RU2793593C1 - Method for manufacturing seamless metal pipes - Google Patents

Abstract

FIELD: pipe manufacturing.

SUBSTANCE: invention is related to a method for manufacturing the first seamless metal pipe with the first wall thickness and the second seamless metal pipe with the second wall thickness by using a three-roll inclined roll mill. The first stage of inclined rolling, the stage of changing the settings and the second stage of inclined rolling are carried out. During the first inclined rolling step, the first billet is rolled by means of an inclined roll mill. During the second inclined rolling step, the second billet is rolled by means of the inclined roll mill in the changed state. When the second wall thickness is smaller than the first wall thickness, the rolling angle of each of the inclined rolls is set to be larger than the rolling angle set for the first inclined rolling step. When the second wall thickness is larger than the first wall thickness, the rolling angle of each of the inclined rolls is set to be smaller than the rolling angle set for the first inclined rolling step.

EFFECT: occurrence of defects in manufacturing of pipes is prevented.

6 cl, 23 dwg, 8 tbl, 6 ex

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

[0001]

The present invention relates to a method for manufacturing a seamless metal pipe by means of the Mannesmann process.

BACKGROUND TO THE INVENTION

[0002]

Basically, a method for manufacturing a seamless metal pipe by the Mannesmann process includes the following steps. A round blank is heated to a predetermined temperature. The round blank is subjected to piercing rolling with molding into a hollow sleeve (seamless metal tube). The hollow sleeve is then subjected to lengthening rolling and diameter-adjusting rolling. In the piercing rolling step, a piercing rolling mill (for example, a piercing mill) is used. In the extension rolling step, an extension rolling mill (for example, a continuous tube mill or an extension bar) is used. The piercing rolling mill is a rolling mill with inclined rolls. In some cases, an inclined roll mill is used as an extension mill.

[0003]

Such inclined roll rolling mills are described, for example, in Japanese Patent Published Application H05-228514 (Patent Document 1), Japanese Patent Published Application H02-263506 (Patent Document 2), Japanese Patent Published Application S64-31505 (Patent Document 3) and Published Japanese Patent Application S59-80716 (Patent Document 4).

[0004]

Such an inclined roll mill includes a mandrel and two inclined rolls as a rolling tool. In some cases, the inclined roll mill includes three inclined rolls. An inclined roll mill having two inclined rolls is called a two-roll type inclined roll mill. An inclined roll mill having three inclined rolls is called a three-roll type inclined roll mill. The inclined rolls are arranged at regular angular intervals around the rolling line. The central axis of each of the inclined rolls is inclined relative to the rolling line. In other words, each of the inclined rolls has a feed angle. In some cases, each of the inclined rolls further has a rolling angle. The mandrel is located on the rolling line between inclined rolls.

[0005]

When the inclined roll mill is used as the piercing mill, the inclined rolling (piercing rolling) is performed as follows. A round solid ingot is used as a blank for piercing rolling. The heated billet is placed on the rolling line. The billet is fed into position between the inclined rolls by means of a pusher and engaged with the inclined rolls. Then, the billet is moved forward on the rolling line while rotating around its own axis, and subjected to piercing rolling by means of inclined rolls and a mandrel. In this way, a hollow sleeve (seamless metal pipe) with a given wall thickness and a given outer diameter can be obtained.

[0006]

When an inclined roll mill is used as the extension mill, the process of the inclined rolling (extension rolling) is the same as the piercing rolling process, except that a hollow sleeve is used as the elongation rolling workpiece.

[0007]

The piercing mill (piercing rolling mill), which performs the first step (piercing rolling step) of the method for manufacturing seamless metal pipe by the Mannesmann process, was put into practical use by the Mannesmann brothers in 1885. At that time, the piercing mill was a two-roll type basic mill. Since the introduction of the piercing mill into practical use, various improvements have been made, and the piercing mill is still in use today in factories around the world. Other piercing machines other than this piercing mill were introduced, but almost all piercing machines other than the piercing mill used in the Mannesmann process were discontinued, except for the Erhardt piercing process and the extrusion process. The reason for this is that the piercing mill provides superior productivity and dimensional accuracy to products. Therefore, it is not an exaggeration to say that only the piercing mill (piercing rolling mill) is the only successful piercing machine in the industry.

[0008]

However, the current two roll type piercing mill has some problems to be solved. There are two main problems, as described below.

[0009]

One of the problems is the occurrence of internal defects as a result of the Mannesmann fracture. Mannesmann fracture means a phenomenon in which the central part of the workpiece becomes brittle and breaks. In the piercing mill of the two-roll type, a guide tool (such as a guide fence or a disk roll) is located between the inclined rolls around the rolling line. The guide tool is designed to limit the formation of bulges in the workpiece. During piercing (tilting) rolling, a compressive stress, which acts in the direction in which the inclined rolls face each other, and a tensile stress, which acts in the direction in which the guide elements face each other, simultaneously occur in the central part of the rolled workpiece. These stresses have an effect that is repeated every quarter of a revolution of the workpiece. This repeated application of these stresses results in the formation of a Mannesmann fracture. When the Mannesmann fracture is severe, the resulting hollow sleeve has defects on the inner surface. These defects are internal defects.

[0010]

For many years the Mannesmann fracture was used to advantage and the mandrel was pressed into the fracture center of the workpiece to pierce the workpiece. This flashing method is simple. However, this method of firmware leads to the formation of internal defects.

[0011]

Recently, some measures have been taken to suppress the occurrence of the Mannesmann break. An example of such measures is the use of conical inclined rolls. However, there are no measures to completely prevent the occurrence of a Mannesmann fracture. Accordingly, the problem of occurrence of internal defects is still not solved. Particularly, when the workpiece is a cast ingot or a material with poor machinability such as stainless steel and the like, it promotes the formation of a Mannesmann fracture and increases the possibility of internal defects.

[0012]

The Mannesmann fracture increases with an increase in the number of repetitions of the application of the stresses described above, that is, the number of revolutions of the workpiece. Therefore, one way to suppress the Mannesmann fracture is to increase the angle of the entry cross surface of each of the inclined rolls to reduce the distance between the point at which the workpiece comes into contact with the inclined rolls and the point at which the workpiece comes into contact with the tip of the mandrel. However, in a two-roll type piercer, each of the inclined rolls typically has an inlet cross-surface angle of about 3°. This has the following reason. During piercing rolling using a two-roll type piercing mill, the billet is prone to shear in a direction perpendicular to the direction in which the inclined rolls face each other, which often leads to misengagement, and therefore, providing a large entrance cross surface angle is problematic.

[0013]

The second problem is the occurrence of external defects as a result of damage to the disk roll. The disc roll is a guiding tool provided in the two-roll type piercing mill. Previously, a fixed guide ruler was used as a guiding tool. During piercing (tilting) rolling, the guide rail slides over the workpiece. When the sliding movement is heavy, the resulting hollow sleeve has defects on the outer surface. These defects are called external defects.

[0014]

Recently, the guide fence has been replaced by a rotating disc roll. The use of a disk roll reduces the incidence of external defects.

[0015]

However, the direction of rotation of the disk roll is not necessarily the same as the direction of rotation of the workpiece. Therefore, it is impossible to prevent galling between the surface of the disk roll and the outer surface of the workpiece. In addition, it is impossible to prevent deformation of the disc roll surface. Therefore, maintenance of the disk roll surface or regular replacement of the disk roll with a new one is required.

LIST OF CITATIONS

PATENT DOCUMENTS

[0016]

Patent Document 1: Published Japanese Patent Application No. H05-228514

Patent Document 2: Published Japanese Patent Application No. H02-263506

Patent Document 3: Published Japanese Patent Application No. S64-31505

Patent Document 4: Published Japanese Patent Application No. S59-80716

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0017]

The rolling mill with inclined rolls of the three-roll type solves the two problems described above. In the three-roll type inclined-roll rolling mill, unlike the two-roll type inclined-roll rolling mill, only the compressive stress is applied to the center portion of the workpiece during the inclined rolling, and hence Mannesmann fracture is not generated. Accordingly, it does not lead to internal defects. The three-roll type inclined roll mill does not use a guiding tool. Accordingly, it does not lead to the appearance of external defects. Thus, in order to solve the quality problems inherent in the inclined rolling of the two roll type inclined roll mill, it is highly effective to use the three roll type inclined roll mill for the inclined rolling.

[0018]

However, such a three-roll type inclined roll mill is not used in practice for manufacturing a seamless metal pipe. For example, it is difficult to manufacture a seamless metal pipe with a small wall thickness by piercing rolling using a three-roll type inclined roll mill. The reason for this is the absence of any guiding tools in the three-roll inclined roll mill. The installation of a guide tool in a three-roll type inclined roll mill has been considered but has not been put into practice. The reason for this is that it is difficult to install a guide tool in a three-roll type inclined roll mill from a structural point of view. At present, the three-roll type inclined roll mill is only used as an extension mill such as an Assel system mill and the like, specially provided to be able to manufacture a seamless metal pipe having a large wall thickness from a hollow sleeve.

[0019]

An inclined roll mill of the three-roll type is preferred for use in every case of manufacturing a seamless metal pipe. Therefore, it is important that any seamless metal pipe, regardless of whether it has a small wall thickness or a large wall thickness, can be produced without quality problems by inclined rolling using a three-roll type inclined roll mill.

[0020]

The aim of the present invention is to provide a method for manufacturing a seamless metal pipe which makes practical use of a three-roll inclined roll mill.

SOLUTION

[0021]

The method for manufacturing a seamless metal pipe according to an embodiment of the present invention is to manufacture a first seamless metal pipe with a first wall thickness and a second seamless metal pipe with a second wall thickness that is different from the first wall thickness by using an inclined roll mill. Rolling mill with inclined rolls includes: a mandrel located on the rolling line; and three inclined rolls spaced at regular angular intervals around the rolling line, each having an inlet lateral surface and an exit lateral surface. The distance between the rolling line and the entrance transverse surface gradually decreases with increasing distance from the entrance of the inclined rolls and with decreasing distance from the exit of the inclined rolls along the rolling line, and the distance between the rolling line and the exit transverse surface gradually increases with increasing distance from the entrance of the inclined rolls and with decreasing distance from the exit of the inclined rolls along the rolling line.

[0022]

The manufacturing method includes a first slant rolling step, a setting change step, and a second slant rolling step. In the inclined rolling step, the first heated billet is rolled by means of an inclined roll mill. In the setting change step, the setting state of the inclined roll mill is changed according to the order (a) or (b) as described later. In the second inclined rolling step, the second heated billet is rolled by the inclined roll mill in the changed state of the installation.

(a) When the second wall thickness is smaller than the first wall thickness, the rolling angle of each of the inclined rolls is set to be larger than the rolling angle of each of the inclined rolls set for the first inclined rolling step.

(b) When the second wall thickness is larger than the first wall thickness, the rolling angle of each of the inclined rolls is set to be smaller than the rolling angle of each of the inclined rolls set for the first inclined rolling step.

ADVANTAGES OF THE INVENTION

[0023]

The manufacturing method according to an embodiment of the present invention enables the manufacture of thin-walled seamless metal pipe and thick-walled seamless metal pipe without causing any quality problems by using a three-roll type inclined roll mill. Thus, the manufacturing method makes practical use of a three-roll inclined roll mill.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]

[Fig. 1] FIG. 1 is a schematic perspective view of a three roll type inclined roll mill.

