Metal tube rolling method, metal tube manufacturing method and rolling equipment

Abstract

To provide a rolling method which materializes suppression of deterioration of circularity of a cross-sectional shape of a metal tube after rolling, a manufacturing method of a metal tube, rolling equipment and a metal tube.SOLUTION: A rolling method of a metal tube includes a rolling step in which diameter reduction rolling is performed on a material tube 1 while the material tube is rotated in a circumferential direction of the tube and advanced in an axial direction of the tube, thereby getting a metal tube 2, wherein in the rolling step, the diameter reduction rolling is controlled based on difference between an outer diameter maximum value of the rolled metal tube and an outer diameter minimum value of the rolled metal tube which are set in advance. The rolling load during the diameter reduction rolling satisfies the following expression (1): L≤1.2×t×Cp+4.5×t+18 (1) in which Cp is the difference [mm] between the outer diameter maximum value set in advance and the outer diameter minimum value in a vertical cross section of the metal tube after cold rolling step; t is wall thickness [mm] of the material tube before the rolling step; and L is rolling load maximum value [ton].SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a rolling method for a metal tube, a manufacturing method for a metal tube, a rolling facility, and a metal tube, in which the yield strength of the metal tube is improved by work hardening to produce a metal tube having high yield strength. [Background technology]

[0002] In fields where seamless metal pipe products are used, particularly where excellent corrosion resistance and high strength are required, seamless steel pipes made of duplex stainless steel (equivalent to JIS G3459 SUS 329J1, 329J3L, 329J4L) and austenitic stainless steel (equivalent to JIS G3459 SUS 301, 302, 304, 305, 309, 310, 312, 315, 316, 317, 836, 890, 321, 347) containing large amounts of corrosion-resistance-improving elements such as Cr, Mo, and Ni to improve corrosion resistance, as well as seamless pipes made of Ni-based alloys (equivalent to JIS H4552 NW4400, NW6007, NW0276, NW6022, NW6002) are used. These steels and alloys contain large amounts of alloying elements that are added to provide excellent corrosion resistance. Therefore, the structure is a single austenite phase or a multi-phase structure that includes the austenite phase. The austenite phase, which has a face-centered cubic (fcc) crystal structure, often has a lower yield strength than the ferrite phase and martensite phase, which have a body-centered cubic (bcc) crystal structure, in low to room temperature usage environments. Therefore, when a higher yield strength is required for a material containing the austenite phase, cold processing is added and dislocation strengthening by processing is used to increase the yield strength.

[0003] For example, in the case of high-strength, highly corrosion-resistant steel pipes with an outer diameter of 3-1 / 2 inches or more used in oil country tubular goods, cold working such as cold drawing and cold pilgering is widely used, and high-strength steel pipes with a yield strength of 125 ksi or more have been put into practical use (see Non-Patent Document 1).

[0004] However, the cold drawing method described in Non-Patent Document 1 is an effective method for improving the strength of the steel pipe in the longitudinal direction and for uniforming the wall thickness distribution in the longitudinal direction of the steel pipe, but many processes such as softening heat treatment of the steel pipe, pickling, chemical conversion treatment for providing a lubricating coating, and pipe end processing for creating a gripping portion during drawing are required before drawing. In addition, the wall thickness reduction rate is only about 20% due to the limitation of the pressure required for drawing and the prevention of seizure on the tool. Furthermore, if the wall thickness reduction amount is insufficient in one drawing, it is necessary to repeat the series of processes from the softening heat treatment described above again. In addition, since the shape of the steel pipe after drawing is uniquely determined by the tool dimensions used for drawing, the tool needs to be replaced when changing the size, and it is not suitable for small-lot, multi-variety production. Furthermore, there is a problem that the equipment investment and energy consumption are large because many processes are required when performing drawing. On the other hand, cold pilger processing does not require preliminary processing of the steel pipe and can obtain a high wall thickness reduction rate, but the feed amount in one pass is small at only several tens of mm, and production efficiency is poor. In addition, the shape of the mill roll is complicated, and the burden of tool manufacturing (the workload and economic burden for manufacturing the mill roll) is large.

