Method for manufacturing seamless metal pipe

Abstract

In the manufacturing method disclosed in the present invention, a three-roll inclined rolling mill is used to manufacture a first seamless metal tube having a first wall thickness and a second seamless metal tube having a second wall thickness different from the first wall thickness, and the manufacturing method includes a first inclined rolling process (#5), a setting change process (#10) and a second inclined rolling process (#15). In the first inclined rolling process, the first rolled piece is rolled using the inclined rolling mill. In the setting change process, the setting conditions of the inclined rolling mill are changed to any one of the following (a) and (b): (a) when the second wall thickness is smaller than the first wall thickness, the crossing angle of the inclined rolls is made larger than that in the first inclined rolling process; (b) when the second wall thickness is larger than the first wall thickness, the crossing angle of the inclined rolls is made smaller than that in the first inclined rolling process. In the second inclined rolling process, the second rolled piece is rolled using the inclined rolling mill with the changed setting conditions.

Description

Technical Field

[0001] The present invention relates to a method for producing a seamless metal pipe using the Mannesmann process. Background Art

[0002] Generally speaking, the method for manufacturing seamless metal tubes using the Mannesmann method includes the following steps. The round steel billet is heated to a predetermined temperature. The round steel billet is subjected to piercing and rolling to manufacture a tube billet (seamless metal tube). The tube billet is further subjected to elongation rolling and further subjected to sizing rolling. In the piercing and rolling process, a piercing and rolling mill (e.g., a piercing and rolling mill (English: piercer)) is used. In the elongation rolling process, an elongation rolling mill (e.g., a mandrel mill (English: mandrel mill), an elongation rolling mill (English: Elongator)) is used. The piercing and rolling mill is an inclined rolling mill. An inclined rolling mill is sometimes also used as an elongation rolling mill.

[0003] Inclined rolling mills are disclosed in, for example, Japanese Patent Application Laid-Open No. 5-228514 (Patent Document 1), Japanese Patent Application Laid-Open No. 2-263506 (Patent Document 2), Japanese Patent Application Laid-Open No. 64-31505 (Patent Document 3), and Japanese Patent Application Laid-Open No. 59-80716 (Patent Document 4).

[0004] The inclined rolling mill is equipped with a plug and two inclined rollers as a rolling tool. Sometimes, the inclined rolling mill also has three inclined rollers. The inclined rolling mill equipped with two inclined rollers is called a two-roller inclined rolling mill. The inclined rolling mill equipped with three inclined rollers is called a three-roller inclined rolling mill. The inclined rollers are arranged at equal intervals around the rolling line. The central axis of each inclined roller is inclined relative to the rolling line. In other words, an inclination angle is given to each inclined roller. Sometimes, a crossing angle is further given to each inclined roller. The plug is arranged on the rolling line between the inclined rollers.

[0005] When the inclined rolling mill is used as a piercing rolling mill, the inclined rolling (piercing rolling) is performed as follows. The rolled piece is a solid round billet. The heated rolled piece is arranged on the rolling line. The rolled piece is conveyed between the rotating inclined rollers by a pusher and is bitten into the inclined rollers. Then, the rolled piece moves forward on the rolling line while rotating around its own axis and is pierced and rolled by the inclined rollers and the plug. Thus, a tube billet (seamless metal tube) with a predetermined wall thickness and outer diameter is obtained.

[0006] When an inclined rolling mill is used as an elongation rolling mill, the conditions of the inclined rolling (elongation rolling) are the same as those of the piercing rolling except that the rolled material is a hollow billet.

[0007] Here, in the manufacturing method of seamless metal tubes using the Mannesmann method, the piercing and rolling mill (piercing and rolling mill) responsible for the first step (piercing and rolling process) in each step was put into practical use by the Mannesmann brothers in 1885. The piercing and rolling mill at that time was a basic two-roller piercing and rolling mill. Various improvements have been implemented to the piercing and rolling mill after its practical use, and it is still operating in factories around the world. So far, a large number of piercing machines other than piercing and rolling mills have been put into practical use, but except for the Ehrlich piercing method and the glass lubricant high-speed extrusion method, piercing machines other than piercing and rolling mills have been almost eliminated. This is because the piercing and rolling mills are excellent in productivity and product dimensional accuracy. Therefore, it is not an exaggeration to say that the only piercing machine that has achieved industrial success is the piercing and rolling mill (piercing and rolling mill).

[0008] However, the existing two-roll piercing mill has problems to be solved. There are mainly the following two problems.

[0009] The first problem is the inner surface defects caused by Mannesmann failure. Mannesmann failure refers to the phenomenon that the center of the rolled piece becomes brittle and damaged. In the case of a two-roll piercing mill, a guide tool (e.g., a guide plate, a disc roll) is provided between the inclined rolls around the rolling line. The guide tool is responsible for limiting the extension of the rolled piece. In piercing rolling (inclined rolling), the compressive stress in the relative directions of the inclined rolls and the tensile stress in the relative directions of the guide members act simultaneously on the center of the rotating rolled piece. Such stress is repeated every 1 / 4 rotation of the rolled piece. Mannesmann failure occurs due to repeated stress loads. When the Mannesmann failure becomes significant, defects are generated on the inner surface of the resulting tube. The defect is an inner surface defect.

[0010] Conventionally, for a long time, the Mannesmann fracture method has been used to press the plug against the damaged center of the rolled material to perform piercing. This is because piercing can be performed easily. However, this method generates inner surface defects.

[0011] In recent years, measures have been taken to suppress Mannesmann damage. For example, a tapered inclined roll is used. However, Mannesmann damage cannot be completely prevented. Therefore, the occurrence of inner surface defects cannot be eradicated. In particular, when the rolled workpiece is a difficult-to-process material such as a cast steel billet or stainless steel, Mannesmann damage is promoted and the frequency of inner surface defects increases.

[0012] In addition, Mannesmann failure becomes significant in accordance with the number of repetitions of the aforementioned stress load, that is, the number of rotations of the rolled piece. Therefore, increasing the entrance side angle of the inclined roll and shortening the distance from the rolled piece contacting the inclined roll to the top of the plug is one of the means to suppress Mannesmann failure. However, inclined rolls with an entrance side angle of about 3° are generally used in two-roll piercing mills. This is because, in the piercing and rolling performed by the two-roll piercing mill, the rolled piece is easily offset in a direction perpendicular to the direction in which the inclined rolls are relative to each other, which easily causes poor bite, so it is difficult to use an excessively large entrance side angle.

[0013] The second problem is the outer surface flaws caused by damage to the disc rolls. The disc rolls are guide tools installed in the two-roll piercing mill. At that time, a fixed guide plate was used as the guide tool. During the piercing and rolling (inclined rolling) process, the guide plate slides relative to the rolled workpiece. When this sliding becomes significant, flaws are generated on the outer surface of the resulting tube. This flaw is an outer surface flaw.

[0014] In recent years, rotating disk rollers have been used instead of guide plates. The use of disk rollers has reduced the frequency of outer surface flaws.

[0015] However, the rotation direction of the disc roller is not necessarily consistent with the rotation direction of the rolled workpiece. Therefore, it is not possible to prevent the surface of the disc roller from sticking to the outer surface of the rolled workpiece. In addition, it is not possible to prevent the surface of the disc roller from deforming. Thus, it is necessary to regularly maintain the surface of the disc roller or replace the disc roller regularly.

[0016] Prior art literature

[0017] Patent Literature

[0018] Patent Document 1: Japanese Patent Application Laid-Open No. 5-228514

[0019] Patent Document 2: Japanese Patent Application Laid-Open No. 2-263506

[0020] Patent Document 3: Japanese Patent Application Laid-Open No. 64-31505

[0021] Patent Document 4: Japanese Patent Application Laid-Open No. 59-80716 Summary of the invention

[0022] Problem that the invention aims to solve

[0023] The three-roller inclined rolling mill can solve the above-mentioned problems. In the three-roller inclined rolling mill, unlike the two-roller inclined rolling mill, during the inclined rolling process, since only compressive stress acts on the center of the rolled piece, Mannesmann damage will not occur. Therefore, inner surface defects will not occur. In addition, no guide tool is used in the three-roller inclined rolling mill. Therefore, outer surface defects will not occur. Therefore, in order to solve the inherent quality problems of inclined rolling using a two-roller inclined rolling mill, inclined rolling using a three-roller inclined rolling mill is extremely useful.

[0024] However, the three-roller inclined rolling mill is not put into practical use in the manufacture of all seamless metal pipes. For example, in the piercing rolling using the three-roller inclined rolling mill, it is difficult to manufacture seamless metal pipes with a smaller wall thickness. This is because no guide tool is used. Therefore, although the setting of the guide tool has been studied, it has not yet been realized. This is because it is structurally difficult to set the guide tool in the three-roller inclined rolling mill. At present, the three-roller inclined rolling mill is only used as an extension rolling mill such as an Assel rolling mill dedicated to manufacturing seamless metal pipes with a larger wall thickness from hollow tube billets.

[0025] Therefore, it is expected that the three-roll inclined rolling mill can be put into practical use in the production of all seamless metal pipes. Therefore, in the inclined rolling using the three-roll inclined rolling mill, it is important to be able to produce both seamless metal pipes with smaller wall thickness and seamless metal pipes with larger wall thickness without quality problems.

[0026] An object of the present invention is to provide a method for producing a seamless metal pipe that can put a three-roll inclined rolling mill into practical use.