[Fig. 2] FIG. 2 is a front view of a three roll type inclined roll mill.

[Fig. 3] FIG. 3 is a plan view of a three roll type inclined roll mill.

[Fig. 4] FIG. 4 is a side view of a three roll type inclined roll mill.

[Fig. 5] FIG. 5 is a schematic view showing an exemplary inclined roll with a convex inlet transverse surface.

[Fig. 6] FIG. 6 is a flow chart showing a method for manufacturing a seamless metal pipe according to an embodiment of the present invention.

[Fig. 7] FIG. 7 is a schematic view showing the change order (a) performed in the setting change step.

[Fig. 8] FIG. 8 is a schematic view showing the change order (b) performed in the setting change step.

[Fig. 9] FIG. 9 is an external view of a mandrel used for piercing testing.

[Fig. 10] FIG. 10 is an external view of an inclined roll used for piercing testing.

[Fig. 11] FIG. 11 is an external view of the inclined roll used for the piercing rolling test.

[Fig. 12] FIG. 12 is a graph showing the occurrence/non-occurrence of disturbances in Example 1.

[Fig. 13] FIG. 13 is a graph showing the occurrence/non-occurrence of violations in Example 3.

[Fig. 14] FIG. 14 is a graph showing the occurrence/non-occurrence of violations in Example 4.

[Fig. 15] FIG. 15 is an external view of the inclined roll used for the piercing rolling test.

[Fig. 16] FIG. 16 is a graph showing the occurrence/non-occurrence of disturbances in Example 5.

[Fig. 17] FIG. 17 is a graph showing the occurrence/non-occurrence of violations in Example 5.

[Fig. 18] FIG. 18 is an external view of an inclined roll used for piercing testing.

[Fig. 19] FIG. 19 is an external view of an inclined roll used for piercing testing.

[Fig. 20] FIG. 20 is a graph showing the occurrence/non-occurrence of disturbances in Example 6.

[Fig. 21] FIG. 21 is a graph showing the occurrence/non-occurrence of disturbances in Example 6.

[Fig. 22] FIG. 22 is a graph showing the engagement angle (α).

[Fig. 23] FIG. 23 is a graph showing the occurrence/non-occurrence of violations with respect to engagement angle (α) and pinch reduction ratio.

Detailed description of embodiments of the present invention

[0025]

To achieve the goal mentioned above, the authors carried out research and, as a result, the authors came to the following conclusions.

[0026]

[Fundamental Structure of Three Roll Type Inclined Roll Mill]

On FIG. 1-4 schematically show the structure of a three-roll inclined roll mill. Among these drawings, Fig. 1 is a perspective view of the inclined roll mill as seen from the exit side of the inclined roll mill. Fig. 2 is a front view of the inclined roll mill as seen along the rolling line PL from the inlet side of the rolling mill. Fig. 3 is a plan view of a rolling mill with inclined rolls. Fig. 4 is a side view of a tilt roll mill. On FIG. 1 does not show mandrel 2. FIG. 3 and 4 show only one inclined roll located above the rolling line PL as the uppermost roll 1, and the other lower two inclined rolls 1 are not shown. FIG. 4, the billet WP is shown in section taken along a plane including the rolling line PL. On FIG. 1-4 show the case in which the blank WP is a round solid blank. In other words, the inclined roll mill shown in FIG. 1-4 is a piercing rolling mill used for piercing rolling. In the present specification, the three-roll type inclined roll mill is sometimes referred to simply as the inclined roll mill.

[0027]

As seen in FIG. 1-4, the inclined roll mill includes a mandrel 2 and three inclined rolls 1 as rolling tools. Three inclined rolls 1 are arranged at regular angular intervals around the rolling line PL. In particular, the three inclined rolls 1 are arranged at an angle of 120° to each other. One of the three inclined rolls 1 is located directly above (in the vertical direction) the rolling line PL. As long as the three inclined rolls 1 are arranged at uniform angular intervals around the rolling line PL, there are no other restrictions on the positions of the three inclined rolls 1. For example, one of the inclined rolls 1 may be located immediately below (in the vertical direction) the rolling line PL. The lateral surface of each of the inclined rolls 1 is divided into an entry lateral surface 1a and an exit lateral surface 1b, which are arranged side by side along the rolling line PL.

[0028]

Each of the inclined rolls 1 has a central axis 1c inclined with respect to the rolling line PL. In other words, each of the inclined rolls 1 has an infeed angle FA (see Fig. 3). Each of the inclined rolls 1 has a rolling angle CA (see Fig. 4). The feeding angle FA and the rolling angle CA are adjustable. In addition, each of the inclined rolls 1 has a gap with respect to the rolling line PL. This roll gap is adjustable.

[0029]

The feed angle FA is the deviation angle of the central axis 1c of the inclined roll 1 from the rolling line PL in the circumferential direction around the rolling line PL. The rolling angle CA is the deviation angle of the central axis 1c of the inclined roll 1 from the rolling line PL in the radial direction from the rolling line PL.

[0030]

The distance between the rolling line PL and the inlet transverse surface 1a gradually decreases as the distance from the inclined roll inlet increases and the distance from the inclined roll outlet along the rolling line PL decreases. On the other hand, the distance between the rolling line PL and the exit lateral surface 1b gradually increases as the distance from the inlet increases and the distance from the outlet decreases along the rolling line PL. The entrance transverse surface 1a is, for example, a conical surface with a constant slope. The exit transverse surface 1b is, for example, a conical surface with a constant slope.

[0031]

The mandrel 2 is located on the rolling line PL between the inclined rolls 1. The mandrel 2 is held by a mandrel bar extending along the rolling line PL.

[0032]

Inclined rolling (piercing rolling) using such an inclined roll mill is carried out as follows. The blank WP, which is a round blank, is heated. The heated billet WP is placed on the rolling line PL. The workpiece WP is pushed by the pusher to be brought into position between the rotating inclined rolls 1 and into contact with the rotating inclined rolls 1. Then, the workpiece WP is moved forward along the rolling line PL while rotating about its axis, and the workpiece PL is pierced and rolled by inclined rolls 1 and mandrel 2. Thus, a hollow sleeve (seamless metal pipe) is produced with a given wall thickness and a given outer diameter.

[0033]

When the slant-roll mill is used as the extension mill, the slant rolling (length-rolling) process is the same as the piercing-rolling process, except that the workpiece subjected to extension rolling is a hollow tube. In particular, the workpiece moves forward while rotating around its axis, and the workpiece is elongated and rolled by inclined rolls 1 and mandrel 2.

[0034]

[Studying the Practical Use of Three Roll Type Inclined Roll Mill]

At first, the inventors focused on piercing rolling, which is one of the above-described types of inclined rolling (piercing rolling and elongation rolling) using a three-roll type inclined roll mill. They did this because the processing conditions for piercing rolling are much more severe than those for elongation rolling. The inventors have attempted to manufacture a seamless metal pipe with a small wall thickness, which is considered a weak point of piercing rolling, using a three-roll inclined roll mill (three-roll piercing mill).

[0035]

In the present specification, the manufacture of a seamless metal pipe with a small wall thickness is sometimes referred to as the manufacture of a thin-walled pipe.

[0036]

If the manufacture of a thin-walled pipe is performed by piercing rolling using a three-roll type inclined roll mill, the workpiece material is partially pushed between adjacent inclined rolls, and the workpiece stops rotating. For this reason, the manufacture of a thin-walled pipe is difficult.

[0037]

As a first countermeasure, the inventors have attempted to reduce the amount of wall thickness processing performed by the mandrel. The inventors have suggested that it is possible to reduce the amount of material being pushed between adjacent inclined rolls by reducing the amount of wall thickness processing. To do this, the authors used a decrease in the roll gap. By reducing the roll gap, the outside diameter of the workpiece is reduced before the workpiece reaches the tip of the mandrel, and thereafter, the outside diameter of the workpiece is increased. Thus, the amount of wall thickness processing performed by the mandrel can be reduced. For example, when two hollow sleeves with the same outer diameter and the same wall thickness are respectively made from two blanks with different cross-sectional areas (different diameters), the blank with a smaller cross-sectional area (smaller diameter) ends up requiring less processing.

[0038]

However, the inventors have found that if the roll gap is too small with respect to the outer diameter of the workpiece, the outer diameter of the hollow core made by piercing becomes non-uniform with respect to the lengthwise direction. In this case, the workpiece must be subjected to significant processing, which leads to an imbalance between the amount of movement of material at the entrance of the inclined rolls and the amount of movement of material at the exit of the inclined rolls. To balance the volume of material movement at the input of the inclined rolls and the volume of material movement at the output of the inclined rolls, it is necessary to reduce the speed of movement of the workpiece at the input. The amount of movement of material in a particular position is expressed as the result of multiplying the cross-sectional area in that position by the speed of movement of the workpiece in that position.

[0039]

Then the authors performed various experiments and numerical analyzes in search of a way to reduce the speed of movement of the workpiece at the entrance of the inclined rolls. As a result, the inventors have found that by increasing the angle of the inlet cross surface of each of the inclined rolls, the speed of movement of the workpiece at the inlet portion of the inclined rolls can be reduced, thereby suppressing fluctuation in the circumference of the workpiece.

[0040]

In the present description, the angle of the entrance transverse surface of the inclined roll is sometimes referred to as the angle of the entrance transverse surface. The angle of the entry transverse surface indicates, in a section including the rolling line, the largest angle between the rolling line and the entry transverse surface in the area in which the entry transverse surface is in contact with the workpiece.

[0041]

Then, as a second countermeasure, the inventors attempted to control the elongation of the billet in each direction during piercing rolling. This is done by increasing the angle of the exit lateral surface of each of the inclined rolls. By increasing the exit cross-surface angle of each of the inclined rolls, it becomes possible to shorten the length, with respect to the lengthwise direction of the workpiece, of the area in which the exit cross-surfaces of the inclined rolls are in contact with the workpiece, and thus the restraining force of the material in length becomes weak. Then the workpiece is elongated easier in the length direction, but more difficult in the circumferential direction. Accordingly, the propensity of the material to be pushed between adjacent inclined rolls is reduced.

[0042]

In the present specification, the angle of the exit lateral surface of the inclined roll is sometimes referred to as the angle of the exit lateral surface. The exit transverse surface angle indicates, in a section including the rolling line, the largest angle between the rolling line and the exit transverse surface in a region in which the exit transverse surface is in contact with the workpiece.

[0043]

By adopting the above-described countermeasures and methods, it becomes possible to manufacture a thin-walled pipe by using a three-roll type inclined-roll rolling mill, which was thought to be impossible.

[0044]

However, the countermeasures and methods described above are only suitable for the manufacture of thin-walled pipe. In the case of applying these measures and countermeasures to the manufacture of a thick-walled pipe, the following problem occurs: the part of the mandrel having the maximum diameter is clamped at the rear end of the manufactured hollow sleeve, and the mandrel cannot be removed from the hollow sleeve. This is a kind of rolling problem, and this problem is called a mandrel jam. Mandrel jam is a phenomenon that occurs when the circumference of the workpiece is too short. To avoid jamming of the mandrel, the circumference of the workpiece must be large enough, which is contrary to the condition required for the manufacture of thin-walled pipe.

