JPH0857506A - Mandrel mill - Google Patents

Abstract

PURPOSE: To prevent hole defects generating in the flange side parts of the grooved rolls of even-number stands at the time of the reduction and the thinning of a tube stock by a mandrel mill. CONSTITUTION: The groove shape of a roll of odd-number roll stands is made a shape satisfying that the ratio of a distance S between the center RO of the radius R1 of curvature of the groove bottom part of the roll caliber and a pass center P and the radius R1 of curvature is <=0.1, or/and the roll caliber shape of even-number roll stand is made a shape satisfying that the peripheral length of the caliber is 0.97 times of the peripheral length of the roll caliber of the odd number place roll stand or less in a just preceding stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mandrel mill for continuously reducing the diameter and reducing the wall thickness of a raw pipe having a mandrel bar inserted in a seamless pipe manufacturing process.

[0002]

2. Description of the Related Art As a continuous rolling machine of a seamless pipe, a roll stand having a pair of upper and lower hole rolls is used for 5 to 8 so that the roll shaft centers of the adjacent roll stands are alternately phased by 90 °. There is a mandrel mill for continuously reducing the wall thickness and reducing the diameter by continuously passing a raw pipe into which a mandrel bar is inserted in a roll hole type array of roll stand groups arranged in a base tandem.

[0003] When rolling a shell with a mandrel mill, a flaw called a "piercing defect" that penetrates in the wall thickness direction of the shell may occur during the rolling. This "piercing defect"
Is a steel type having poor hot workability such as high alloy steel and stainless steel, and is often generated when rolling a thin-walled pipe having a wall thickness of 10 mm or less.

There are various mechanisms for the occurrence of "perforated defects", which are explained as follows, for example.

FIG. 6 is a schematic view showing a deformed state of a raw pipe during rolling in a roll stand for applying a wall thickness reduction of a mandrel mill. The raw pipe 1 is a roll roll 2A, 2B having an elliptical hole shape. Hole-shaped groove bottom portion of the hole-type roll restrained by the mandrel bar 3 (hereinafter, simply referred to as groove bottom portion)
In the region I, which is thinned, reduced in diameter and stretched in the axial direction of the pipe,
Along with this, the hole type rolls 2A and 2B and the mandrel bar 3
The tension in the direction of the pipe axis also acts on the region II, which is the hole type flange portion (hereinafter simply referred to as the flange portion) of the hole type roll that is not constrained by Thinned and stretched.

At this time, since the stress state of the tension acting on the base pipe 1 in the portion corresponding to the flange portion of each roll stand is close to the uniaxial tension, when the deformability of the base pipe 1 is small, It is said that the tension causes a "perforated defect" penetrating in the thickness direction in the portion of the raw pipe 1 corresponding to the flange portion.

As a method for preventing this "drilling defect", Japanese Patent Laid-Open No. 63-154205 discloses that a diameter reduction rate of a test piece made of the same material as the pipe material to be rolled is 40% by a high temperature tensile test prior to rolling. A method has been proposed in which a temperature range in which the temperature is at least% is obtained and rolling is performed in this temperature range.

Further, in Japanese Unexamined Patent Publication No. 5-50115, unlike the above-mentioned mechanism, "a hole is formed in a blank pipe portion corresponding to a central portion of the groove bottom portion of each roll stand having the maximum rolling reduction. "Defect" occurs, and the hole shape of the hole type roll of the first roll stand is set such that the groove bottom radius of the groove bottom portion is 0.54 times or less than the interval between the roll groove bottoms, and / or There is also proposed a method in which the hole type roll of the second roll stand is used, in which the groove bottom radius of curvature of the groove bottom portion is set to 0.52 times or less the gap between the roll groove bottoms.

