Incline rolling method, inclined rolling equipment, and method for manufacturing metal pipes

Abstract

The purpose is to provide a method for inclined rolling, inclined rolling equipment, and a method for manufacturing metal pipes. [Solution] The present invention relates to an inclined rolling method in which a raw tube is rotated in the circumferential direction and advanced in the axial direction of the tube while being rolled to reduce the diameter of the raw tube with two or more rolling rolls, comprising a first step of lightly reducing the tip region of the raw tube and a second step of reducing the diameter of the region excluding the tip region, wherein the roll gap of the two or more rolling rolls is controlled to satisfy equation (2) when Gap is the roll gap and Di is the outer diameter of the raw tube, and light reduction of the tip region is performed. Di≧Gap≧Di×0.92 …(2)

Description

Technical Field

[0001] The present invention relates to an inclined rolling method for reducing the diameter of a metal tube, inclined rolling equipment, 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) or austenitic stainless steels (specifically equivalent to JIS G3459 SUS 301, 302, 304, 305, 309, 310, 312, 315, 316, 317, 836, 890, 321, 347) containing a large amount of alloying elements for improving corrosion resistance such as Cr, Mo, Ni, etc., and 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 austenite phase or a multiphase structure containing a large amount of austenite phase. The austenite phase with a face-centered cubic lattice (fcc) structure generally has a lower yield strength than the ferrite phase or martensite phase with a body-centered cubic lattice (bcc) structure in a use environment from low temperature to normal temperature. Therefore, when higher yield strength is required for materials containing the austenite phase, cold working is applied to the material, and dislocation strengthening by processing is used to increase the yield strength.

[0004] For example, in high-strength and high-corrosion-resistant steel pipes 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 pipes 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 mentioned above, the cold rolling method described in Patent Document 1 has numerous advantages. However, the inclined rolling mill used in Patent Document 1 may cause excessive deformation of the pipe material due to the large stress applied in the circumferential direction of the pipe by the rolling load from the rolling rolls during rolling. Therefore, Patent Document 1 is not yet sufficient as a technique to bring the cross-sectional shape of the metal pipe after rolling closer to a perfect circle, that is, to improve the roundness of the metal pipe.

[0011] Furthermore, the provision of technology to improve the roundness of metal pipes after rolling is required not only for cold rolling but also for hot rolling and warm rolling. In addition, it is required that this technology be applicable not only to seamless metal pipes but also to welded steel pipes and forge-welded steel pipes.

[0012] This invention has been made in view of the above problems, and aims to provide an inclined rolling method, inclined rolling equipment, and a method for manufacturing metal tubes that can 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 to improve the roundness of metal pipes and a method for manufacturing metal pipes, as well as inclined rolling equipment that would make this method feasible. As a result, they found that by setting the processing conditions for the tip of the rolled pipe, which is performed prior to diameter reduction rolling, in a way that does not worsen the roundness, the cross-sectional shape of the entire length of the metal pipe after rolling can be made closer to a perfect circle.

[0014] Through further investigation, the inventors have completed an invention comprising the following gist. [1] An inclined rolling method in which a raw tube is rotated in the circumferential direction and advanced in the axial direction of the tube, and the raw tube is rolled to reduce its diameter using two or more rolling rolls, A first step involves lightly reducing the tip region of the aforementioned raw tube, The process includes a second step of reducing the diameter of the raw tube region excluding the tip region, In the first step described above, When the leading edge of the aforementioned tube is taken as the starting point, and the position located a distance L in the axial direction of the tube from the starting point is taken as the ending point, the region within the range from the starting point to the ending point is defined as the leading edge region. An inclined rolling method that controls the roll gap of the two or more rolling rolls to satisfy equation (2) and performs a light reduction of the leading edge region. L = Di × 6.00 …(1) Di≧Gap≧Di×0.92 …(2) Here, as shown in equation (2), Gap: Roll gap [mm] Di: The outer diameter of the raw tube [mm]. [2] Two or more rolling rolls arranged at an angle on the circumference of a pass line, It includes control means for controlling the roll gap of the two or more rolling rolls, The control means is When the leading edge of the tube is taken as the starting point, and the endpoint is taken at a position separated from the starting point by a value L expressed by equation (1) in the direction of the tube axis, the region within the range from the starting point to the endpoint is defined as the tip region. The tilt rolling equipment controls the roll gap to satisfy formula (2) when lightly pressing the tip region. L = Di × 6.00 …(1) Di ≧ Gap ≧ Di × 0.92 …(2) Here, as shown in formula (2), Gap: roll gap [mm], Di: outer diameter of the base pipe [mm]. [3] A method for manufacturing a metal pipe, comprising a pipe manufacturing process of performing diameter reduction rolling on a base pipe using the tilt rolling method according to [1] above to obtain a metal pipe.