[0027] Solutions for solving problems

[0028] In a method for manufacturing a seamless metal pipe according to an embodiment of the present invention, a first seamless metal pipe having a first wall thickness and a second seamless metal pipe having a second wall thickness different from the first wall thickness are manufactured using an inclined rolling mill. The inclined rolling mill includes: a plug disposed on a rolling line; and three inclined rolls disposed at equal intervals around the rolling line, each having an inlet side surface and an outlet side surface. The distance between the rolling line and the inlet side surface gradually decreases from the inlet side of the rolling line toward the outlet side, and the distance between the rolling line and the outlet side surface gradually increases from the inlet side of the rolling line toward the outlet side.

[0029] The manufacturing method includes a first inclined rolling process, a setting change process, and a second inclined rolling process. In the first inclined rolling process, the heated first rolled workpiece is rolled using an inclined rolling mill. In the setting change process, the setting conditions of the inclined rolling mill are changed to any one of the following (a) and (b): (a) when the second wall thickness is smaller than the first wall thickness, the crossing angle of the inclined rollers is larger than that in the first inclined rolling process; (b) when the second wall thickness is larger than the first wall thickness, the crossing angle of the inclined rollers is smaller than that in the first inclined rolling process. In the second inclined rolling process, the heated second rolled workpiece is rolled using the inclined rolling mill with the changed setting conditions.

[0030] Effects of the Invention

[0031] According to the manufacturing method of the embodiment of the present invention, both a thin-walled seamless metal pipe and a thick-walled seamless metal pipe having no quality problems can be manufactured by the tilt rolling using a three-roller tilt rolling mill. Therefore, the three-roller tilt rolling mill can be put into practical use. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a three-roller inclined rolling mill perspective view.

[0033] Figure 2 This is the front view of the three-roller inclined rolling mill.

[0034] Figure 3 It is a top view of a three-roll inclined rolling mill.

[0035] Figure 4 It is a side view of a three-roller inclined rolling mill.

[0036] Figure 5 This is a diagram showing an example of an inclined roller having a convexly curved inlet side surface.

[0037] Figure 6 This is a flowchart showing the method for producing the seamless metal pipe according to the present embodiment.

[0038] Figure 7 This is a schematic diagram showing the situation of option (a) in the setting change process.

[0039] Figure 8 This is a schematic diagram showing the situation of option (b) in the setting change process.

[0040] Fig. 9 This is a diagram showing the appearance of a plug used in a piercing and rolling test.

[0041] Fig.10 This is a diagram showing the appearance of an inclined roll used in a piercing and rolling test.

[0042] Fig.11 This is a diagram showing the appearance of an inclined roll used in a piercing and rolling test.

[0043] Fig.12 This is a diagram summarizing whether or not any malfunction has occurred in Example 1.

[0044] Fig.13 This is a diagram summarizing whether or not any malfunctions occurred in Example 3.

[0045] Fig.14 This is a diagram summarizing whether or not any malfunctions occurred in Example 4.

[0046] Fig.15 This is a diagram showing the appearance of an inclined roll used in a piercing and rolling test.

[0047] Fig.16 This is a diagram summarizing whether or not any malfunctions occurred in Example 5.

[0048] Fig.17 This is a diagram summarizing whether or not any malfunctions occurred in Example 5.

[0049] Fig.18 This is a diagram showing the appearance of an inclined roll used in a piercing and rolling test.

[0050] Fig.19 This is a diagram showing the appearance of an inclined roll used in a piercing and rolling test.

[0051] Fig. 20 This is a diagram summarizing whether or not any malfunctions occurred in Example 6.

[0052] Fig.21 This is a diagram summarizing whether or not any malfunctions occurred in Example 6.

[0053] Fig. 22 It is a diagram for explaining the bite angle (α).

[0054] Fig.23 This graph summarizes the presence or absence of malfunctions based on the relationship between the bite angle (α) and the gouge draft ratio. DETAILED DESCRIPTION

[0055] The present inventors have conducted intensive studies to solve the above-mentioned problems and have obtained the following findings.

[0056] [Basic structure of three-roller inclined rolling mill]

[0057] Figure 1 to Figure 4 are diagrams showing the structure of a three-roller inclined rolling mill. In these diagrams, Figure 1 This is a perspective view of the inclined rolling mill as viewed from the exit side of the rolling line PL. Figure 23 shows a front view of the inclined rolling mill viewed along the pass line PL from the entrance side of the pass line PL. Figure 3 A top view of the inclined rolling mill is shown. Figure 4 FIG. 4 shows a side view of the inclined rolling mill. Figure 1 The illustration of the plug 2 is omitted. Figure 3 and Figure 4 In FIG. 1 , only one inclined roll 1 arranged above the pass line PL in the vertical direction is shown, and the two inclined rolls 1 arranged below are omitted. Figure 4 In FIG, the rolled workpiece WP is represented by a cross section including the rolling line PL. Figure 1 to Figure 4 The example in FIG. 1 shows the situation when the rolled workpiece WP is a solid round billet. Figure 1 to Figure 4 The inclined rolling mill exemplified in FIG. 1 is a piercing rolling mill used for piercing rolling. In this specification, a three-roll inclined rolling mill may be simply referred to as an inclined rolling mill.

[0058] Reference Figure 1 to Figure 4 The inclined rolling mill has a plug 2 and three inclined rollers 1 as a rolling tool. The three inclined rollers 1 are arranged at equal intervals around the rolling line PL. That is, the three inclined rollers 1 are arranged at intervals of 120° from each other. One of the three inclined rollers 1 is arranged directly above the rolling line PL (above in the vertical direction). However, as long as the three inclined rollers 1 are arranged at equal intervals around the rolling line PL, the positions of the three inclined rollers 1 are not limited. For example, one inclined roller 1 may be arranged directly below the rolling line PL (below in the vertical direction). The surface of each inclined roller 1 is divided into an entrance side 1a and an exit side 1b along the rolling line PL.

[0059] The central axis 1c of each inclined roll 1 is inclined with respect to the pass line PL. That is, each inclined roll 1 is given an inclination angle FA (see Figure 3 ). Each inclined roller is given a crossing angle CA (refer to Figure 4 ). The inclination angle FA and the cross angle CA are adjustable. In addition, each inclined roll 1 is given an opening relative to the rolling line PL. The roll opening is also adjustable.

[0060] The tilt angle FA is the swing angle of the central axis 1c of the inclined roll 1 in the circumferential direction around the pass line PL. The cross angle CA is the swing angle of the central axis 1c of the inclined roll 1 in the radial direction around the pass line PL.

[0061] The distance between the rolling line PL and the inlet side surface 1a gradually decreases from the inlet side of the rolling line PL toward the outlet side. On the other hand, the distance between the rolling line PL and the outlet side surface 1b gradually increases from the inlet side of the rolling line PL toward the outlet side. The inlet side surface 1a is, for example, a conical surface with a constant gradient. The outlet side surface 1b is, for example, a conical surface with a constant gradient.

[0062] The plug 2 is arranged on the pass line PL between the inclined rolls 1. The plug 2 is held by a mandrel 3 extending along the pass line PL.

[0063] The inclined rolling (piercing rolling) using such an inclined rolling mill is performed as follows. The round billet, i.e., the rolled piece WP, is heated. The heated rolled piece WP is arranged on the rolling line PL. The rolled piece WP is conveyed between the rotating inclined rolls 1 by a pusher and is bitten into the inclined roll 1. Then, the rolled piece WP moves forward on the rolling line PL while rotating around its own axis and is pierced and rolled by the inclined roll 1 and the plug 2. Thus, a pipe billet (seamless metal pipe) having a predetermined wall thickness and outer diameter is obtained.

[0064] When the inclined rolling mill is used as an elongation rolling mill, the conditions of the inclined rolling (elongation rolling) are the same as those of the piercing rolling except that the rolled material is a hollow tube. That is, the rolled material moves forward on the rolling line PL while rotating around its own axis, and is elongated and rolled by the inclined rolls 1 and the plug 2.

[0065] [Study on the practical application of three-roller inclined rolling mill]

[0066] The inventors first focused on piercing rolling among the inclined rolling (piercing rolling and elongation rolling) using a three-roll inclined rolling mill. This is because the processing conditions of piercing rolling are much stricter than those of elongation rolling. In addition, the inventors studied the production of seamless metal pipes with a small wall thickness, which is considered to be a weak point in piercing rolling using a three-roll inclined rolling mill (three-roll piercing mill).

[0067] In this specification, the production of a seamless metal tube with a small wall thickness may be referred to as thin-walled tube production, and the production of a seamless metal tube with a large wall thickness may be referred to as thick-walled tube production.

[0068] In the piercing and rolling process using a three-roll inclined rolling mill, if a thin-walled pipe is produced, part of the rolled material is squeezed out between the inclined rolls, and the rotation of the rolled material stops. This makes thin-walled pipe production difficult.

[0069] As a first countermeasure, the inventors tried to reduce the wall thickness processing amount performed by the plug. This is because if the wall thickness processing amount performed by the plug is small, the extrusion of the material between the inclined rollers is also reduced. As a means of this, a method of reducing the roll opening is adopted. By reducing the roll opening, the outer diameter of the rolled piece is reduced until the rolled piece reaches the top of the plug, and then the outer diameter of the rolled piece is increased. In this way, the wall thickness processing amount performed by the plug can be reduced. For example, in the case of obtaining a pipe blank with the same outer diameter and wall thickness from two rolled pieces with different cross-sectional areas (outer diameters), the overall processing degree of the rolled piece with the smaller cross-sectional area (outer diameter) can be smaller.

[0070] However, it is known that when the roll opening is too small relative to the outer diameter of the rolled product, the outer diameter of the tube obtained by piercing and rolling becomes uneven in the length direction. This is because the overall processing degree of the rolled product increases, and the balance between the material flow rate on the inlet side of the inclined roll and the material flow rate on the outlet side of the inclined roll is destroyed. In order to make the material flow rate on the inlet side of the inclined roll equal to the material flow rate on the outlet side of the inclined roll, it is necessary to reduce the running speed of the rolled product on the inlet side. In addition, the material flow rate is represented by the product of the cross-sectional area of ​​the rolled product at the target position and the running speed.