[0045]

Thus, the steps taken to make a thick-walled pipe are the opposite of those taken to make a thin-walled pipe. In particular, in the manufacture of a thick-walled pipe, in order to ensure that the billet has a sufficiently large cross-sectional area of material at a position at the inlet side of the mandrel, the roll gap must be increased. In this case, in order to ensure that the driving force is applied to the workpiece, it is necessary to increase the length of the area in which the workpiece is in contact with the inclined rolls at the entrance of the inclined rolls. As a means for this, the angle of the inlet lateral surface of each of the inclined rolls is reduced. In addition, in order to move the material of the workpiece in the circumferential direction, the angle of the exit cross surface of the inclined rolls must be reduced to increase the restraining force of the material in length.

[0046]

In order to manufacture a thick-walled pipe and a thin-walled pipe using the same inclined roll mill, the inclined rolls differing from each other in shape and size must be used in the rolling mill in different cases. Therefore, the limits of the wall thickness to be produced can be widened in the manufacture of a thick-walled pipe and in the manufacture of a thin-walled pipe.

[0047]

However, if the inclined rolls are changed according to the wall thickness of the seamless metal pipe to be produced, the production efficiency inevitably decreases. Therefore, if possible, the exclusion of the change of inclined rolls is required.

[0048]

The inventors further investigated a method for eliminating the need to change inclined rolls. As a result, they have found that a possible means for this is to make the inlet lateral surface of each of the inclined rolls in the form of a convex surface. The entrance transverse surface of each of the inclined rolls is typically a conical surface with a constant slope. As a means for producing seamless metal pipes with different wall thicknesses using the same inclined rolls, the inventors proposed the idea of using inclined rolls each having a convex inlet transverse surface and adjusting the rolling angle CA of each of the inclined rolls.

[0049]

The following describes the structure of the rolling mill with inclined rolls of the three-roll type. The three-roll type inclined roll mill includes an inlet casing and an exit casing which support both ends of the center rolls of the respective inclined rolls. Each shroud or both shrouds is rotatable, and thus the feed angle FA of the inclined rolls can be adjusted. The inlet casing and the outlet casing are configured to adjust the inlet anchor points at which the inlet shell supports the central shafts of the respective inclined rolls at the inlet side, and the outlet anchor points at which the outlet casing supports the central shafts of the respective inclined rolls at the outlet side, respectively, independently of each other. from friend. By separately adjusting these reference points, the angles of the rolling of the inclined rolls are controlled. In addition, separate adjustment of the roll gap at the inlet side and the roll gap at the exit side is provided. However, both ends of the central shafts of the respective inclined rolls may support one shroud.

[0050]

On FIG. 5 schematically shows an exemplary inclined roll 1 with a convex inlet transverse surface 1a. As seen in FIG. 5, at the boundary between the inlet transverse surface 1a and the outlet transverse surface 1b, there is a nip G. The inlet transverse surface 1a of the inclined roll 1 is not a simple cone surface, that is, not a cone surface with a constant slope, but a convex surface. In the present description, the convex surface means a conical surface, the slope of which is continuously changing, a cone surface, the slope of which is discontinuously changing, or a cone surface, which is a combination of them. For example, in the view of the input transverse surface 1a of the inclined roll 1 in the section taken along the Central axis 1c, visible convex curved line. This convexly curved line is expressed, for example, by means of a function expressing a circular arc with a constant radius of curvature. This curved line can be expressed in terms of a higher order polynomial function. Alternatively, in a sectional view of the inlet transverse surface 1a of the inclined roll 1 taken along the central axis 1c, a combination of a convex curved line and a straight line may be seen, or a combination of a plurality of straight lines with different slopes may be seen. On the other hand, the exit lateral surface 1b of the inclined roll 1 is a tapered surface with a constant slope.

[0051]

Next, a method for manufacturing a thin-walled pipe and manufacturing a thick-walled pipe by piercing rolling using inclined rolls with convex inlet transverse surfaces will be described.

[0052]

To produce a thin-walled pipe, the angles CA of the rolling of the inclined rolls are increased. In addition, the roll gap is reduced. Thus, inclined rolls are installed closer to the rolling line. As a result, the point of contact at which the billet comes into contact with the inclined rolls is shifted towards the inlet along the rolling line. Accordingly, the workpiece comes into contact with the inlet lateral surface of the inclined rolls at a position where the inlet lateral surfaces of the inclined rolls have a large slope. This regulation is also advantageous when a workpiece having a large diameter is to be greatly reduced.

[0053]

By increasing the rolling angles CA of the inclined rolls, the angles of the exit transverse surface of the inclined rolls are increased. However, the angles of the inlet transverse surface of the inclined rolls remain virtually unchanged. This is due to the fact that the input transverse surfaces of the inclined rolls are convex surfaces. In this case, if the entrance cross surfaces of the inclined rolls are simply tapered surfaces and have a completely constant slope, the lengths of the body of the entry transverse surfaces must be preliminarily increased. As the roll angles CA of the inclined rolls increase, the angles of the inlet transverse surface of the inclined rolls decrease, and it is necessary to increase the roll gap at the inlet ends of the inlet transverse surfaces to a level greater than or equal to the diameter of the workpiece. However, when the input transverse surfaces are convex surfaces, this is not necessary. In addition, even if the diameter of the workpiece is large, excessive squeezing of the material does not occur because the inlet lateral surfaces of the inclined rolls are convex surfaces, and the workpiece is machined to obtain a constant circumference along its entire length. When changing the rolling angles CA, the mandrel can be changed.

[0054]

The roll gap adjustment may be performed after adjusting the skew roll angles CA or before adjusting the skew roll angles CA. The roll gap adjustment and the tilt roll angle adjustment CA can be repeated for fine adjustment.

[0055]

For the production of thick-walled pipe, the regulation is carried out in the opposite way to the regulation for the production of thin-walled pipe. In particular, firstly, the rolling angles CA of the inclined rolls are reduced. Then the roll gap increases. In this way, the inclined rolls are placed farther away from the rolling line. As a result, the contact start point at which the billet comes into contact with the inclined rolls is shifted towards the exit along the rolling line. Accordingly, the workpiece comes into contact with the inclined rolls at a position in which the entry lateral surfaces of the inclined rolls have slight slopes.

[0056]

By reducing the rolling angles CA of the inclined rolls before adjusting the roll gap, the angles of the outward transverse surface of the inclined rolls are reduced. In this case, if the inlet lateral surfaces of the inclined rolls are simply tapered surfaces and have a completely constant slope, as the inclination roll forming angles CA decrease, the angles of the inlet lateral surface increase. As a result, the length of the contact area in which the workpiece is in contact with the inlet lateral surfaces of the inclined rolls becomes shorter, and the workpiece cannot come into stable contact with the inclined rolls. However, when the entry transverse surfaces are convex surfaces, the angle of the entry transverse surface hardly changes, and there is no disturbance in bringing the workpiece into contact with the inclined rolls. When changing the rolling angles CA, the mandrel can be changed.

[0057]

As described above, when the inlet lateral surfaces of the inclined rolls are convex surfaces, it becomes possible to manufacture seamless metal pipes with different wall thicknesses by using the same inclined rolls only by changing the installation states of the inclined rolls. During the manufacturing process, even when the workpiece has a large diameter, fluctuation of the circumference of the material is prevented, and there is no rolling problem such as pushing of the material, jamming of the mandrel, and so on.

[0058]

By adopting the above-described appropriate measures and countermeasures, it is possible to produce a seamless metal pipe, whether it is a thick-walled metal pipe or a thin-walled metal pipe, by inclined rolling (piercing rolling or elongation rolling) using a three-roll type inclined roll mill. . The degree of wall thickness of a seamless metal pipe is expressed by the ratio of wall thickness and outer diameter. This ratio is also referred to as the ratio of wall thickness to outside diameter. The small value of the ratio of wall thickness to outer diameter means that the degree of wall thickness of the seamless metal pipe is small, and this means that the metal pipe has a thin wall. It has been difficult to manufacture a thin-walled seamless metal pipe with a ratio of wall thickness to outer diameter of 0.07 or less by conventional piercing rolling using a three-roll type inclined roll mill. By adopting the above-described countermeasures and methods, it becomes possible to manufacture not only a thick-walled seamless metal pipe, but also a thin-walled seamless metal pipe with a ratio of wall thickness to outer diameter of 0.07 or less.

[0059]

By adopting the countermeasures and methods described above, it becomes possible to perform piercing rolling for a high degree of processing by using a round blank with a large outer diameter as a workpiece. It then becomes possible to reduce the blanks to round blanks with a large outer diameter, resulting in the advantageous effect of greatly reducing the cost of steel production and blooming rolling.

[0060]

A method for manufacturing a seamless metal pipe according to the present invention has been developed based on the findings described above.

[0061]

In the method for manufacturing a seamless metal pipe according to the present invention, a first seamless metal pipe with a first wall thickness and a second seamless metal pipe with a second wall thickness that is different from the first wall thickness are produced by using an inclined roll mill. The inclined roll mill includes a mandrel and three inclined rolls. The mandrel is located on the rolling line. Three inclined rolls are arranged at regular angular intervals around the rolling line, and each of the three inclined rolls has an inlet transverse surface and an outlet transverse surface. The distance between the rolling line and the inlet transverse surface gradually decreases with increasing distance from the inlet and with decreasing distance from the outlet along the rolling line. The distance between the rolling line and the exit transverse surface gradually increases as the distance from the inlet increases and as the distance from the exit decreases along the rolling line.

[0062]

The above-mentioned manufacturing method includes a first oblique rolling step, a setting change step, and a second oblique rolling step. In the first slant rolling step, the first heated billet is rolled by means of a slant roll mill. In the setting change step, the setting state of the inclined roll mill is changed according to the order (a) or (b) as described later. In the second step of oblique rolling, the second heated billet is rolled in the changed state. (The structure of a manufacturing process having these features is called the first process structure.)

(a) When the second wall thickness is smaller than the first wall thickness, the rolling angle of each of the inclined rolls is set to a larger value than the rolling angle of each of the inclined rolls set for the first oblique rolling step.

(b) When the second wall thickness is larger than the first wall thickness, the rolling angle of each of the inclined rolls is set to a smaller value than the rolling angle of each of the inclined rolls set for the first inclined rolling step.

[0063]

The manufacturing method with the first process structure can produce both a thin-walled seamless metal pipe and a thick-walled seamless metal pipe without causing any quality problem by using a three-roll type inclined roll mill. Thus, a three-roll type inclined roll mill can be brought into practical use.

[0064]

The first wall thickness and the second wall thickness are target wall thicknesses, and the actual wall thicknesses obtained after oblique rolling may differ slightly from the target values indicated as the first and second wall thicknesses.

[0065]

In a typical example, the slant mill is a piercing mill. In this case, the first oblique rolling step and the second oblique rolling step are piercing rolling steps. The first workpiece and the second workpiece are round solid ingots.

[0066]

In another typical example, the slant mill is an extension mill. In this case, the first bevel rolling step and the second bevel rolling step are elongation rolling steps. The first blank and the second blank are round hollow shells.

[0067]

In the first slant rolling step and the second slant rolling step of the manufacturing method with the first process structure, the exit cross surface angle in the contact region in which the exit cross surface is in contact with the workpiece (exit cross surface angle) is preferably greater than or equal to 0°, and less than or equal to 9°. In the first bevel 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 exit cross surface angle is set to be greater than 3°. In the same way, in the second bevel rolling step, when the ratio of the second wall thickness to the outer diameter of the second seamless metal pipe is less than or equal to 0.07, the exit cross surface angle is set to be greater than 3°. (The structure of the manufacturing method, additionally having this feature, is called the second structure of the process.)