[0005] In response to these problems, the technology described in Patent Document 1 proposes a cold rolling method characterized in that a metal tube is passed through the roll gap of an inclined rolling mill having two or more rolling rolls whose rotation axes are inclined with respect to the center line of the rolling pass direction of the metal tube to reduce the diameter. This makes it possible to improve the strength of the metal tube by cold working with high processing efficiency without requiring pretreatment such as providing a surface coating to the metal tube before processing or processing the tube end, and provides good effects in terms of environmental protection and industry. In addition, the outer diameter can be reduced by simply changing the roll gap of the inclined rolling mill to produce different product sizes. In addition, the free deformation of the inner surface prevents the surface pressure generated on the tool from becoming excessive, and the desired processing strain can be added without the occurrence of surface defects such as seizure that occur in cold drawing, making it suitable for small-lot production of a wide variety of products. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6432614 [Non-patent literature]

[0007] [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, Inc., May 6, 1986, pp.115-145 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the inventors have found through their studies that in the cold rolling method described in Patent Document 1, depending on the rolling conditions, a cross-sectional shape that is close to a perfect circle before rolling may be deformed into an ellipse or the like after rolling, resulting in a decrease in roundness. Thus, there has been a demand for the establishment of a technology for suppressing the decrease in roundness after rolling such as cold rolling.

[0009] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a rolling method, a manufacturing method for a metal pipe, a rolling facility, and a metal pipe that suppress a decrease in the roundness of the cross-sectional shape after rolling. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the present inventors have conducted extensive research into rolling methods and manufacturing methods for improving the roundness of metal tubes, and as a result, have completed the invention as outlined below. [1] A rolling process includes a rolling process in which a blank tube is rolled to reduce its diameter while being rotated in a tube circumferential direction and rolled in an axial direction to obtain a metal tube, a rolling load during the diameter reduction rolling being controlled based on a difference between a predetermined maximum outside diameter value and a predetermined minimum outside diameter value of the metal tube after the rolling step. [2] The method for rolling a metal tube according to [1] above, wherein in the rolling step, a rolling load during the diameter reduction rolling satisfies the following formula (1): L≦1.2×t×Cp+4.5×t+18...Equation (1) Here, in formula (1), Cp: ​​the difference [mm] between the preset maximum and minimum outer diameters in a cross section perpendicular to the axial direction of the metal tube after the rolling process; t: wall thickness of the raw tube before rolling process [mm], L: Maximum rolling load [ton]. [3] The method for rolling a metal tube according to [2] above, wherein, in the rolling step, when the diameter reduction rolling is performed in a plurality of passes, the maximum rolling load L in each pass satisfies the formula (1). [4] A method for producing a metal tube, comprising the steps of: performing diameter-reducing rolling by the rolling method according to any one of [1] to [3] above to obtain a metal tube. [5] A rolling facility for producing a metal tube by rolling a blank tube into a reduced diameter while rotating the blank tube in a circumferential direction and advancing the blank tube in an axial direction, comprising: a control unit that controls a rolling load during the diameter reduction rolling, based on a difference between a predetermined maximum outer diameter and a predetermined minimum outer diameter of the metal tube after the rolling process. [6] The rolling equipment according to [5], wherein the control unit controls the rolling load during the diameter reduction rolling so as to satisfy the following formula (1): L≦1.2×t×Cp+4.5×t+18...Equation (1) Here, in formula (1), Cp: ​​the difference [mm] between the preset maximum and minimum outer diameters in a cross section perpendicular to the axial direction of the metal tube after the rolling process; t: wall thickness of the raw tube before rolling process [mm], L: Maximum rolling load [ton]. [7] A metal pipe in which the difference between the maximum and minimum outer diameters in a cross section perpendicular to the axial direction is 3.5 mm or less. Effect of the Invention

[0011] According to the present invention, it is possible to suppress a decrease in the roundness of the cross-sectional shape after rolling. [Brief description of the drawings]

[0012] [Figure 1]FIG. 1 is a schematic diagram illustrating an example of a rolling method for a metal tube according to the present invention. [Diagram 2] FIG. 2 is a schematic diagram illustrating the cross-sectional shape of a metal tube after rolling. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