The workability (area reduction rate) during tube rolling in the mandrel mill depends on the finished wall thickness.
That is, in mandrel mill rolling, outer diameter Di x wall thickness T
When finishing from a pipe of i to a pipe of outer diameter Do x wall thickness To, a mandrel bar with outer diameter Dm of (Do -2 x To) is used to set the wall thickness reduction amount (Ti-To) to a constant value. By rolling, different wall thicknesses To1 and T from the same outer diameter element pipe Di
It is general to adopt a so-called same outer diameter setup rolling method in which a tube having o2, ... And having the same outer diameter Do is rolled, and in this case, the finished pipes have finished wall thicknesses To1, To2 ,. The thicknesses Ti1 and Ti at which the thickness reduction amount becomes a constant value according to
2, ..., and the mandrel bar has an outer diameter Dm
Of (Do −2 × To1), (Do −2 × To2) ...

Therefore, when rolling a pipe having a thin finish wall, the workability becomes high, and conversely, when rolling a pipe having a thick finish wall, the workability becomes low, so that the finish meat of the pipe is finished. The workability differs depending on the thickness To.

Further, the deformability of the work material required to prevent the "drilling defect" that occurs during pipe rolling in the mandrel mill differs depending on the size of the workability, and as the workability increases, It is required to have high deformability.

Further, the maximum workability in the same outer diameter setup rolling method is usually the thin-wall piercing rolling limit of the piercing and rolling machine for producing the shell, that is, the same outer diameter raw tube which can be pierced by the piercing and rolling machine. Since it is determined from the relationship between the minimum wall thickness and the minimum wall thickness to be finished by mandrel mill rolling, "hole defect"
In order to reduce the above-mentioned maximum workability for the purpose of preventing the occurrence of the above, it is considered that the piercing and rolling machine pierces and rolls into a further thin-walled pipe.

In this measure, it is necessary to change the rotational speed control pattern of the hole type roll of each roll stand of the mandrel mill with the change of the wall thickness reduction which is a constant value, and the control becomes complicated. In addition to that, an excessive load is applied to the piercing and rolling machine to cause a failure and stoppage, and an extremely short life of the piercing plug and frequent defects such as seizure flaws are caused, which is difficult to employ.

Although it is conceivable to increase the power of the roll driving motor of the piercing and rolling mill, it is uneconomical because a large amount of cost is required.

In the method disclosed in Japanese Patent Laid-Open No. 63-154205, since the influence of the workability, which changes depending on the finished wall thickness, is not taken into consideration at all, a "drilling defect" that occurs during high workability rolling. However, there is a problem in that it cannot be reliably prevented.

In the method disclosed in Japanese Unexamined Patent Publication No. 5-50115, according to the results of experiments and research conducted by the inventor of the present application, a necking phenomenon occurs at the center of the groove bottom portion of a hole-type roll, which causes The fact that "defects" occurred was not found.

[0017]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and does not require the provision of manufacturing a thin-walled raw tube by a piercing and rolling machine, and is a "drilling defect" even in high-workability rolling. An object of the present invention is to provide a new mandrel mill capable of reliably preventing the occurrence of the above.

[0018]

The summary of the present invention is as follows.
It is in (1)-(3).

(1) A tube is continuously rolled by a plurality of roll stands having a pair of upper and lower hole type rolls and a mandrel bar arranged in a roll hole type array formed by the hole type rolls of the roll stand group. It is a mandrel mill and the hole type of the odd number roll stand
The ratio (S / R1) of the distance S between the center Ro of the groove bottom curvature radius R1 and the path center P and the groove bottom curvature radius R1 is
A mandrel mill characterized by having a hole shape of 0.1 or less.

(2) A tube is continuously rolled by a plurality of roll stands having a pair of upper and lower hole type rolls and a mandrel bar arranged in a roll hole type array formed by the hole type rolls of the roll stand group. It is a mandrel mill and the hole type of the even type roll stand
The ratio (Li / Li-1) of the hole circumference L i to the hole circumference L i-1 of the hole roll of the odd-numbered roll stand in the immediately preceding stage.
) Has a hole shape of 0.97 or less.

