US20110023569A1

US20110023569A1 - Method for Producing Seamless Pipe

Info

Publication number: US20110023569A1
Application number: US12/884,983
Authority: US
Inventors: Takateru Inage; Tomio Yamakawa; Kazuhiro Shimoda
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2008-03-27
Filing date: 2010-09-17
Publication date: 2011-02-03
Also published as: CN101980802A; BRPI0910031A2; EP2281641A4; BRPI0910031B1; JP4471134B2; JPWO2009119245A1; WO2009119245A1; MX2010010439A; EP2281641A1

Abstract

Disclosed is a method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of a pair of skew rolls disposed to face each other around a pass line, a pair of guide devices disposed to face each other around the pass line, and a plug disposed along the pass line, between the pair of the skew rolls and also between the pair of the guide devices, wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1) to (3) to prevent wrinkle flaws of the seamless pipe:

−1.0<Δθ (1)

Δθ=θ_p−θ_r (2)

−0.37×Δθ+1.47≦R _n≦0.37×Δθ+2.67 (3)

- wherein the meanings of the individual symbols in the above-described formulas are as follows:
- θ_r: The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zero
- θ_p: The half angle (°) between the pass line and the reeling section of the plug
- R_n: The number of times of the reeling of the plug

Description

The disclosure of International Application No. PCT/JP2009/053652 filed Feb. 27, 2009 including specification, drawings and claims is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for producing a seamless pipe.

BACKGROUND ART

As a method for producing a seamless pipe, the Mannesmann process is known in which a seamless pipe is obtained by subjecting a heated round billet to piercing-rolling with a piercing mill, then to elongation rolling with a mandrel mill, a plug mill or the like, and further to sizing with a sizing mill to yield the seamless pipe.
The piercing mill is usually a piercing machine having rolling rolls consisting of a pair of barrel-shaped or cone-shaped main rolls (also referred to as skew rolls), guide devices such as a guide shoe, disk rolls, a roller-type guide or the like, and an inner surface regulating tool referred to as a plug.
FIG. 1 is a schematic view illustrating an example of a skew piercing mill using cone-shaped skew rolls, and FIG. 2 is a schematic view taken in the direction of A-A in FIG. 1. FIG. 3 is a view schematically illustrating the shape of the plug.
As shown in FIG. 1, in a piercing mill, for example, a pair of main rolls 1 is disposed to face each other so as for each of the axial centerlines of the rolls to form a cross angle of γ in relation to the pass line X-X of the round billet B as a workpiece. Additionally, as shown in FIG. 2, one of the main rolls 1 is disposed to form a feed angle β in relation to the pass line X-X. The other main roll 1 not shown in FIG. 2 is disposed to face the one of the main rolls 1, with the pass line X-X interposed therebetween, at the feed angle β in relation to the pass line X-X. The main rolls 1 to exert spiral movement to the round billet B are directly connected to driving devices 4 respectively, so as to be rotated about the axial centerlines of the rolls as the rotational centerlines.