[0068]

The manufacturing method with the second process structure is advantageous when the first seamless metal pipe is a thin-walled seamless metal pipe or when the second seamless metal pipe is a thin-walled seamless metal pipe. Only if the exit cross surface angle is set in the above-mentioned range of values, it is possible to produce a thin-walled metal pipe as the first seamless metal pipe or the second seamless metal pipe without generating any quality problems. From the point of view of occurrence of this effect, the angle of the exit cross surface is preferably larger than 3°. Any upper limit for the angle of the outlet transverse surface is not specifically set. However, considering the design of the mandrel, the upper limit for the exit lateral surface angle is preferably 9°, and more preferably 6°.

[0069]

In the manufacturing method with the first process structure or the second process structure, preferably, the inlet transverse surface is a convex surface, and the inclined rolls used in the first oblique rolling step are also used in the second oblique rolling step. (The structure of the manufacturing method, additionally having these features, is called the third structure of the process.)

[0070]

In the manufacturing method with the third process structure, it is not necessary to change the inclined rolls for the first inclined rolling step and the second inclined rolling step. Thus, it becomes possible to manufacture the first seamless metal pipe and the second seamless metal pipe, which have different wall thicknesses, by using the same inclined roll mill without changing the inclined rolls. Accordingly, excellent manufacturing efficiency is ensured.

[0071]

In the manufacturing method with the first process structure or with the second process structure, for example, the first blank and the second blank are solid. In other words, the first bevel rolling step and the second bevel rolling step may be piercing rolling steps. In this case, in the first bevel rolling step and the second bevel rolling step, when the target is the piercing ratio of 3.5 or more, it is preferable that the inlet cross surface angle is set to be greater than or equal to 8° and less than or equal to 15°, and pinch reduction ratio is set to 30% or more. (The structure of a manufacturing method additionally having these features is called the fourth process structure.)

[0072]

The manufacturing method with the fourth process structure is advantageous when the first bevel rolling step and the second bevel rolling step are piercing rolling steps. When piercing rolling with a high degree of processing, such as piercing rolling in which a piercing ratio of 3.5 or more is targeted, is performed with the inlet cross surface angle and the pinch reduction ratio set in the above ranges, it is possible to manufacture the first seamless metal pipe and a second seamless metal pipe without any quality problems. From the point of view of occurrence of this effect, the angle of the inlet transverse surface is preferably greater than or equal to 8° and less than or equal to 15°. From the same point of view, the nip reduction ratio is preferably 30% or more. There is no specific upper limit for the pinch reduction ratio. However, if the nip reduction ratio is large and if the cross-sectional area of the material is small, the reaction force acting on the inclined rolls becomes large. In order to achieve sufficient bearing capacity to support the shafts of the inclined rolls, the shafts of the inclined rolls must be large enough, and accordingly the diameters of the rolls must be large enough. When the number of inclined rolls is three or more, the inclined rolls may come into contact with each other, and hence the upper limit for the nip reduction ratio is preferably 60%.

[0073]

In the present specification, the piercing ratio refers to the ratio of the length of the seamless metal pipe obtained by piercing rolling to the length of the billet before piercing rolling. Specifically, the first bevel rolling piercing ratio means the ratio of the length of the first seamless metal pipe to the length of the first billet. The piercing ratio in the second bevel rolling indicates the ratio of the length of the second seamless metal pipe to the length of the second billet. From another point of view, the piercing ratio means the ratio of the cross-sectional area of the workpiece before piercing rolling to the cross-sectional area of the seamless metal pipe obtained by piercing rolling. Thus, the piercing ratio is an indication of the degree of processing by piercing rolling. When the oblique rolling is elongation rolling, the piercing ratio is called the elongation ratio.

[0074]

In the present description, the pinch reduction ratio (GD) [%] is defined by Formula (A) below, which uses the outer diameter (DB) of a round blank used as a blank and the clearance (RO) of a roll as parameters.

GD=(DB - RO)/DB × 100 (A)

[0075]

In Formula (A), roll gap (RO) denotes the degree of roll gap in the pinch region of the roll, that is, at the boundary between the inlet lateral surface and the exit lateral surface of the roll. More precisely, the gap (RO) of the roll is a value that is equal to two shortest distances between the surface (eg, the entrance transverse surface) of the roll and the rolling line.

[0076]

In the manufacturing method with any of the first to third process structures, for example, the first blank and the second blank are solid. In other words, the first bevel rolling step and the second bevel rolling step may be piercing rolling steps. In this case, preferably, the inlet transverse surface is a convex surface, and in a section of the inclined roll including the central axis of the inclined roll, a circular arc is visible as a line forming the inlet transverse surface. The value calculated by dividing the radius of curvature of the circular arc by the outer diameter of the first workpiece or the second workpiece is greater than or equal to 1.67 and less than or equal to 6.67. (The structure of the manufacturing method, additionally having this feature, is called the fifth structure of the process).

[0077]

The manufacturing method with the fifth process structure is advantageous when the inclined roll mill is a piercing rolling mill and when the first inclined rolling step and the second inclined rolling step are piercing rolling steps. In the present description, when the entrance transverse surface is a convex surface, and when the circular arc is seen as a line forming the entrance transverse surface in a section of the inclined roll along the central axis of the inclined roll, the value calculated by dividing the radius of curvature of the circular arc by the outer diameter of the circular blank, used as a workpiece, sometimes referred to as the curved surface index.

[0078]

In the first bevel rolling step and the second bevel rolling step, if the curved surface index of the inlet transverse surface with respect to the first billet and the curved surface index of the inlet transverse surface with respect to the second billet are in the above range, it is possible to produce the first seamless metal pipe and the second seamless metal pipe without any quality problems. From the viewpoint of occurrence of this effect, the curved surface index of the inlet transverse surface is preferably greater than or equal to 1.67 and less than or equal to 6.67. However, if the radius of curvature from which the curved surface index of the inlet transverse surface is obtained is small, the material is rolled over a short contact area and the inclined roll surface is subject to noticeable abrasion. If the radius of curvature is large, the length of the body of the entrance transverse surface of each of the inclined rolls must be increased to ensure that the workpiece can be engaged with the inclined rolls, thereby increasing equipment costs and manufacturing costs of the inclined rolls. Therefore, when a round blank with an outer diameter of 60 mm is used, the radius of curvature is preferably greater than or equal to 150 mm and less than or equal to 350 mm. In this case, the preferred range for the curved surface index is calculated to be greater than or equal to 2.50 and less than or equal to 5.83.

[0079]

In the manufacturing method with any of the first to third process structures, for example, the first blank and the second blank are solid. In other words, the first bevel rolling step and the second bevel rolling step may be piercing rolling steps. In this case, preferably, in the first bevel rolling step and the second bevel rolling step, the pinch reduction ratio (GD) and the engagement angle (α) satisfy the condition expressed by Formula (1) below. (The structure of the manufacturing method further having this feature is called the sixth process structure.)

0.12×GD+1.5≤α≤0.25×GD+6 (1)

[0080]

The pinch reduction ratio (GD) in Formula (1) is determined by Formula (A) above. The engagement angle (α) is defined as follows. The plane including both the central axis of the inclined roll and the rolling line is based on the assumption that the infeed angle FA of the inclined roll is 0°. In the plane, a line is drawn connecting the point of contact, at which the workpiece (round blank) comes into contact with the inclined roll, and the pinch point. The start point of contact at which the workpiece comes into contact with the inclined roll corresponds to the engagement point at which the workpiece engages with the inlet transverse surface of the inclined roll. The angle between the line and the rolling line is the engagement angle (α).

[0081]

The manufacturing method with the sixth process structure is advantageous when the inclined roll mill is a piercing rolling mill and when the first inclined rolling step and the second inclined rolling step are piercing rolling steps. When the pinch reduction ratio (GD) and the engagement angle (α) satisfy the conditions expressed by Formula (1), it is possible to manufacture the first seamless metal pipe and the second seamless metal pipe without generating any quality problems. To obtain this effect, as shown in Formula (1), the engagement angle (α) is preferably greater than or equal to "0.12×GD+1.5" and less than or equal to "0.25×GD+6". From another point of view, the slope of the inlet transverse surface is preferably set to increase with decreasing distance from the inlet end of the inlet transverse surface, whereby the nip reduction ratio (GD) and engagement angle (α) can satisfy the conditions expressed by Formula (1) .

[0082]

Next, with reference to the drawings, specific examples of the manufacturing method according to the present invention will be described. Fig. 6 is a block diagram showing a method for manufacturing a seamless metal pipe according to the present invention. As seen in FIG. 6, the manufacturing method according to the present invention includes a first bevel rolling step (#5), a setting change step (#10), and a second bevel rolling step (#15).

[0083]

[First Stage (#5) Slant Rolling]

In the first oblique rolling step (#5), the first billet is rolled by using a three-roll type inclined roll mill to manufacture a first seamless metal pipe with a first wall thickness. Before this, the first billet is heated to a predetermined temperature in a heating furnace. The workpiece is heated to a temperature, for example, lying in the range of 1150-1250°C.

[0084]

The first blank is a round blank. In this case, the inclined roll mill is a piercing mill, and the round bar is pierced and rolled by the piercing mill. However, the first blank may be a hollow sleeve. The hollow sleeve may be a hollow sleeve made by piercing or may be made by any other method. In this case, the inclined roll mill is an extension mill, and the hollow sleeve is elongated and rolled by the extension mill.

[0085]

[Second Stage (#15) Slant Rolling]

In the second bevel rolling step (#15), the second billet is rolled by a three-roll type inclined roll mill to produce a second seamless metal pipe with a second wall thickness different from the first wall thickness. As in the case of the first bevel rolling step, before the second bevel rolling step, the second billet is heated to a predetermined temperature.

[0086]

In the second inclined rolling stage, the same inclined roll mill used in the first inclined rolling stage is used. The inclined roll mill used in the first inclined rolling step and the second inclined rolling step includes inclined rolls whose entry lateral surfaces are convex surfaces, as seen in FIG. 5. However, the entry lateral surfaces of the inclined rolls may be tapered surfaces with a constant slope, as seen in FIG. 1-4. The exit cross surfaces of the inclined rolls are tapered surfaces with a constant slope, as seen in FIG. 1-5. The distance between the rolling line and the inlet transverse surface of each of the inclined rolls gradually decreases with increasing distance from the inlet and decreasing distance from the outlet along the rolling line. The distance between the rolling line and the exit cross surface of each of the inclined rolls gradually increases with increasing distance from the inlet and decreasing distance from the exit along the rolling line.

[0087]

If the first workpiece is a round blank, then the second workpiece is a round blank. If the first blank is a hollow sleeve, then the second blank is also a hollow sleeve. When the second billet is a round blank, the round blank is pierced and rolled by a piercing rolling mill. When the second billet is a hollow sleeve, the hollow sleeve is elongated and rolled by an extension rolling mill.

[0088]

The shape and dimensions of the second blank are the same as the shape and dimensions of the first blank. However, the shape and dimensions of the second blank may differ from the shape and dimensions of the first blank. The material of the second workpiece is the same as the material of the first workpiece. However, the material of the second workpiece may be different from the material of the first workpiece.