[0071] Therefore, the inventors conducted various experiments and numerical analysis to study a method of reducing the running speed of the rolled material on the entrance side of the inclined rolls. As a result, it was confirmed that if the angle of the entrance side of the inclined rolls is increased, the running speed of the rolled material is reduced on the entrance side of the inclined rolls, thereby suppressing the circumferential length variation of the rolled material in the circumferential direction.

[0072] In this specification, the angle of the entrance side surface of the inclined roll is sometimes referred to as the entrance side surface angle. The entrance side surface angle refers to the maximum angle formed by the entrance side surface and the rolling line in the range of contact with the rolled material in the cross section including the rolling line.

[0073] Next, as a second countermeasure, the inventor tried to control the direction in which the rolled piece extends during the piercing and rolling process. This is because this method increases the angle of the outlet side of the inclined roller. If the angle of the outlet side of the inclined roller is increased, the distance in the length direction within the range of contact with the rolled piece on the outlet side of the inclined roller is shortened, weakening the material constraint in the length direction. In this way, the rolled piece is easy to extend in the length direction, but the extension of the rolled piece in the circumferential direction is correspondingly reduced. This reduces the extrusion amount.

[0074] In this specification, the angle of the exit side surface of the inclined roll is sometimes referred to as the exit side surface angle. The exit side surface angle refers to the maximum angle formed by the exit side surface and the rolling line in the range of contact with the rolled material in the cross section including the rolling line.

[0075] By adopting the above-mentioned countermeasures and means for the countermeasures, it is possible to perform thin-wall pipe production by a three-roll inclined rolling mill, which has been difficult in the past.

[0076] However, the above countermeasures and means are only for thin-walled pipe making. When the above countermeasures and means are used for thick-walled pipe making, the maximum diameter portion of the plug remains at the rear end of the obtained tube blank, and the plug cannot be completely pulled out of the tube blank. This problem is one of the rolling defects and is called plug clogging. Plug clogging is a phenomenon caused by insufficient circumferential length of the rolled piece in the circumferential direction. In order to eliminate plug clogging, the circumferential length of the rolled piece in the circumferential direction must be increased, which is opposite to the case of thin-walled pipe making.

[0077] Therefore, the means used in the case of thick-walled pipe making are opposite to those used in the case of thin-walled pipe making. That is, in the case of thick-walled pipe making, the roll opening is increased in order to make the material cross-sectional area of ​​the rolled workpiece on the inlet side larger than the plug. In this case, in order to obtain the propulsive force on the rolled workpiece, it is necessary to lengthen the distance at which the inlet side of the inclined roller contacts the rolled workpiece. As a means for this, the inlet side angle of the inclined roller must be reduced. In addition, in order to release the material of the rolled workpiece in the circumferential direction, the outlet side angle of the inclined roller must be reduced, thereby strengthening the material constraint in the length direction.

[0078] When a thick-walled pipe and a thin-walled pipe are produced by a single inclined rolling mill, inclined rolls of different shapes and sizes may be used. This can increase the wall thickness limits of the thick-walled pipe and the thin-walled pipe.

[0079] However, if the inclined rolls are replaced according to the wall thickness of the seamless metal pipe to be obtained, it cannot be denied that the working efficiency will be reduced. Therefore, it is desirable to avoid the replacement of the inclined rolls if possible.

[0080] The inventors have further conducted intensive research on a means of eliminating the need to replace the inclined rolls. As a result, the means of achieving this is to set the inlet side surface of the inclined roll to a convex surface. Generally speaking, the inlet side surface of the inclined roll is a tapered surface with a constant gradient. The inventors have come up with the idea of ​​using an inclined roll whose inlet side surface is a convex surface as a means of manufacturing seamless metal pipes with different wall thicknesses using the same inclined roll, and then adjusting the crossing angle CA of the inclined roll.

[0081] Here, the structure of a three-roller inclined rolling mill is described. In the three-roller inclined rolling mill, the brackets (housing) supporting the two ends of the central axis of the inclined roller are divided into an entrance side and an exit side. The inclination angle FA of the inclined roller is adjusted by rotating one or both of the two brackets. In addition, in each bracket on the entrance side and the exit side, the support position of the inclined roller is configured to be adjustable. By adjusting the respective support positions, the crossing angle CA of the inclined roller is adjusted. In coordination with this, the roller openings of the entrance side and the exit side are adjusted. In addition, sometimes both ends of the central axis of the inclined roller are also supported by one bracket.

[0082] Figure 5 1 is a diagram showing an example of an inclined roller 1 having a convex inlet side surface 1a. Figure 5, the boundary between the inlet side surface 1a and the outlet side surface 1b is a gouge G. The inlet side surface 1a of the inclined roller 1 is not a simple conical surface, that is, it is not a conical surface with a constant gradient, but a convex surface. In this specification, a convex surface refers to a conical surface with a continuously changing gradient, a conical surface with a step-by-step gradient change, or a conical surface obtained by combining them. The line that appears when the inlet side surface 1a is cut along the center axis 1c of the inclined roller 1 is, for example, a convex curve. The convex curve is represented by, for example, a function that depicts an arc with a constant radius of curvature. The convex curve can also be represented by a polynomial function. In addition, the line that appears when the inlet side surface 1a is cut along the center axis 1c of the inclined roller 1 can be a combination of a convex curve and a straight line, or a combination of multiple straight lines with different slopes. On the other hand, the outlet side surface 1b of the inclined roller 1 is a conical surface with a constant gradient.

[0083] A method of producing a thin-walled pipe and a thick-walled pipe by piercing and rolling using an inclined roll having a convexly curved inlet side surface will be described below.

[0084] In the case of thin-walled pipe making, the crossing angle CA of the inclined rolls is increased. Then the roll opening is reduced. In other words, the inclined rolls are brought closer to the rolling line. As a result, the starting contact position between the rolled piece and the inclined rolls moves toward the entrance side of the rolling line. Therefore, the rolled piece contacts the entrance side surface at a position where the gradient of the entrance side surface of the inclined roll is large. This also brings about the same effect when the diameter of the rolled piece is increased and a high reduction is performed.

[0085] The exit side angle of the inclined roller is increased by increasing the crossing angle CA of the inclined roller. At this time, the entrance side angle of the inclined roller hardly changes. This is because the entrance side of the inclined roller is a convex surface. In this case, assuming that the entrance side of the inclined roller is a simple conical surface and the gradient of the entire area of ​​the entrance side is constant, it is necessary to lengthen the main body length of the entrance side in advance. This is because as the crossing angle CA of the inclined roller increases, the entrance side angle decreases, so the roller opening at the entrance side end of the entrance side must be expanded to a diameter greater than the diameter of the rolled piece. In contrast, if the entrance side is a convex surface, there is no such necessity. In addition, even if the diameter of the rolled piece is large, as long as the entrance side is a convex surface, the circumference of the rolled piece in the length direction will not change but will be stable because there is no excessive pressure on the material. In addition, the plug can also be replaced in coordination with the change in the crossing angle CA.

[0086] The roller opening can be adjusted after or before the inclined roller crossing angle CA is adjusted. For fine adjustment, the roller opening and the inclined roller crossing angle CA can be adjusted repeatedly.

[0087] In the case of thick-walled pipe making, the opposite operation is performed to that of thin-walled pipe making. Specifically, the cross angle CA of the inclined rolls is first reduced. Then the roll opening is increased. In other words, the inclined rolls are moved away from the rolling line. As a result, the starting contact position between the rolled piece and the inclined rolls moves toward the exit side of the rolling line. Therefore, the rolled piece contacts the entrance side of the inclined rolls at a position where the gradient of the entrance side is smaller.

[0088] The exit side angle of the inclined roller is reduced by reducing the crossing angle CA of the inclined roller before adjusting the roller opening. In this case, assuming that the entrance side of the inclined roller is a simple conical surface and the gradient of the entire area of ​​the entrance side is constant, the entrance side angle increases as the crossing angle CA of the inclined roller decreases. Therefore, the contact distance between the entrance side and the rolled workpiece is shortened, and the bite of the rolled workpiece becomes unstable. In contrast, if the entrance side is a convex surface, the entrance side angle hardly changes, so there will be no poor bite of the rolled workpiece. In addition, the plug can also be replaced in coordination with the change of the crossing angle CA.

[0089] In this way, if the inlet side surface of the inclined roll is a convex curved surface, the same inclined roll can be used to produce seamless metal pipes with different wall thicknesses simply by changing the setting conditions of the inclined roll. In this case, even when the diameter of the rolled material is large, the circumference of the material is suppressed, and rolling defects such as material extrusion and plug clogging will not occur.

[0090] If the above countermeasures and means are properly applied, seamless metal tubes can be manufactured regardless of the size of the wall thickness by using the inclined rolling (piercing rolling and stretching rolling) of a three-roll inclined rolling mill. The degree of wall thickness of a seamless metal tube is expressed by the ratio of the wall thickness to the outer diameter. The value of this ratio is also called the wall thickness to outer diameter ratio. This means that the smaller the wall thickness to outer diameter ratio, the smaller the degree of wall thickness of the seamless metal tube (i.e., thin-walled). In the piercing rolling using a conventional three-roll inclined rolling mill, it is difficult to manufacture a thin-walled seamless metal tube with a wall thickness to outer diameter ratio of less than 0.07. Through the application of the above-mentioned countermeasures and means, thin-walled seamless metal tubes with a wall thickness to outer diameter ratio of less than 0.07 can be manufactured, and of course thick-walled seamless metal tubes can also be manufactured.