[0014] The method for rolling a metallic tube of the present invention includes a rolling step in which a blank tube is subjected to diameter-reducing rolling while being rotated in the tube circumferential direction and advanced in the tube axial direction to obtain a metallic tube, and in the rolling step, the rolling load during diameter-reducing rolling is controlled based on a preset difference between a maximum outer diameter and a minimum outer diameter of the metallic tube after the rolling step, thereby making it possible to suppress a decrease in the roundness of the cross-sectional shape after rolling of the metallic tube. In addition, in this rolling step, it is preferable that the rolling load during diameter reduction rolling satisfies the following formula (1). L≦1.2×t×Cp+4.5×t+18...Equation (1) Here, in formula (1), Cp: ​​the difference [mm] between the preset maximum and minimum outer diameters in a cross section perpendicular to the axial direction of the metal tube after the rolling process; t: wall thickness of the raw tube before rolling process [mm], L: Maximum rolling load [ton].

[0015] In addition, when rolling is performed in a plurality of passes, the rolling process in the present invention may be for only some of the passes, or may include the processing of all passes. For example, when rolling is performed in a total of three passes, the rolling process in the present invention may be for only any one pass, or may be for only the first two passes or the last two passes, or may include the processing of all three passes.

[0016] The rolling equipment of the present invention is for obtaining a metallic tube by performing diameter reduction rolling on a mother tube while rotating the mother tube in the circumferential direction and advancing the mother tube in the axial direction, and has a control unit that controls the rolling load during the diameter reduction rolling based on a preset difference between a maximum outer diameter and a minimum outer diameter of the metallic tube after the rolling process. It is preferable that the control unit controls the rolling load during the diameter reduction rolling so as to satisfy the above formula (1).

[0017] Hereinafter, a rolling method for rolling a metal pipe using the rolling equipment of the present invention will be described. As an example of the rolling equipment, an inclined rolling mill having two or more rolls whose rotation axes are inclined with respect to a center line (pipe axis direction) in the rolling pass direction of the metal pipe will be described.

[0018] FIG. 1 is a schematic diagram for explaining a case where rolling is performed by a two-roll type tilt rolling mill (a tilt rolling mill having two rolling rolls) as an example of the rolling method of the present invention. FIG. 1(b) is a front view seen from the rolling direction, FIG. 1(a) is a side view of the AA section of the tube to be rolled (metal tube) and the rolling roll in FIG. 1(b), and FIG. 1(c) is a view of the section of the tube to be rolled (metal tube) and the rolling roll as seen from the BB arrow in FIG. 1(a). Hereinafter, a case where cold rolling is performed as rolling will be explained. As an example of the rolling equipment, a cold rolling equipment is taken. Note that in the present invention, not only cold rolling but also hot rolling and warm rolling can be performed.

[0019] As described above, in the present invention, the tilt rolling mill for cold rolling a metal pipe has two or more rolling rolls 3 for rolling the mother pipe 1 (metal pipe 2) arranged in the circumferential direction of the mother pipe 1. In the present invention, as described later, the adjustment of the rolling load (maximum rolling load) of the mother pipe 1 can be automatically controlled by a control unit of the tilt rolling mill. The control unit can control the rolling load during diameter reduction rolling so as to satisfy formula (1) described later. The control unit is not particularly limited, but may be an information processing device such as a computer having a CPU (Central Processing Unit).

[0020] As shown in FIG. 1, in the present invention, a mother tube 1 is supplied from the entry side (right side in FIG. 1(a)) of a tilt rolling mill into a roll gap in which the rotation axes of rolls 3 form an inclination angle β (angle of inclination with respect to the tube axial direction when viewed in a direction perpendicular to the tube axial direction and in a direction in which a rolling load is applied to the mother tube 1: β) with respect to a rolling pass direction center line (tube axial direction) or further form a cross angle (cross angle) γ (angle of inclination with respect to the tube axial direction when viewed in a direction perpendicular to the tube axial direction and in a direction in which a rolling load is applied to the mother tube 1: γ) at the rolling exit side. The mother tube 1 is then subjected to diameter reduction rolling by passing the mother tube 1 in the rolling direction shown in FIG. 1 while being pressed between a pair of rolls 3, thereby producing a metal tube (cold-rolled tube) 2. Here, although not particularly limited, the inclination angle β is preferably 0.5 to 40.0°. The cross angle γ is preferably 0 to 45.0°. In the pair of rolling rolls 3, the directions in which the inclination angle β and the cross angle γ are formed with respect to the tube axial direction may be opposite to each other.