[0089]

[Stage (#10) Setting Changes]

The setting change step (#10) is performed after the first bevel rolling step (#5) and before the second bevel rolling step (#15) to change the setting state of the inclined roll mill according to the order (a) or (b) as described later. Thus, in the setting change step (#10), based on the setting state of the inclined roll mill for the first step of inclined rolling, the setting state of the inclined roll mill is changed to the state corresponding to the second step of inclined rolling.

(a) When the second wall thickness is smaller than the first wall thickness, the rolling angle of each of the inclined rolls is set to be larger than the rolling angle set for the first inclined rolling step.

(b) When the second wall thickness is larger than the first wall thickness, the rolling angle of each of the inclined rolls is set to be smaller than the rolling angle set for the first inclined rolling step.

[0090]

On FIG. 7 and 8 schematically show specific examples of the setting change step. On FIG. 7 shows the order of (a) change. On FIG. 8 shows the order (b) of the change. On FIG. 7 and 8 show a case in which the inlet lateral surface 1a of each of the inclined rolls 1 is a convex surface.

[0091]

As seen in FIG. 7 and 8, the inlet lateral surface 1a of the inclined roll 1 is a convex surface. When looking at the input transverse surface 1a in a section taken along the central axis 1c of the inclined roll 1, a convexly curved line is visible. This convexly curved line can be expressed as a function that defines a circular curve with a constant radius of curvature. On the other hand, the exit lateral surface 1b of the inclined roll 1 is a tapered surface.

[0092]

The distance between the rolling line PL and the inlet lateral surface 1a of the inclined roll 1 gradually decreases as the distance from the inlet increases and the distance from the outlet decreases along the rolling line PL. On the other hand, the distance between the rolling line PL and the exit lateral surface 1b of the inclined roll 1 gradually increases with increasing distance from the inlet and decreasing the distance from the outlet along the rolling line PL.

[0093]

[Order (a) Changes]

Order (a) applies when the second wall thickness is less than the first wall thickness. From another point of view, order (a) is applied when the manufacture of a thin-walled seamless metal pipe is required in the second step of oblique rolling. In this case, the rolling angle CA of each of the inclined rolls 1 is set to be larger than the rolling angle CA set for the first bevel rolling step. Thus, the exit cross surface angle Ɵb of each of the inclined rolls 1 is increased.

[0094]

By changing the installation state of the inclined roll mill according to the order (a), in the second step of inclined rolling to produce the second seamless metal pipe, excessive pressure does not occur, and the second billet WP is machined with a constant circumference along its entire length. Thus, it becomes possible to manufacture a thin-walled seamless metal pipe without causing any quality problems.

[0095]

[Order (b) Changes]

Order (b) applies when the second wall thickness is greater than the first wall thickness. From another point of view, order (b) is applied when the manufacture of a thick-walled seamless metal pipe is required in the second step of oblique rolling. In this case, the rolling angle CA of each of the inclined rolls 1 is set to be smaller than the rolling angle CA set for the first bevel rolling step. Thus, the exit cross surface angle Ɵb of each of the inclined rolls 1 is reduced.

[0096]

By changing the setting state of the inclined roll mill according to the order (b), no misengagement with the inclined rolls occurs in the second inclined rolling step to produce the second seamless metal pipe. Thus, it becomes possible to manufacture a thick-walled seamless metal pipe without generating any quality problems.

[0097]

Also, when the first blank and the second blank have the same shape and dimensions, it is advantageous from the following point of view. The installation state of the equipment (eg, conveyor system) located upstream of the inclined roll mill may be the same for the first inclined rolling step and the second inclined rolling step. Accordingly, the manufacturing efficiency is excellent.

[Example 1]

[0098]

In Example 1, a piercing rolling test was carried out. In the piercing rolling test, a carbon steel round bar was used as a rolling blank, and a carbon steel round bar was pierced and rolled into a hollow tube (seamless metal tube) for processing. As possible components for the inclined roll mill, a plurality of mandrels with different sizes and shapes (Mandrels Nos. A-F) have been prepared. A variety of inclined swaths with different sizes and shapes are also prepared (Rolls No. R60-R600 and O-Z).

[0099]

Fig. 9 is an external view of a mandrel that was used for piercing testing. As seen in FIG. 9, each of the mandrels 2 of Mandrels Nos. A to F is shaped like an ordinary artillery shell. Table 1 below shows the dimensions of the mandrels of 2 Mandrels Nos. A-F. In Table 1, "L" denotes the length from the tip to the point of maximum diameter along the axis of mandrel 2 as seen in FIG. 9. In Table 1, "D" denotes the maximum diameter of the mandrel body 2 as seen in FIG. 9.

[0100]

[Table 1]

TABLE 1

Mandrel 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

[0101]

Fig. 10 and 11 are external views of the inclined rolls that were used for the piercing test. On FIG. 10 shows inclined rolls 1 Rolls No. R60-R600. On FIG. 11 shows inclined rolls 1 Rolls No. O-Z.

[0102]

As seen in FIG. 10, the lateral surface of each inclined roll 1 of Roll Nos. R60-R600 1 is divided into an entry lateral surface 1a and an exit lateral surface 1b by a nip G. The entry lateral surface 1a is a convex surface. When looking at the input transverse surface 1a in a section taken along the central axis 1c of the inclined roll 1, a convex curved line is visible. This convexly curved line is an arc with a constant radius (RG) of curvature. The exit transverse surface 1b is a tapered surface. Inclined rolls 1 Rolls no. R60-R600 have the same total length in the axial direction. Inclined rolls 1 Rolls Nos. R60-R600 have the same length of the exit lateral surface 1b in the axial length direction. Inclined rolls 1 Rolls No. R60-R600 have different lengths of the inlet transverse surface 1a in the axial direction, depending on the radius of curvature of the arc forming the convex surface (RG). Therefore, an auxiliary cylinder 1aa is attached to the inlet end of the inlet transverse surface 1a if necessary.

[0103]

Next, Table 2 shows the dimensions of Roll Nos. R60-R600, which are used as inclined rolls 1. FIG. 10 shows the dimensions that are common to Roll Nos. R60-R600 of the inclined rolls 1. In Table 2, "RG" denotes the radius of curvature of the arc forming the convex surface of the inlet transverse surface 1a as seen in FIG. 10. In Table 2, "H" indicates the length of the auxiliary cylinder 1aa in the axial direction, as seen in FIG. 10.

[0104]

[Table 2]

TABLE 2

Roll 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

[0105]

As seen in FIG. 11, the lateral surface of each inclined roll 1 of Roll Nos. O-Z is divided into an entry lateral surface 1a and an exit lateral surface 1b by a nip G. The entry lateral surface 1a is a tapered surface. The exit transverse surface 1b is a tapered surface. The inclined rolls 1 Rolls No. O-Z have the same length of the inlet transverse surface 1a in the axial direction. Inclined rolls 1 Rolls No. O-Z have the same length of the exit transverse surface 1b in the axial direction. Accordingly, the inclined rolls 1 Rolls No. O-Z have the same overall length. The total length of the inclined rolls 1 Rolls No. O-Z is the same as the total length of the inclined rolls 1 Rolls No. R60-R600.

[0106]

Table 3 below shows the dimensions of Rolls No. O-Z. On FIG. 11 shows the dimensions that are common to Rolls No. O-Z used as inclined rolls 1. In Table 3, "αa" denotes the slope of the inlet transverse surface 1a as seen in FIG. eleven.

[0107]

[Table 3]

TABLE 3

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

[0108]

In the piercing test in Example 1, mandrels of Mandrel Nos. A to D and inclined rolls of Roll Nos. O, P, R, and S were used in various combinations for piercing rolling. In the test, the round billet was heated to 1200° C. The infeed angle FA of each of the inclined rolls was 10°. Rolling angle CA and roll gap are changed.

[0109]

An attempt was made to manufacture hollow sleeves (seamless metal pipes) having the same outer diameter and different wall thicknesses. This is done for the following reason. In practice, hollow cores made by piercing are transferred to an extension mill and, in most cases, the hollow cores must have the same outside diameter.

[0110]

Table 4 below shows the test conditions and test results in Example 1.

[0111]

[Table 4]

TABLE 4

State Roll Pinch reduction ratio [%] Mandrel blank Seamless Metal Pipe Firmware factor Rolling problem Grade No. Rolling angle [°] Surface angle [°] Gap tip [mm] No. Diameter [mm] Length [mm] Outer diameter [mm] Outer diameter D [mm] Wall thickness t [mm] t/D At the entrance At the exit 1 O -3.0 4.0 0.0 47.0 14.5 14.0 A 33.0 60 55 50.0 7.3 0.146 2.43

2 O -3.0 4.0 0.0 41.0 18.0 14.0 A 33.0 60 50 50.0 4.3 0.086 3.18 push through 3 P 0.0 4.0 3.0 36.0 10.0 14.0 B 42.0 125 40 52.2 4.5 0.086 1.86

4 P 0.0 4.0 3.0 35.0 12.5 14.0 B 42.0 125 40 52.3 4.0 0.076 2.07

5 P 0.0 4.0 3.0 34.0 15.0 14.0 B 42.0 125 40 51.4 3.5 0.068 2.39 push through 6 R 3.0 4.0 6.0 34.0 15.0 14.0 C 42.0 81 40 51.2 3.5 0.068 2.40

7 R 3.0 4.0 6.0 32.8 18.0 14.0 C 42.0 81 40 51.2 2.9 0.057 2.86

8 R 3.0 4.0 6.0 32.5 18.8 14.0 C 42.0 81 40 51.2 2.7 0.053 3.05 push through 9 S 6.0 4.0 9.0 32.6 18.5 14.0 D 42.0 60 40 51.4 2.8 0.054 2.94

10 S 6.0 4.0 9.0 32.2 19.5 14.0 D 42.0 60 40 51.2 2.7 0.053 3.05 push through eleven S 6.0 4.0 9.30 32.4 19.0 14.0 D 42.0 60 40 51.2 3.4 0.066 2.46

12 S 6.0 4.0 9.0 35.0 12.5 14.0 D 42.0 60 40 51.3 4.0 0.078 2.11 Mandrel jam 13 R 3.0 4.0 6.0 35.0 12.5 14.0 C 42.0 81 40 51.2 4.1 0.080 2.07 Mandrel jam 14 P 0.0 4.0 3.0 37.0 7.5 14.0 B 42.0 125 40 52.8 5.1 0.097 1.64

15 P 0.0 4.0 3.0 37.0 7.5 10.0 B 42.0 125 40 52.0 5.4 0.104 1.59 Mandrel jam 16 O -3.0 4.0 0.0 48.7 11.5 10.0 A 33.0 60 55 50.5 8.2 0.162 2.18

" в столбце «Оценка» означает превосходно. (Note)"

" in the Grade column means excellent.

[0112]

First, in State 1, in which the inclined rolls of Roll No. O and the mandrel of Mandrel No. A are used, piercing rolling is performed. Then, in State 2, in which the roll gap is smaller than State 1, piercing rolling was carried out. State 2 is provided for manufacturing a seamless metal pipe with a target wall thickness less than the target wall thickness in State 1. However, during piercing performed in State 2, material push occurs. Therefore, the inclined rolls are replaced by the inclined rolls of Roll No. P, the exit lateral surface angle is changed from 0° to 3°, and the mandrel is replaced with the Mandrel No. B mandrel. In this state (State 3), piercing rolling is performed. During the piercing rolling in State 3, material pushing or any other problem does not occur, and the production of a seamless metal pipe with a wall thickness less than that obtained in State 1 is ensured.

