, and both the inclination angle β and the intersection angle γ are 0°, from the viewpoint of processing stability, it is preferable that the entry angle M be 0.2 to 20.0°, and the exit angle N be 0.2 to 20.0°.

[0035] Furthermore, for example, if the shape of the rolling roll 3 is such that the diameter is linearly or continuously changed in the longitudinal direction of the roll in order to control the contact position with the raw pipe 1 as described above, and both the inclination angle β and the intersection angle γ are 0°, then from the viewpoint of processing stability, it is preferable that the entry side angle M is 0.2 to 20.0°, and the exit side angle N is also 0.2 to 20.0°. The same applies when a conical roll is used.

[0036] Next, we will explain the control means 4.

[0037] The control means 4 controls the roll gap of two or more rolling rolls 3 based on the rolling conditions of the inclined rolling method of the present invention, which will be described later. The control means 4 uses two or more rolling rolls adjusted to a predetermined roll gap to perform multiple passes of diameter reduction rolling on the raw pipe. This makes it possible to suppress the decrease in the roundness of the cross-sectional shape of the metal pipe after diameter reduction rolling. The control means 4 can be controlled, for example, by a control device (not shown) for the inclined rolling mill 10. This control device can control the operation of the inclined rolling mill 10 based on commands from a process computer (not shown) that manages the operation of the metal pipe manufacturing process.

[0038] As described above, a two-roll type inclined rolling mill has been explained using Figure 1, etc., but according to the present invention, similar effects can be obtained even when rolling a raw pipe in an inclined rolling mill in which three or more rolling rolls are arranged in the circumferential direction of the pipe.

[0039] Although Figure 1 and other diagrams illustrate an inclined rolling mill with a single set of rolling rolls, when performing multiple passes using an inclined rolling mill of this structure, the rolling pipe should be repeatedly passed through a roll gap that has been adjusted for each pass. For example, although not shown in the diagram, when using an inclined rolling mill with a structure in which multiple sets of rolling mills are arranged in a row in the rolling direction, the pipe to be rolled can be passed through the roll gap adjusted for each rolling mill.

[0040] Next, the roundness in the present invention will be explained with reference to Figure 5. Figure 5 shows an example of a metal tube after inclined rolling, specifically one that has been deformed into an elliptical shape perpendicular to the tube axis.

[0041] As shown in Figure 5, in a cross-section perpendicular to the axial direction of the metal pipe 21 after inclined rolling, let Dmax be the maximum value of the outer diameter of the metal pipe 21 (i.e., the major axis of the ellipse) and Dmin be the minimum value of the outer diameter of the metal pipe 21 (i.e., the minor axis of the ellipse). Then, let C be the difference between these two values, i.e., the difference between the maximum and minimum outer diameters in the cross-section perpendicular to the axial direction of the metal pipe 21, i.e., "Dmax-Dmin". These maximum and minimum outer diameters may be measured values ​​or set values. If measured values ​​are used, they can be measured using the method described later.

[0042] Substituting the above values ​​of C and Dmax into equation (5), the resulting value (R) is referred to as the roundness. In this invention, a roundness (R) value is defined as "good roundness" (i.e., "excellent roundness") if it is less than or equal to a predetermined C / Dmax value (target value). R = C / Dmax …(5) Here, in equation (5), R is the roundness, C is the difference between the maximum and minimum outer diameters in the cross section perpendicular to the axial direction of the metal pipe after rolling, and Dmax is the maximum outer diameter in the cross section perpendicular to the axial direction of the metal pipe after rolling.

[0043] The outer diameter of a metal pipe (in mm) can be measured using, for example, a caliper. When measuring the outer diameter of the pipe end, a ruler may also be used.