Also, as shown in FIG. 2, a pair of disk rolls 2 is disposed to face each other, around the pass line with a phase shift of 90° from the pair of main rolls 1. The pair of disk rolls 2 is rotationally driven in the same direction as the traveling direction of the workpiece at a predetermined speed, to serve an important role in making the workpiece round in shape through suppressing the increase of the circumferential length of the workpiece during wall thickness processing.
A plug 3 has a bombshell-like shape whose base end is supported by the front end of a mandrel bar M, and the plug 3 and the mandrel bar M are disposed on the pass line X-X. As the material for the plug 3, Cr—Ni low alloy steels are usually used, and an oxide layer is formed on the plug by heat pretreatment in order to enhance the durability.
For example, as shown in FIG. 3, the plug 3 is mainly composed of a rolling section 31, a reeling section 32 and a relief section 33, and has the maximum diameter of Pd at the boundary between the reeling section 32 and the relief section 33. The rolling section 31 mainly plays a role of piercing the solid billet, and the reeling section 32 plays a role of equalizing the wall thickness of the hollow shell and at the same time a role of smoothing the inner surface of the hollow shell. The reeling section 32 has a half angle θp in relation to the axial center of the plug, namely, the pass line of the round billet (see FIG. 5).
In a piercing mill constituted as described above, the heated round billet B is fed rightward on the pass line X-X in the figure, and rolled while the round billet is being subjected to wall thickness processing with the main rolls 1 and the plug 3 during the passage of the round billet through the gap between the skew rolls. In this case, the round billet B spirally moves on the pass line X-X, and the axial center portion of the round billet is pierced with the plug 3 to be converted into a hollow shell.
During the piercing-rolling with the piercing mill, the asperities formed on the inner surface of the hollow shell are flattened out by the inner surface regulating tools in the lower process such as the plug of the elongator, the bar of the mandrel mill and the plug of a plug mill, and then the asperities develop into wrinkle flaws (eruption flaws). In other words, the occurrence of the fine inner surface flaws in final products is attributed to the properties and conditions (roughness) of the inner surface of the hollow shell after the piercing with a piercing mill.
In particular, in the seamless steel pipes undergoing high pressures on the inner surfaces thereof, such as a fuel injection pipe, the occurrence of fine rice-grain-like flaws could lead to a serious accident through the blowout of the pipe initiated by such flaws. When the inner diameter of the pipe is large, it is possible to mechanically remove the inner surface flaws with a grinder or the like, however, when the inner diameter of the pipe is small, it is difficult to completely remove the inner surface flaws. Even if the removal of the inner surface flaws of a pipe having a small inner diameter is possible, the number of work steps is naturally increased, and problems from the viewpoint of a product may be left unsolved in such a way that the wall thickness of the flaw-removed portion is thin.
For the purpose of solving such problems as described above, the present inventor has disclosed in Patent Document 1 a method for producing a seamless steel pipe by using a plug where the scale coated layer of the reeling section is made thinner than that of the rolling section.
Patent Document 1: JP10-249412A