[0113]

In State 4 and State 5, the roll gap is set smaller than in State 3, and piercing rolling is performed to produce a seamless metal pipe with a smaller wall thickness. By piercing in State 4, a seamless metal pipe with a wall thickness smaller than the wall thickness obtained in State 3 is obtained without problems. On the other hand, during piercing in State 5, pushing of material occurs. Therefore, the inclined rolls are replaced by the inclined rolls of Roll No. R, the exit cross-surface angle is changed to 6°, and the mandrel is replaced with the Mandrel No. C mandrel. In this state (State 6), piercing rolling is performed. By piercing in State 6, a seamless metal pipe with a wall thickness less than the wall thickness obtained in State 4 is provided.

[0114]

In State 7 and State 8, the roll gap is set smaller than in State 6, and piercing rolling is performed to produce a seamless metal pipe with a smaller wall thickness. Through the piercing rolling in State 7, a seamless metal pipe with a wall thickness less than the wall thickness obtained in State 6 is obtained. On the other hand, during piercing rolling in State 8, material pushing occurs. Therefore, the inclined rolls are replaced with the inclined rolls of Roll No. S, the exit cross-surface angle is changed to 9°, and the mandrel is replaced with the Mandrel No. D mandrel. In this state (State 9), piercing rolling is performed. By piercing in State 9, a seamless metal pipe with a wall thickness less than the wall thickness obtained in State 7 is provided. in State 10, material push-through occurs.

[0115]

Then, in State 11 and State 12, the roll gap is set larger than in State 9, and piercing rolling is performed to produce a seamless metal pipe with a larger wall thickness. A seamless metal pipe with a wall thickness larger than the wall thickness obtained in State 9 was drilled through the piercing rolling in State 11. During the piercing rolling in State 12, a mandrel jam occurs. Therefore, the inclined rolls are replaced by the inclined rolls of Roll No. R, the exit lateral surface angle is changed from 9° to 6°, and the mandrel is replaced with the Mandrel No. C mandrel. In this state (State 13), piercing rolling is performed. However, State 13 cannot solve the jammed mandrel problem.

[0116]

Therefore, the inclined rolls are replaced by the inclined rolls of Roll No. P, the exit cross-surface angle is reduced, and the mandrel is replaced with a Mandrel No. B. In this state (State 14), piercing rolling is performed. By piercing rolling in State 14, a seamless metal pipe with a wall thickness greater than the wall thickness obtained in State 11 is obtained. In State 15, in which the tip of the mandrel is reduced, piercing rolling is carried out to produce a seamless metal pipe with a larger wall thickness. During piercing rolling in State 15, a mandrel jam occurs. Therefore, the inclined rolls are replaced by the inclined rolls of Roll No. O, the exit cross-surface angle is reduced, and the mandrel is replaced with the Mandrel No. A mandrel. In this state (State 16), piercing rolling is performed. By piercing rolling in State 16, a seamless metal pipe with a wall thickness greater than the wall thickness obtained in State 14 is obtained.

[0117]

From the results described above, the following conclusions can be drawn. The following discussion relates to a process for manufacturing a first seamless metal pipe with a first wall thickness by piercing rolling, and thereafter manufacturing a second seamless metal pipe with a second wall thickness different from the first wall thickness by piercing rolling. When the second wall thickness of the second seamless metal pipe is smaller than the first wall thickness, it is possible to produce the second thin-walled seamless metal pipe without causing quality problems by increasing the exit cross surface angle of each of the inclined rolls. The exit cross surface angle can be increased by increasing the rolling angle of each of the inclined rolls. On the other hand, when the second wall thickness is larger than the first wall thickness, it is possible to produce a second thick-walled seamless metal pipe without causing quality problems by reducing the exit cross-surface angle of each of the inclined rolls. The exit cross surface angle can be reduced by decreasing the rolling angle of each of the inclined rolls. Thus, by the manufacturing method with the first process structure described above, it is possible to manufacture a seamless metal pipe, whether it has a small wall thickness or a large wall thickness, without generating any quality problems.

[0118]

It is determined in which of the States 1-16 any violation occurs (material push or mandrel jam) during piercing rolling, and in which it does not occur. The determination is made with respect to the angle of the exit transverse surface of each of the inclined rolls and the ratio of the wall thickness to the outer diameter of the seamless metal pipe. Fig. 12 is a graph showing the occurrence/non-occurrence of violations in Example 1. In the graph of FIG. 12, the horizontal axis shows the exit cross surface angle [˚] of each of the inclined rolls, and the vertical axis shows the ratio (t/d) of the wall thickness to the outside diameter [dimensionless] of the seamless metal pipe. On FIG. 12, "○" indicates no occurrence of a violation, and "×" indicates an occurrence of a violation. On FIG. 12, "A", "B", "C", and "D" denote mandrel numbers.

[0119]

From the results shown in FIG. 12, the following conclusions are drawn. As the exit cross-surface angle of each of the inclined rolls increases, the ratio of wall thickness to outer diameter decreases, which leads to the formation of a push. This means that the increase in the angle of the exit cross surface leads to the prevention of the occurrence of push through of the material and expands the range of wall thickness in the manufacture of thin-walled pipe. On the other hand, as the exit cross-surface angle of each of the inclined rolls decreases, the ratio of the wall thickness to the outer diameter increases, which causes the mandrel to get stuck. This means that the decrease in the angle of the outlet transverse surface leads to the prevention of the occurrence of mandrel sticking and expands the range of wall thickness in the manufacture of thick-walled pipe.

[0120]

In order to produce a thin-walled seamless metal pipe with a wall thickness to outside diameter ratio of less than or equal to 0.07 without problems (no material push through), exit transverse surface angles must be set greater than 3°. No upper limit has been set for the exit cross surface angle. The limiting wall thickness in the manufacture of a thin-walled pipe is practically the same when the angle of the outlet transverse surface is 6° or 9°. When the exit cross face angle is set greater than 9°, the mandrel geometry must be shortened. In this case, the design of the mandrel is complicated and there is a risk of deterioration in dimensional accuracy, in particular, the risk of increasing the wall thickness inhomogeneity. Therefore, the upper limit for the exit cross surface angle is preferably 9°, and more preferably 6°.

[0121]

Both in the manufacture of a thin-walled pipe and in the manufacture of a thick-walled pipe, when the exit cross-surface angle is set to be less than 0°, the roll gap decreases with decreasing distance from the exit of the inclined rolls. In this case, the clearance between the inner circumference of the manufactured seamless metal pipe and the outer circumference of the mandrel is too small, and it is difficult to extract the mandrel from the seamless metal pipe. Therefore, the angle of the exit transverse surface is preferably greater than or equal to 0°. Thus, the manufacturing method with the above-described second process structure makes it possible to manufacture a seamless metal pipe, whether it has a small wall thickness or a large wall thickness, without generating any quality problems.

[Example 2]

[0122]

Change of inclined rolls requires a long downtime of the equipment and leads to a decrease in manufacturing efficiency. Therefore, every effort must be made to avoid tilt roll changes.

[0123]

In Example 2, the piercing rolling test was carried out in the same order as in Example 1. In the piercing rolling test of Example 2, mandrels of Mandrels Nos. A-D and inclined rolls of Rolls Nos. V, P, and R220 were used in various combinations for piercing rolling.

[0124]

Table 5 below shows the test conditions and test results in Example 2.

[0125]

[Table 5]

TABLE 5

State Roll Pinch reduction ratio [%] Mandrel blank Seamless Metal Pipe Firmware factor Rolling problem Grade No. Rolling angle [°] Surface angle [°] Gap tip [mm] No. Diameter [mm] Length [mm] Outer diameter [mm] Outer diameter D [mm] Wall thickness t [mm] t/D At the entrance At the exit 17 V -3.0 17.0 0.0 47.0 14.5 14.0 A 33.0 60 550 Engagement failure 18 P -3.0 7.0 0.0 47.0 14.5 14.0 A 33.0 60 55 50.1 7.3 0.146 2.42

19 P 0.0 4.0 3.0 36.0 10.0 14.0 B 42.0 125 40 52.4 4.5 0.086 1.86

20 P 0.0 4.0 3.0 35.0 12.5 14.0 B 42.0 125 40 52.6 4.0 0.076 2.06

21 P 3.0 1.0 6.0 34.0 15.0 14.0 C 42.0 81 40 Roll body length too short 22 R220 -3.0 R220 0.0 47.0 14.5 14.0 A 33.0 60 55 50.1 7.2 0.144 2.45

23 R220 0.0 R220 3.0 36.0 10.0 14.0 B 42.0 125 40 52.4 4.4 0.084 1.89

24 R220 0.0 R220 3.0 35.0 12.5 14.0 B 42.0 125 40 52.5 4.0 0.076 2.06

25 R220 3.0 R220 6.0 34.0 15.0 14.0 C 42.0 81 40 51.3 3.6 0.070 2.33

26 R220 3.0 R220 6.0 32.8 18.0 14.0 C 42.0 81 40 51.4 3.0 0.058 2.75

27 R220 6.0 R220 9.0 32.6 18.5 14.0 D 42.0 60 40 51.4 2.9 0.056 2.84

28 R220 6.0 R220 9.0 32.4 19.0 14.0 D 42.0 60 40 51.2 3.4 0.066 2.46

29 R220 0.0 R220 3.0 37.0 7.5 14.0 B 42.0 125 40 52.8 5.0 0.095 1.67

thirty R220 -3.0 R220 0.0 48.7 11.5 10.0 A 33.0 60 55 50.5 8.2 0.162 2.18

" в столбце «Оценка» означает превосходно. (Note)"

" in the Grade column means excellent.

[0126]

First, piercing was carried out in State 17, in which the inclined rolls of Roll No. V and the mandrel of Mandrel A were used. State 17 is the same as State 1 in Example 1, except for the angle of the inlet cross surface of each of the inclined rolls used. However, during piercing in State 17, the round bar does not engage with the inclined rolls.

[0127]

Therefore, the piercing rolling in States 18-21 is carried out. In Conditions 18-21, the inclined rolls of Roll P are used, and the results of piercing in Conditions 18-21 show that a billet having substantially the same dimensions as the piercing billet in State 1 can be pierced and rolled in any of the Conditions 18 -21. State 19 and State 20 correspond to State 3 and State 4 in Example 1, respectively. The results of the piercing rolling in State 19 and State 20 show that the billet can be pierced and rolled to a thinner wall thickness and an even thinner wall thickness. In Example 1, piercing in State 5, in which the inclined rolls of Roll No. P and the mandrel of Mandrel No. B are used in combination, does not allow the billet to be machined to an even thinner wall thickness. Therefore, for State 21, a combination of the inclined rolls of Roll P and the mandrel of Mandrel C is used, and the exit cross surface angle is set to be the same as the exit cross surface angle for State 6 in Example 1, in order to obtain the same wall thickness as the target wall thickness. in piercing rolling in State 6 in Example 1. However, during piercing rolling in State 21, the round billet hits the leading ends of the inclined rolls and cannot engage with the inclined rolls. The reason for this is the following. The angle of the entrance lateral surface of each of the inclined rolls is 1°, and therefore the body length (length in the axial direction) of each of the inclined rolls is too small.