[0044] Furthermore, when measuring the outer diameter other than at the pipe ends, the metal pipe is cut perpendicular to the pipe axis at the measurement point, and the shape of the cut surface is measured. In this invention, the maximum and minimum values ​​of the outer diameter are obtained by measuring the outer diameter of the pipe at 24 points at equal intervals in the circumferential direction of the pipe, with the maximum value being Dmax and the minimum value being Dmin. When measuring the outer diameter using calipers or a ruler, the distance in the circumferential direction between two points that serve as measurement positions (i.e., the positions where the calipers or ruler are placed) on the pipe circumference in a cross section perpendicular to the pipe axis is set to be half the pipe circumference.

[0045] The outer diameter of a metal pipe can be measured at any point along the pipe axis, but since transient deformation is likely to occur in the region including the leading and trailing ends of the metal pipe, it is desirable to measure the outer diameter at points 20 mm away from each end, more preferably 40 mm away from each end. Alternatively, multiple measurement points can be used along the pipe axis, and the average of the obtained values ​​can be used. For example, 10 measurement points can be used, and the average of the measurements taken at these 10 points can be used.

[0046] [Inclined rolling method] Next, the inclined rolling method of the present invention will be described. This inclined rolling method can be applied to the inclined rolling equipment 10 of the present invention described above. The description of the rolling rolls and control means has already been described and will therefore be omitted.

[0047] First, we will explain why the inventors focused on using multi-pass rolling for inclined rolling.

[0048] One example of a metal pipe manufacturing method involves applying inclined rolling to the pipe material in a single pass. However, in the case of single-pass rolling, the circumferential curvature imparted during processing becomes large (i.e., the overhang between the rolls becomes large), resulting in insufficient roundness of the resulting metal pipe.

[0049] Therefore, through our investigations, we have found that by performing inclined rolling of the pipe material in multiple passes, it is possible to suppress the decrease in the roundness of the cross-sectional shape of the metal pipe after rolling. The reason for this is as follows.

[0050] During rolling, the rolled tube (raw tube) is subjected to circumferential bending by the rolling rolls. This bending causes the rolled tube to buckle in the circumferential direction, which can result in a deterioration of its roundness. However, when the rolling process is changed from single-pass to multi-pass rolling, the amount of reduction in each rolling pass (i.e., the difference between the outer diameter of the rolled tube before rolling and the roll gap) becomes smaller, thus reducing the curvature added by the bending in each rolling pass. As a result, circumferential buckling becomes less likely, and the decrease in roundness can be suppressed. On the other hand, the inventors have also found that increasing the number of passes increases the variation in circumferential hardness, and this variation in hardness can become the starting point for circumferential buckling, leading to a deterioration in roundness.

[0051] Furthermore, further investigations by the inventors revealed that the following effects can also be obtained.

[0052] When rolling is performed with a small reduction amount, bending is not applied to the point of degrading roundness, but the yield strength of the metal pipe increases due to work hardening. This increase in yield strength reduces the bending curvature applied in the next rolling pass, making circumferential buckling less likely. It has been found that this allows for increased strength while suppressing the decrease in roundness. Based on these findings, the inventors have identified appropriate rolling conditions, specifically conditions related to setting the roll gap, that can obtain the above-mentioned effects.

[0053] The present invention relates to a method for inclined rolling of metal pipes, in which a raw pipe is rotated in the circumferential direction and advanced in the axial direction while being rolled to reduce its diameter. This inclined rolling method for metal pipes includes a rolling step in which a metal pipe is obtained by performing multiple passes of diameter reduction rolling on the raw pipe using two or more rolling rolls. In this rolling step, the roll gap of each pass is set to be less than or equal to the roll gap of the previous pass. By performing rolling in multiple passes in this way, the effect of not deteriorating the roundness of the cross-sectional shape of the metal pipe after the rolling step is completed (i.e., the shape of the cross-section perpendicular to the axial direction of the pipe) can be obtained (i.e., suppressing the decrease in roundness). Furthermore, it is preferable that the roll gap of each pass in the above rolling process be less than the roll gap of the immediately preceding pass.

