JP4196991B2 - Method of piercing and rolling in the manufacture of seamless pipes - Google Patents

Description

  The present invention relates to a method for piercing and rolling a billet in a manufacturing process of a seamless pipe. In particular, the present invention relates to a piercing-rolling method capable of producing a thin-walled raw tube (hollow piece) with high workability from a billet.

  The most commonly employed methods for producing seamless pipes include the Mannesmann-plug mill method and the Mannesmann-mandrel mill method. In these methods, a solid billet heated to a predetermined temperature in a heating furnace is pierced by a piercing and rolling machine to form a hollow rod-shaped hollow piece, which is mainly reduced in thickness by a rolling mill such as a plug mill and a mandrel mill to reduce the thickness of the hollow shell. And Next, the outer diameter is mainly reduced by a drawing mill such as a sizer or stretch reducer to obtain a seamless pipe having a predetermined size. The present invention relates to the first piercing and rolling step in the above steps.

First, the inventions proposed by the present inventors in Patent Document 1 and Patent Document 2 will be described as conventional techniques.
Japanese Patent Publication No. 5-23842 Japanese Patent Publication No. 8-4811

The invention of Patent Document 1 (hereinafter referred to as “first prior invention”) includes an inclination angle β of a cone-type main roll supported on both ends, which is opposed to left and right or up and down across a pass line through which a billet and a hollow piece pass, The crossing angle γ of the main roll is maintained in the range of the following formulas (1) to (3), and the diameter d _{0 of the} solid billet, the outer diameter d of the hollow piece after piercing and rolling, and the wall thickness t are It is an invention of a method for producing a seamless pipe that satisfies the following formula (4) and has a perforation ratio of 4.0 or more, a tube expansion ratio of 1.15 or more, or a “wall thickness / outer diameter” ratio of 6.5 or less.

The inclination angle β is an angle formed by the roll axis and the horizontal or vertical plane of the pass line. Further, the crossing angle γ is an angle formed by the roll axis line with respect to a vertical plane or a horizontal plane of the pass line.
8 ° ≦ β ≦ 20 ° (1)
5 ° ≦ γ ≦ 35 ° (2)
15 ° ≦ β + γ ≦ 50 ° (3)
1.5 ≦ −Ψ _r / Ψ _θ ≦ 4.5 (4)
However, Ψ _r = ln (2t / d ₀ )
Ψ _θ = ln {2 (dt) / d ₀ }

In the method of the first prior invention, the roll forging effect that is noticeably generated in the piercing and rolling process, especially the thin piercing and rolling process of high workability, by maintaining the roll inclination angle β and the crossing angle γ within an appropriate range. And a method of suppressing the additional shear deformation as much as possible. In addition, it prevents internal flaws and lamination (double cracks that occur at the center of the wall thickness) that occur in stainless steel and high alloy steel pipes, and the circumferential strain Ψ _θ and the thickness direction strain Ψ _r By optimizing the distribution and satisfying the relationship of the above formula (4), operational troubles such as flare and peeling of the flesh or clogging of the bottom are reduced.

  The above-mentioned first prior invention has made it possible to perform the pipe making of difficult-to-process materials, which has conventionally been forced to be piped by the Eugene extrusion pipe making process, by the Mannesmann pipe making method. In addition, since thin piercing and rolling with a high workability is possible, it is possible to omit or shorten processes in the subsequent drawing and drawing processes. Therefore, the present invention has greatly contributed to rationalization of the seamless pipe manufacturing process.

  For example, Mannesmann piercers and rotary longatores used in the Mannesmann-plug mill process can replace single piercing and rolling mills with single piercing instead of double piercing. The Mannesmann-plug mill method is a method in which Mannesmann piercer → rotary longator → plug mill → reeler → sizer process.