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

According to the invention described in Patent Document 1, the occurrence of rice-grain shaped eruption can be prevented. However, the removal amount of the scale coated layer is required to be strictly regulated, and when the control of the coated layer thickness is not performed properly to make the coated layer thickness too thin, problems such as the scoring of the reeling section and shortening of the tool life of the plug may arise.
The present invention takes as its object the provision of a method for producing a seamless pipe, capable of improving the properties and conditions of the inner surface of a hollow shell after piercing-rolling and suppressing the wrinkle flaws of a seamless pipe.

Means for Solving the Problems

The present inventor made a diligent study in order to solve such problems as described above, and consequently has discovered the following findings.
FIG. 4 is a schematic view illustrating an example of the piercing-rolling process in the cross section perpendicular to the pass line. As shown in FIG. 4, in the piercing-rolling, for example, a workpiece 5 is pressed into the space formed by the pair of main rolls 1 and the plug 3, and thus the workpiece 5 is subjected to the wall thickness processing. Thereafter, the outer diameter increase is suppressed at a half-turned position by the disk rolls 2, and the wall thickness processing is conducted again by the pair of main rolls 1 and the plug 3. By repeating such operations, a hole is pierced in the workpiece 5 and at the same time, the wall thickness of the workpiece 5 is controlled.
Here, the inner surface of the workpiece 5 having reached the region indicated by “a” in FIG. 4 undergoes the action of a contraction force in the circumferential direction, and thus wrinkles can occur. Subsequently, when the workpiece 5 spirally moves to reach the region indicated by “b” in FIG. 4, the outer surface of the workpiece is brought into contact with the main rolls 1. At this time, the workpiece 5 undergoes the outer diameter processing, and hence the inner surface wrinkles formed in the region “a” are deepened. Subsequently, when the workpiece 5 reaches the region indicated by “c” in FIG. 4, the inner surface of the workpiece 5 is brought into contact with the plug 3. At this time, the wrinkles are stretched in the circumferential direction to develop into fine flaws.
The present inventor investigated the factors degrading the properties and conditions of the inner surface of workpieces, and consequently has discovered the following findings with respect to the occurrence of the piercing troubles such as the roughening of the inner surface of a pipe and the tail clogging of the workpiece (meaning the condition in which on completion of piercing-rolling, the workpiece is not yet detached from the main rolls or the plug remains in the bottom portion of the workpiece).
(a) With the increase of the number of times R_nof the reeling of the plug, the driving force in the direction of the forward movement of the workpiece is lowered. Consequently, the speed of the workpiece on completion of the piercing-rolling is decreased and the piercing troubles such as the tail clogging tends to occur. However, with the increase of Δθ (=θ_p−θ_r, where θ_r: the half angle between the pass line and the main roll face in the condition where the feed angle of the main rolls is zero, θ_p: the half angle between the pass line and the reeling section of the plug), the rolling reduction on the exit side of the gorge is increased, and the interfacial pressure is increased to minimize the piercing trouble. Consequently, the degree of freedom (mainly, the degree of freedom of the upper limit) of the number of times R_nof the reeling of the plug is increased.
(b) With the increase of the number of times R_nof the reeling of the plug, the number of times of the rolling applied to the workpiece is increased, and hence the roughness of the inner surface of the pierced shell tends to be reduced. Such roughness reduction effect is enhanced with the increase of Δθ (=θ_p−θ_r). Consequently, the degree of freedom (mainly, the degree of freedom of the lower limit) of the number of times R_nof the reeling of the plug is increased.
(c) With the increase of D₂/D₁, the circumferential speed of the roll on the exit side of the gorge is increased, and hence the outer diameter expansion in the region indicated by “b” in FIG. 4 can be suppressed. Consequently, even when the number of times R_nof the reeling of the plug is decreased, it is also possible to prevent the occurrence of wrinkles, and the degree of freedom (mainly, the degree of freedom of the lower limit) of the number of times R_nof the reeling of the plug is increased.