[0054] In the rolling process of the present invention, in addition to setting the roll gap of each pass to be less than or equal to the roll gap of the previous pass, it is preferable to set the roll gap of the final pass to satisfy equation (1). 0 < (D - Gb) / (D - Gf) ≤ 0.60 …(1) Here, as shown in equation (1), D: Outer diameter of the raw tube before diameter reduction rolling [mm] Gf: Roll gap in the final pass [mm] Gb: Roll gap [mm] in the pass immediately preceding the final pass. That is the case.

[0055] As described above, by increasing the yield strength of the rolled pipe through work hardening to a degree that does not degrade roundness before the pass immediately preceding the final pass, the effect of reducing the bending curvature imparted in the next pass (i.e., the final pass) can be obtained. In order to achieve reduction without degrading roundness, it is more effective for the value of "(D-Gb) / (D-Gf)" shown in equation (1) above to be 0.60 or less. This value is more preferably 0.50 or less.

[0056] Furthermore, in order to increase the yield strength of the metal pipe by work hardening to an extent that does not worsen the roundness by the pass immediately preceding the final pass, thereby obtaining the effect of reducing the bending curvature imparted in the next rolling pass, the lower limit of the value of "(D-Gb) / (D-Gf)" shown in equation (1) should be greater than 0. This value is more preferably 0.001 or greater, and even more preferably 0.002 or greater.

[0057] Increasing the number of passes increases the variation in circumferential hardness, and this variation in hardness can become the starting point for circumferential buckling, potentially leading to a deterioration in roundness. Therefore, a pass count in the range of 2 to 10 is desirable.

[0058] In the metal pipe inclination rolling method of the present invention described above, an inclination rolling facility 10 is used, which is equipped with two or more rolling rolls 3, the rotation axis 7 of the rolling rolls 3 of which are arranged at an inclination angle β with respect to the pass line 6 (pipe axis direction). Rolling conditions are set by a control means 4 that controls the roll gap of the rolling rolls 3 of the inclination rolling facility, and the raw pipe 1 is rolled while rotating in the circumferential direction and advancing in the pipe axis direction. As shown in Figure 4, the raw pipe 1 is supplied to the roll gap of the inclination rolling facility 10 from the entry side of the inclination rolling facility 10 (i.e., the right side of the paper as shown in Figure 4). The raw pipe 1 is subjected to inclination rolling by passing the raw pipe 1 in the rolling direction while being squeezed by each rolling roll 3. As a result, a metal pipe with a reduced outer diameter is obtained.

[0059] According to the present invention, it is possible to suppress the decrease in the roundness of the cross-sectional shape after rolling. Furthermore, by improving the yield strength of the metal tube through work hardening, it is possible to obtain a metal tube with high yield strength.

[0060] As mentioned above, for example, the control means 4 can be controlled by the control device (not shown) of the inclined rolling equipment 10. Furthermore, the rolling rolls 3 may be arranged with an intersection angle γ. The entry side angle M and exit side angle N may also be set appropriately so that they fall within the above-mentioned numerical ranges.

[0061] [Method of manufacturing metal pipes] Next, a method for manufacturing metal tubes using the inclined rolling equipment of the present invention described above will be explained.

[0062] The present invention relates to a method for manufacturing a metal tube, which involves subjecting a raw tube to inclined rolling using the above-described inclined rolling method for metal tubes. This manufacturing method includes a tube-making step in which the raw tube 1 is rotated in the circumferential direction and advanced in the axial direction of the tube, and the raw tube 1 is passed through the roll gaps of two or more rolling rolls 3, thereby obtaining a metal tube by inclined rolling in multiple passes.

[0063] In the pipe manufacturing process, the control means 4 controls the roll gap of the rolling rolls 3, and the raw pipe 1 is rolled using the rolling rolls 3 set to the roll gap. Note that the explanation of the roll gap control means has already been provided in the explanation of the inclined rolling equipment and inclined rolling method, so it will be omitted here.