[0021] In the present invention, two or more rolls 3 are arranged with their rotation axes inclined at an inclination angle β with respect to the rolling pass direction center line (tube axial direction). With such an inclined arrangement, the rolls 3 rotating about the rotation axes draw the mother tube 1 supplied to the roll gap in the rolling direction by utilizing frictional force generated by contact between the rolls 3 and the mother tube 1. Therefore, the mother tube 1 is spirally rolled while being rotated by the rolls 3. That is, the mother tube 1 is rolled while rotating in the tube circumferential direction and proceeding in the tube axial direction.

[0022] Such a rolling mode can be realized by making the roll gap between at least two of the mill rolls 3 smaller than the outer diameter of the mother tube 1 and by arranging each of the mill rolls 3 at an incline as described above. Although a two-roll type inclined rolling mill is shown in Fig. 1, the mother tube 1 can be similarly reduced in diameter by rolling using an inclined rolling mill having three or more mill rolls arranged in the circumferential direction of the tube. There are no particular limitations on the shape of the mill rolls 3, and in addition to the barrel-shaped rolls shown in Fig. 1, mill rolls having a shape in which the diameter changes linearly or continuously in the longitudinal direction of the roll in order to control the contact position with the mother tube 1 can also be used. Here, when the rolling rolls are barrel-shaped rolls, although there are no particular limitations, the entrance side angle is preferably 0.2 to 10.0° when the inclination angle and cross angle are both 0°, and the exit side angle is preferably 0.2 to 10.0°. The inlet side angle refers to the angle of inclination with respect to the tube axis direction when viewed in a direction perpendicular to the tube axis direction and perpendicular to the direction in which the rolling load is applied to the mother tube 1 on the tapered side of the rolling roll 3, whose cross-sectional shape gradually becomes smaller toward the rolling inlet side. The above-mentioned exit side angle refers to the angle of inclination with respect to the tube axis direction when viewed in a direction perpendicular to the tube axis direction and perpendicular to the direction in which the rolling load is applied to the mother tube 1 on the tapered side of the rolling roll 3, whose cross-sectional shape gradually becomes smaller toward the rolling exit side.

[0023] L≦1.2×t×Cp+4.5×t+18...Equation (1) The formula (1) specified in the present invention will be described in detail below. In the present invention, the rolling conditions are set so as not to deteriorate the roundness of the cross-sectional shape (shape of a cross section perpendicular to the tube axis direction) of the rolled metal tube 2. An example of the cross-sectional shape of the rolled metal tube 2 is shown in Fig. 2. Fig. 2 shows an example in which the cross-sectional shape is deformed into an ellipse. Here, among the outer diameters after rolling, the maximum value (longer diameter of the ellipse) is defined as Dmax, and the minimum value (minor diameter of the ellipse) is defined as Dmin, and the difference between the two is defined as C (Cp): the difference between the maximum outer diameter and the minimum outer diameter in a cross section perpendicular to the axial direction of the metal tube 2.

[0024] The outer diameter can be measured using a vernier caliper, and a ruler can be used when measuring the end of the pipe. When measuring a part other than the end of the pipe, the measurement point can be cut and the shape of the cut surface can be measured. The maximum and minimum values ​​of the outer diameter can be obtained by measuring 24 points at equal intervals around the circumference of the pipe. Here, when measuring the outer diameter using a vernier caliper or ruler, the distance in the circumferential direction of the pipe between two measurement points (caliper installation positions) on the circumference of the pipe on a cross section perpendicular to the pipe axial direction is set to be 1 / 2 of the pipe circumference. The outer diameter may be measured at any position in the axial direction of the tube, but it is preferable to measure the positions excluding 20 mm from the leading and trailing ends, and more preferably, the positions excluding 40 mm from the leading and trailing ends. The wall thickness can be measured using a micrometer. When measuring at a place other than the end of the pipe, the measurement point may be cut and the wall thickness of the cut portion may be measured.