The above-described θ_rmeans the half angle (see “θ_r” in FIG. 5) between the pass line and the main roll face in the condition where the feed angle of the main roll is zero, and the above-described θ_pmeans the half angle (see “θ_r” in FIG. 5) between the pass line and the reeling section of the plug. It is to be noted that the number of times R_nof the reeling of the plug is obtained from the following formula:
R _n =L _p/(π×d×tan β/2)
In this formula, L_pmeans the length (mm) of the reeling section, d means the value obtained from the following formula, and β means the feed angle (°) of the main rolls:
d=(d ₁ +d ₂)/2
where, d₁is the outer diameter (mm) of the round billet, and d₂is the outer diameter of the hollow shell.
The present invention has been achieved on the basis of such findings as described above, and involves a method for producing a seamless pipe shown in the following [1] to [4].
[1] A method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of:
a pair of skew rolls disposed to face each other around a pass line;
a pair of guide devices disposed to face each other around the pass line; and
a plug disposed along the pass line, between the pair of the skew rolls and also between the pair of the guide devices,
wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1) to (3);
−1.0≦Δθ (1)
Δθ=θ_p−θ_r (2)
−0.37×Δθ+1.47≦R _n≦0.37×Δθ+2.67 (3)
wherein the meanings of the individual symbols in the above-described formulas are as follows;
θ_r; The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zero
θ_p; The half angle (°) between the pass line and the reeling section of the plug
R_n; The number of times of the reeling of the plug
[2] A method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of:
a pair of skew rolls disposed to face each other around a pass line;
a pair of guide devices disposed to face each other around the pass line; and
a plug disposed along the pass line, between the pair of the skew rolls and also between the pair of the guide devices,
wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1), (2) and (4):
−1.0<Δθ (1)
Δθ=θ_p−θ_r (2)
−0.24×Δθ+1.73≦R _n≦0.37×Δθ+2.67 (4)
wherein the meanings of the individual symbols in the above-described formulas are as follows:
θ_r: The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zero
θ_p: The half angle (°) between the pass line and the reeling section of the plug
R_n: The number of times of the reeling of the plug
[3] The method for producing a seamless pipe according to [1] or [2], wherein the piercing-rolling is performed under the conditions further satisfying the following formula (5):
−1.37×D ₂ /D ₁+2.74≦R _n (5)
wherein the meanings of the individual symbols in the above-described formula are as follows:
D₁: The roll diameter (mm) in the gorge portion of the main rolls
D₂: The main roll outer diameter (mm) at the position of the maximum-diameter portion of the plug
R_n: The number of times of the reeling of the plug
[4] The method for producing a seamless pipe according to [1] or [2], wherein the piercing-rolling is performed under the conditions further satisfying the following formula (6):
−1.25×D ₂ /D ₁+2.88≦R _n (6)
wherein the meanings of the individual symbols in the above-described formula are as follows:
D₁: The roll diameter (mm) in the gorge portion of the main rolls
D₂: The main roll outer diameter (mm) at the position of the maximum-diameter portion of the plug
R_n: The number of times of the reeling of the plug
In the present invention, the reeling section means the part satisfying any one of the following conditions:
(A) The part in which the wall thickness working ratio obtained from the following formula is 5% or less:
Wall thickness working ratio=(G ₁ −G ₂)/G ₁×100(%)
wherein the meanings of the individual symbols in the formula are as follows:
G₁: The distance (mm) between the plug and the roll at the starting position in the corresponding part of the plug
G₂: The distance (mm) between the plug and the roll at the completion position in the corresponding part of the plug
(B) The part, in the vicinity of the entrance side, of the maximum-diameter portion of the plug
(C) The part in which the face angle difference obtained from the following formula is 2° or less when the section corresponding to the reeling section has no curvature:
Face angle difference (°)=face angle of the concerned part of the plug-face angle of the exit side of the rolls:
wherein “the starting position in the corresponding part of the plug” means, for example, the position of the border line between the sections indicated by reference numerals 31 and 32 in FIG. 3, and “the completion position in the corresponding part of the plug” means, for example, the position of the border line between the sections indicated by reference numerals 32 and 33 in FIG. 3.