[0091] In addition, by applying the above-mentioned countermeasures and means, a round steel billet having a larger outer diameter can be used as a rolled product, and high-processing degree piercing and rolling can be performed. In this case, since the outer diameter of the round steel billet can be concentrated on the larger side, there is a beneficial advantage that the manufacturing cost required for steelmaking and initial rolling can be greatly reduced.

[0092] The method for producing a seamless metal pipe of the present invention has been accomplished based on the above findings.

[0093] In a method for manufacturing a seamless metal tube according to an embodiment of the present invention, a first seamless metal tube having a first wall thickness and a second seamless metal tube having a second wall thickness different from the first wall thickness are manufactured using an inclined rolling mill. The inclined rolling mill includes a plug and three inclined rollers. The plug is arranged on a rolling line. The three inclined rollers are arranged at equal intervals around the rolling line, and each has an inlet side surface and an outlet side surface. The distance between the rolling line and the above-mentioned inlet side surface gradually decreases from the inlet side of the rolling line toward the outlet side. The distance between the rolling line and the above-mentioned outlet side surface gradually increases from the inlet side of the rolling line toward the outlet side.

[0094] The manufacturing method includes a first inclined rolling process, a setting change process, and a second inclined rolling process. The first inclined rolling process uses an inclined rolling mill to roll the first heated rolled workpiece. The setting change process changes the setting conditions of the inclined rolling mill to any one of the following (a) and (b): (a) when the second wall thickness is smaller than the first wall thickness, the crossing angle of the inclined rollers is larger than that in the first inclined rolling process; (b) when the second wall thickness is larger than the first wall thickness, the crossing angle of the inclined rollers is smaller than that in the first inclined rolling process. The second inclined rolling process uses the inclined rolling mill with changed setting conditions to roll the second heated rolled workpiece (first structure).

[0095] According to the manufacturing method of the first structure, both a thin-walled seamless metal pipe and a thick-walled seamless metal pipe having no quality problems can be manufactured by the tilt rolling using the three-roller tilt rolling mill. Therefore, the three-roller tilt rolling mill can be put into practical use.

[0096] The first wall thickness and the second wall thickness are target wall thicknesses after the inclined rolling, and may be slightly different from the actual wall thicknesses after the inclined rolling.

[0097] In a typical example, the inclined rolling mill is a piercing rolling mill. In this case, the first inclined rolling step and the second inclined rolling step are piercing rolling steps. The first rolled material and the second rolled material are solid round billets.

[0098] In another typical example, the inclined rolling mill is an elongation rolling mill. In this case, the first inclined rolling step and the second inclined rolling step are elongation rolling steps. The first rolled material and the second rolled material are hollow shell pipes.

[0099] In the manufacturing method of the first structure, it is preferred that, in the first inclined rolling step and the second inclined rolling step, the angle of the outlet side surface within the contact range with the first rolled workpiece and the second rolled workpiece in the outlet side surface (exit side surface angle) is set to be greater than 0° and less than 9°. In particular, in the first inclined rolling step, when the ratio of the first wall thickness to the outer diameter of the first seamless metal pipe is less than 0.07, the angle of the outlet side surface is greater than 3°. Similarly, in the second inclined rolling step, when the ratio of the second wall thickness to the outer diameter of the second seamless metal pipe is less than 0.07, the angle of the outlet side surface is greater than 3° (second structure).

[0100] The manufacturing method of the second structure is useful when the first seamless metal tube is a thin-walled seamless metal tube or the second seamless metal tube is a thin-walled seamless metal tube. If the outlet side angle is set to the above range, it is possible to manufacture a thin-walled first seamless metal tube and a thin-walled second seamless metal tube with no quality problems. From the viewpoint of embodying this effect, the outlet side angle is preferably greater than 3°. The upper limit of the outlet side angle is not particularly limited. However, in order to take into account the design of the plug, the upper limit of the outlet side angle is preferably 9°, more preferably 6°.

[0101] In the manufacturing method of the first or second structure, it is preferable that the inlet side surface is a convex curved surface, and the inclined rolls used in the first inclined rolling step are used in the second inclined rolling step (third structure).

[0102] According to the manufacturing method of the third structure, it is not necessary to replace the inclined rolls between the first inclined rolling process and the second inclined rolling process. That is, it is possible to manufacture the first seamless metal pipe and the second seamless metal pipe having different wall thicknesses using one inclined rolling mill without replacing the inclined rolls. Therefore, the operation efficiency is excellent.

[0103] In the manufacturing method of the first structure or the second structure, for example, the first rolled workpiece and the second rolled workpiece are solid. That is, the first inclined rolling process and the second inclined rolling process are piercing rolling processes. In this case, it is preferred that, in the first inclined rolling process and the second inclined rolling process, the piercing ratio is set to be greater than 3.5, the angle of the entrance side surface (entrance side surface angle) is set to be greater than 8° and less than 15°, and the round gouge drafting ratio is set to be greater than 30% (the fourth structure).

[0104] The manufacturing method of the fourth structure is useful when the inclined rolling mill is a piercing rolling mill and the first inclined rolling process and the second inclined rolling process are piercing rolling processes. In the case of high-processing piercing rolling with a piercing ratio of 3.5 or more, if the entrance side angle and the draft ratio are set to the above-mentioned range, the first seamless metal tube and the second seamless metal tube with no quality problems can be manufactured. From the viewpoint of embodying this effect, the entrance side angle is preferably set to be greater than 8° and less than 15°. From the same viewpoint, it is preferred to set the round gouge draft ratio to be greater than 30%. The upper limit of the round gouge draft ratio is not particularly limited. However, when the round gouge draft ratio is increased and the cross-sectional area of ​​the material is reduced, the reaction force to the inclined roller increases. In order to ensure the bearing strength supporting the inclined roller shaft, the roller shaft is increased, so the roller diameter also needs to be increased. However, when the number of inclined rollers is three or more, the inclined rollers interfere with each other, so the upper limit of the round gouge draft ratio is preferably set to 60%.

[0105] In this specification, the perforation ratio refers to the ratio of the length of the seamless metal tube after perforation rolling to the length of the rolled piece before perforation rolling. That is, the perforation ratio of the first inclined rolling process refers to the ratio of the length of the first seamless metal tube to the length of the first rolled piece. The perforation ratio of the second inclined rolling process refers to the ratio of the length of the second seamless metal tube to the length of the second rolled piece. According to another viewpoint, the perforation ratio refers to the ratio of the cross-sectional area of ​​the rolled piece before perforation rolling to the cross-sectional area of ​​the seamless metal tube after perforation rolling. In short, the perforation ratio is an indicator of the degree of processing of perforation rolling. In addition, in the case of extension rolling, the perforation ratio is called the extension ratio.

[0106] In this specification, the gouge draft ratio (GD) [%] is expressed by the following formula (A) using the outer diameter (DB) of a round billet as a rolled material and the roll opening (RO) as parameters.

[0107] GD=(DB-RO) / DB×100(A)

[0108] The roll opening (RO) in formula (A) refers to the roll opening at the boundary between the roll gouge, i.e., the roll inlet side and the roll outlet side. More precisely, the roll opening (RO) is twice the shortest distance between the surface of the inclined roll (e.g., the inlet side) and the rolling line.

[0109] In the manufacturing method of the first to third structures, for example, the first rolled workpiece and the second rolled workpiece are solid. That is, the first inclined rolling process and the second inclined rolling process are piercing rolling processes. In this case, it is preferred that the inlet side surface is a convex curved surface, and the line of the inlet side surface appearing on the cross section including the central axis of the inclined roller is an arc. The value obtained by dividing the radius of curvature of the arc by the outer diameter of the first rolled workpiece and the second rolled workpiece is greater than 1.67 and less than 6.67 (the fifth structure).

[0110] The manufacturing method of the fifth structure is useful when the inclined rolling mill is a piercing rolling mill and the first inclined rolling step and the second inclined rolling step are piercing rolling steps. In this specification, when the inlet side surface is a convex curved surface and the line of the inlet side surface appearing in the cross section including the central axis of the inclined roll is an arc, the value obtained by dividing the radius of curvature of the arc by the outer diameter of the round billet as the rolled product is sometimes referred to as the curvature index of the inlet side surface.

[0111] In the first inclined rolling process and the second inclined rolling process, if the curvature index of the inlet side surface associated with the first rolled workpiece and the second rolled workpiece is set to the above-mentioned range, the first seamless metal tube and the second seamless metal tube with no quality problems can be manufactured. From the viewpoint of embodying this effect, the curvature index of the inlet side surface is preferably greater than 1.67 and less than 6.67. However, when the curvature radius of the curvature index of the inlet side surface is small, the wear of the surface of the inclined roller becomes significant when the material is rolled with a shorter contact length. In addition, when the curvature radius is large, in order to allow the rolled workpiece to bite, the main body length of the inlet side surface of the inclined roller has to be lengthened, and the equipment cost and the manufacturing cost of the inclined roller increase. Therefore, when a round billet with an outer diameter of 60 mm is used, the curvature radius is preferably greater than 150 mm and less than 350 mm. In this case, by conversion, the curvature index of the inlet side surface is preferably greater than 2.50 and less than 5.83.

[0112] In the manufacturing method of the first to third structures, for example, the first rolled workpiece and the second rolled workpiece are solid. That is, the first inclined rolling process and the second inclined rolling process are piercing rolling processes. In this case, it is preferred that, in the first inclined rolling process and the second inclined rolling process, the gouge drafting ratio (GD) and the bite angle (α) satisfy the condition represented by the following formula (1) (the sixth structure).