(3) A tube is continuously rolled by a plurality of roll stands having a pair of upper and lower hole type rolls and a mandrel bar arranged in a roll hole type array formed by the hole type rolls of the roll stand group. It is a mandrel mill and the hole type of the odd number roll stand
The ratio (S / R1) of the distance S between the center Ro of the groove bottom curvature radius R1 and the path center P and the groove bottom curvature radius R1 is
In addition to the hole type of 0.1 or less, the hole type of the even type roll stand is the hole type circumferential length Li and the hole type circumferential length L of the odd number roll stand of the immediately preceding roll stand.
A mandrel mill having a hole shape with a ratio (Li / Li-1) to i-1 of 0.97 or less.

As a result of investigating the cause of the occurrence of the "drilling defect" in the mandrel mill, the present inventor newly found the mechanism described below, which is different from the conventional mechanism,
The invention was made.

That is, in the deformed state of the raw pipe in the mandrel mill rolling, since the roll rotation axes of the hole type rolls of the front and rear roll stands are alternately phased by 90 ° and arranged in tandem, the odd roll stand and the even roll are arranged. It is the fact that, unlike the stands, the "perforated defects" occur during rolling on even roll stands and not during rolling on odd roll stands.

The mechanism will be described below.

In the odd-numbered roll stand, since the inlet side wall thicknesses of the shell corresponding to the groove bottom portion and the flange portion of the hole type roll are almost the same, that is, the wall thickness in the circumferential direction is almost uniform, the hole type roll of Only the wall thickness of the portion corresponding to the groove bottom portion is mainly reduced. For this reason, as for the exit side wall thickness of the raw pipe, the portion corresponding to the groove bottom portion of the hole type roll is thin and the portion corresponding to the flange portion is rolled to be thick.

On the other hand, in the even roll stand, as described above, the rotation axis of the odd roll stand is 90 degrees.
° Since the hole rolls are arranged in phase with each other, the wall thickness on the inlet side of the raw pipe is thick at the part corresponding to the groove bottom part of the hole roll and thin at the part corresponding to the flange part. Only the wall thickness of the portion corresponding to the groove bottom portion is mainly reduced. For this reason, the outlet side wall thickness of the raw pipe is rolled so that the groove bottom portion of the hole type roll and the portion corresponding to the flange portion have substantially the same wall thickness.

Here, paying attention to the tension acting on the raw pipe portion corresponding to the flange portion of the hole type roll, in the odd roll stand, only the raw pipe portion corresponding to the groove bottom portion of the hole type roll is mainly stretched. As a result, tension also acts on the base pipe part corresponding to the flange part, but since the wall thickness of the inlet side base pipe corresponding to the flange part is thicker than the wall thickness corresponding to the groove bottom part, a large tensile force is applied. No stress. On the other hand, in the even-numbered roll stand, since the wall thickness of the inlet side pipe corresponding to the flange of the hole type roll is thinner than the wall thickness corresponding to the groove bottom, the tensile stress applied to the pipe corresponding to the flange Is excessive and causes a large amount of metal loss, and when this metal loss is extreme, a necking phenomenon occurs.

Therefore, the occurrence of the "perforated defect" due to the necking phenomenon does not occur in the odd roll stand, but occurs in the portion corresponding to the flange portion of the hole roll of the even roll stand.

As a result of various experimental studies, the inventor of the present invention has prevented the "perforation defect" caused by the necking phenomenon occurring in the raw pipe portion corresponding to the flange portion of the hole type roll of the even roll stand. In order to achieve this, it was found that it is effective to adopt the following two means.

That is, the first method is such that even if the same tension as in the conventional case is applied to the raw pipe portion corresponding to the roll hole type flange portion of the even roll stand, it is difficult to reduce the wall thickness of the raw pipe portion. The second method is to reduce the tension in the pipe axis length direction that acts on the raw pipe portion corresponding to the roll hole type flange portion of the even roll stand.

[0031]

The present invention will be described in more detail with reference to the accompanying drawings.