ADVANTAGES OF THE INVENTION

According to the present invention, the properties and conditions of the inner surface of a hollow shell after piercing-rolling can be improved, and the wrinkle flaws in a seamless pipe which is obtained by performing elongation rolling and sizing after piercing-rolling can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an example of a skew piercing mill using cone-shaped skew rolls;

FIG. 2 is a schematic view showing a view taken in the direction of A-A in FIG. 1;

FIG. 3 is a view schematically illustrating the shape of a plug;

FIG. 4 is a schematic view illustrating an example of a piercing-rolling process in a cross section perpendicular to a pass line;

FIG. 5 is a schematic view illustrating main rolls and the plug under the conditions that the feed angle β is zero;

FIG. 6 is a graph showing the relation between Δθ and R_n; and

FIG. 7 is a graph showing the relation between D₂/D₁and R_n.

DESCRIPTION OF SYMBOLS

1 Main roll
2 Disk roll
3 Plug
31 Rolling section
32 Reeling section
33 Relief section
4 Driving device
5 Workpiece
B Round billet

BEST MODE FOR CARRYING OUT THE INVENTION

Existing piercing mills can be employed in the method for producing seamless steel pipes according to the present invention. In other words, a piercing mill can be used in which a plug is disposed along a pass line, between a pair of skew rolls and also between a pair of guide devices, the members in each of these pairs being disposed to face each other around the pass line, and which has usual guide devices such as guide shoes, disk rolls or roller-type guides. The shape of the plug is also not particularly limited. For example, there can be used a plug having a structure which consists of the rolling section 31, the reeling section 32 and the relief section 33 as shown in FIG. 3 and which has a maximum diameter at the boundary between the reeling section 32 and the relief section 33.
It is preferable to use disk rolls as the guide device because the disk rolls can increase the speed of the material in the axial direction. It is also preferable to use cone-shaped rolls as the main rolls.
The round billet to be subjected to piercing-rolling is required to be heated to 1300° C. or lower. When the temperature of the round billet exceeds 1300° C., inner surface flaws occur due to the melting of the inner surface of the round billet to degrade the properties and conditions of the inner surface of the pipe. On the other hand, the resistance to deformation of the round billet becomes greater with the considerable decrease of the temperature to make it impossible to perform piercing-rolling or remarkably shorten the operating lives of the plug and other production facilities. Therefore, it is preferable to set the temperature of the round billet at 1150° C. or higher.
Here, Δθ defined by the following formula (a) is required to be set at −1.0 or more:
Δθ=θ_p−ƒ_r (a)
wherein the meanings of the individual symbols in the above-described formula are as follows:
θ_r: The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zero
θ_p: The half angle (°) between the pass line and the reeling section of the plug
Specifically, the inner surface of the hollow shell after piercing-rolling is smoothed by increasing the number of times R_nof the reeling of the plug; however, depending on the value of the plug face angle θ_prelative to that of the exit-side face angle θ_rof the main rolls, some problems may occur including the insufficient smoothness and the failures such as the tail clogging and the unevenness of the wall thickness. This tendency is enhanced with the decrease of the above-described Δθ, and when Δθ is less than −1.0, even the increase of the number of times Rr, of the reeling of the plug fails in smoothing the inner surface of the hollow shell after piercing-rolling. Therefore, Δθ is set at −1.0 or more.
When the number of times R_nof the reeling of the plug is too small, the surface roughness of the inner surface of the hollow shell after piercing-rolling is high, and when the number of times R_nof the reeling of the plug is too large, the problem of the tail clogging tends to occur. However, with the increase of Δθ, all these problems hardly occur. This is because the increase of Δθ pushes up the rolling reduction at the exit side of the gorge, and hence the interfacial pressure is increased to minimize the piercing trouble. In other words, the processing with the reeling section can be concentrated in the second half of the operation, and hence even with the same number of times of the reeling of the plug, a hollow shell satisfactory in the properties and conditions of the inner surface can be obtained. Accordingly, the number of times R_nof the reeling of the plug is required to satisfy the relation represented by the following formula (b) in terms of Δθ. More preferable is the case where the following formula (b1) is satisfied:
−0.37×Δθ+1.47≦R _n≦0.37×Δθ+2.67 (b)
−0.24×Δθ+1.73≦R _n≦0.37×Δθ+2.67 (b1)
The number of times R_nof the reeling of the plug preferably further satisfies the following formula (c). More preferable is the case where the following formula (d) is satisfied:
−1.37×D ₂ /D ₁+2.74≦R _n (c)
−1.25×D ₂ /D ₁+2.88≦R _n (d)
wherein the meanings of the individual symbols in the above-described formulas are as follows:
D₁: The roll diameter (mm) in the gorge portion of the main rolls
D₂: The main roll outer diameter (mm) at the position of the maximum-diameter portion of the plug
The inner surface roughness of the hollow shell after piercing-rolling tends to be affected by the roll diameter at the position on the exit side of the main rolls. When the main roll outer diameter D₂at the maximum diameter position of the plug is set to be larger than the gorge portion diameter D₁of the main rolls, the compressive strain in the circumferential direction acting on the inner surface of the workpiece tends to be relaxed, and consequently the suppression of the wrinkles on the inner surface of the pipe is facilitated. As described above, the small Δθ value make it difficult to smooth the inner surface of the pipe, however, by setting the relation between D₂/D₁and R_nso as to satisfy the conditions represented by the above-described formula (c), the surface roughness of the inner surface of the pipe can be improved. The surface roughness of the inner surface of the pipe is further improved by setting the relation between D₂/D₁and R_nso as to satisfy the above-described formula (d).
The method for producing a seamless pipe according to the present invention can be applied to any pipes such as metal pipes, ordinary steel pipes, low-alloy steel pipes and high-alloy steel pipes, and is particularly suitable for steel pipes with smooth inner surfaces which are used for automobile components.