[0113] 0.12×GD+1.5≤α≤0.25×GD+6 (1)

[0114] The gouge drafting ratio (GD) in the above formula (1) is expressed by the above formula (A). On the other hand, the bite angle (α) is determined as described below. The inclination angle FA of the inclined roller is assumed to be 0°. Determine an imaginary plane containing both the center axis of the inclined roller and the rolling line at this time. In this imaginary plane, draw a straight line connecting the starting contact position of the rolled piece (round billet) with the inclined roller and the gouge position. The starting contact position of the rolled piece with the inclined roller and the bite position of the rolled piece on the entrance side of the inclined roller correspond to each other. The angle between the straight line and the rolling line is the bite angle (α).

[0115] The manufacturing method of the sixth structure is useful when the inclined rolling mill is a piercing rolling mill and the first inclined rolling process and the second inclined rolling process are piercing rolling processes. If the gouge drafting ratio (GD) and the bite angle (α) satisfy the conditions of formula (1), the first seamless metal tube and the second seamless metal tube with no quality problems can be manufactured. In order to achieve this effect, as shown in formula (1), the bite angle (α) is preferably greater than "0.12×GD+1.5" and less than "0.25×GD+6". According to another viewpoint, in order to satisfy the conditions shown in formula (1), it is sufficient to set the inlet side surface in such a way that the gradient becomes larger as it approaches the inlet side end.

[0116] A specific example of the production method of the present embodiment will be described below with reference to the drawings. Figure 6 2 is a flow chart showing a method for producing a seamless metal pipe according to the present embodiment. Figure 6 The manufacturing method of this embodiment includes a first inclined rolling step (#5), a setting change step (#10), and a second inclined rolling step (#15).

[0117] [First inclined rolling process (#5)]

[0118] In the first inclined rolling process (#5), the first rolled product is rolled using a three-roll inclined rolling mill to produce a first seamless metal tube having a first wall thickness. At this time, the first rolled product is preheated to a predetermined temperature by a heating furnace. The heating temperature of the first rolled product is, for example, in the range of 1150°C to 1250°C.

[0119] The first rolled product is a round billet. In this case, the inclined rolling mill is a piercing rolling mill, and the round billet is pierced and rolled by the piercing rolling mill. However, the first rolled product may also be a tube billet. The tube billet may be a tube billet manufactured by piercing and rolling, or a tube billet manufactured by other methods. In this case, the inclined rolling mill is an elongation rolling mill, and the tube billet is elongated and rolled by the elongation rolling mill.

[0120] [Second inclined rolling process (#15)]

[0121] In the second inclined rolling process (#15), the second rolled product is rolled using a three-roll inclined rolling mill to produce a second seamless metal tube having a second wall thickness different from the first wall thickness. At this time, the second rolled product is preheated to a predetermined temperature as in the first inclined rolling process.

[0122] The same inclined rolling mill as that used in the first inclined rolling process is used in the second inclined rolling process. In the inclined rolling mill used in the first inclined rolling process and the second inclined rolling process, the inlet side surface of the inclined roll is as described above. Figure 5However, the inlet side of the inclined roller can also be as described above. Figure 1 to Figure 4 The exit side of the inclined roller is as shown in FIG. Figure 1 to Figure 5 The tapered surface with a constant gradient is shown. The distance between the rolling line and the inlet side surface gradually decreases from the inlet side of the rolling line toward the outlet side. The distance between the rolling line and the outlet side surface gradually increases from the inlet side of the rolling line toward the outlet side.

[0123] If the first rolled product is a round billet, the second rolled product is also a round billet. If the first rolled product is a tube billet, the second rolled product is also a tube billet. When the second rolled product is a round billet, the round billet is pierced and rolled by a piercing rolling mill. When the second rolled product is a tube billet, the tube billet is elongated and rolled by an elongation rolling mill.

[0124] The shape and size of the second rolled piece are the same as those of the first rolled piece. However, the shape and size of the second rolled piece may be different from those of the first rolled piece. The material of the second rolled piece is the same as that of the first rolled piece. However, the material of the second rolled piece may be different from that of the first rolled piece.

[0125] [Setting change process (#10)]

[0126] The setting change step (#10) changes the setting conditions of the inclined rolling mill to either (a) or (b) after the first inclined rolling step (#5) and before the second inclined rolling step (#15). That is, the setting change step sets the setting conditions of the inclined rolling mill suitable for the second inclined rolling step based on the setting conditions of the inclined rolling mill in the first inclined rolling step.

[0127] (a) When the second wall thickness is smaller than the first wall thickness, the crossing angle of the inclined rolls is made larger than that in the first inclined rolling step,

[0128] (b) When the second wall thickness is larger than the first wall thickness, the crossing angle of the inclined rolls is made smaller than that in the first inclined rolling step.

[0129] Figure 7 and Figure 8 are schematic diagrams for explaining a specific example of a setting change process. Figure 7 Indicates the situation of option (a). Figure 8 Indicates the situation of option (b). Figure 7 and Figure 8 2 shows a case where the inlet side surface 1 a of the inclined roller 1 is a convex curved surface.

[0130] Reference Figure 7 and Figure 8, the inlet side surface 1a of the inclined roller 1 is a convex curved surface. The line that appears when the inlet side surface 1a is cut along the central axis 1c of the inclined roller 1 is a convex curve. The convex curve is represented by a function that depicts an arc with a constant radius of curvature. On the other hand, the outlet side surface 1b of the inclined roller 1 is a tapered surface.

[0131] In such an inclined roll 1, the distance between the pass line PL and the inlet side surface 1a gradually decreases from the inlet side of the pass line PL toward the outlet side. On the other hand, the distance between the pass line PL and the outlet side surface 1b gradually increases from the inlet side of the pass line PL toward the outlet side.

[0132] [Option (a)]

[0133] When the second wall thickness is smaller than the first wall thickness, option (a) is selected. According to another viewpoint, when a thin-walled seamless metal pipe is manufactured by the second inclined rolling process, option (a) is selected. In this case, the crossing angle CA of the inclined roll 1 is made larger than that in the first inclined rolling process. As a result, the exit side angle θb of the inclined roll 1 is increased.

[0134] As described above, if the setting conditions of the inclined rolling mill are changed to option (a), when the second seamless metal pipe is manufactured by the second inclined rolling process, there is no excessive pressing of material, so the circumference of the second rolled workpiece WP in the longitudinal direction does not change but is stable. As a result, a seamless metal pipe with a small wall thickness and no quality problems can be manufactured.

[0135] [Option (b)]

[0136] When the second wall thickness is greater than the first wall thickness, option (b) is selected. According to another viewpoint, when a thick-walled seamless metal pipe is manufactured by the second inclined rolling process, option (b) is selected. In this case, the crossing angle CA of the inclined roll 1 is made smaller than that in the first inclined rolling process. As a result, the exit side angle θb of the inclined roll 1 is reduced.

[0137] Thus, if the setting conditions of the inclined rolling mill are changed to option (b), the second seamless metal pipe will not have any bite defects when the rolled material is manufactured by the second inclined rolling process, thereby manufacturing a thick seamless metal pipe with no quality problems.

[0138] In particular, when the second rolled product has the same shape, size and material as the first rolled product, this is useful in the following aspects: Setting conditions of equipment (eg, transport equipment) disposed upstream of the inclined rolling mill can be shared, thereby achieving excellent work efficiency.

[0139] Example 1

[0140] In Example 1, a piercing and rolling test was conducted. In this piercing and rolling test, a round billet (raw material, rolled product) of carbon steel was pierced and rolled using a three-roll inclined rolling mill to produce a tube billet (seamless metal tube). As a structural element of the inclined rolling mill, a plurality of plugs (plug No. A to F) having different sizes and shapes were prepared. In addition, a plurality of inclined rolls (roll No. R60 to R600, O to Z) having different sizes and shapes were prepared.

[0141] Fig. 9 This is a diagram showing the appearance of a plug used in a piercing and rolling test. Fig. 9 The shape of the plug 2 of plug No. A to F is a general cannonball shape. The following Table 1 shows the dimensions of the plug 2 of plug No. A to F. L in Table 1 is Fig. 9 The axial length of the portion from the top to the maximum diameter of the plug 2 shown. D in Table 1 is Fig. 9 The maximum diameter of the main body of the plug 2 is shown.

[0142] [Table 1]

[0143] Table 1

[0144] Top No. D[mm] L[mm] A 33.0 60 B 42.0 125 C 42.0 81 D 42.0 60 E 48.0 105 F 46.0 125

[0145] Fig.10 and Fig.11 are diagrams showing the appearance of the inclined rolls used in the piercing and rolling test. Fig.10 The inclined rollers 1 of roller Nos. R60 to R600 are shown in FIG. Fig.11 The inclined roller 1 of roller Nos. O to Z is shown in FIG.

[0146] Reference Fig.10 The inclined roller 1 of roller No. R60 to R600 is divided into an inlet side surface 1a and an outlet side surface 1b with a gouge G as a boundary. The inlet side surface 1a is a convex surface. The line that appears when the inlet side surface 1a is cut along the center axis 1c of the inclined roller 1 is a convex curve. The convex curve is an arc with a constant radius of curvature (RG). The outlet side surface 1b is a tapered surface. The total length of the inclined roller 1 is constant. The axial length of the outlet side surface 1b is constant. The axial length of the inlet side surface 1a changes according to the radius of curvature (RG) of the arc forming the convex surface. Therefore, an auxiliary cylindrical portion 1aa is provided at the inlet side end of the inlet side surface 1a as needed.