Advantages of the Invention

[0015] According to the present invention, since it is possible to make the cross-sectional shape of the entire length of the rolled metal pipe closer to a perfect circle, it is possible to suppress a decrease in the roundness of the metal pipe.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a schematic diagram for explaining an embodiment of the tilt rolling equipment of the present invention. [Figure 2] FIG. 2 is a schematic side view of the tilt rolling equipment shown in FIG. 1. [Figure 3] FIG. 3 is a schematic top view of the tilt rolling equipment shown in FIG. 1. [Figure 4] FIG. 4 is a schematic diagram for explaining the surface angle of the rolling roll in the tilt 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 rolled metal pipe. [Figure 6] FIG. 6(A) is a schematic diagram for explaining the "contact location" of the present invention, and FIG. 6(B) is a schematic diagram for explaining the "tip region" of the present invention.

Embodiments for Carrying Out the Invention

[0017] The inclined rolling equipment, inclined rolling method, and metal pipe manufacturing method of the present invention will be described with reference to the figures. The following description illustrates preferred embodiments of the present invention, and the present invention is not limited to these embodiments.

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

[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 rolling is being performed on a pipe to be 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. Figure 3 is a schematic diagram of the inclined rolling equipment and pipe to be rolled shown in Figure 1, viewed from above. 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 the raw pipe 1 and the pipe to be rolled 2 as cross-sectional views.

[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. Furthermore, 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 equipped with two or more rolling rolls 3 arranged at an inclination on the circumference of a circle centered on a pass line 6. This inclined rolling mill 10 has a control means 4 for controlling the roll gap of the rolling rolls 3. As shown in Figure 1, two or more rolling rolls 3 are arranged in the circumferential direction of the raw pipe 1 and the pipe to be rolled 2. The raw pipe 1 is supplied to the roll gap of this inclined rolling mill 10 from the entry side of the inclined rolling mill (i.e., the rolling entry side). The raw pipe 1 is then passed through the rolling direction 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 a metal pipe with a reduced outer diameter is obtained. This raw pipe includes seamless steel pipes, welded steel pipes, forge-welded steel pipes, and UOE pipes before diameter reduction rolling.

[0022] 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 compressed between two rolling rolls 3 as it passes through in the rolling direction. In this example of a cold rolling mill, the metal pipe obtained after rolling is 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 rolls 3. Figure 2 is a cross-sectional view along line AA shown in Figure 1, and shows the rolling rolls 3 and the rolled pipe 2 as viewed from the side. Figure 3 is a view along arrow BB shown in Figure 2, and shows the rolling rolls 3 and the rolled pipe 2 as viewed from above.

[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, which performs inclined rolling on the raw pipe. The shape of the rolling roll 3 can be, for example, a barrel-shaped roll or a conical roll. Figures 1 to 4 show an example in which a barrel-shaped roll is used as the rolling roll 3.