[0128]

To manufacture a thin-walled pipe, first of all, the angle of the outlet transverse surface of each of the inclined rolls is increased, as a result of which the angle of the inlet transverse surface is inevitably reduced. In addition, for the manufacture of a thin-walled pipe, the roll gap must be small, for which the length of the body of the inlet transverse surface must be increased. For piercing rolling in State 23, the entry lateral surface of each of the inclined rolls must have a body length greater than or equal to 170 mm to engage the round bar with the inclined rolls. The entrance transverse surface of each of the inclined rolls used in the test has a body length of 150 mm (see Fig. 11).

[0129]

If the inlet lateral surfaces of the inclined rolls are too large in body length, this results in an increase in construction costs of the inclined roll rolling mill and in tool manufacturing costs. Therefore, increasing the length of the body of the entrance transverse surface of each of the inclined rolls is undesirable.

[0130]

Then, piercing rolling was carried out in States 22-30. For piercing rolling in Conditions 22-30, inclined rolls are replaced with inclined rolls Roll No. R220, and different mandrels are used. The rolling angle CA of each of the inclined rolls and the roll gap are set to different values. Piercing rolling in States 22 to 30 was carried out in the same order as in Example 1. In particular, in piercing rolling in the same states, a seamless metal pipe with a small wall thickness was produced, and then a seamless metal pipe with a large wall thickness was manufactured. During piercing rolling in any of the States 22 to 30, the seamless metal pipe can be produced without the occurrence of an engagement failure or any other failure.

[0131]

From these results, the following conclusions can be drawn. When the inlet lateral surface of each of the inclined rolls is a convex surface, it is possible to produce a thin-walled seamless metal pipe and a thick-walled seamless metal pipe without causing any quality problem without having to change the inclined rolls.

[Example 3]

[0132]

In Example 3, the piercing rolling test was carried out in the same manner as in Example 1. In the piercing rolling test of Example 3, mandrels of Mandrels Nos. C and E and inclined rolls of Rolls Nos. S-Z and R220 were used in various combinations, and piercing rolling was performed to high degree of processing with a firmware coefficient greater than or equal to 3.5. Two types of round blanks are used, one of which has an outer diameter of 50 mm and the other has an outer diameter of 60 mm.

[0133]

The rolling angle CA of each of the inclined rolls is 3°, and the feeding angle FA of each of the inclined rolls is 10°. The angle of the output transverse surface of each of the inclined rolls is 6°. Checked to see if there is any disturbance or problem (material pushing, misengagement, or material circumference fluctuation) when the roll gap is changed. This check is made with respect to the angle of the inlet transverse surface of each of the inclined rolls and to the nip reduction ratio.

[0134]

Table 6 below shows the test conditions and test results in Example 3.

[0135]

[Table 6]

TABLE 6

State Roll Pinch reduction ratio [%] Mandrel blank Seamless Metal Pipe Firmware factor Rolling problem Grade No. Rolling angle [°] Surface angle [°] Gap tip [mm] No. Diameter [mm] Length [mm] Outer diameter [mm] Outer diameter D [mm] Wall thickness t [mm] t/D At the entrance At the exit 31 S 3.0 7.0 6.0 33.5 44.2 18.0 E 48.0 105 60 59.5 2.9 0.049 5.48 Large swing around the circumference 32 S 3.0 7.0 6.0 34.0 32.0 14.0 C 42.0 81 50 51.4 3.6 0.070 3.63 Large swing around the circumference 33 T 3.0 8.0 6.0 34.0 32.0 14.0 C 42.0 81 50 51.5 3.5 0.068 3.72 OK

34 U 3.0 9.0 6.0 29.8 50.3 4.0 E 48.0 105 60 59.5 2.9 0.049 5.48 OK

35 V 3.0 11.0 6.0 35.5 29.0 18.0 C 42.0 81 50 51.3 3.9 0.076 3.38 push through 36 V 3.0 11.0 6.0 33.5 33.0 18.0 C 42.0 81 50 51.3 2.9 0.057 4.45 OK

37 W 3.0 12.0 6.0 29.8 40.4 18.0 C 42.0 81 50 51.2 3.4 0.066 3.85 OK

38 X 3.0 14.0 6.0 35.0 30.0 18.0 C 42.0 81 50 51.3 3.6 0.070 3.64 OK

39 X 3.0 14.0 6.0 36.0 28.0 22.0 C 42.0 81 50 51.3 3.7 0.072 3.55 push through 40 Y 3.0 15.0 6.0 29.8 50.3 4.0 E 48.0 105 60 59.6 2.9 0.049 5.47 OK

41 Y 3.0 15.0 6.0 34.0 32.0 14.0 C 42.0 81 50 51.4 3.5 0.068 3.73 OK

42 Z 3.0 16.0 6.0 34.0 32.0 14.0 C 42.0 81 50 51.2 3.6 0.070 3.65 Engagement failure 43 Z 3.0 16.0 6.0 33.5 44.2 18.0 E 480, 105 60 59.6 2.9 0.049 5.47 Engagement failure

TABLE 6

State Roll Pinch reduction ratio [%] Mandrel blank Seamless Metal Pipe Firmware factor Rolling problem Grade No. Rolling angle [°] Surface angle [°] Gap tip [mm] No. Diameter [mm] Length [mm] Outer diameter [mm] Outer diameter D [mm] Wall thickness t [mm] t/D At the entrance At the exit 44 R220 3.0 R220 6.0 34.0 32.0 14.0 C 42.0 81 50 51.2 3.6 0.070 3.65 OK

45 R220 3.0 R220 6.0 36.0 28.0 22.0 C 42.0 81 50 51.4 3.7 0.072 3.54 push through

" в столбце «Оценка» означает превосходно.(Note)"

" in the Grade column means excellent.

[0136]

Fig. 13 is a graph showing the occurrence/non-occurrence of violations in Example 3. In the graph of FIG. 13, the horizontal axis shows the angle of the inlet cross surface [˚] of each of the inclined rolls, and the vertical axis shows the nip reduction ratio [%]. On FIG. 13, "○" indicates no occurrence of a violation, and "×" indicates an occurrence of a violation.

[0137]

As seen in Table 6 and FIG. 13, when the angle of the inlet cross surface of each of the inclined rolls is less than or equal to 7°, the circumference of the material fluctuates greatly depending on the position with respect to the lengthwise direction. On the other hand, when the angle of the inlet lateral surface of each of the inclined rolls is greater than or equal to 16°, misengagement occurs. When the pinch reduction ratio is less than 30%, material push-through occurs. Therefore, for piercing rolling with a piercing ratio less than or equal to 3.5, the entry cross surface angles must be set greater than or equal to 8° and less than or equal to 15°, and the reduction ratio must be set greater than or equal to 30%.

[0138]

It has been confirmed that even when using inclined rolls of Roll No. R220, that is, even when using inclined rolls each having a convex entry lateral surface, a nip reduction ratio of less than or equal to 30% causes pushing to occur during piercing rolling with a ratio firmware greater than or equal to 3.5.

[Example 4]

[0139]

In Example 4, the piercing rolling test was carried out in the same manner as in Example 1. In the piercing rolling test of Example 4, mandrels Nos. A, E, and F and five types of inclined rolls (Rolls Nos. R60-R600) were used in various combinations. , and piercing rolling was carried out. Three different states are set as base states, States 46-48. In addition, in each of the States 46 to 48, all five types of rolls (Rolls Nos. R60 to R600) are alternately used. Conditions 46, 47, and 48 are each for making thick-walled seamless metal pipe, medium-thickness seamless metal pipe, and thin-walled seamless metal pipe. In State 46, State 47, and State 48, the exit lateral surface angle is 0°, 3°, and 6°, respectively.

[0140]

Checked for any violation during piercing in each State. The test is made with respect to the point of contact, at which the round blank comes into contact with the inclined rolls. The contact start point can be calculated by the 2D geometry based on the assumption that the feed angle FA of each of the inclined rolls is 0°. The contact start point is determined by a first distance along the rolling line and a second distance in a direction perpendicular to the rolling line. The first distance is the distance from the pinch point to the start point of contact. The second distance is a distance calculated by subtracting half of the roll gap from the distance between the rolling line and the contact start point. In the present specification, the first distance is sometimes referred to as the pinch distance. The second distance is sometimes called (distance from the rolling line) - (roll gap/2).

[0141]

Table 7 below shows the results of the test in Example 4.

[0142]

[Table 7]

TABLE 7

State Roll Pinch reduction ratio [%] Mandrel blank Seamless Metal Pipe Firmware factor Rolling angle [°] Surface angle [°] Gap tip [mm] No. Diameter [mm] Length [mm] Outer diameter [mm] Outer diameter D [mm] Wall thickness t [mm] t/D At the entrance At the exit 46 -3.0 R 0.0 54.6 9.0 10.0 A 33.0 60 60 56.0 11.0 0.196 1.82 47 0.0 R 3.0 42.3 29.5 15.5 F 46.0 125 60 57.8 5.0 0.087 3.41 48 3.0 R 6.0 30.0 50.0 3.0 E 48.0 81 60 58.7 3.1 0.053 5.22

[0143]

Fig. 14 is a graph showing the occurrence/non-occurrence of violations in Example 4. In the graph of FIG. 14, the horizontal axis shows the pinch distance [mm], and the vertical axis shows (distance from the rolling line)-(roll gap/2) [mm]. On FIG. 14, "○" indicates no occurrence of a violation, and "×" indicates an occurrence of a violation.

[0144]

As seen in FIG. 14, when the rolls of Roll No. R60 are used as inclined rolls, in any of the States 46 to 48, misengagement occurs. When the rolls of Rolls No. R600 are used as inclined rolls, in any of the States 47-48, there is a significant oscillation of the material along the circumference.

[0145]

From these results, the following conclusions can be drawn. To prevent the occurrence of disturbances, it is preferable to select Rolls No. R100-R400 from Rolls No. R60-R600 as inclined rolls. When the rolls of Roll No. R100, which is the smallest type among Roll Nos. R100 to R400, are used as inclined rolls, the curvature radius of the arc forming the convex inlet lateral surface of each of the inclined rolls is 100 mm. When the rolls of Roll No. R400, which is the largest type among Roll Nos. R100 to R400, are used as inclined rolls, the curvature radius of the arc forming the convex inlet lateral surface of each of the inclined rolls is 400 mm. The outer diameter of the round blank is 60 mm. Accordingly, when the amount calculated by dividing the radius of curvature of the arc forming the convex entry lateral surface by the outer diameter of the round blank (the curved surface index of the entry lateral surface) is greater than or equal to 1.67 and less than or equal to 6.67, the occurrence of disturbances can be suppressed. .

[Example 5]

[0146]

In Example 5, the piercing test was carried out in the same manner as in Example 1. In the piercing test of Example 5, two more types of inclined rolls (Rolls Nos. A and B) were used.

[0147]

On FIG. 15 shows the appearance of the inclined roll used in the piercing test. The inclined rolls 1 of Roll Nos. A and B are the same as the inclined rolls shown in FIG. 15.