[0147] Table 2 below shows the dimensions of the inclined rolls 1 of roll Nos. R60 to R600. Fig.10 The dimensions common to these inclined rollers 1 are shown in Table 2. RG in Table 2 is Fig.10 The radius of curvature of the arc forming the convex surface of the inlet side surface 1a is shown. H in Table 2 is Fig.10 The axial length of the auxiliary cylindrical portion 1aa is shown.

[0148] [Table 2]

[0149] Table 2

[0150] Roller No. RG[mm] H[mm] R60 60 93.8 R100 100 68.0 R220 220 20.3 R400 400 0.0 R600 600 0.0

[0151] Reference Fig.11 The inclined roller 1 of roller No. O to Z is divided into an inlet side surface 1a and an outlet side surface 1b with a round gouge G as a boundary. The inlet side surface 1a is a tapered surface. The outlet side surface 1b is a tapered surface. The axial length of the inlet side surface 1a is constant. The axial length of the outlet side surface 1b is constant. Therefore, the total length of the inclined roller 1 is constant. The total length of the inclined roller 1 of roller No. O to Z is the same as the total length of the inclined roller 1 of roller No. R60 to R600.

[0152] Table 3 below shows the dimensions of the inclined rolls 1 of roll Nos. O to Z. Fig.11 The dimensions common to these inclined rollers 1 are shown in Table 3. Fig.11 The gradient of the inlet side 1a is shown.

[0153] [Table 3]

[0154] Table 3

[0155] Roller No. αa[°] O 1 P 4 Q 5 R 7 S 10 T 11 U 12 V 14 W 15 × 17 Y 18 Z 19

[0156] In the piercing and rolling test of Example 1, various combinations of plugs No. A to D and inclined rolls No. O, P, R, and S were used to perform piercing and rolling. At this time, the heating temperature of the round billet was 1200° C. The inclination angle FA of the inclined rolls was 10°. In addition, various changes were made to the crossing angle CA of the inclined rolls and the roll opening.

[0157] In addition, various changes were made to the wall thickness of the tube (seamless metal tube) to be obtained while aiming to keep the outer diameter constant. This is because in actual operation, the tube after piercing and rolling is supplied to the elongation rolling mill, so in many cases the outer diameter needs to be kept constant.

[0158] The test conditions and test results of Example 1 are shown in Table 4 below.

[0159] [Table 4]

[0160]

[0161] First, piercing and rolling were performed under condition 1 using an inclined roll of roll No.O and a plug of plug No.A. Next, piercing and rolling were performed under condition 2 in which the roll opening was smaller than that of condition 1. Condition 2 is a condition in which the target wall thickness of the seamless metal pipe to be obtained is smaller than that of condition 1. However, extrusion of the material occurred during the piercing and rolling under condition 2. Therefore, piercing and rolling were performed under condition 3 in which the inclined roll was replaced with an inclined roll of roll No.P, the exit side angle was changed from 0° to 3°, and the plug was replaced with a plug of plug No.B. In the piercing and rolling under condition 3, a seamless metal pipe with a smaller wall thickness than that of condition 1 can be obtained without causing undesirable conditions such as extrusion of the material.

[0162] Piercing and rolling were performed under conditions 4 and 5 to reduce the roll opening and wall thickness compared with condition 3. In the piercing and rolling under condition 4, the wall thickness was thinner than that under condition 3 without any problem. On the other hand, material extrusion occurred in the piercing and rolling under condition 5. Therefore, piercing and rolling was performed under condition 6 in which the inclined roll was replaced with an inclined roll of roll No. R, the exit side angle was changed to 6°, and the plug was replaced with a plug of plug No. C. In the piercing and rolling under condition 6, a seamless metal pipe with a smaller wall thickness than that under condition 4 was obtained.

[0163] Piercing and rolling were performed under conditions 7 and 8 to make the roll opening smaller than that under condition 6 and further reduce the wall thickness. In the piercing and rolling under condition 7, the wall was thinner than that under condition 6. On the other hand, extrusion of material occurred in the piercing and rolling under condition 8. Therefore, piercing and rolling were performed under condition 9 in which the inclined roll was replaced with an inclined roll of roll No. S, the exit side angle was changed to 9°, and the plug was replaced with a plug of plug No. D. In the piercing and rolling under condition 9, a seamless metal pipe with a smaller wall thickness than that under condition 7 was obtained. Next, piercing and rolling was performed under condition 10 in which the roll opening was smaller than that under condition 9. Extrusion of material occurred in the piercing and rolling under condition 10.

[0164] Next, piercing and rolling were performed under conditions 11 and 12, with the roll opening being larger than that under condition 9 and the wall thickness being increased. In the piercing and rolling under condition 11, a seamless metal pipe with a larger wall thickness than that under condition 9 was obtained. Plug clogging occurred in the piercing and rolling under condition 12. Therefore, piercing and rolling was performed under condition 13, in which the inclined roll was replaced with an inclined roll of roll No. R, the exit side angle was changed from 9° to 6°, and the plug was replaced with a plug of plug No. C. However, plug clogging was not eliminated under condition 13.

[0165] Therefore, piercing and rolling were performed under condition 14, in which the inclined roll was replaced with the inclined roll of roll No. P, the exit side angle was reduced, and the plug was replaced with the plug of plug No. B. In the piercing and rolling under condition 14, a seamless metal pipe with a larger wall thickness than that under condition 11 was obtained. Piercing and rolling was performed under condition 15, in which the plug extension amount under condition 14 was reduced and the wall thickness was further increased. Plug clogging occurred in the piercing and rolling under condition 15. Therefore, piercing and rolling were performed under condition 16, in which the inclined roll was replaced with the inclined roll of roll No. O, the exit side angle was further reduced, and the plug was replaced with the plug of plug No. A. In the piercing and rolling under condition 16, a seamless metal pipe with a larger wall thickness than that under condition 14 was obtained.

[0166] The following is shown based on the above results. Consider a process in which, after a first seamless metal tube having a first wall thickness is manufactured by piercing and rolling, a second seamless metal tube having a wall thickness different from the first is manufactured by piercing and rolling in the same manner. When manufacturing the second seamless metal tube, if the outlet side angle of the inclined roller is increased when the second wall thickness is smaller than the first wall thickness, a second thin-walled seamless metal tube having no quality problems can be manufactured. In order to increase the outlet side angle of the inclined roller, it is sufficient to increase the crossing angle of the inclined roller. On the contrary, if the outlet side angle of the inclined roller is reduced when the second wall thickness is larger than the first wall thickness, a second thick-walled seamless metal tube having no quality problems can be manufactured. In order to reduce the outlet side angle of the inclined roller, it is sufficient to reduce the crossing angle of the inclined roller. In short, according to the manufacturing method of the first structure described above, both seamless metal tubes having a smaller wall thickness and seamless metal tubes having a larger wall thickness can be manufactured without quality problems.

[0167] In addition, the occurrence of defects (material extrusion and plug clogging) was checked for the piercing and rolling conditions 1 to 16. The exit side angle of the inclined roll and the wall thickness to outer diameter ratio of the seamless metal pipe were used as indices for this checking. Fig.12 This is a diagram summarizing whether any adverse events occurred in Example 1. Fig.12 In the figure, the horizontal axis represents the exit side angle of the inclined roll [°], and the vertical axis represents the wall thickness to outer diameter ratio (t / d) of the obtained seamless metal tube [unit: dimensionless]. Fig.12 In the figure, the “○” mark indicates that no adverse conditions occurred, and the “×” mark indicates that adverse conditions occurred. Fig.12 A, B, C and D refer to the plug numbers.

[0168] according to Fig.12The results show the following. As the exit side angle of the inclined roll increases, the wall thickness to outer diameter ratio of the extruded pipe decreases. This means that if the exit side angle of the inclined roll is increased, the extrusion is suppressed, and the wall thickness limit in thin-walled pipe making is expanded. On the contrary, as the exit side angle of the inclined roll decreases, the wall thickness to outer diameter ratio that causes plug clogging increases. This means that if the exit side angle of the inclined roll is reduced, plug clogging is suppressed, and the wall thickness limit in thick-walled pipe making is expanded.

[0169] In addition, in order to manufacture a thin-walled seamless metal tube with a wall thickness to outer diameter ratio of less than 0.07 without causing adverse conditions (extrusion), it is sufficient to make the outlet side angle larger than 3°. The upper limit of the outlet side angle is not particularly limited. However, when the outlet side angle is 6° and when it is 9°, the wall thickness limit in thin-walled pipe making is almost the same. In addition, when the outlet side angle is larger than 9°, the plug has to be geometrically shortened. In this case, the design of the plug itself becomes difficult, and there is a concern about the deterioration of dimensional accuracy, especially uneven wall thickness. Therefore, the upper limit of the outlet side angle is preferably 9°, and more preferably 6°.

[0170] In addition, in any case of thin-walled pipe making and thick-walled pipe making, if the exit side angle is made smaller than 0°, the roll opening decreases as the exit side of the inclined roll is approached. In this case, the gap between the inner periphery of the manufactured seamless metal pipe and the outer periphery of the plug becomes smaller, and it becomes difficult to pull the plug out of the seamless metal pipe. Therefore, the exit side angle is preferably 0° or more. In short, according to the manufacturing method of the second structure described above, both seamless metal pipes with smaller wall thickness and seamless metal pipes with larger wall thickness, which have less quality problems, can be manufactured.

[0171] Example 2

[0172] The replacement of the tilted roller requires a long downtime, which reduces the production output. Therefore, the replacement of the tilted roller should be avoided as much as possible.

[0173] In Example 2, a piercing and rolling test was carried out in the same manner as in Example 1. In the piercing and rolling test of Example 2, piercing and rolling were carried out using various combinations of plugs No. A to D and inclined rolls No. V, P, and R220.