First, the first method, that is, even if the same tension as in the conventional case is applied to the raw pipe portion corresponding to the roll hole type flange portion of the even roll stand, it is difficult to reduce the wall thickness of the raw pipe portion. The method of setting is explained.

FIG. 5 shows a first example of an odd roll stand that uses a conventional mandrel mill to form a shell having a uniform wall thickness in the circumferential direction.
After rolling with a roll stand and then rolling with a second stand, which is an even roll stand, when the "hole hole defect" occurs in the blank pipe part corresponding to the roll hole type flange part of the even roll stand FIG. 3 is a diagram showing an example of the inlet-side wall thickness distribution and the outlet-side wall thickness distribution in the circumferential direction about 1/2 of the circumference, with the vertical axis indicating the wall thickness and the horizontal axis indicating the hole-type circumferential position of the hole-type roll. In the figure, the 0 ° position on the horizontal axis is the center of the groove bottom of the hole-type roll, the range of 60 ° to −60 ° is the groove bottom portion (corresponding to the region I in FIG. 6), 60 ° to 90 ° and −. The range of 60 ° to −90 ° corresponds to the flange portion (corresponding to the area II in FIG. 6).

As is apparent from FIG. 5, in the even number roll stand of the conventional mandrel mill, the wall thickness of the portion of the hole type roll corresponding to the groove bottom portion (hereinafter, simply referred to as the region I) is greatly reduced. However, along with this, the wall thickness of the portion corresponding to the flange portion (hereinafter, simply referred to as region II) is also reduced. Then, paying attention to the wall thickness reduction rate of the portion corresponding to the region II, the wall thickness reduction rate at the circumferential position is 90%.
It is maximum at ° and −90 °, becomes smaller as it deviates from this position, and becomes smallest at positions at 60 ° and −60 °. II
It was found that as a result of concentrated occurrence in the end portion on the side opposite to the inner region I, a necking phenomenon occurred in this portion and a "perforated defect" occurred.

To clarify the reason why this phenomenon occurs, the present inventor conducted the following experiment.

That is, the influence of the wall thickness distribution on the inlet side of the raw pipe on the wall thickness reduction rate in the portion corresponding to the area II of the hole type roll of the even roll stand was investigated. The results are shown in FIGS. 2 and 3.

2 and 3, the wall thickness distribution in the circumferential direction of the raw pipe before and after rolling on the second roll stand, which is an even roll stand, is plotted along the vertical axis for 1/4 of the thickness distribution, as in FIG. FIG. 2 is a diagram showing the wall thickness by taking the hole type circumferential position on the horizontal axis, and FIG. In the case of a non-uniform wall thickness distribution that becomes thin toward the end side, it corresponds to the partially enlarged view of FIG. 5, and FIG. 3 shows that the inlet side wall thickness of the raw pipe is the groove bottom of the region I of the hole type roll. It is a figure which respectively shows the case of a substantially equal uniform distribution toward the end side of the area | region II opposite to the area | region II.

As can be seen from FIG. 2 and FIG. 3, when the wall thickness on the inlet side of the raw tube in the portion corresponding to the area II of the hole-type roll has a substantially uniform distribution in the circumferential direction, the wall thinning rate of the raw tube Is almost uniform in the circumferential direction as compared with the case where the wall thickness on the inlet side of the element pipe is non-uniform, and it is 90 ° corresponding to the end of the region II opposite to the region I.
The position is small, and in the case of FIG. 2, the “perforated defect” occurred, but in the case of FIG. 3, the occurrence of the “perforated defect” was not recognized.

From the results of this experiment, if the wall thickness distribution on the inlet side of the plain tube in the portion corresponding to the area II of the hole type roll of the even roll stand is made substantially uniform in the circumferential direction, the "perforated defect" is generated. Therefore, the hole shape of the odd-numbered roll stand can be prevented so that the wall thickness of the hollow pipe of the odd-numbered roll stand, which is the immediately preceding stand, is rolled in the circumferential direction in a substantially uniform manner. It can be seen that it is sufficient to determine the pore shape of the roll.