[0064] In the method for manufacturing metal tubes of the present invention, for example, the metal tubes after the above-described tube manufacturing process may be subjected to heat treatment. Alternatively, for example, the metal tubes after the above-described tube manufacturing process may be subjected to pickling to remove scale from the surface of the metal tubes. The conditions for heat treatment and pickling should be set appropriately according to the composition of the metal tubes, etc.

[0065] In this invention, the manufacturing conditions for the raw tube before the above-described tube manufacturing process are not particularly limited, and commonly known manufacturing conditions can be used. Furthermore, the raw tube before the tube manufacturing process is not particularly limited, and may, for example, be a hollow tube.

[0066] Furthermore, even if there is a difference between the maximum and minimum outer diameters of the raw tube before rolling, the desired effect can be obtained in the present invention. In the present invention, the raw tube before rolling has an elliptical or circular cross-section perpendicular to the tube axis, and it is preferable that the value obtained by dividing the difference between the maximum and minimum outer diameters by the maximum outer diameter is 10% or less, and more preferably 7% or less.

[0067] In this invention, the metal tube obtained by the rolling method described above can have a yield strength of 700 MPa or more. Furthermore, although not particularly limited, the outer diameter of the metal tube is preferably 8 to 600 mm. The wall thickness of the metal tube is preferably 0.2 to 40.0 mm, and more preferably 1.0 to 32.0 mm. [Examples]

[0068] The following describes embodiments of the present invention. However, the present invention is not limited by the embodiments described below, and modifications can be made as appropriate within the scope that is consistent with the spirit of the present invention.

[0069] First, raw pipes with an outer diameter of 100 mm (difference between maximum and minimum outer diameter: 0 mm), a wall thickness of 5 mm, and a length of 250 mm were taken from stainless steel rods conforming to the JIS G 4303:2012 standard (material: SUS329J3L) shown in Table 1 by machining.

[0070] [Table 1]

[0071] Next, the sampled raw tubes were subjected to multi-pass diameter reduction rolling under the conditions shown in Table 2, based on the inclined rolling method described above. For this rolling, equipment with the number of rolls shown in Table 2 (i.e., a 2-roll inclined rolling mill and a 3-roll inclined rolling mill) was used as the cold rolling equipment. These rolling rolls were barrel-shaped rolls with an entry side angle M of 2.5° and an exit side angle N of 3.0° when both the inclination angle and crossing angle were 0°. During rolling, the crossing angle γ of the rolling rolls was set to 0° and the inclination angle β to 4.5°. Cold rolling was performed on the raw tubes at room temperature with the number of passes and roll gap shown in Table 2 to obtain metal tubes.

[0072] Next, the roundness (R) was evaluated using the method described above with the metal tube after rolling (in this case, a cold-rolled tube). A caliper was used to measure the outer diameter of the metal tube. As described above, the difference C was calculated from the maximum outer diameter Dmax (measured value) and minimum outer diameter Dmin (measured value) obtained by measuring the outer diameter at 24 points at equal intervals in the circumferential direction of the tube. The value of C / Dmax was calculated using equation (5) and is shown in Table 2.

[0073] In the examples, the C / Dmax value measured after rolling was evaluated as "acceptable (i.e., excellent roundness)" if it was 0.014 or less, which was a predetermined target value for C / Dmax. Specifically, if the value was 0.010 or less, the symbol "◎" was written in the evaluation result column of Table 2, and if it was greater than 0.010 but 0.014 or less, the symbol "○" was written. On the other hand, if the C / Dmax value was greater than 0.014, it was evaluated as "unacceptable," and the symbol "×" was written in the evaluation result column of Table 2.

[0074] The measurement position in the axial direction of the pipe was set to the center of the metal pipe (i.e., half the total length of the pipe from the end of the pipe).

[0075] The results are shown in Table 2.