[0174] The test conditions and test results of Example 2 are shown in Table 5 below.

[0175] [Table 5]

[0176]

[0177] First, piercing and rolling were performed under Condition 17 using the inclined roll No. V and the plug No. A. Condition 17 corresponds to Condition 1 of Example 1 except for the entrance side angle of the inclined rolls used. However, in the piercing and rolling under Condition 17, the round billet was not bitten by the inclined rolls.

[0178] Therefore, piercing rolling was performed under conditions 18 to 21. In conditions 18 to 21, the inclined roll was replaced with roll No. P, and it was confirmed that a material of almost the same size as that of condition 1 could be pierced. In particular, conditions 19 and 20 are conditions corresponding to conditions 3 and 4 of Example 1, respectively. Through the piercing rolling of these conditions 19 and 20, it was confirmed again that piercing was possible even if the wall thickness was gradually reduced. Here, under condition 5 of Example 1, the wall thickness could not be further reduced in the combination of the inclined roll of roll No. P and the plug of plug No. B. Therefore, under condition 21, the combination of the inclined roll of roll No. P and the plug of plug No. C was used, and the exit side angle of the inclined roll was set to the same as that of condition 6 of Example 1, and the wall thickness was the same as that of condition 6 of Example 1. However, in the piercing rolling of condition 21, the round billet collided with the end face on the entrance side of the inclined roll, and the round billet could not be bitten into the inclined roll. This is because the inlet side surface angle of the inclined roller is 1°, and therefore the main body length (axial length) of the inlet side surface of the inclined roller is insufficient.

[0179] In the case of the original thin-walled pipe making, the entrance side angle is inevitably reduced due to the increase in the exit side angle of the inclined roll. In addition, the roll opening is small in thin-walled pipe making, so it is necessary to further lengthen the main length of the entrance side. In the piercing rolling of condition 23, the main length of the entrance side required for the round billet to bite into the inclined roll is 170 mm or more. The main length of the entrance side of the inclined roll used in the actual test is 150 mm (refer to Fig.11 ).

[0180] If the main body length of the inlet side of the inclined roller is too long, the construction cost of the inclined rolling mill and the manufacturing cost of the tool increase. Therefore, it is not preferable to lengthen the main body length of the inlet side of the inclined roller.

[0181] Therefore, piercing and rolling were performed under conditions 22 to 30. Under conditions 22 to 30, the inclined roll was replaced with an inclined roll of roll No. R220, and the plug was replaced with another plug. Furthermore, various changes were made to the cross angle CA of the inclined roll and the roll opening. Piercing and rolling under conditions 22 to 30 were performed in the same order as in Example 1. That is, in the order of each piercing and rolling, the wall thickness was temporarily reduced and then increased. In any piercing and rolling under conditions 22 to 30, a seamless metal pipe can be obtained without causing defects such as poor biting.

[0182] The above results show that if the inlet side surface of the inclined roll is a convex curved surface, both a thin-walled seamless metal pipe and a thick-walled seamless metal pipe can be manufactured without replacing the inclined rolls.

[0183] Example 3

[0184] In Example 3, a piercing and rolling test was carried out in the same manner as in Example 1. In the piercing and rolling test of Example 3, various combinations of plugs No. C and E and inclined rolls No. S to Z and R220 were used to perform high processing degree piercing and rolling with a piercing ratio of 3.5 or more. The outer diameters of the round billets were 50 mm and 60 mm.

[0185] The cross angle CA of the inclined rolls was 3°, and the inclined angle FA was 10°. In addition, the exit side angle of the inclined rolls was 6°. The roll opening was changed in various ways to investigate whether there were any problems (material extrusion, poor bite, and change in the circumference of the material). The entrance side angle of the inclined rolls and the gouge draft ratio were used as indicators.

[0186] The test conditions and test results of Example 3 are shown in Table 6 below.

[0187] [Table 6]

[0188]

[0189] Fig.13 This is a diagram summarizing whether there are any adverse events in Example 3. Fig.13 In the figure, the horizontal axis represents the inlet side angle of the inclined roll [°], and the vertical axis represents the gouge draft ratio [%]. Fig.13 In the figure, the “○” mark indicates that no adverse conditions occurred, and the “×” mark indicates that adverse conditions occurred.

[0190] Refer to Table 6 and Fig.13 If the inlet side angle of the inclined roll is less than 7°, the circumference of the material in the longitudinal direction changes greatly. On the other hand, if the inlet side angle of the inclined roll is more than 16°, poor biting occurs. In addition, if the round gouge draft is less than 30%, material extrusion occurs. Therefore, it is known that in the case of piercing and rolling with a perforation ratio of 3.5 or more, the inlet side angle of the inclined roll is set to be more than 8° and less than 15°, and the draft is set to be more than 30%.

[0191] In addition, it was confirmed that even when using an inclined roll of roll No. R220, that is, when using an inclined roll with a convex entrance side, extrusion will occur if the gouge draft ratio is set to less than 30% during piercing and rolling with a piercing ratio of 3.5 or more.

[0192] Example 4

[0193] In Example 4, the piercing and rolling test was carried out in the same manner as in Example 1. In the piercing and rolling test of Example 4, various combinations of plugs No. A, E and F and five inclined rolls No. R60 to R600 were used to carry out piercing and rolling. As basic conditions, three conditions 46 to 48 were set. In each of conditions 46 to 48, inclined rolls No. R60 to R600 were used. Conditions 46, 47 and 48 were used to manufacture thick-walled, medium-walled and thin-walled seamless metal pipes, respectively. The exit side angles of conditions 46, 47 and 48 were 0°, 3° and 6°, respectively.

[0194] In the piercing and rolling under various conditions, whether or not any defect occurs is investigated. The investigation uses the starting contact position between the round billet and the inclined roll as an indicator. The starting contact position can be calculated by two-dimensional geometry when the inclination angle FA of the inclined roll is assumed to be 0°. The indicators investigated are the first distance in the direction along the rolling line and the second distance in the direction perpendicular to the rolling line. The first distance is the distance from the gouge position to the starting contact position. The second distance is the distance obtained by subtracting half of the roll opening from the distance from the rolling line to the starting contact position. In this specification, the first distance is sometimes referred to as the distance from the gouge. The second distance is sometimes referred to as (distance from the rolling line)-(roll opening / 2).

[0195] The test conditions of Example 4 are shown in Table 7 below.

[0196] [Table 7]

[0197]

[0198] Fig.14 This is a diagram summarizing whether or not any adverse events occurred in Example 4. Fig.14 In the figure, the horizontal axis represents the distance from the gouge [mm], and the vertical axis represents (the distance from the rolling line) - (roller opening / 2) [mm]. Fig.14 In the figure, the “○” mark indicates that no adverse conditions occurred, and the “×” mark indicates that adverse conditions occurred.

[0199] Reference Fig.14 In the piercing and rolling using the inclined roll of roll No. R60, the biting failure occurred in all conditions 46 to 48. In addition, in the piercing and rolling using the inclined roll of roll No. R600, the failure of large variation in the circumference of the material occurred in the case of conditions 47 and 48.

[0200] The above results show the following. In order to suppress the occurrence of adverse conditions, it is preferred to use inclined rollers of rollers No. R100 to R400 among rollers No. R60 to R600. In the smallest inclined roller No. R100 among these rollers No. R100 to R400, the radius of curvature of the arc forming the convex surface of the inlet side surface is 100 mm. On the other hand, in the largest inclined roller No. R400, the radius of curvature of the arc forming the convex surface of the inlet side surface is 400 mm. The outer diameter of the round billet is 60 mm. Therefore, as long as the value obtained by dividing the radius of curvature of the arc forming the convex surface of the inlet side surface by the outer diameter of the round billet (curvature index of the inlet side surface) is greater than 1.67 and less than 6.67, the occurrence of adverse conditions can be suppressed.

[0201] Example 5

[0202] In Example 5, the piercing and rolling test was carried out in the same manner as in Example 1. In Example 5, two types of inclined rolls (rollers No. A and B) were also prepared.

[0203] Fig.15 : is a figure which shows the appearance of the inclined roll used in the piercing and rolling test. Fig.15 The inclined roller 1 of roller No. A and B is shown in FIG.

[0204] Reference Fig.15 The inclined roller 1 of roller No. A and B is divided into an inlet side 1a and an outlet side 1b with a gouge G as a boundary. The inlet side 1a is a convex surface. The line that appears when the inlet side 1a is cut along the center axis 1c of the inclined roller 1 is a convex curve. The convex curve is represented by a polynomial function. The outlet side 1b is a conical surface. The total length of the inclined roller 1 is constant. The axial length of the outlet side 1b is constant. The axial length of the inlet side 1a changes according to the polynomial function of the convex curve that forms the convex surface. Therefore, an auxiliary cylindrical portion 1aa is provided at the inlet side end of the inlet side 1a as needed. The total length of the inclined roller 1 of roller No. A and B is the same as the total length of the inclined roller 1 of the above-mentioned roller No. O~Z and R60~R600.

[0205] The following Table 8 shows the dimensions of the inclined rollers 1 of roller Nos. A and B. Fig.15 The dimensions common to these inclined rollers 1 are shown in Table 8. a and b in Table 8 are Fig.15 The coefficients of the multinomial function of the convex curve forming the convex surface of the inlet side surface 1a are shown. H in Table 8 is Fig.15 The axial length of the auxiliary cylindrical portion 1aa is shown.

[0206] [Table 8]

[0207] Table 8

[0208] Roller No. a b H[mm] A 0.0020 0.0083 0.00 B 0.0021 0.0852 13.13

[0209] In the piercing and rolling test of Example 5, inclined rolls of roll Nos. A and B were used under each of conditions 46 to 48 in Example 4. In the piercing and rolling under each condition, the occurrence of a defect was investigated in the same manner as in Example 4.