FIG. 1 shows an example of an odd roll stand, specifically, a hole roll of a first roll stand.
Since it is vertically symmetrical, only the upper half is shown. In the figure, 1 is a blank tube, 2 is a hole type roll, 3 is a mandrel bar, R1 is the radius of curvature of the groove bottom part, and R2 is the curvature of the flange part. Radius, R3 is the radius of curvature of the flange clearance, R4
Is the radius of curvature of the virtual tube outside of the roll gap, Ro is the center of curvature of R1, P is the path center, Mr is the mandrel bar radius, θ1 is the forming half angle of R1, θ2 is the forming angle of R2, and θ3 is Forming angle at R3, θ4 is forming angle at R4,
S is a distance between the path center P and the center Ro.

As shown in FIG. 1, generally, in the hole type of the hole type roll of the odd number roll stand, with the center Mo of the mandrel bar 3 aligned with the path center P, the wall thickness at the center of the groove bottom has a predetermined value. So that the center of curvature Ro of the radius of curvature R1 is set at a position separated from the path center P by the separation distance S, and the groove bottom portion is formed.

Therefore, if the distance S is increased,
Since it is necessary to set the wall thickness at the center of the groove bottom to a predetermined value, the radius of curvature R
Since 1 also has to be increased, the distance between the outer surface of the mandrel bar 3 and the hole type of the hole type roll 2 increases from the center of the groove bottom toward the flange end side, and the wall thickness of the raw tube after rolling is reduced. However, the ratio of the separation distance S to the radius of curvature R1 (S / R1) is 0.1 or less, and when it is formed into a hole shape having a groove bottom portion with a radius of curvature R1, the material after rolling is It was found that the wall thickness of the pipe can be made almost uniform in the circumferential direction.

Incidentally, the hole type roll of the conventional odd number roll stand is formed so that the hole type has a groove bottom portion having a radius of curvature R1 at which the (S / R1) value exceeds 0.1.

Next, the second method will be described.

This second method is, as described above, the flange portion (region II) of the hole roll of the even roll stand.
Is a method of reducing the tension in the axial direction of the tube that acts on the base tube portion corresponding to.

First, when rolling with an even roll stand,
In order to reduce the tension in the tube axis length direction that acts on the raw tube portion corresponding to the region II of the hole-type roll, for example, the wall thickness reduction amount in the even roll stand is reduced to reduce the workability. However, if the degree of machining is reduced, the total degree of machining on all roll stands will not reach the prescribed degree of machining, or at the part corresponding to the groove bottom part (region I) of the hole roll of the even roll stand. Since the wall thickness reduction amount of 1 does not reach a predetermined value, it may not be possible to manufacture a thin-walled raw pipe, which cannot be adopted.
Further, in the region II of the hole-type roll, since the raw pipe is not in contact with the hole-type roll and the mandrel bar, it is not possible to control the stress distribution in the thickness direction.

Therefore, as a result of various studies on the method of reducing the tension in the axial direction of the tube corresponding to the region II of the hole-type roll with the same degree of processing as the conventional method, the compressive stress was applied in the circumferential direction. We have found that giving is effective.

FIG. 4 shows the same degree of peripheral workability as that of the conventional method when rolling a raw pipe with an even number roll stand having a hole roll having an elliptical hole shape, specifically, a second roll stand. It is a figure showing the thinning behavior of the raw pipe portion corresponding to the area II of the hole-type roll in the case where is changed, and the horizontal axis shows the peripheral length working ratio (%) obtained by the following equation, and the vertical axis shows the following equation. The thickness reduction rate (%) calculated in step 1 is taken and shown.