[0026] The rolling roll 3 is positioned so that its axis of rotation 7 is inclined at an angle β with respect to the pass line 6. The angle β refers to the angle (in degrees) between the straight line in the direction of the pipe axis (pass line 6) and the axis of rotation 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 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 pipe 1 to pull the raw pipe 1 supplied to the roll gap in the rolling direction (i.e., the rolling path direction). Here, the end of the raw pipe being pulled in is considered the tip. Therefore, the raw pipe 1 is rolled in a helical shape while being rotated by the rolling rolls 3. That is, the raw pipe 1 is rolled to reduce its diameter while rotating in the circumferential direction and advancing in the axial direction of the pipe. This type of rolling can be achieved by making the roll gap of the rolling rolls 3 of the inclined rolling mill smaller than the outer diameter of the raw pipe 1, and by arranging each of the rolling rolls 3 at an inclination as described above (see Figure 1).

[0028] In this invention, prior to the diameter reduction rolling, the first step described later is performed on the tip of the raw tube 1.

[0029] From the viewpoint of ensuring that the raw pipe 1 and the pipe to be rolled 2 advance stably in the rolling direction, it is preferable to set the inclination angle β in the range of 0.5 to 40.0°.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The entry angle M of the rolling roll 3 refers to the angle (in degrees) between the side of the rolling roll 3 that is on the rolling entry side relative to the rolling direction of the pipe (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) when the rolling roll 3 is viewed from the side, as shown in Figure 4. The exit angle N of the rolling roll 3 refers to the angle (in degrees) between the side of the rolling roll 3 that is on the rolling exit side relative to the rolling direction of the pipe (i.e., the tapered side of the rolling roll 3 where the cross-sectional shape gradually decreases toward the rolling exit side) and a straight line 6a parallel to the pipe axis direction when the rolling roll 3 is viewed from the side, as shown in Figure 4. The above "when the rolling roll 3 is viewed from the side" means when viewed 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.

[0034] The entry angle M and exit angle N are not specifically defined. The entry angle M and exit angle N should be set as appropriate from the viewpoint of the gripping ability of the raw pipe and the stability of the progress of the rolled pipe. Depending on the manufacturing equipment, it is preferable to set the entry angle M in the range of 0.2 to 20.0° and the exit angle N in the range of 0.2 to 20.0°.

[0035] Next, we will explain the control means 4.

[0036] The control means 4 controls the roll gap of each rolling roll 3 based on the conditions of the inclined rolling method of the present invention, which will be described later. In the present invention, in particular, by setting the processing conditions of the first step using the control means 4 so as not to worsen the roundness of the tip of the rolled pipe 2, the cross-sectional shape of the metal pipe after diameter reduction rolling (i.e., after the completion of the second step) can be made closer to a perfect circle over its entire length.

[0037] 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.

[0038] 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 that has been deformed into an elliptical shape, perpendicular to the tube axis.

[0039] As shown in Figure 5, in a cross section perpendicular to the axial direction of the metal pipe 21 after inclined rolling, the maximum value of the outer diameter of the metal pipe 21 (i.e., the major axis of the ellipse) is Dmax, and the minimum value of the outer diameter (i.e., the minor axis of the ellipse) is Dmin. 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 (Dmax - Dmin), is defined as Cp. The target value of the outer diameter is D. Substituting the value of Cp and the value of D into equation (5), the resulting value (R) is called the roundness. In this invention, a value of this roundness (R) that is less than or equal to a predetermined Cp / D value is defined as "good roundness" (i.e., "excellent roundness"). R = Cp / D …(5) Here, in equation (5), R is the roundness, Cp 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 D is the target outer diameter in the cross section perpendicular to the axial direction of the metal pipe after rolling.

[0040] 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.

[0041] 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 equally spaced points in the circumferential direction of the pipe at the cut surface, 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.

[0042] The measurement position in the axial direction of the metal pipe can be any point except for the area rolled by the roll gap shown in equation (2) (i.e., the lightly reduced area). However, since transient deformation is likely to occur in the area including the leading and trailing ends of the metal pipe, it is desirable to measure the outer diameter at a point 20 mm away from each end, more preferably 40 mm away from each end. In addition, multiple measurement positions may be set in the axial direction of the pipe, and the average value of the obtained values ​​may be used. For example, the number of measurement points may be set to 10, and the average value of the measurements taken at these 10 points may be used.