  In the Mannesmann-Mandrel mill system, the Mandrel Mill can be made into a small number of stands by replacing the Mannesmann Piercer with a cross-piercing and rolling mill. The Mannesmann-mandrel mill method is a method in which a process of Mannesmann piercer → mandrel mill → stretch reducer is performed.

  Furthermore, the introduction of the cross piercing rolling mill has been succeeded even in the process of Mannesmann-Assel mill system, that is, the process of Mannesmann piercer → Assel mill → stretch reducer. According to the cross piercing and rolling mill, so-called “size-free rolling” is possible, in which multi-size hollow pieces can be manufactured from a single-sized billet by simply changing the plug, so billet size integration, shortening of setup change time, etc. There are significant operational advantages.

The invention of Patent Document 2 (hereinafter referred to as “second prior invention”) is an invention made for the purpose of further optimizing the relationship between the diameter of the cone-type main roll and the diameter of the solid billet. In the present invention, rotary forging effect was minimized, and to be suppressed as much as possible redundant shear strain, cone-type main roll gorge portion diameter (i.e., roll gorge diameter) formula and D _g and billet diameter d ₀ of the following It is characterized by satisfying (a).
2.5 ≦ D _g / d ₀ ≦ 4.5 (a)

  In the second prior invention, in order to stably perforate difficult-to-work materials such as stainless steel and high alloy steel without generating internal flaws or lamination, the roll gorge diameter is made as small as possible relative to the billet diameter. It should be. However, in order to reduce the roll gorge diameter, the shaft diameters of the entrance and exit rolls must be reduced due to the roll structure. If it does so, the intensity | strength of the bearing which supports a roll axis | shaft will be insufficient, especially in the case of a cone type roll, the fatigue strength of the bearing of an entrance side is insufficient, and durability becomes a problem. Therefore, excessive reduction of the roll gorge diameter cannot be recommended in actual operation.

  An object of the present invention is to provide a piercing-rolling method capable of suppressing the rotational forging effect as much as possible and suppressing the additional shear deformation as much as possible without reducing the roll gorge diameter so much.

  The present inventor has repeatedly studied to achieve the above-described object, and has arrived at the invention of the following piercing and rolling method. In addition, the meaning of the code | symbol in the following description was shown in FIG.

Maintaining the inclination angle β and crossing angle γ of the cone-type main roll supported at both ends across the pass line in the range satisfying the following formulas (1) to (3) The relationship between the outer diameter d ₀ of the actual billet and the outer diameter d and the wall thickness t of the hollow piece after piercing and rolling satisfies the following formula (4), and further, the inlet diameter D ₁ and the outlet diameter D _{2 of the} main roll A piercing-rolling method in seamless pipe production, wherein the above d ₀ , d and γ satisfy the following expression (5).
8 ° ≦ β ≦ 20 ° (1)
5 ° ≦ γ ≦ 35 ° (2)
15 ° ≦ β + γ ≦ 50 ° (3)
1.5 ≦ −Ψ _r / Ψ _θ ≦ 4.5 (4)
(D / d ₀ ) / (0.75 + 0.025γ) ≦ D ₂ / D ₁ (5)
However, in Formula (4), Ψ _r = ln (2t / d ₀ )
Ψ _θ = ln {2 (dt) / d ₀ }
It is.

  As described above, the inclination angle β is an angle formed by the roll axis center line with respect to the horizontal plane or vertical plane of the pass line, and the crossing angle γ is the roll axis axis line with respect to the vertical plane or horizontal plane of the pass line. It is the angle to be made.

In the above-described method of the present invention, it is desirable that the relationship between the inlet diameter D ₁ and outlet diameter D ₂ of the main roll and the above d ₀ , d and γ satisfies the following expression (6).
D ₂ / D ₁ ≦ (d / d ₀ ) / (1.00−0.027γ) (6)
Further, the effect of the above-described method of the present invention is that the rotary forging effect and additional deformation become remarkable, the piercing-rolling ratio is 4.0 or more, the tube expansion ratio is 1.15 or more, or the hollow piece “thickness / outer diameter ratio” is 6.5 or less. Sufficiently obtained in rolling.