[(Pre-rolling circumference-post-rolling circumference) / pre-rolling circumference] × 100 ・ ・ ・ ・ ・ [(wall thickness before rolling-wall thickness after rolling) / wall thickness before rolling] × 100 As is clear from FIG. 4, it is understood that the greater the peripheral length working degree is, the smaller the wall thinning rate of the raw pipe portion corresponding to the region II of the hole type roll is. This is because as the degree of peripheral length processing increases, peripheral length processing in the pipe circumferential direction is also applied in the region II of the hole-shaped roll to cause elongation in the pipe axis length direction. The tension becomes smaller.

Then, when the peripheral length working ratio is 3% or more, that is, when the above-mentioned (Li / Li-1) value is 0.97 or less, when the roll is an even roll stand, the roll of the hole roll is It was found that the necking phenomenon that occurs in the blank tube portion corresponding to the region II can be eliminated and the "perforated defect" does not occur.

The hole type of the hole type roll of the conventional even number roll stand had a hole type peripheral length with the above-mentioned (Li / Li-1) value of more than 0.97.

As a concrete means for making the hole type of the hole type roll of the even number roll stand such that the (Li / Li-1) value is 0.97 or less, the center of the groove bottom of the hole type roll is used. In the state where the distance from the outer surface of the mandrel bar at, that is, the wall thickness after rolling is maintained at a predetermined value, the radius of curvature of each part is changed to change the hole shape of the hole roll of the even roll stand. Or / and changing the hole shape of the hole roll of the odd roll stand immediately preceding the even roll stand.

Further, the perforation circumference Li of each of the perforation rolls is the perforation specifications R1, R2, R4, θ1 and θ2 shown in FIG.
And θ4 can be used to obtain the following equation.

Li = 4 (R1..theta.1 + R2..theta.2 + R4..theta.4) ... As detailed above, according to the present invention, in the hole type of the hole type roll in the mandrel mill, The hole type of the odd number roll stand has a groove type having a groove bottom portion whose groove radius of curvature R1 is 0.1 or less of the separation distance S between the curvature center Ro and the path center P. Since the inlet side wall thickness distribution in the circumferential direction of the blank pipe rolled at the flange part of the hole roll of the roll stand becomes almost uniform, excessive local thinning phenomenon does not occur and "hole defect" occurs. Can be prevented. In addition, since the hole circumference of the hole roll of the even roll stand is set to 0.97 times or less of the hole circumference of the hole roll of the odd roll stand in the immediately preceding stage, the hole circumference of the even roll stand is not increased. Since it is possible to apply a compressive strain in the circumferential direction to the pipe, and the thin wall thickness at the flange portion of the hole-type roll does not become excessive, the "hole drilling" that occurs at the raw pipe portion corresponding to this flange portion It is possible to prevent the occurrence of "defects".

The present invention can obtain the same effect when the above-mentioned first method and second method are carried out individually or in combination. In addition, the first and second methods apply a thickness reduction substantially in the roll stands constituting the mill, except for the perfect circular rolling and the final 2-3 stands that loosen the close contact between the tube and the mandrel bar. It goes without saying that it can be applied to all stands.

The effects of the present invention will be described below with reference to examples.

[0057]

EXAMPLE In order to confirm the effect of the present invention, the following rolling test was conducted using an actual seven-stand mandrel mill. In addition, since the latter three stands do not substantially generate a "perforated defect" in the case where the thickness reduction is not applied, the method of the present invention was applied to the first to fourth stands.

Table 1 shows the dimensional specifications and rolling conditions of the conventional holed roll, and Table 2 shows the test conditions of the present invention together with the results thereof.

The method of the present invention is described in claim 1.
In order to confirm the effect of the first invention, the first stand and the third stand
In the present invention, the radius of curvature R1 of the groove bottom of the groove bottom portion of one or both of the stands is changed so that the ratio between the center of curvature Ro and the distance S between the path centers P is 0.05 and 0.1. The example (test number: 2 to 6) and the conventional example (test number: 1) in which none of the stands were changed were rolled.