[0076] [Table 2]

[0077] The pipes No. 1 to 17 shown in Table 2 were obtained by rolling using a two-roll inclined rolling mill with two rolls. Pipe No. 1 was not rolled in multiple passes. As a result, the measured C / Dmax value is greater than the target value, and the decrease in roundness was not suppressed, resulting in an evaluation result of "×". Pipe No. 17 underwent multiple passes of rolling, but the roll gap was larger in the second pass than in the first pass, and the measured C / Dmax was also greater than the target value. Therefore, the evaluation result is "×".

[0078] On the other hand, pipes No. 2 to 16 were rolled using multiple passes. As a result, the measured C / Dmax values ​​are smaller than the target values, indicating that the decrease in roundness has been suppressed. Note that if equation (1) is satisfied, the evaluation result is "◎".

[0079] The pipes No. 18 to 34 shown in Table 2 were obtained by rolling using a 3-roll type inclined rolling mill with 3 rolls. Pipe No. 18 was not rolled in multiple passes. As a result, the measured C / Dmax value is greater than the target value, and the decrease in roundness was not suppressed, resulting in an evaluation result of "×". Pipe No. 34 underwent multiple passes of rolling, but the roll gap was larger in the second pass than in the first pass, and the measured C / Dmax was also greater than the target value. Therefore, the evaluation result is "×".

[0080] On the other hand, pipes No. 19 to 33 were rolled using multiple passes. As a result, the measured C / Dmax values ​​are smaller than the target values, indicating that the decrease in roundness has been suppressed. Note that if equation (1) is satisfied, the evaluation result is "◎".

[0081] From the above, it was found that the decrease in roundness can be suppressed in the present invention example. [Explanation of Symbols]

[0082] 1. Raw tube 2 Rolled pipe 3 Rolling Rolls 3a Rolling section 4. Control means 6 Pass Lines 7. Rotation axis of the rolling mill rolls 10. Inclined rolling mill equipment 21 Metal tube β Tilt angle γ intersection angle M Entrance angle N Exit angle Gap Roll Gap Dmax: Maximum outer diameter Dmin Minimum outer diameter

Claims

1. A method for inclined rolling of a metal pipe, in which the raw pipe is rotated in the circumferential direction and advanced in the axial direction of the pipe while being rolled to reduce its diameter, The process includes a rolling step to obtain a metal tube by performing multiple passes of diameter reduction rolling on the raw tube using two or more rolling rolls. A method for inclined rolling of a metal pipe, wherein in the rolling process, the roll gap of each pass is set to be less than or equal to the roll gap of the immediately preceding pass.

2. In the aforementioned rolling process, The method for inclined rolling of a metal pipe according to claim 1, wherein the roll gap of the final pass is set to satisfy equation (1). 0<(D-Gb) / (D-Gf)≦0.60...(1) Here, as shown in equation (1), D: Outer diameter of the raw tube before reduction rolling [mm] Gf: Roll gap [mm] in the final pass. Gb: Roll gap [mm] in the pass immediately preceding the final pass. That is the case.

3. A metal pipe inclined rolling mill that reduces the diameter of a raw pipe while rotating it in the circumferential direction and advancing it in the axial direction, Two or more rolling rolls are arranged at an inclination on the circumference centered on the pass line, The system includes control means for controlling the roll gap of the two or more rolling rolls, The control means, when performing multiple passes of diameter reduction rolling on the raw tube using the two or more rolling rolls, A metal pipe inclined rolling mill that sets the roll gap of each pass to be less than or equal to the roll gap of the previous pass.

4. The control means is Furthermore, the metal pipe inclined rolling apparatus according to claim 3 is configured such that the roll gap of the final pass satisfies equation (1). 0<(D-Gb) / (D-Gf)≦0.60...(1) Here, as shown in equation (1), D: Outer diameter of the raw tube before reduction rolling [mm] Gf: Roll gap [mm] in the final pass. Gb: Roll gap [mm] in the pass immediately preceding the final pass. That is the case.

5. A method for manufacturing a metal tube, comprising a tube manufacturing step of applying diameter reduction rolling to a raw tube using the inclined rolling method for metal tubes described in claim 1 or 2 to obtain a metal tube.

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