  The ranges of the values of the inclination angle β and the crossing angle γ in the method of the present invention are the same as the ranges in the inventions of Patent Document 1 and Patent Document 2 described above. These ranges were determined from the viewpoint of reducing the rotary forging effect and suppressing the additional shear deformation as much as possible.

The ratio of the logarithmic strain Ψ _{r in the} radial direction and the logarithmic strain Ψ _{θ in} the circumferential direction, that is, the range of “−Ψ _r / Ψ _θ ” is the same as that in the invention of Patent Document 1. This is determined from the principle and principle of how to distribute the amount of reduction in piercing and rolling in the longitudinal direction and circumferential direction. If the principle and principle are not met, flare ringing (peeling phenomenon) and peeling will occur. Or clogging occurs, and piercing and rolling itself stops.

  A major feature of the present invention is that attention is paid to the fact that the roll shape with respect to the billet diameter largely affects the rotary forging effect. Hereinafter, this point will be described.

First, the ratio of the inlet diameter D ₁ and the outlet diameter D ₂ of the cone type roll at the contact limit position between the pipe material and the main roll, that is, the expansion ratio “D ₂ / D ₁ ”, the outer diameter d of the hollow piece the ratio of the billet outside diameter d ₀ and, i.e., the pipe expansion ratio "d / d _0" and relationships cross angle γ of the tubing was investigated from the viewpoint of suppressing the rotary forging effects and the redundant shear deformation.

Prior to the experiment, an index (index) representing the roll shape was selected. Then, it was examined whether or not various possible indexes could be an index indicating the relationship with the rotary forging effect and the additive shear deformation. As a result, the ratio between the tube material expansion ratio “d / d ₀ ” and the cone-type roll expansion ratio “D ₂ / D ₁ ”, that is, (d / d ₀ ) / (D ₂ / D ₁ ) I decided to use it as an index.

Barrel width L ₁ of the input side across the gorge position of the roll shown in FIG. 1, i.e., barrel width ratio of the barrel width L ₂ of the distance and the exit side from the starting point biting roll tube material to the roll gorge "L ₂ / L ₁ ”is also considered as an index, but this is not directly related to the rotary forging effect and the applied shear deformation, and the appropriate range thereof was determined from another viewpoint. In general, an unnecessary extra length is added to the barrel width, and it is difficult to define the barrel width ratio itself.

In general, as the roll crossing angle γ increases, the roll expansion ratio “D ₂ / D ₁ ” increases, resulting in a more conical shape. However, when the barrel width L ₂ on the outlet side is the same, if the comparison is made on the assumption that they have the same roll crossing angle, the larger the tube expansion ratio “d / d ₀ ” of the tube material, the larger the roll expansion ratio “ D ₂ / D ₁ "is inevitably reduced, it is necessary to perform a made roll design" proper in consideration of the d / d ₀ "" D ₂ / D ₁ ", the difficulty of the roll design here It is in.

The roll design must be made from the viewpoint of _{reducing the} rotational forging effect before the plug during piercing rolling and suppressing the additional shear deformation represented by the circumferential shear strain γ _rθ after the plug piercing rolling as much as possible. This is because the embrittlement of the pipe material due to the rotary forging effect is the cause of the internal flaws of the pipe, and the additional shear deformation is the cause of the propagation of the internal flaws.

  The present inventor conducted an experiment of piercing and rolling using a carbon steel billet as a test material using an experimental cross piercing and rolling mill, changing the roll shape, and detailed the influence of the roll shape on the rotary forging effect and the additional shear deformation. It was examined. Experimental conditions are shown in Tables 1 and 2. The thickness t of the hollow piece after piercing and rolling was set so that the “thickness / outer diameter” ratio, that is, (t / d) × 100 was 2.5 to 3%.