[0025] Through experimental investigations by the inventors, it was found that C is highly correlated with the load applied to the rolling rolls, and that the correlation between C and the load is affected by the wall thickness of the mother pipe, taking the values ​​shown in formulas (2) and (3). Here, L is the maximum value of the loads generated during the time that the mother pipe (metal pipe) is being rolled, and is the load that showed the largest value among multiple rolls. When L≧4.5×t+18 C=L / (1.2×t)-(4.5×t+18) / (1.2×t) Formula (2) If L<4.5×t+18, C=0 Equation (3) Here, in equations (2) and (3), C: Difference between the maximum and minimum outer diameters of the metal tube in a cross section perpendicular to the tube axis direction measured after rolling (after the final pass) [mm], t: wall thickness of the raw tube before rolling process [mm], L: Maximum rolling load generated on each roll [ton].

[0026] Based on this knowledge, in the present invention, the rolling conditions are set so that the rolling load falls within the range shown in formula (1), which makes it possible to further suppress the decrease in the roundness of the cross-sectional shape after rolling. L≦1.2×t×Cp+4.5×t+18...Equation (1) Here, in formula (1), Cp: ​​Tolerance for the difference between the maximum and minimum outer diameters [mm] (the difference between the preset maximum and minimum outer diameters in the cross section perpendicular to the axial direction of the metal tube after the cold rolling process [mm]) t: wall thickness of the raw tube before rolling process [mm], L: Maximum rolling load [ton].

[0027] Here, before starting rolling, it is necessary to set L in advance so as to satisfy the above formula (1). For this purpose, a value of the rolling load predicted by using a numerical analysis such as the finite element method may be used, or if possible, a preliminary experiment may be carried out under the same conditions and the load actual value may be used. The setting of L can be performed by the above-mentioned control unit. For example, when the above-mentioned load actual value is used, the load actual value can be stored in the above-mentioned control unit, and the control unit can adjust the rolling load on the rolling roll based on the data of the load actual value. Specifically, the rolling conditions can be set by setting the gap between the rolls (Gap in FIG. 1). If there is concern that L will exceed the upper limit value shown in formula (1), the gap can be set wider to reduce L.

[0028] Regarding the above formula (1), L is preferably (1.2×t×Cp+4.5×t+18)×0.85 or less, and more preferably (1.2×t×Cp+4.5×t+18)×0.70 or less. Also, L is preferably 2.0 tons or more, and more preferably 4.0 tons or more. L can be measured by a load cell installed adjacent to the rolls of a tilt rolling mill or the like.

[0029] In addition, when the rolling conditions are set by the above method, there is a possibility that the specified material strength may not be obtained after rolling. In such a case, rolling that satisfies the condition of formula (1) may be performed multiple times until the material strength is obtained. In this case, when performing multiple passes of diameter reduction rolling, it is preferable that the maximum rolling load L in each pass satisfies the above formula (1). When performing multiple passes, Cp is the difference between a preset maximum outer diameter and a preset minimum outer diameter in a cross section perpendicular to the axial direction of the metal pipe after the cold rolling process (after the final pass). For t, the value of the wall thickness before the first pass may be used. As described above, when performing multiple passes of rolling, the rolling process in the present invention may target only some of the passes, or may include all passes. For example, when performing a total of three passes of rolling, if the rolling process in the present invention targets the latter two passes, the time before the first pass that the wall thickness t targets is immediately before the second pass out of the total three passes.

[0030] The above has described the method for rolling a metal tube of the present invention. According to the method for rolling a metal tube of the present invention, it is possible to suppress a decrease in the roundness of the cross-sectional shape after rolling. In particular, no preparatory treatment such as providing a surface coating or machining the tube ends is required for the raw tube before rolling, and the hardness can be improved by rolling while suppressing a decrease in the roundness of the cross-sectional shape after rolling.

[0031] The present invention also provides a method for producing a metal tube, which comprises performing diameter-reducing rolling by the above-mentioned rolling method to obtain a metal tube. The method for producing a metal tube is not particularly limited, but after diameter-reducing rolling, a heat treatment may be performed to obtain the metal tube. Furthermore, after diameter-reducing rolling, a treatment may be performed to remove scale from the surface of the metal tube by pickling.