Example 1

A slab produced by continuous casting from the steel having the chemical composition shown in Table 1 was subjected to blooming and finished into a round billet of 225 mm in outer diameter, and from the central portion of the round billet, a round billet of 70 mm in outer diameter was machined to prepare a sample material. As the guide device, disk rolls were used; the main roll shape and the plug shape were varied, and thus piercing-rolling was performed under the production conditions shown in Table 2 or 3; and the inner surface roughness (the maximum height Rz defined by JIS-0601) of each of the obtained hollow shells was measured.

TABLE 1

Chemical composition of the sample material
(in mass %, balance: Fe and impurities)

C	Si	Mn	P	S	Ca	Nb

0.20	0.35	1.35	0.012	0.01	0.001	0.04

	TABLE 2

	Heating temperature	1180° C.-1240° C.
	Feed angle β	7°-16°
	Gorge diameter D₁of the main rolls	350φ-410φ mm
	Exit-side face angle θ_rof the main	3°-5.5°
	rolls
	Roll diameter ratio D₂/D₁	0.9-1.3
	Δθ = θ_p− θ_r	−0.25°-1.0°
	Hollow shell outer diameter	70.0-75.0 mm
	Hollow shell wall thickness	4.6-10.1 mm

	TABLE 3

	Heating temperature	1180° C.-1240° C.
	Feed angle β	7°-16°
	Gorge diameter D₁of the main rolls	350φ-410φ mm
	Exit-side face angle θ_rof the main	3°-5.5°
	rolls
	Roll diameter ratio D₂/D₁	0.95-1.15
	Δθ = θ_p− θ_r	−1.5°-1.25°
	Hollow shell outer diameter	70.0-75.0 mm
	Hollow shell wall thickness	4.6-10.1 mm

FIG. 6 shows the properties and conditions of the inner surface of the hollow shell produced under the conditions shown in Table 2 with respect to Δθ and R_n, and FIG. 7 shows the properties and conditions of the inner surface of the hollow shell produced under the conditions shown in Table 3 with respect to D₂/D₁and R_n.
In FIGS. 6 and 7, ▴, Δ and ◯ mean that the inner surface roughness of the hollow shell is such that Rz>150 μm, 100 μm≦Rz≦150 μm, and Rz<100 μm, respectively. In FIG. 6, x means that piercing troubles such as the tail clogging occurred.
As shown in FIG. 6, in the region where Δθ was less than −1.0, the inner surface roughness Rz of the hollow shell exceeded 150 μm or piercing trouble occurred. Although Δθ was −1.0, in the region where R_nexceeded “0.37×Δθ+2.67,” piercing trouble occurred, and in the region where R_nwas less than “−0.37×Δθ+1.47,” the surface roughness Rz was increased. When the production conditions were regulated in such a way that R_nfell in the region equal to or larger than “−0.24×Δθ+1.73,” the surface roughness Rz was able to be made smaller.
As shown in FIG. 7, in the region where Rr, was less than “−1.37×D₂/D₁+2.74,” the surface roughness was increased, and in the region where R_nwas equal to or larger than “−1.37×D₂/D₁+2.74,” the surface roughness was within a satisfactory range. In the region where Rr, was equal to or larger than “−1.25×D₂/D₁+2.88,” the surface roughness was able to be further decreased.

Example 2

A continuous cast material having the chemical composition shown in Table 1 was converted into round billets of 191 mm in outer diameter by blooming, then each of the round billets was subjected to piercing-rolling under the conditions shown in Table 4 and from each of the round billets, 100 seamless steel pipes of 73 mm in outer diameter and 5.51 mm in wall thickness were produced, and the properties and conditions of the inner surface of the obtained seamless steel pipes were investigated. The results thus obtained are shown in Table 5.