[0043] [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. Note that the description of the rolling rolls and control means has already been described and will therefore be omitted.

[0044] First, with reference to Figure 6, we will explain why the inventors focused on controlling the roll gap during rolling in the tip region of the rolled pipe. Figure 6(A) is an enlarged view of the rolling portion and its surroundings during rolling of the rolled pipe, and Figure 6(B) is a diagram illustrating the tip region of the rolled pipe.

[0045] As shown in Figure 6(A), the area in contact between the tapered side surface 3b of the rolling roll 3 and the surface of the rolled pipe 2 during rolling is referred to as the "contact area." In the example shown in Figure 6(A), the rolling portion 3a of the rolling roll 3 is in contact with the rolling entry side and the rolling exit side. Through our investigation, we have found that the constraint between this contact area and the longitudinal portion of the rolled pipe adjacent to the contact area on the rolling exit side (the area indicated by the white arrow in Figure 6(A)) affects the deterioration of roundness. The reason is as follows: When the roundness of the cross-sectional shape of the contact area deteriorates, the adjacent portion (i.e., the longitudinal portion of the rolled pipe mentioned above) changes to the same or a similar cross-sectional shape as the contact area because it is a single continuous pipe with the contact area. At this time, if the cross-sectional shape of the longitudinal portion of the rolled pipe has already deteriorated in roundness before the contact area deforms, only the cross-sectional shape of the contact area will change to a state of deteriorated roundness. In contrast, if the cross-sectional shape of the longitudinal portion of the rolled pipe has not deteriorated in roundness before the contact point deforms, then the cross-sectional shape of the contact point must change to a state of deteriorated roundness, and simultaneously, the cross-sectional shape of the longitudinal portion of the rolled pipe must also change to a state of deteriorated roundness. In other words, since a wider range of the cross-sectional shape needs to change, it can be said that the roundness is less likely to deteriorate. Furthermore, it was found that when the roundness is high in the longitudinal portion of the rolled pipe, the roundness after processing at the contact point is also good.

[0046] In other words, we found that if the roundness of the tip of a metal pipe is good, the constrainment of the tip will also improve the roundness of the longitudinal portion of the pipe following the tip, resulting in good roundness of the entire pipe after processing. The "roundness of the entire pipe after processing" mentioned above refers to the roundness of the metal pipe after processing, including the steady-state portion other than the tip.

[0047] Based on the above findings, we have discovered a method for processing the tip of a metal pipe under processing conditions that do not degrade its roundness.

[0048] Now, with reference to Figure 6(B), the tip portion described above will be explained. In this invention, as shown in Figure 6(B), the leading edge of the rolled pipe (i.e., the leading edge of the raw pipe) is taken as the starting point, and the endpoint is a position separated from the starting point by a value L in the pipe axis direction represented by equation (1). The specific region within the range from the starting point to the endpoint is defined as the "tip region." The beginning of this tip region coincides with the aforementioned leading edge (i.e., the starting point). On the other hand, the end of the tip region is a single point within the specified region. L = Di × 6.00 …(1) Here, Di is the outer diameter of the tube (mm) as shown in equation (1).

[0049] The first step is performed at the roll gap shown in the equations described below. The range in the longitudinal direction of the pipe is a specific area within the range from the leading edge (starting point) of the metal pipe to a position (endpoint) separated in the axial direction by the value of L represented by equation (1), i.e., "a value calculated as 600% of the outer diameter Di of the raw pipe before rolling," as shown in Figure 6(B). Although the effect can be obtained by performing the first step over a wide range in the longitudinal direction of the pipe, the effect is equivalent even if a wider range than that of equation (1) is targeted. Furthermore, if the portion that is not processed (lightly reduced) becomes large, it will lead to a decrease in yield. In other words, the narrower the leading edge region that is lightly reduced in the first step, the less the yield decreases, which is preferable from the viewpoint of yield. On the other hand, in order to obtain the effect of preventing deterioration of roundness by constraining the longitudinal portion of the rolled pipe, it is undesirable for the leading edge region to be too narrow.