An example of the influence of the expansion ratio “D ₂ / D ₁ ” and the expansion ratio “d / d ₀ ” on the rotary forging effect is shown in FIGS. An example of the influence of the expansion ratio “D ₂ / D ₁ ” and the expansion ratio “d / d ₀ ” on the applied shear deformation is shown in FIGS. 3 (a) and 3 (b).

  The effect of the roll shape on the rotary forging effect is that the main roll and disk roll are stopped in the middle of piercing and rolling to create an "intermediate stop material", and the diameter direction (guide direction) perpendicular to the axial direction from the tip position of the plug A plate-like minute tensile test piece having a parallel part of 25 mm and a thickness of 3 mm was collected and subjected to a tensile test at room temperature, and the influence of the roll shape on the drawing value (%) was examined and evaluated. The rotary forging effect appears more clearly in the drawing value (%) than the elongation value (%) in the tensile test.

As the additional shear deformation, attention was paid to the circumferential shear strain γ _rθ , and the measurement was performed by a pin embedding method. That is, a plurality of pins were embedded parallel to the axis along the diameter of the solid billet, and the circumferential shear strain γ _rθ was measured across the hollow piece after piercing and rolling.

As apparent from FIG. 2, for example, when the roll crossing angle γ is fixed, the aperture value decreases as the tube expansion ratio “d / d ₀ ” decreases and as the diameter expansion ratio “D ₂ / D ₁ ” increases. Can be increased. That is, the rotary forging effect can be reduced. In other words, the range of the inclination angle β in which the aperture value of the tube material before plug is larger than the aperture value of the base material becomes wider.
Further, as can be seen from FIG. 3, the circumferential shear strain can be reduced as the tube expansion ratio is smaller and the diameter expansion ratio is larger. That is, additional shear deformation can be suppressed. Therefore, even when the pipe expansion ratio is increased, if the roll crossing angle γ is sufficiently increased so that the diameter expansion ratio is increased and the roll shape is made appropriate, the circumferential shear deformation does not become too large.

Incidentally, if the roll shape is inappropriate, that is, when the roll cross angle is smaller than the pipe expansion ratio, the expansion ratio becomes too small to take the pipe expansion ratio, out of the roll diameter D ₂ is gorge diameter D _g The action of pulling out the pipe material to the outlet side is weakened due to the decrease in the peripheral speed of the outlet roll at the pipe material separation point. Thereby, the slip phenomenon between the roll and the pipe material becomes remarkable. This slip phenomenon is also affected by the billet diameter, the slip becomes larger on the entry side, the rotary forging effect begins to appear with the increase in the number of rotary forging, and the slope angle β at which the tube material before the plug becomes more brittle than the base metal The range expands. The number of rotation forgings is the number of rotations of the billet from when the billet is bitten into the roll until it reaches the plug tip.

Of course, additional shear deformation also appears greatly. If the extreme is when the diameter D ₂ out of the roll approaches the inlet diameter D _1. The additive shear deformation is a general term for circumferential shear strain γ _rθ , surface torsional shear strain γ _θl, and longitudinal shear strain γ _lr .

4 and 5 show the relationship between the tube expansion ratio “d / d ₀ ”, the roll diameter expansion ratio “D ₂ / D ₁ ”, and the roll crossing angle γ. These figures also show the results of the roll shape quality determination. That is, a circle indicates that the roll shape is appropriate, and a circle indicates that it is inappropriate.
Whether the roll shape is appropriate or not needs to be determined by the rotary forging effect. Therefore, whether or not the ductility (drawing value) of the tube material before the plug can be made larger than the drawing value of the base material (billet) was used as a criterion for determination. Then, piercing and rolling was performed with an inclination angle (β) of 12 °, and as described above, a tensile test was performed using a plate-like micro-tensile test piece having a parallel part of 25 mm and a thickness of 3 mm taken from the cross section of the tube material before the plug. A test was conducted to investigate whether or not the aperture value of the tube material before the plug was larger than the aperture value of the base material. The case where it is large is the above-mentioned circle mark, and the case where it is not is the circle mark. From FIG. 4 and FIG. 5, the conditions for an appropriate roll shape are as follows.
(5/6) + (1/3) (d / d ₀ ) ≦ (D ₂ / D ₁ )
1 + 0.03γ ≦ (D ₂ / D ₁ )