	TABLE 4

	Heating temperature	1240° C.
	Feed angle β	6°-16°
	Gorge diameter D₁of the main rolls	1400φ mm
	Exit-side face angle θ_rof the main rolls	3°-4°
	Roll diameter ratio D₂/D₁	1.05-1.15
	Δθ = θ_p− θ_r	−1.5°-1.25°
	Number of times R_nof reeling of the plug	0.8-3.5

TABLE 5

								Rate of occurrence of
Δθ	D₂/D₁	A	B	C	D	E	Rn	inner surface flaws

Comparative	−1.50	1.1	−0.53	−0.59	0.62	0.27	−0.00	1.50	100
Examples	−1.50	1.1	0.98	0.91	−0.89	1.77	1.50	3.00	Piercing trouble
	−1.00	1.1	1.16	1.03	−0.70	1.77	1.50	3.00	Piercing trouble
	1.00	1.1	−0.10	−0.49	2.04	−0.23	−0.51	1.00	70
	1.00	1.1	0.15	−0.24	1.79	0.02	−0.26	1.25	10
	1.25	1.1	−0.01	−0.43	2.13	−0.23	−0.51	1.00	15
	1.00	0.9	0.15	−0.24	1.79	−0.26	−0.51	1.25	90
	1.00	1.0	0.15	−0.24	1.79	−0.12	−0.38	1.25	50
	1.00	1.2	0.15	−0.24	1.79	0.15	−0.13	1.25	8
	1.00	1.25	0.15	−0.24	1.79	0.22	−0.07	1.25	5
Examples of	−1.00	1.1	0.16	0.03	0.30	0.77	0.50	2.0	3
the present	−0.50	1.1	0.35	0.15	0.49	0.77	0.50	2.0	2
invention	0.00	1.1	0.53	0.27	0.67	0.77	0.50	2.0	1
	0.50	1.1	1.22	0.89	0.36	1.27	1.00	2.5	0
	1.25	1.1	1.49	1.07	0.63	1.27	1.00	2.5	0
	1.00	1.3	0.15	−0.24	1.79	0.29	−0.00	1.25	2

It is to be noted that the meanings of A to E in the above-described table are as follows:
A: Calculated value of “R_n− (−0.37 × Δθ + 1.47)”
B: Calculated value of “R_n− (−0.24 × Δθ + 1.73)”
C: Calculated value of “0.37 × Δθ + 2.67 − R_n”
D: Calculated value of “R_n− (−1.37 × D₂/D₁+ 2.74)”
E: Calculated value of “R_n− (−1.25 × D₂/D₁+ 2.88)”

As shown in Table 5, the rates of occurrence of the inner surface flaws were able to be remarkably reduced in Examples of the present invention compared to those in Comparative Examples.
Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.

INDUSTRIAL APPLICABILITY

Claims

1. A method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of:

a pair of skew rolls disposed to face each other around a pass line;

a pair of guide devices disposed to face each other around the pass line; and

a plug disposed along the pass line, between the pair of the skew rolls and also between the pair of the guide devices,

wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1) to (3):

−1.0<Δθ (1)

Δθ=θ_p−θ_r (2)

−0.37×Δθ+1.47≦R _n≦0.37×Δθ+2.67 (3)

wherein the meanings of the individual symbols in the above-described formulas are as follows:

θ_r: The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zero

θ_p: The half angle (°) between the pass line and the reeling section of the plug

R_n: The number of times of the reeling of the plug

2. A method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of:

a pair of skew rolls disposed to face each other around a pass line;

a pair of guide devices disposed to face each other around the pass line; and

wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1), (2) and (4):

−1.0<Δθ (1)

Δθ=θ_r−θ_r (2)

−0.24×Δθ+1.73≦R _n≦0.37×Δθ+2.67 (4)

R_n: The number of times of the reeling of the plug

3. The method for producing a seamless pipe according to claim 1, wherein the piercing-rolling is performed under the conditions further satisfying the following formula (5):

−1.37×D ₂ /D ₁+2.74 (5)

wherein the meanings of the individual symbols in the above-described formula are as follows:

D₁: The roll diameter (mm) in the gorge portion of the main rolls

D₂: The main roll outer diameter (mm) at the position of the maximum-diameter portion of the plug

R_n: The number of times of the reeling of the plug

4. The method for producing a seamless pipe according to claim 1, wherein the piercing-rolling is performed under the conditions further satisfying the following formula (6):

−1.25×D ₂ /D ₁+2.88≦R _n (6)

D₁: The roll diameter (mm) in the gorge portion of the main rolls

R_n: The number of times of the reeling of the plug.