[0032] In the present invention, the metal tube obtained by the above-mentioned rolling method can have a difference between the maximum and minimum outer diameters in a cross section perpendicular to the tube axis direction of 3.5 mm or less. This difference is preferably 1.0 mm or less, and more preferably 0.5 mm or less. Moreover, the metal tube obtained by the above-mentioned rolling method can have a yield strength of 700 MPa or more. Although not particularly limited, the outer diameter of the metal tube is preferably 8 to 600 mm. The wall thickness of the metal pipe is preferably 0.2 to 40.0 mm, and more preferably 1.0 to 32.0 mm.

[0033] The manufacturing conditions up to the rolling of the raw pipe described above are not particularly limited, and generally known manufacturing conditions can be adopted. There are no particular limitations on the raw pipe before rolling, and it may be any commonly known hollow pipe material. There are no particular limitations on the material or mechanical properties, but it is preferable that the yield strength is 220 MPa or more. In addition, the desired effects can be obtained in the present invention even if there is a difference between the maximum and minimum outside diameters of the shape of the mother tube before rolling. In the present invention, the mother tube before rolling has an elliptical or circular cross section perpendicular to the tube axis direction, and the difference between the maximum and minimum outside diameters is preferably 10 mm or less, and more preferably 7 mm or less. EXAMPLES

[0034] Examples of the present invention will now be described. A blank tube with an outer diameter of 70 mm (difference between the maximum and minimum outer diameters: 0 mm), a wall thickness of 5 mm or 8 mm, and a length of 250 mm was obtained by machining from a stainless steel bar (material b: SUS329J3L) conforming to the JIS G 4303:2012 standard shown in Table 1, and was subjected to diameter reduction rolling under the conditions shown in Table 2. The tilt rolling mill (cold rolling equipment) used for diameter reduction rolling was a two-roll type tilt rolling mill or a three-roll type tilt rolling mill having barrel-shaped rolls with an entry side angle of 2.5° and an exit side angle of 3.0° when the tilt angle and cross angle were both 0°. The tilt angle β was set to 3° and the cross angle γ was set to 0° during rolling, and the blank tube was rolled at room temperature. Cold rolling was performed with the roll gap and number of rolls for each pass shown in Table 2.

[0035] [Table 1]

[0036] The outer diameter of the rolled metal tube (cold-rolled tube) was measured at 24 equally spaced points around the circumference using a vernier caliper to obtain the maximum value Dmax and the minimum value Dmin. The difference between the maximum and minimum outer diameters was evaluated as C: the difference between the maximum and minimum outer diameters. The measurement position in the axial direction of the tube was the center position (a position half the length from the tube end). Before the first pass, the wall thickness at the tip of the metal tube was measured with a micrometer at four points at 90° intervals in the circumferential direction, and the average value (sum of the wall thicknesses at the four points / 4) was defined as the wall thickness of the metal tube before the rolling process. Furthermore, full thickness tensile test pieces were taken from each cold rolled pipe in the pipe longitudinal direction, and a tensile test was performed at room temperature in accordance with JIS Z 2241 (2011) to evaluate the yield strength YS (0.2% proof stress). The yield strength YS of the blank pipe before cold rolling was 368 MPa. The rolling load was measured by a load cell installed adjacent to the roll unit of the tilt rolling mill.

[0037] The results are shown in Table 2.

[0038] [Table 2]

[0039] Pipes No. 1 to 5 and 8 to 11 were obtained by rolling in only one pass using a rolling mill with two rolls. For pipes Nos. 1 to 5 and 8 to 11, the rolling load during diameter reduction was controlled in consideration of the difference between the maximum and minimum outer diameters of the metal pipes after the rolling process that was set in advance, so that the decrease in roundness could be suppressed. In pipes No. 3, 4, 10, and 11, the yield strength could be improved by reducing the roll gap. Furthermore, as shown in Table 2, when the difference between the preset maximum and minimum outer diameters after rolling was set to 0.7 mm, the differences between the maximum and minimum outer diameters after rolling were 0.7 mm or less for tubes Nos. 1, 2, 5, 8, and 9 of the present invention, in which the maximum rolling load was equal to or less than the upper limit calculated by formula (1), and the decrease in roundness was further suppressed.