In order to confirm the effect of the second invention of claim 2, the hole type peripheral length of the even-numbered stand or the immediately preceding stand is changed so that either one or both of the second stand and the fourth stand. In the present invention example (test number: 8, 10, 12, 14, 15) in which the hole circumference ratio to the hole roll of the immediately preceding odd-numbered stand is 0.97, and the circumference ratio is 0.
Comparative example changed to 99 (Test numbers: 7, 9, 1
1, 13) and rolled.

Further, in order to confirm the effect of the third invention of claim 3, the hole types of the first and third stands are changed so that the S / R1 value is 0.10. The hole-type circumference ratio for each of the previous stands is 0.97
Rolling was performed using the example of the present invention (test number: 16) that was changed to

[0062]

[Table 1]

[0063]

[Table 2]

As is clear from the results shown in Table 2, in the case of the method of the present invention, no "perforated defect" is observed in the applied even roll stand. On the other hand, in the even number roll stands of the conventional example and the comparative example, which are out of the conditions of the present invention, "perforated defects" occur.

Although the description of the results is omitted, when the same rolling test was conducted for other materials and dimensions,
Similar results were obtained.

[0066]

According to the present invention, by changing the hole shape of the hole type roll of the odd number roll stand and / or the even number roll stand, the "hole drilling" generated in the flange portion of the hole type roll of the even number roll stand. It is possible to reliably prevent "defects".

[Brief description of drawings]

FIG. 1 is a view showing a hole type roll of an even number roll stand of the present invention.

FIG. 2 is a diagram showing an example of an inlet-side wall thickness distribution and an outlet-side wall thickness distribution in a second roll stand of a raw pipe having a substantially uniform circumferential wall thickness corresponding to a flange portion of a hole-type roll.

FIG. 3 is a diagram showing an example of an inlet-side wall thickness distribution and an outlet-side wall thickness distribution in a second roll stand of a raw pipe having a non-uniform circumferential wall thickness corresponding to a flange portion of a hole-type roll.

FIG. 4 is a diagram showing a relationship between a peripheral length working degree in a second roll stand and a wall thinning rate of a shell corresponding to a flange portion of a hole-type roll.

FIG. 5 is a diagram showing an example of an inlet-side wall thickness distribution and an outlet-side wall thickness distribution in a circumferential direction of a raw pipe in a conventional second roll stand.

FIG. 6 is a schematic view showing a rolled state in a mandrel mill.

[Explanation of symbols]

 1 Element tube 2 Hole type roll 3 Mandrel bar

Claims (3)

[Claims] 1. A pipe is continuously rolled by a plurality of roll stands having a pair of upper and lower hole rolls and a mandrel bar arranged in a roll hole array formed by the hole rolls of the roll stand group. It is a mandrel mill and the hole type of the odd number roll stand
The center of curvature Ro of the groove bottom radius of curvature R1 and the path center P
The ratio (S / R1
) Has a hole shape of 0.1 or less. 2. A pipe is continuously rolled by a plurality of roll stands having a pair of upper and lower hole rolls, and a mandrel bar arranged in a roll hole array formed by the hole rolls of the roll stand group. It is a mandrel mill and the hole type of the even type roll stand
The ratio (Li / Li-1) of the hole circumference L i to the hole circumference L i-1 of the hole roll of the odd-numbered roll stand in the immediately preceding stage.
) Has a hole shape of 0.97 or less. 3. A tube is continuously rolled by a plurality of roll stands having a pair of upper and lower hole rolls and a mandrel bar arranged in a roll hole array formed by the hole rolls of the roll stand group. It is a mandrel mill and the hole type of the odd number roll stand
The ratio (S / R1) of the separation distance S between the center Ro of the groove bottom curvature radius R1 and the path center P and the groove bottom curvature radius R1 is
In addition to the hole type of 0.1 or less, the hole type of the even type roll stand is the hole type circumferential length Li and the hole type circumferential length L of the odd number roll stand of the immediately preceding roll stand.
A mandrel mill having a hole shape with a ratio (Li / Li-1) to i-1 of 0.97 or less.