[0210] Fig.16 and Fig.17 It is a diagram summarizing whether there are any adverse events in Example 5. In these diagrams, Fig.16 The results using the inclined roller of roller No. A are shown. Fig.17 The results using the inclined roller of roller No. B are shown. Fig.16 and Fig.17 In the figure, the horizontal axis represents the distance from the gouge [mm], and the vertical axis represents (the distance from the rolling line) - (roller opening / 2) [mm]. Fig.16 and Fig.17 In the figure, the "◇" mark indicates that no adverse conditions have occurred, and the "×" mark indicates that adverse conditions have occurred. Fig.16 and Fig.17 The conditions and ranges thereof under which no failure occurs in the piercing and rolling of Example 4 are also shown.

[0211] Reference Fig.16 In the piercing rolling using the inclined roll of roll No. A, in any of the cases where the exit side angle is 0°, 3° and 6°, the contact between the round billet and the inclined roll begins within the range where no defect occurs in Example 4, and no defect occurs. Fig.17 In the piercing and rolling using the inclined rolls of roll No. B, when the exit flank angle was 0°, the round billet and the inclined rolls sometimes started to contact outside the range where no failure occurred in Example 4. At this time, a bite failure occurred.

[0212] Example 6

[0213] In Example 6, the piercing and rolling test was carried out in the same manner as in Example 1. In Example 6, two types of inclined rolls (rollers No. C and D) were also prepared.

[0214] Fig.18 and Fig.19 are diagrams showing the appearance of the inclined rolls used in the piercing and rolling test. Fig.18 The inclined roller 1 of roller No. C is shown in FIG. Fig.19 The inclined roller 1 of roller No. D is shown in FIG.

[0215] Reference Fig.18 and Fig.19The inclined roller 1 of roller No.C and D is divided into an inlet side 1a and an outlet side 1b with a gouge G as a boundary. The inlet side 1a is composed of a three-level conical surface. That is, the inlet side 1a is a conical surface (convex surface) whose gradient changes in three stages along the center axis 1c. The outlet side 1b is a conical surface with a constant gradient. The axial length of the inlet side 1a is constant. The axial length of the outlet side 1b is constant. Therefore, the total length of the inclined roller 1 is constant. The total length of the inclined roller 1 of roller No.C and D is the same as the total length of the inclined roller 1 of the above-mentioned rollers No.A, B, O~Z and R60~R600.

[0216] In the piercing and rolling test of Example 6, the inclined rolls of roll Nos. C and D were used under each of conditions 46 to 48 of Example 4, as in Example 5. In the piercing and rolling under each condition, the occurrence of a defect was investigated as in Example 4.

[0217] Fig. 20 and Fig.21 This is a diagram summarizing whether or not there are any adverse events in Example 6. In these diagrams, Fig. 20 The results using the inclined roller of roller No. C are shown. Fig.21 The results using the inclined roller No. D are shown. Fig. 20 and Fig.21 In the figure, the horizontal axis represents the distance from the gouge [mm], and the vertical axis represents (the distance from the rolling line) - (roller opening / 2) [mm]. Fig. 20 and Fig.21 In the figure, the "◇" mark indicates that no adverse conditions have occurred, and the "×" mark indicates that adverse conditions have occurred. Fig. 20 and Fig.21 The conditions and ranges thereof under which no failure occurs in the piercing and rolling of Example 4 are also shown.

[0218] Reference Fig. 20 In the piercing rolling using the inclined roll of roll No. C, in any of the cases where the exit side angle is 0°, 3° and 6°, within the range where no defect occurs in Example 4, the contact between the round billet and the inclined roll begins, and no defect occurs. Fig.21 In the piercing and rolling process using the inclined roll of roll No. D, when the exit side angle was 6°, the round billet and the inclined roll sometimes started to contact outside the range where no problem occurred in Example 4. At this time, the circumference of the material varied.

[0219] The relationship between the above-mentioned bite angle (α) and the gouge draft ratio was summarized based on the results of Examples 4 to 6.

[0220] Fig. 22 This is a diagram for explaining the bite angle (α). Fig. 22In the figure, the horizontal axis represents the distance from the gouge [mm], and the vertical axis represents (the distance from the rolling line) - (roller opening / 2) [mm]. Fig. 22 In the graph, the “0” mark indicates the starting contact position. The origin corresponds to the gouge position. The horizontal axis corresponds to the rolling line. Draw a straight line connecting the starting contact position and the gouge position. The angle between the straight line and the rolling line is the bite angle (α). This is as described above.

[0221] Fig.23 This graph summarizes whether or not a problem has occurred based on the relationship between the bite angle (α) and the gouge draft ratio. Fig.23 In the figure, the horizontal axis represents the gouge draft ratio [%], and the vertical axis represents the bite angle (α) [°]. Fig.23 In the figure, the “○” mark indicates that no adverse conditions occurred, and the “×” mark indicates that adverse conditions occurred.

[0222] according to Fig.23 The results show the following. The larger the gouge drafting ratio, the larger the bite angle (α) is required. In order to increase the gouge drafting ratio, the roll opening should be reduced. If the roll opening is small, it becomes a piercing rolling with a relatively high perforation. In contrast, the smaller the gouge drafting ratio, the smaller the bite angle (α) is required. In order to reduce the gouge drafting ratio, the roll opening should be increased. If the roll opening is large, it becomes a piercing rolling with a relatively low perforation.

[0223] Therefore, as shown in the above formula (1), the bite angle (α) is preferably greater than "0.12×GD+1.5" and less than "0.25×GD+6". From another point of view, in order to satisfy the condition shown in formula (1), it is sufficient to set the inlet side surface in such a way that the gradient becomes larger as it approaches the inlet side end of the inlet side surface.

[0224] In addition, the present invention is not limited to the above-mentioned embodiment, and various changes can be made without departing from the scope of the present invention.

[0225] Industrial Applicability

[0226] The production method of the present invention can be effectively used for the production of seamless metal pipes based on the Mannesmann process.

[0227] Description of Reference Numerals

[0228] 1. Inclined roll; 1a. Inlet side; 1b. Outlet side; 1c. Center axis; 2. Plug; PL. Pass line; WP. Workpiece; CA. Cross angle; FA. Incline angle.

Claims

1. A method for producing a seamless metal pipe, comprising producing a first seamless metal pipe having a first wall thickness and a second seamless metal pipe having a second wall thickness different from the first wall thickness using an inclined rolling mill, wherein: The inclined rolling mill comprises: A plug, which is arranged on the rolling line; as well as three inclined rollers, which are arranged at equal intervals around the rolling line, and respectively have an inlet side surface and an outlet side surface, in which the distance between the rolling line and the inlet side surface gradually decreases from the inlet side of the rolling line toward the outlet side, and the distance between the rolling line and the outlet side surface gradually increases from the inlet side of the rolling line toward the outlet side, The manufacturing method comprises: a first inclined rolling step in which the heated first rolled piece is rolled using the inclined rolling mill to produce the first seamless metal pipe; a setting changing step, in which the setting conditions of the inclined rolling mill are changed to any one of the following (a) and (b): (a) when the second wall thickness is smaller than the first wall thickness, the crossing angle of the inclined rolls is made larger than that in the first inclined rolling step; (b) when the second wall thickness is larger than the first wall thickness, the crossing angle of the inclined rolls is made smaller than that in the first inclined rolling step; as well as a second inclined rolling step in which the heated second rolled material is rolled using the inclined rolling mill with changed setting conditions to produce the second seamless metal pipe; The crossing angle is a swing angle of the central axis of the inclined roll in the radial direction centered on the rolling line.

2. The method for manufacturing a seamless metal pipe according to claim 1, wherein: In the first inclined rolling step and the second inclined rolling step, the angle of the outlet side surface within the contact range of the outlet side surface with the first rolled workpiece and the second rolled workpiece is set to be greater than 0° and less than 9°, In the first inclined rolling step, when the ratio of the first wall thickness to the outer diameter of the first seamless metal pipe is 0.07 or less, the angle of the outlet side surface is made larger than 3°, In the second inclined rolling step, when the ratio of the second wall thickness to the outer diameter of the second seamless metal pipe is 0.07 or less, the angle of the outlet side surface is set to be larger than 3°.

3. The method for producing a seamless metal pipe according to claim 1 or 2, wherein: The inlet side surface is a convex surface, In the second inclined rolling step, the inclined rolls used in the first inclined rolling step are used.

4. The method for producing a seamless metal pipe according to claim 1 or 2, wherein: The first rolled workpiece and the second rolled workpiece are solid. In the first and second inclined rolling steps, the perforation ratio is set to 3.5 or more, the angle of the inlet side surface is set to 8° or more and 15° or less, and the gouge draft ratio is set to 30% or more.

5. The method for producing a seamless metal pipe according to claim 1 or 2, wherein: The first rolled workpiece and the second rolled workpiece are solid. The inlet side surface is a convex curved surface, and the line of the inlet side surface appearing in the cross section including the central axis of the inclined roller is an arc, A value obtained by dividing the radius of curvature of the circular arc by the outer diameters of the first rolled workpiece and the second rolled workpiece is greater than or equal to 1.67 and less than or equal to 6.

67.

6. The method for producing a seamless metal pipe according to claim 1 or 2, wherein: The first rolled workpiece and the second rolled workpiece are solid. In the first inclined rolling step and the second inclined rolling step, the gouge draft ratio GD and the bite angle α satisfy the condition represented by the following formula (1): 0.12×GD+1.5≤α≤0.25×GD+6 (1).

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