[0040] In addition, in the case of tubes No. 6 and No. 7, which are examples of the invention, in which multiple passes were performed, the maximum rolling load in all passes was below the upper limit calculated by formula (1), and the difference between the maximum and minimum outer diameters after rolling was 0.7 mm or less, which further suppressed the decrease in roundness.

[0041] Pipes No. 12 to 16 and 19 to 22 were rolled in only one pass using a rolling mill with three rolls. For pipes Nos. 12 to 16 and 19 to 22, the rolling load during diameter reduction was controlled in consideration of the difference between the maximum and minimum outer diameters of the metal pipes after the rolling process that was previously set, and therefore the decrease in roundness could be suppressed. In pipes No. 14, 15, 21, and 22, the yield strength could be improved by reducing the roll gap. Furthermore, as shown in Table 2, when the difference between the preset maximum and minimum outside diameters after rolling was set to 0.7 mm, the differences between the maximum and minimum outside diameters after rolling were 0.7 mm or less for tubes No. 12, 13, 16, 19, and 20, in which the maximum rolling load was equal to or less than the upper limit calculated by formula (1), and thus the decrease in roundness was further suppressed.

[0042] In addition, tubes No. 17 and No. 18 are shown as examples of tubes that were subjected to multiple passes. The maximum rolling load in all passes was below the upper limit calculated by formula (1), and the difference between the maximum and minimum outer diameters after rolling was 0.7 mm or less, which further suppressed the decrease in roundness.

[0043] Moreover, tubes No. 23 and 24 are examples where the difference between the maximum and minimum outer diameters of the metal tubes after the rolling process was not taken into consideration (was not set in advance). Tube No. 23 shows a case where rolling was performed in only one pass using a rolling mill with two rolls. Moreover, tube No. 24 shows a case where rolling was performed in only one pass using a rolling mill with three rolls. In these cases, the difference between the maximum and minimum outer diameters after rolling exceeded 3.5 mm, and the decrease in roundness could not be suppressed.

[0044] From the above, it was found that the example of the present invention can suppress the decrease in roundness. [Explanation of symbols]

[0045] 1. Raw tube 2 Metal pipes (rolled pipes) 3. Rolling β Tilt angle γ intersection angle Gap Roll Gap Dmax Maximum outer diameter Dmin Minimum outer diameter

Claims

1. a rolling step of performing diameter-reducing rolling on a blank tube in one or more passes while rotating the blank tube in a circumferential direction and moving the blank tube in an axial direction to obtain a metal tube, in the rolling step, a rolling load during the diameter-reducing rolling is controlled based on a difference between a predetermined maximum outer diameter value and a predetermined minimum outer diameter value of the metal tube after the rolling step so as to satisfy the following formula (1): the maximum rolling load L in each pass of the diameter reduction rolling satisfies formula (1). L≦1.2×t×Cp+4.5×t+18...Formula (1) Here, in formula (1), Cp: ​​the difference [mm] between a predetermined maximum outer diameter and a predetermined minimum outer diameter in a cross section perpendicular to the axial direction of the metal tube after the rolling process; t: wall thickness of the raw pipe before rolling process [mm], L: Maximum rolling load [ton].

2. A method for producing a metal tube, comprising the steps of: performing diameter-reducing rolling by the rolling method according to claim 1 to obtain a metal tube.

3. 1. A rolling facility for producing a metal tube by performing diameter-reducing rolling on a blank tube in one or more passes while rotating the blank tube in a circumferential direction and moving the blank tube in an axial direction, comprising: a control unit that controls a rolling load during the diameter-reducing rolling based on a difference between a maximum outer diameter and a minimum outer diameter of the metal tube after a rolling process that is set in advance, so as to satisfy the following formula (1): The control unit controls the rolling load maximum value L in each pass of the diameter reduction rolling so that the maximum value L satisfies the formula (1). L≦1.2×t×Cp+4.5×t+18...Formula (1) Here, in formula (1), Cp: ​​the difference [mm] between a predetermined maximum outer diameter and a predetermined minimum outer diameter in a cross section perpendicular to the axial direction of the metal tube after the rolling process; t: wall thickness of the raw pipe before rolling process [mm], L: Maximum rolling load [ton].

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