[0050] For the reasons stated above, the upper limit of the endpoint based on formula (1) is set to "600% of the outer diameter Di of the raw pipe". It is more preferable that the endpoint be set to "500% of the outer diameter Di of the raw pipe". Furthermore, it is preferable that the lower limit of the endpoint be set to "10% of the outer diameter Di of the raw pipe", and more preferable to "25% of the outer diameter Di of the raw pipe". In other words, the endpoint is preferably located within a range of 10% or more of the outer diameter Di of the raw tube and 600% or less of the outer diameter Di of the raw tube. Furthermore, it is preferable that it be 25% or more of the outer diameter Di of the raw tube and 500% or less of the outer diameter Di of the raw tube.

[0051] This section describes an inclined rolling method that processes the tip of a pipe under conditions that do not degrade its roundness.

[0052] The inventors investigated processing conditions that would prevent deterioration of the roundness of the tip portion of a metal pipe. As a result, they found that by processing (light reduction) the tip region of the pipe to be rolled with the roll gap shown in equation (2) before diameter reduction rolling, deterioration of the roundness of the tip portion of the resulting metal pipe can be prevented. In this invention, "light reduction" refers to reduction under conditions that satisfy equation (2).

[0053] Specifically, the method involves a gradient rolling process that includes a first step of lightly reducing the tip region of the raw tube and a second step of reducing the diameter of the region excluding the tip region. In this first step, the roll gap of two or more rolling rolls is controlled to satisfy equation (2) to perform a slight reduction in the tip region. Di≧Gap≧Di×0.92 …(2) Here, as shown in equation (2), Gap: Roll gap [mm] Di: The outer diameter [mm] of the metal tube before rolling (i.e., the raw tube).

[0054] The reason for specifying equation (2) as the processing conditions for the first step is as follows:

[0055] The lower limit of the roll gap must be "Di × 0.92" or greater, as shown in equation (2). This is because machining with a roll gap smaller than this lower limit will worsen the roundness of the part. It is preferable that the lower limit of the roll gap be "Di × 0.95" or greater. In the first step, the purpose is to perform machining under light pressure, so the upper limit of the roll gap should be "Di" or less. It is preferable that the upper limit of the roll gap be "Di × 0.99" or less.

[0056] As described above, if the roll gap is set by the control means 4 in the first step so as to satisfy equation (2), the above effects can be obtained.

[0057] Furthermore, the present invention improves the roundness of the metal pipe along its entire length as a final product by performing a light reduction of the tip of the rolled pipe prior to diameter reduction rolling under the processing conditions specified in the first step described above.

[0058] In this invention, the rolling conditions for the second step, in which diameter reduction rolling is performed from the tip region to the rear end (i.e., the region excluding the tip region), are not specified, and the rolling conditions may be set as appropriate according to the product dimensions and strength. However, from the viewpoint of obtaining sufficient strength for the metal tube, it is preferable that the rolling conditions for the second step be set to at least the following condition, for example, a value calculated as "outer diameter of the metal tube after processing / outer diameter of the raw tube" of 0.92 or less. It is preferable that this value be 0.76 or more.

[0059] In addition to this condition, it is more preferable to set the roll gap in the second step of diameter reduction rolling to be smaller than the roll gap in the first step. That is, it is more preferable to set the roll gap in the second step to be less than the roll gap in the first step. Even more preferable is to set the roll gap in the second step to a range of 76 to 99% of the roll gap in the first step, with the roll gap in the first step being the reference value.

[0060] As described above, for example, the control means 4 can also be controlled by a control device (not shown) for the inclined rolling equipment 10. In this case, the control device can control the operation of the inclined rolling equipment 10 by receiving commands from a process computer (not shown) that manages the operation of the metal pipe manufacturing process.