If “D ₂ / D ₁ ” is adopted as the roll shape index as described above, the correlation between “D ₂ / D ₁ ”, “d / d ₀ ” and γ becomes clear in the graph, but three variables It becomes difficult to formulate the relationship of In order to avoid this problem, the present inventor has calculated the ratio of the pipe material expansion ratio “d / d ₀ ” and the roll diameter expansion ratio “D ₂ / D ₁ ” as the roll shape index, ie, “(d / d ₀ )”. / (D ₂ / D ₁ ) ”was selected.

FIG. 6 is a diagram showing the relationship between the roll shape index “(d / d ₀ ) / (D ₂ / D ₁ )”, the tube expansion ratio “d / d ₀ ”, and the crossing angle γ. Even if “(d / d ₀ ) / (D ₂ / D ₁ )” is plotted on the vertical axis and “γ” is plotted on the horizontal axis, “d / d ₀ ” remains as a parameter. Can be represented by two inequalities. That is,
(D / d ₀ ) / (D ₂ / D ₁ ) ≦ 0.75 + 0.025γ
, And the than this _{(d / d 0) / (} 0.75 + 0.025γ) ≦ (D 2 / D 1) ··· (5)
It becomes.

Here, in order to overcome the strength of the bearing, the problems in equipment life, etc., without excessively reducing the entry side roll diameter, in order to obtain an optimum roll shape, the roll gorge diameter D _g billet diameter d ₀ If it is 4.5 times or more,
1.00−0.027γ ≦ (d / d ₀ ) / (D ₂ / D ₁ )
Than this,
_{D 2 / D 1 ≦ (d} / d 0) / (1.00-0.027γ) ··· (6)
It becomes. From this equation (6) and the above equation (5),
(d / d ₀ ) / (0.75 + 0.025γ) ≦ (D ₂ / D ₁ ) ≦ (d / d ₀ ) / (1.00−0.027γ) (7)
It is a desirable roll shape condition to satisfy.

In Tables 1 and 2 and the graphs of FIGS. 2 to 6, (a) shows the case where the roll gorge diameter D _g = 400 mm, and (b) shows the case where D _g = 500 mm. Therefore, the comparison between (a) and (b) will discuss the content of the second prior invention disclosed in Patent Document 2. Note that the upper limit of the above inequality (formula (7)) can be easily derived by performing the same calculations as in Tables 1 and 2 with D _g = 315 mm.

In other words, D ₁ and D ₂ are the inlet diameter and outlet diameter of the cone-type main roll, but it is assumed that the pipe material is caught at the inlet face of the main roll and leaves the roll at the outlet face. the diameter of the main roll in position billet is bitten roll is the D _1, the main roll diameter at the position where hollow piece leaves the rolls is D _2.

Finally, the barrel width of the roll will be described. The barrel width L is the sum of L ₁ and L ₂ in FIG. Adding an extra length to the barrel width more than necessary leads to an unnecessarily large overall structure of the rolling mill. Therefore, the entry-side barrel width L ₁ should be determined within the range that does not impair the biting stability, and the exit-side barrel width L ₂ should be determined in consideration of the number of finishing reelings, and the barrel width ratio “L ₂ / L ₁ ”should be within the following range.
1.0 ≦ L ₂ / L ₁ ≦ 2.0

[Example 1]
Using a 60 mm diameter billet of 18% Cr-8% Ni austenitic stainless steel as a test material, high-workability thin-wall piercing rolling with a tube expansion ratio of 1.5 was performed using a guide shoe. The heating temperature of the billet was 1250 ° C. The hot workability of stainless steel is much worse than that of carbon steel.