[0148]

As seen in FIG. 15, the lateral surface of each of Rolls Nos. A or B, which are used as the inclined rolls 1, is divided into an entry lateral surface 1a and an exit lateral surface 1b by a nip G. The entry lateral surface 1a is a convex surface. When looking at the input transverse surface 1a in a section taken along the central axis 1c of the inclined roll 1, a convex curved line is visible. This convex curved line is expressed by a higher order polynomial function. The exit transverse surface 1b is a tapered surface. Inclined rolls 1 Rolls No. A and B have the same total length. Inclined rolls 1 Rolls Nos. A and B have the same length in the axial direction of the exit lateral surface 1b. Inclined Rolls 1 Rolls Nos. A and B have different lengths in the axial direction of the inlet transverse surface 1a, depending on a higher-order polynomial function expressing a convex curved line forming a convex surface. Therefore, if necessary, an auxiliary cylinder 1aa is attached to the inlet end of the inlet lateral surface 1a. The total length of the inclined rolls 1 of Roll Nos. A and B is the same as the total length of the above-described inclined rolls 1 of Rolls Nos. O-Z and R60-R600.

[0149]

Table 8 below shows the dimensions of Roll Nos. A and B used as inclined rolls 1. FIG. 15 shows the dimensions that are common to Rolls No. A and B used as inclined rolls 1. In Table 8, "a" and "b" are coefficients in a higher-order polynomial function expressing a convex curved line forming a convex surface of the inlet transverse surface 1a as seen in FIG. 15. In Table 8, "H" indicates the length of the auxiliary cylinder 1aa in the axial direction, as seen in FIG. 15.

[0150]

[Table 8]

TABLE 8

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

[0151]

In the piercing rolling test of Example 5, piercing rolling was performed in States 46 to 48 in Example 4 by using the rolls of Roll Nos. A and B. As in Example 4, a check was made to see if any disturbance occurred during piercing rolling in each of the States. 46-48.

[0152]

Fig. 16 and 17 are graphs showing the occurrence/non-occurrence of disturbances in Example 5. FIG. 16 shows the results using inclined rolls Roll No. A. FIG. 17 shows the results using inclined rolls Roll #B. FIG. 16 and 17, the horizontal axis shows the distance from the nip [mm], and the vertical axis shows (distance from the rolling line) - (roll gap/2) [mm]. On FIG. 16 and 17, "◇" indicates no occurrence of a violation, and "×" indicates an occurrence of a violation. On FIG. 16 and 17 further show the piercing rolling states and the range for the contact start point at which no violation occurs in Example 4.

[0153]

As seen in FIG. 16, in piercing rolling using inclined rolls 1 Roll No. A, in any of the cases in which the exit cross surface angle is 0°, 3°, or 6°, the round bar comes into contact with the inclined rolls at a point in the range in which no violation occurs in Example 4, and no violation occurs. As seen in FIG. 17, in piercing rolling using inclined rolls of Roll No. B, when the exit cross surface angle is 0°, the round billet comes into contact with the inclined rolls at a point outside the range at which no violation occurs in Example 4. At this point, an engagement violation occurs. .

[Example 6]

[0154]

In Example 6, the piercing test was carried out in the same manner as in Example 1. In the piercing test of Example 6, two more types of inclined rolls (Rolls Nos. C and D) were used.

[0155]

Fig. 18 and 19 are external views of the inclined rolls used in the piercing test. On FIG. 18 shows an inclined roll 1 of Roll No. C. FIG. 19 shows an inclined swath 1 Roll no. D.

[0156]

As seen in FIG. 18 and 19, the lateral surface of each of the inclined rolls 1 of Roll No. C and Roll No. D is divided into an entry lateral surface 1a and an exit lateral surface 1b by a nip G. The entry lateral surface 1a is a three-step tapered surface. The inlet transverse surface 1a is a conical surface (convex surface), the slope of which varies in three steps along the central axis 1c. The exit transverse surface 1b is a conical surface with a constant slope. Inclined rolls 1 Rolls Nos. C and D have the same length of the inlet transverse surface 1a in the axial direction. Inclined rolls 1 Rolls Nos. C and D have the same length of the exit lateral surface 1b in the axial direction. Accordingly, the inclined rolls 1 of Rolls Nos. C and D have the same total length. The total length of the inclined rolls 1 of Roll Nos. C and D is the same as the total length of the above-described inclined rolls 1 of Rolls Nos. A, B, O-Z and R60-R600.

[0157]

As in Example 5, in the piercing rolling test of Example 6, piercing rolling was carried out in States 46 to 48 in Example 4 using Rolls No. C and D. rolling in each of States 46-48.

[0158]

Fig. 20 and 21 are graphs showing the occurrence/non-occurrence of disturbances in Example 6. FIG. 20 shows the results using inclined rolls Roll No. C. FIG. 21 shows the results using inclined rolls Roll No. D. FIG. 20 and 21, the horizontal axis shows the distance from the pinch [mm] and the vertical axis shows (distance from the rolling line) - (roll gap/2) [mm]. On FIG. 20 and 21, "◇" indicates no occurrence of a violation, and "×" indicates an occurrence of a violation. On FIG. 20 and 21 further show the piercing rolling states and the range for the contact start point in which no violation occurs in Example 4.

[0159]

As seen in FIG. 20, in piercing rolling using inclined rolls of Roll No. C, in any of the cases in which the exit cross-surface angle is 0°, 3°, or 6°, the round bar comes into contact with the inclined rolls at a point in a range in which no a violation occurs in Example 4 and no violation occurs. As seen in FIG. 21, in piercing rolling using the inclined rolls of Roll No. D, when the exit cross surface angle is 6°, the round billet comes into contact with the inclined rolls at a point outside the range at which no disturbance occurs in Example 4. At this point, material wobble occurs. along the circumference.

[0160]

Based on the results of Examples 4-6, the relationship between the engagement angle (α) and the pinch reduction ratio is derived.

[0161]

Fig. 22 is a graph for explaining the engagement angle (α). On FIG. 22, the horizontal axis shows the nip distance [mm] and the vertical axis shows (distance from the rolling line)-(roll gap/2). On FIG. 22, "○" indicates the contact start point. The origin of coordinates corresponds to the pinch point. The horizontal axis corresponds to the rolling line. Straight lines are drawn between each of the contact points and the pinch point. The angle between each of the lines and the rolling line is the engagement angle (α). This has already been described above.

[0162]

Fig. 23 is a graph showing the occurrence/non-occurrence of disturbances based on the relationship between the engagement angle (α) and the pinch reduction ratio. On FIG. 23, the horizontal axis shows the pinch reduction ratio [%], and the vertical axis shows the engagement angle (α) [˚]. On FIG. 23, "○" indicates no occurrence of a violation, "×" indicates an occurrence of a violation.

[0163]

From the results shown in FIG. 23, the following conclusions are drawn. A larger pinch reduction ratio requires a larger angle (α) of engagement. To increase the nip reduction ratio, the roll gap is reduced. When the roll gap is small, the piercing rolling has a large piercing ratio. On the other hand, a smaller pinch reduction ratio requires a smaller engagement angle (α). To reduce the nip reduction ratio, the roll gap is reduced. When the roll gap is large, the piercing rolling has a small piercing ratio.

[0164]

Therefore, as expressed by Formula (1) above, the engagement angle (α) is preferably greater than or equal to "0.12×GD+1.5" and less than or equal to "0.25×GD+6". From another point of view, the inlet transverse surface should have a slope that becomes larger with decreasing distance from the inlet end, whereby the condition expressed by Formula (1) can be satisfied.

[0165]

It is quite clear that the present invention is not limited to the above-described embodiment, and various changes can be made without departing from the scope of the present invention.

INDUSTRIAL APPLICABILITY

[0166]

The method of manufacture according to the present invention has the possibility of advantageous application to the manufacture of a seamless metal pipe by means of the Mannesmann process.

LIST OF REFERENCES

[0167]

1: inclined swath

1a: inlet transverse surface

1b: exit transverse surface

1c: central axis

2: mandrel

PL: rolling line

WP: blank

CA: roll angle

FA: feed angle

Claims (28)

1. A method for manufacturing a seamless metal pipe for manufacturing a first seamless metal pipe with a first wall thickness and a second seamless metal pipe with a second wall thickness that differs from the first wall thickness by using a rolling mill with inclined rolls, wherein: rolling mill with inclined rolls contains: a mandrel located on the rolling line; And three inclined rolls arranged at uniform angular intervals around the rolling line, each having an inlet transverse surface and an outlet transverse surface, the distance between the rolling line and the inlet transverse surface gradually decreasing with increasing distance from the inlet of the inclined rolls and with decreasing distance from the outlet of the inclined rolls rolls along the rolling line, and the distance between the rolling line and the exit transverse surface gradually increases with increasing distance from the inlet of the inclined rolls and with decreasing distance from the exit of the inclined rolls along the rolling line; And wherein the manufacturing method includes: a first inclined rolling step for rolling the first heated billet by means of the inclined roll mill; a setting change step for changing the setting state of the inclined roll mill as follows (a) or (b): (a) when the second wall thickness is smaller than the first wall thickness, the rolling angle of each of the inclined rolls is set to be larger than the rolling angle of each of the inclined rolls set for the first inclined rolling step; or (b) when the second wall thickness is larger than the first wall thickness, the rolling angle of each of the inclined rolls is set to be smaller than the rolling angle of each of the inclined rolls set for the first inclined rolling step; And a second tilt rolling step for rolling the second heated billet by the tilt roll mill in the changed state. 2. A method for manufacturing a seamless metal pipe according to claim 1, in which: in the first slant rolling step and the second slant rolling step, the angle of the exit lateral surface in the contact area in which the exit lateral surface is in contact with the first workpiece or the second workpiece is greater than or equal to 0° and less than or equal to 9°; And in the first step of oblique rolling, when the ratio of the first wall thickness to the outer diameter of the first seamless metal pipe is less than or equal to 0.07, the angle of the exit cross surface in the contact region is greater than 3°; And in the second step of oblique rolling, when the ratio of the second wall thickness to the outer diameter of the second seamless metal pipe is less than or equal to 0.07, the angle of the exit cross surface in the contact region is greater than 3°. 3. A method for manufacturing a seamless metal pipe according to claim 1 or 2, in which: the input transverse surface is a convex surface and the oblique rolls used in the first oblique rolling step are used in the second oblique rolling step. 4. A method for manufacturing a seamless metal pipe according to claim 1 or 2, in which: the first blank and the second blank are solid; And in the first bevel rolling step and the second bevel rolling step, the target is a piercing ratio greater than or equal to 3.5, an inlet cross surface angle greater than or equal to 8° and less than or equal to 15°, and a nip reduction ratio greater than or equal to 30%. 5. A method for manufacturing a seamless metal pipe according to claim 1 or 2, in which: the first blank and the second blank are solid; the entrance transverse surface is a convex surface, and in a cross section including the central axis of the inclined roll, a circular arc is visible as a line forming the entrance transverse surface; And the value calculated by dividing the radius of curvature of the circular arc by the outer diameter of the first workpiece and the value calculated by dividing the radius of curvature of the circular arc by the outer diameter of the second workpiece are greater than or equal to 1.67 and less than or equal to 6.67. 6. A method for manufacturing a seamless metal pipe according to claim 1 or 2, in which: the first blank and the second blank are solid; And in the first bevel rolling step and the second bevel rolling step, the nip reduction ratio (GD) and the engagement angle (α) satisfy the condition expressed by the following formula (1) 0.12×GD+1.5≤α≤0.25×GD+6 (1).

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