[0061] In the inclined rolling method of the present invention described above, an inclined rolling facility 10 is used, which is equipped with two or more rolling rolls 3, each of which has a rotation axis 7 of the rolling roll 3 positioned at an inclination angle β with respect to the pass line 6 (pipe axis direction). The conditions for the first step are set by a control means 4 that controls the roll gap of the rolling rolls 3 in the inclined rolling facility, and the raw pipe 1 is rolled while rotating in the circumferential direction and advancing in the pipe axis direction. The rolling rolls 3 may also be positioned with an intersection angle γ. The entry side angle M and exit side angle N may also be set appropriately so as to fall within the above-mentioned numerical ranges.

[0062] As shown in Figure 4, a raw pipe 1 is supplied to the roll gap of the inclined rolling mill 10 from the entry side of the inclined rolling mill 10 (i.e., the right side of the paper as shown in Figure 4). The raw pipe 1 is subjected to inclined rolling by passing it through the rolling direction while being compressed between each rolling roll 3. This results in a metal pipe with a reduced outer diameter to the desired size.

[0063] [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. That is, this manufacturing method is a method of producing metal tubes by rolling a tube to be rolled using the inclined rolling method described above. Note that the explanation of the roll gap control means has already been given in the explanation of the inclined rolling equipment and the inclined rolling method, so it will be omitted here.

[0064] The present invention provides a method for manufacturing a metal pipe, which includes a pipe-making step in which a raw pipe 1 is rotated in the circumferential direction and advanced in the axial direction of the pipe, and the raw pipe 1 is passed through the roll gap of two or more rolling rolls 3 and inclined to obtain a metal pipe.

[0065] In the pipe manufacturing process, the control means 4 controls the roll gap of the rolling rolls 3. Specifically, the control means sets the roll gap of the rolling rolls 3 based on a preset outer diameter of the raw pipe, and the rolling rolls 3 set to this roll gap perform the first and second processes on the raw pipe 1. This suppresses a decrease in the roundness of the cross-sectional shape after rolling along the entire length of the metal pipe. Furthermore, from the viewpoint of improving roundness, the control means controls the roll gap to satisfy the equations of the first process in the first or second embodiment described above.

[0066] The outer diameter of the raw pipe mentioned above can be determined, for example, based on product standards for metal pipes.

[0067] 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.

[0068] 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.

[0069] As described above, according to the present invention, by performing inclined rolling on a raw pipe using an inclined rolling mill equipped with a control means that can appropriately control the roll gap of the rolling rolls, it is possible to suppress the decrease in the roundness of the cross-sectional shape of the resulting metal pipe after rolling. In particular, there is no need for pretreatment such as applying a surface coating or processing the pipe ends to the raw pipe before inclined rolling. Furthermore, it is possible to improve the hardness of the metal pipe through inclined rolling while suppressing the decrease in the roundness of the cross-sectional shape after inclined rolling. [Examples]

[0070] The following describes embodiments of the present invention. However, the present invention is not limited by the embodiments described below, and can be modified as appropriate within the scope that is consistent with the spirit of the present invention.

[0071] First, raw pipes with an outer diameter of 100 mm (difference between maximum and minimum outer diameter: 0 mm), a wall thickness of 8 mm, and a length of 1000 mm were taken from stainless steel rods conforming to the JIS G 4303:2012 standard (material: SUS329J3L) shown in Table 1 by machining.

[0072] [Table 1]

[0073] Next, the sampled raw pipes were rolled according to 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 cold rolling equipment. These rolling rolls were barrel-shaped rolls with an entry side angle M of 8.0° and an exit side angle N of 4.0°. Here, rolling was performed by changing the roll gap Gap of the rolling rolls in various ways as shown in Table 2. During rolling, the inclination angle β of the rolling rolls was set to 3° and the intersection angle γ was set to 0°. The roll gap of the rolling rolls was set to the value shown in Table 2 in the tip region of the pipe to be rolled, and to 89.0 mm in the other parts (i.e., the region other than the tip region). The processing conditions for the first step were as shown in Table 2, and the rolling conditions for the second step were within the numerical range described above. The "length in the axial direction from the tip of the pipe" shown in Table 2 corresponds to the tip region described above.