1. Roll conditions Crossing angle ... γ = 25 °
Gorge diameter ... D _g = 400mm
Inclination angle ... β = 12 °
Inlet diameter ... D ₁ = 240mm
Exit diameter ... D ₂ = 550mm
Roll expansion ratio ... _D 2 / _{D 1} = 2.29
Entry side barrel width ... L ₁ = 300mm
Outer barrel width ... L ₂ = 460mm
Barrel width: L ₁ + L ₂ = 760mm
Barrel width ratio ... L ₂ / L ₁ = 1.53

2. Piercing rolling conditions plug diameter ... d _p = 80 mm
Billet diameter ... d ₀ = 60mm
Hollow shell diameter d = 90mm
Hollow shell thickness ... t = 2.7mm
Pipe expansion ratio ... d / d ₀ = 1.50
Punch rolling ratio ... d ₀ ^{2 /} 4t (dt) ₌ 3.82
“Thickness / outer diameter” ratio (t / d) × 100 = 3.0%
Roll shape index (d / d ₀ ) / (D ₂ / D ₁ ) = 0.655
Logarithmic strain in the thickness direction: Ψ _r = ln (2t / d ₀ ) = ln0.09 = −2.408
Logarithmic strain in the circumferential direction: Ψ _θ = ln {2 (dt) / d ₀ } = ln2.91 = 1.068
Reduction distribution ratio… −Ψ _r / Ψ _θ = 2.255

  As described above, since the rolling distribution ratio in the circumferential direction and the wall thickness direction, that is, the rolling distribution ratio in the longitudinal direction and the circumferential direction was appropriate, piercing and rolling could be performed without causing flaring and peeling. Since the roll shape is also optimized, there was no occurrence of internal flaws or lamination even in the high workability ultra-thin wall piercing and rolling of difficult-to-work materials.

[Example 2]
The hot workability of high alloy steel is still inferior to that of stainless steel, and lamination often occurs when the piercing and rolling temperature exceeds 1275 ° C. Therefore, in this example, a 70% diameter billet of high alloy steel of 25% Cr-35% Ni-3Mo was used as a test material, and a high rollability thin hole drilling with a tube expansion ratio of 2 at a temperature of 1200 ° C. Rolled.

1. Roll conditions Crossing angle ... γ = 30 °
Inclination angle ... β = 12 °
Gorge diameter ... D _g = 500mm
Inlet diameter ... D ₁ = 300mm
Exit diameter ... D ₂ = 670mm
Roll expansion ratio ... _D 2 / _{D 1} = 2.23
Entry side barrel width ... L ₁ = 300mm
Outer barrel width ... L ₂ = 460mm
Barrel width: L ₁ + L ₂ = 760mm
Barrel width ratio ... L ₂ / L ₁ = 1.53

2. Piercing rolling conditions plug diameter ... _d p = 130 mm
Billet diameter ... d ₀ = 70 mm
Hollow shell diameter d = 140mm
Hollow shell thickness t = 3.5mm
Tube expansion ratio: d / d ₀ = 2.00
Punch rolling ratio ... d ₀ ^{2 /} 4t (dt) ₌ 2.56
“Thickness / outer diameter” ratio (t / d) × 100 = 2.5%
Roll shape index (d / d ₀ ) / (D ₂ / D ₁ ) = 0.897
Logarithmic strain in the thickness direction: Ψ _r = ln (2t / d ₀ ) = ln0.10 = −2.303
Logarithmic strain in the circumferential direction: Ψ _θ = ln {2 (dt) / d ₀ } = ln3.90 = 1.361
Reduction distribution ratio… −Ψ _r / Ψ _θ = 1.692

  As described above, the rolling distribution in the circumferential direction and the wall thickness direction is appropriate, and the roll shape is also optimized, so this is a high workability thin wall piercing and rolling of high alloy steel with poor hot workability. However, piercing and rolling was possible without any problems.

  In the piercing and rolling method of the present invention, the relative relationship between the tube material expansion ratio and the cone-type main roll expansion ratio is optimized. Therefore, the rotary forging effect in the piercing and rolling process is suppressed by editing, and it is possible to more reliably suppress internal flaws and lamination that are likely to occur in high-workability thin-wall piercing rolling of difficult-to-work materials such as stainless steel and high alloy steel. . According to the method of the present invention, pipe piercing and rolling up to a pipe expansion ratio of 2.0 is possible.

  As described above, the present inventor has proposed a high cross angle piercing rolling method from the viewpoint of killing the rotary forging effect and suppressing the additive shear deformation, and has made several inventions so far. However, the high crossover angle is a necessary condition for killing the rotary forging effect and suppressing the additive shear deformation, but it is not a sufficient condition. The necessary and sufficient condition is the optimization of the roll shape, and the high crossing angle is a necessary condition for the optimization of the roll shape.

It is a figure which shows the aspect of piercing-rolling. It is a diagram showing the effect of rotary forging effect expansion ratio on the (aperture of micro tensile test) (D ₂ / D ₁₎ and the pipe expansion ratio (d / d _p). It is a diagram showing the effect of the redundant shear strain expansion ratio on the (circumferential shear strain) (D ₂ / D ₁₎ and the pipe expansion ratio (d / d _p). Expansion ratio (D ₂ / D _1), is a diagram showing the relationship between the pipe expansion ratio (d / d _p) and roll inclination angle (gamma). Expansion ratio (D ₂ / D _1), is a diagram showing the relationship between the pipe expansion ratio (d / d _p) and roll cross angle (gamma). Roll shape index, i.e., a diagram showing the relationship between _{(d / d 0) / (} D 2 / D 1) and the roll cross angle (gamma).

Explanation of symbols

γ: roll crossing angle D ₁ : roll inlet diameter D ₂ : roll outlet diameter D _g : roll gorge diameter L ₁ : roll inlet barrel width L ₂ : roll outlet barrel width d ₀ : billet outer diameter d: hollow piece Outside diameter t: Thickness of hollow piece

Claims (3)

Maintaining the inclination angle β and crossing angle γ of the cone-type main roll supported at both ends across the pass line in the range satisfying the following formulas (1) to (3) The relationship between the outer diameter d ₀ of the actual billet, the outer diameter d of the hollow piece after piercing and rolling, and the wall thickness t is set so as to satisfy the following formula (4). Further, the inlet diameter D ₁ and the outlet diameter D _{2 of the} main roll A piercing-rolling method in seamless pipe production, wherein the above d ₀ , d and γ satisfy the following expression (5).
8 ° ≦ β ≦ 20 ° (1)
5 ° ≦ γ ≦ 35 ° (2)
15 ° ≦ β + γ ≦ 50 ° (3)
1.5 ≦ −Ψ _r / Ψ _θ ≦ 4.5 (4)
(D / d ₀ ) / (0.75 + 0.025γ) ≦ D ₂ / D ₁ (5)
However, in Formula (4), Ψ _r = ln (2t / d ₀ )
Ψ _θ = ln {2 (dt) / d ₀ }
It is. _2. The piercing and rolling method according to claim 1, wherein the relationship between the inlet diameter D ₁ and the outlet diameter D _{2 of the} main roll and the above d ₀ , d, and γ satisfies the following expression (6).
D ₂ / D ₁ ≦ (d / d ₀ ) / (1.00−0.027γ) (6)   The piercing and rolling method according to claim 1 or 2, wherein the piercing and rolling ratio is 4.0 or more, the expansion ratio is 1.15 or more, or the "thickness / outer diameter ratio" of the hollow piece is 6.5 or less.

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