[0074] A metal tube was obtained by subjecting a raw tube at room temperature to a single-pass rolling process.

[0075] 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 Cp 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 Cp / D was calculated using equation (5) and is shown in Table 2.

[0076] In this example, the Cp / D value measured after rolling was evaluated as "acceptable (i.e., excellent roundness)" if it was 0.009 or less, which was a predetermined target value. Specifically, if the value was between 0 and 0.006, the symbol "◎" was written in the evaluation result column of Table 2, and if it was greater than 0.006 and 0.009 or less, the symbol "○" was written. On the other hand, if the Cp / D value exceeded 0.009, it was evaluated as "unacceptable," and the symbol "×" was written in the evaluation result column of Table 2.

[0077] 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).

[0078] The results are shown in Table 2.

[0079] [Table 2]

[0080] The pipes No. 1 to 16 shown in Table 2 were obtained by rolling using a two-roll inclined rolling mill with two rolls. For pipes No. 1 to 3, the roll gap value in the tip region does not satisfy equation (2). As a result, the roundness does not meet the target value. In contrast, for pipes No. 4 to 16, the roll gap value in the tip region satisfies equation (2). As a result, the target value for roundness is met.

[0081] The pipes No. 17 to 32 shown in Table 2 were obtained by rolling using a 3-roll type inclined rolling mill with 3 rolls. For pipes No. 17 to 19, the roll gap value in the tip region does not satisfy equation (2). As a result, the roundness does not meet the target value. In contrast, for pipes No. 20 to 32, the roll gap value in the tip region satisfies equation (2). As a result, the target value for roundness is met.

[0082] From the above, it was found that the decrease in roundness can be suppressed in the present invention example. [Explanation of Symbols]

[0083] 1. Raw tube 2 Rolled pipe 3 Rolling Rolls 3a Rolling section 3b Tapered side 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. An inclined rolling method in which a raw tube is rotated in the circumferential direction and advanced in the axial direction of the tube while being rolled to reduce its diameter using two or more rolling rolls, A first step involves lightly reducing the tip region of the aforementioned raw tube, The process includes a second step of reducing the diameter of the raw tube region excluding the tip region, In the first step described above, When the leading edge of the tube is defined as the starting point, and the endpoint is defined as a position separated from the starting point by a value L represented by equation (1) in the direction of the tube axis, the region within the range from the starting point to the endpoint is defined as the leading edge region. A tilt rolling method that controls the roll gap of the two or more rolling rolls to satisfy equation (2) and performs a light reduction of the leading edge region. L=Di×6.00…(1) Di ≥ Gap ≥ Di × 0.92 …(2) Here, as shown in equation (2), Gap: Roll gap [mm] Di: The outer diameter of the raw tube [mm].

2. Two or more rolling rolls are arranged at an inclination on the circumference centered on the pass line, It includes control means for controlling the roll gap of the two or more rolling rolls, The control means is When the leading edge of the tube is taken as the starting point, and the endpoint is taken at a position separated from the starting point by a value L expressed by equation (1) in the direction of the tube axis, the region within the range from the starting point to the endpoint is defined as the tip region. An inclined rolling mill that controls the roll gap to satisfy equation (2) when lightly reducing the tip region. L=Di×6.00…(1) Di ≥ Gap ≥ Di × 0.92 …(2) Here, as shown in equation (2), Gap: Roll gap [mm] Di: The outer diameter of the raw tube [mm].

3. A method for manufacturing a metal pipe, comprising a pipe manufacturing step of reducing the diameter of a raw pipe using the inclined rolling method described in claim 1 to obtain a metal pipe.

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